The dark night goes quietly

The dark night goes quietly

Fight light pollution with the International Dark Sky Association.

Matt Quandt

Bad lighting hurts everyone. The loss of the dark star-filled sky is of tragic consequences for the environment and for the human soul, akin to the loss of our forested landscapes and other natural treasures.

This view of North and Central America at night is part of a nightly world view constructed by the Defense Meteorological Satellites Program (DMSP). C. Mayhew & R. Simmon (NASA / GSFC) / NOAA / NGDC / DMSP Digital Archive [View Larger Image] As you can tell by this excerpt from their Lighting Code Handbook, the International Dark Sky Association (IDA) takes their responsibility of preserving the darkness of the night sky very seriously. And for good reason. According to a popular 2001 study by Pierantonio Cinzano and Fabio Falchi of Italy's University of Padua, two-thirds of the world's population is blind to the night sky — with stars vanishing from sight at an endangered species-type pace. Obviously, myriad differences between the two issues exist, and the preservation of life should remain at the forefront of civilization's collective attention, but, like any natural resource, the majesty of the night sky need not be compromised by industry or commerce. So what can be done to preserve the fading jewels of our night sky? Since 1988, the IDA — a non-profit organization — has dedicated itself to providing pragmatic answers to that question. It has lobbied state and local government for an increased awareness to the consequences of light pollution. Although founded and primarily managed by astronomers, the IDA does not base its activism on an observational astronomer's point of view. Rather, it emphasizes the gratuitous amount of energy — and money — wasted due to reckless lighting practices. A simple glance at a car dealership, highway-side billboard, or a corporate building at night will demonstrate the light-spewing the IDA is trying to clean up. Without question, lights help us feel secure. Whether in our houses, our cars, or on our sidewalks, we bask in the protective glow of lights. The IDA does not seek to eliminate such useful and necessary forms of lighting. Instead, it just hopes to modify the current excessive lighting practices. Following through with such efforts can conserve energy, reduce harmful glare on the road, and of course, allow for a purer view of the night sky. The IDA, though, is trying to convince local and state governments to light intelligently and, coincidentally, economically. Arizona, Connecticut, Maine, New Mexico, and Texas already have enacted anti-light-pollution laws. The IDA's web site, www.darksky.org, provides material to help you in all levels of commitment to their cause. Darksky.org can help you find a local affiliate; it provides suggestions on how to begin local campaigns; it provides examples of glare-reduction lighting; and much more. For more information on light pollution, see "Reclaim the night sky," by Astronomy Senior Editor Michael E. Bakich, which appeared in the June 2004 issue of Astronomy.

Comets

Comets

Snowballs from the solar system's edge create one of grandest spectacles visible to the naked eye.

Francis Reddy

Comets are dark, solid bodies a few kilometers across that orbit the Sun in eccentric paths. Comets can be described as "dirty snowballs" containing a mixture of dust and frozen gases. Some of the icy material — perhaps less than 1 percent — evaporates as the comet nears the Sun, creating an envelope of gas and dust that enshrouds the solid body. This envelope, called the coma, may be up to 620,000 miles (1,000,000 kilometers) across. Swept back by the solar wind and the radiation pressure of sunlight, this material forms the comet's tail. Comet tails can span a distance greater than that separating the Earth from the Sun. That such a small amount of material could create visible features so large has led some to describe comets as "the closest thing to nothing anything can be and still be something." To the naked eye, the coma of a bright comet looks star-like, a tiny ball of light set within a milky glow. The comet's tail or tails fan out from the coma. If present, a broad dust tail may be the most striking visual feature. The glowing gas tail is straighter, narrower and often fainter than the dust tail. Within the coma, and invisible to both the naked eye and the most powerful telescopes, lies the small icy body responsible for this grand apparition — the comet's nucleus.

Bushy stars The ancient Chinese names for comets reflect their visual appearance. A comet with a prominent tail was called a "broom star" (huixing), while one with no obvious tail was a "bushy star" (poxing). Until the mid-1400s, the Chinese made the most detailed and complete observations of comets. As early as 200 b.c., they employed official skywatchers to record and interpret any new omens in the heavens. These officials recognized, some nine centuries before their European counterparts, that comet tails always point away from the Sun. The Chinese interest in comets, however, was for their astrological importance as signs of coming change. The Greeks likewise recognized a comet with an extended tail as a "bearded star" (aster pogonias) and one without a tail as a "long-haired star" (aster kometes), from which our modern word derives. Aristotle regarded them as a fiery atmospheric phenomenon, to be lumped together with meteors and the aurora. They could not be planets, he reasoned, because comets can appear far from the ecliptic. He thought of comets as being whipped up by the motion of the Sun and stars around the Earth. Their appearance was a warning of coming droughts and high winds. As these ideas were extended in the Middle Ages, comets became viewed less as a portent of disaster than as a cause. They were viewed as a fiery corruption of the air, pockets of hot contaminated vapor that could bring earthquakes, disease, and famine.

Shown in this 1910 photo of Comet Halley are the comet's head and the beginning of its long tail. NOAO [View Larger Image] Some of these ideas were questioned seriously when the great comet of 1577 attracted the attention of Danish observer Tycho Brahe. He could see no reason why comet tails should always point away from the Sun if they were products of the weather. He measured the position of the comet with respect to the stars at different times during the night in an effort to find its parallax — a clue to the object's true distance from Earth. His observations, which indicated that the comet lay beyond the Moon but not as far off as Venus, helped invigorate the scientific study of comets. More than a century later, Isaac Newton showed that comets obeyed Johannes Kepler's laws of planetary motion and concluded "comets are a sort of planet revolved in very eccentric orbits around the Sun." Future observations of the comet of 1682 would eventually remove any lingering doubts. Newton's friend Edmond Halley began collecting accurate cometary observations in 1695 to compare the orbits of many comets. Halley noticed that several comet orbits seemed similar and shared roughly the same period, between 75 and 76 years. "Many considerations incline me to believe the comet of 1531 observed by Apianus to have been the same as that described by Kepler … in 1607 and which I again observed in 1682," Halley wrote. "Whence I would venture confidently to predict its return, namely in the year 1758. And if this occurs, there will be no further cause for doubt that the other comets ought to return also." Halley's confidence proved well founded — the first comet ever predicted to return was again spotted on December 25, 1758. It has been known as Halley's Comet ever since.

Naming comets Comets are more commonly named for their discoverers; up to three independent co-discoverers may share the credit. Increasingly, those discoverers are not individuals, but dedicated small-body discovery programs or solar-observing satellites. Numerous comets have been named for the Lincoln Near Earth Asteroid Research (LINEAR) project of the Massachusetts Institute of Technology in Boston, the Near Earth Asteroid Tracking (NEAT) program operated by the Jet Propulsion Laboratory in Pasadena, California, and the Lowell Observatory Near-Earth Object Search (LONEOS) run by Lowell Observatory in Flagstaff, Arizona. The pace of comet discovery has more than doubled in recent decades, up from an average of about a dozen per year in the late 1980s to about 30 per year in this century's opening years. The Sun-monitoring Solar and Heliospheric Observatory (SOHO) satellite has found 850 comets so far. This tally increases by an average of 80 per year, making SOHO history's most prolific, if unintended, comet discoverer.

Accurate polar-alignment and a short-focus wide-angle lens may allow piggyback exposures of up to an hour. David Healy [View Larger Image] Because the names of discoverers don't allow for a unique identification, comets receive a more prosaic official name. This consists of a one-letter prefix, usually a C for "comet" or a P for "periodic," followed by the year of discovery and an uppercase letter that indicates the half-month in which the discovery occurred. For example, an A represents January 1 though 15, B is January 16 through 31, and so on. (The letter I isn't used to avoid confusion with earlier nomenclature that used Roman numerals, and the letter Z isn't necessary.) After this letter comes a number that represents the order of discovery during the half-month. Halley's Comet, which was the first comet discovered or recovered in the second half of October 1982, therefore receives the designation P/1982U1. When the return of a comet is well established, either through a recovery or by observing a second passage through perihelion, astronomers add a number to the prefix. Since Halley was the first comet whose return was identified, its full designation becomes 1P/1982U1. Astronomers have accumulated detailed orbital information on more than 1,500 individual comets. Of those, only about 10 percent complete an orbit around the Sun in less than 200 years. A typical "short-period" comet travels once around the Sun every 7 years in an orbit inclined to Earth's by some 13°, passing no closer to the Sun than about 1.5 AU, or just within the mean distance of Mars. Halley's Comet is the brightest and most active member of this group. The remaining population consists of long-period comets, those that take at least 200 years to return to the inner solar system. So comet aficionados pin their hopes to the unanticipated arrival of an as-yet-unknown long-period comet.

How bright will it be? The two most important considerations in assessing the visibility of a comet are its distance from the Sun at closest approach, which controls the comet's activity, and its distance from Earth, preferably after the intense heating of it closest approach to the Sun. Halley, for example, was an impressive sight in 1910, but anemic in 1986 — a disappointment even to those who traveled far from city lights. The main difference between the two apparitions was the comet's distance from Earth. Halley reached perihelion at a time when Earth was on the opposite side of the Sun, and the comet never came closer to Earth than 0.417 AU (38.7 million miles or 62.4 million km), which is about three times the distance of its 1910 approach.

Kaoru Ikeya and Tsutomu Seki independently discovered this comet on September 18, 1965, within nearly 15 minutes of each other. Roger Lynds / NOAO / AURA / NSF [View Larger Image] Another example of the importance of proximity was the 1983 display of comet IRAS-Araki-Alcock (C/1983 H1). A small and relatively inactive comet, it was discovered first by the Infrared Astronomical Satellite (IRAS) in late April and originally identified as an asteroid. In early May, amateurs Genichi Araki of Japan and George Alcock of England independently discovered the object. It soon became an obvious sight to the unaided eye high in the northern sky, and on May 12 the comet brushed past Earth at 0.0312 AU (2.9 million miles or 4.7 million km) — closer than any comet since 1770. A typical comet might move across the sky by a degree or so a day, too slowly for the eye to notice. IRAS-Araki-Alcock was so close that its motion was clearly evident to observers, who compared its movement to that of the minute hand on a clock. At its best, the comet was about twice the apparent diameter of the Moon and looked like a star nestled within a puff of smoke. It showed no evidence of a tail — a fine example of a "bushy star" — and faded from view by the third week of May. Intrinsically larger or more active comets can produce a spectacle without getting quite so close to us. Comet West (C/1975 V1) improved dramatically within a week of its very close approach to the Sun, aided in large part by the breakup of its nucleus into four fragments. West dominated the morning sky of early March 1976 with complex gas and dust tails extending 25° or more. A decade earlier, an even more spectacular comet, Ikeya-Seki (C/1965 S1), could be seen even during the daylight as it raced past the Sun, skimming its surface by less than one solar diameter. This intense heating led to the breakup of the nucleus into at least two fragments and a corresponding increase in brightness. During the days around perihelion, Ikeya-Seki could be seen as a star-like object in broad daylight just by blocking the Sun with a hand — the brightest comet of the 20th century. It emerged from the Sun's glare in the last week of October 1965 sporting a bright tail about 25° long. Any list of "great comets" must include both West and Ikeya-Seki.

Sungrazers Ikeya-Seki's punishing orbit places it into a category of comets known as the "sungrazers." Heinrich Kreutz extensively examined the orbits of sungrazing comets and suggested that they shared a common ancestry. Kreutz argued that the comets he studied are possibly fragments of some much larger comet that fell apart at a close approach to the Sun. Sungrazers have perihelion distances less than 0.02 AU, orbital periods of a few centuries, and other distinguishing orbital characteristics, but they were also apparently rare. Brian Marsden of the Harvard-Smithsonian Center for Astrophysics identified eight members, and suspected three others, in his 1965 and 1989 studies of the Kreutz group. By his second study, 15 apparent sungrazing comets had been discovered by the SOLWIND and Solar Maximum Mission satellites, and Marsden noted these "discoveries suggest that members may in fact be coming back to the Sun more or less continuously." Like these fragments, most of the comets so far discovered by comet-champion SOHO also do not survive their passage. Marsden believes that nearly all of them belong to the Kreutz group, although there are too few observations to uniquely determine their orbits. The SOHO sungrazers are probably just a few meters across. Marsden speculates that a historical sungrazer, one the Greek Ephorus reported to have split in two pieces in the winter of 372 b.c., might even be the granddaddy of them all.

Comet duds Even when orbital geometry promises a good display, the comet itself may simply fail to cooperate. Comet Kohoutek (C/1973 E1), which was widely predicted to be the "comet of the century" in 1973, did manage to become a naked-eye object but never lived up to its publicity. Another example is Comet Austin (C/1989 X1), discovered in December 1989 by New Zealand amateur Rodney Austin. The comet's orbit was favorable, but as Austin closed on the Sun, it failed to maintain its rapid brightening and, in the end, proved a bigger dud than Kohoutek. Both Austin and Kohoutek appear to have been new comets, those making their first close pass by the Sun. Astronomers believe that comets originate from two "cold storage" zones that surround the planetary system. The inner portion of this comet cloud is a thick disk centered on the ecliptic that begins near the orbit of Neptune (about 30 AU) and extends beyond the orbit of Pluto to 50 AU. Often called the Kuiper Belt, it contains a few tens of thousands of icy objects larger than about a half-mile across; at least 800 are currently known. A much larger and more diffuse component, called the Oort cloud and containing perhaps a trillion comets, forms a Sun-centered spherical shell extending from the outer Kuiper Belt to about one-third of a light-year or more into space. Many astronomers believe that the Kuiper Belt is the source for the short-period comets and that the Oort cloud, from which comets are more easily dislodged, is the source for the long-period comets. Feeble gravitational disturbances from passing stars and interstellar gas clouds remove enough orbital energy from Oort cloud comets that they begin their million-year-long fall toward the Sun. Long-period comets may arrive from any direction, their elongated orbits randomly oriented to the orbits of the planets, while the short-period comets are confined closer to the ecliptic. New arrivals from the comet cloud probably retain a coating of highly volatile ices, such as frozen carbon dioxide, that begins to evaporate at much lower temperatures than frozen water. Such comets "turn on" at relatively large distances from the Sun, but brighten only until the coating evaporates.

Recent great comets Comet Hyakutake (C/1996 B2) was, in the words of Brooks Observatory comet expert John Bortle, "one of the grandest of the millennium." It was discovered visually by Japanese amateur Yuji Hyakutake when at a distance of 2.0 AU — and only 55 days before its closest approach to Earth (March 25, 1996, 0.102 AU). By late March, mid-northern observers could see it directly overhead before dawn with a tail at least 30°long. In the days around closest approach it was an easy object even from cities, and its motion against the stars, like that of IRAS-Araki-Alcock, was evident in minutes. On March 27, as it moved near Polaris, Hyakutake was visible all night long and could easily be seen from the suburbs. From a reasonably dark sky the comet was truly something special, showing a tail that spanned some 70° or longer — all the more impressive because it seemed to contain relatively little dust. Hyakutake took us by complete surprise, upstaging the appearance of another comet that was already widely anticipated.

Chuck Claver of the National Optical Astronomy Observatories in Tucson, Arizona, captured Comet Hale-Bopp from his backyard in Oro Valley, Arizona, on the evening of April, 9 1997. He combined multiple exposures to create this image. Chuck Claver / NOAO [View Larger Image] That comet was Hale-Bopp (C/1995 O1). What made Hyakutake a great comet was its unusually close pass, which turned a faint and relatively inactive comet into an apparently bright one. But Hale-Bopp was another matter. It was the brightest and most active comet to pass inside Earth's orbit since the one Tycho Brahe examined in 1577. Hale-Bopp showed unusually high activity even at great distance from the Sun and was widely expected to be the one that would end the bright comet drought. It was discovered July 23, 1995, by Alan Hale in New Mexico and Thomas Bopp in Arizona within minutes of one another. After perihelion on April 1, 1997, Hale-Bopp became a striking object in the northwestern sky, cruising through Cassiopeia and Perseus with a pair of tails. The straight, faint gaseous tail was easy to see from a moderately dark site, but the comet's most striking aspect was its dramatically curved 25-degree-long dust tail. Observers in the Northern Hemisphere could see Hale-Bopp with the naked eye, even from urban sites, and it remained well-placed for viewing throughout April and into May. As an indication of the comet's unusual activity, consider that it was never closer to Earth than 122 million miles (197 million km) and passed no closer to the Sun than 91 percent of Earth's distance.

Exploring comets This image of Comet Ikeya-Zhang was taken on March 13, 2002 with a Meade STC 10" and a SBIG ST7-E CFW-8. Denis Bergeron [View Larger Image] Astronomers believe comets may be the best-preserved remnants of the cloud of dust and gas in which the Sun and planets formed. In the deep-freeze of the outermost solar system, they have remained largely unchanged during the 4 billion years the solar system has existed. Planetary scientists study comets for the same reason paleontologists study fossils: to catch a glimpse of the most ancient past. And what better way to scrutinize comets than by visiting them directly? Japan, the European Space Agency (ESA), and the Soviet Union began the direct exploration of comets in 1985 by sending separate missions past Halley's Comet. The ESA probe, Giotto, returned the first detailed images of a comet's nucleus, revealing a dark, peanut-shaped body, a hint of hills and craters, and several bright jets spewing streams of gas and dust. Another burst of comet exploration is now under way: ESA has launched its ambitious mission for Rosetta, which will rendezvous with and orbit the inbound Comet 67P/Churyumov-Gerasimenko in 2014. It will also place a small lander on the comet's surface. The Discovery mission New Exploration of Tempel 1 (NExT) is scheduled to fly by Comet Tempel 1 on February 14, 2011. The mission will reuse NASA's Stardust spacecraft to examine the changes to a comet's nucleus after its close approach to the Sun. The Comet Sample Return Mission, a Design Reference Mission, is scheduled to launch in 2013 and collect samples from the surface of an organic-rich comet nucleus. Researchers will study the samples' chemical composition in order to learn more about the chemical origins of our solar system

Comet C/1996 B2 Hyakutake

On January 31, 1996, IAU circular 6299 reported the visual discovery of a comet by Yuji Hyakutake of Japan. Comet Hyakutake, also designated Comet C/1996 B2, approached within 0.1 AU of the Earth (about 15 million km) on March 25. Perihelion on May 1 saw the comet at a distance of 0.23 AU from the Sun. After perihelion, the comet should be visible in the southern hemisphere from the middle of May to early June.

Information on Comet Hyakutake

Perihelion distance: 0.23019 AU
Perihelion date: 01 May 1996 UT 09:30 (5:30 AM EDT)
Closest approach to Earth: 0.1 AU
Date of closest approach to Earth: 25 March 1996
Orbital inclination: 124.924 deg.
Orbital eccentricity: > 0.999784
Argument of perihelion: 130.165 deg.
Longitude of ascending node: 188.046 deg.
Pre-perihelion Orbital period: ~ 8,000 years
Post-perihelion Orbital period: ~ 14,000 years
Original Semi-major axis: ~ 400 AU

Epoch 2450270.50000 = 1996 July 6.00000
Ref. solution 46, 11 May 1996

Comet Hale-Bopp

Comet Hale-Bopp (C/1995 O1) was discovered on 23 July 1995 by two independent observers, Alan Hale (Cloudcroft, N.M.) and Thomas Bopp (Stanfield, AZ), and is showing potential of putting on a spectacular display as it nears its 1997 perihelion. The image above was taken by the Hubble Space Telescope, and shows material ejected from the rotating comet in a "pinwheel" pattern. More information on this image is available in the caption.

The nucleus of Hale-Bopp is estimated to be about 30 to 40 km across - Comet Halley's nucleus was estimated at 8 x 8 x 16 km. The nucleus is exhibiting sudden brief eruptions and a complex mottled surface. Its absolute magnitude of -1 makes it one of the brightest comets to reach the inner solar system in history. Closest approach to Earth will occurred on 22 March 1997 at a distance of 1.3 A.U. It made for a spectacular view in the March morning sky, and will be in the evening skies from mid-March to early May. Closest approach to the Sun was on 31 March at a distance of .91 A.U. The comet is estimated to have last passed by the Sun about 4200 years ago.

Hale-Bopp was visible low in the northern hemisphere pre-dawn sky in February to the ENE just below the constellation Cygnus. By the end of March the comet moved from Cygnus to Lacerta to Andromeda in the NE pre-dawn sky. The comet will be disappearing from the pre-dawn sky at the beginning of April. Since mid-March, however, the comet has also been visible in the early evening sky to the NW to WNW, at the bottom of Perseus. The comet will become higher in the sky through mid-April, and then move down towards the horizon by early May. The comet is currently one of the brightest objects in the sky and the tail is spectacular.

Information on Hale-Bopp

Perihelion distance: 0.9141 AU
Perihelion date: 01 April 1997 UT 03:19 (31 March 22:19 EST)
Closest approach to Earth: 1.3 AU
Date of closest approach to Earth: 22 March 1997
Next Perihelion: ~2380 years
Previous Perihelion: ~4200 years ago
Orbital inclination: 89.43 deg.
Orbital eccentricity: 0.9951
Argument of perihelion: 130.59 deg.
Longitude of ascending node: 282.47 deg.

Heliocentric coordinates of Hale-Bopp - for any given dates

Results of IUE and Hubble observations of Hale-Bopp - 27 March 1997

NASA plans to observe Hale-Bopp - 13 March 1997

Planetary System Formation

Planetary System Formation

"The created world is but a small parenthesis in eternity."

Planetary system formation coincides with the process of star formation in which our Sun belongs to the generation of stars created 4.6 billion years ago, when our galaxy was roughly half its present age. A cloud of interstellar gas, dust, and ices containing several generations of material collapsed to form the nebula from which the Sun and the rest of our solar system grew. This collapse may have been triggered by a nearby supernova. Cosmologists believe that because the material in the nebula was rotating to some degree, not all of the nebular material fell directly into the central mass that would become the Sun. Instead, some of the material was confined to a flat, spinning disk, called a protoplanetary disk, around a young Sun. As time went on, the grains and ices in the disk bumped into and stuck to one another forming macroscopic objects with sizes on of order 0.01-10 meters, all orbiting in the same direction and same plane analogus to the rings around Saturn. As the objects grew larger, their gravitational forces increased, attracting more matter from the disk and gradually building kilometer-sized bodies called planetesimals. These planetesimals further collided and either shattered into fragments or merged producing larger objects. The gravitational pull of the largest planetesimals produced rapid growth to the size of small planets and formed the nuclei of the planets as we know them today.

Thin disk around Beta Pictoris. [more]	HD 141569: Gap in stellar dust disk. [more]	HR 4796A: Dust ring around star. [more]
These three images show dust rings around a newly formed stars. Beta Pictoris' disk is slightly warped possibly due to the gravitational pull of a planet. The other two images also show gaps, bright and dark areas similar to Saturn's rings.

Some planetesimals in the outer solar system became large enough to accrete gas forming the giant planets Juipter, Saturn, Uranus, and Neptune. Because of the higher temperatures in the inner solar system, accretion of ice and gas was inhibited so the planetesimals grew into what is known as the rocky terrestrial planets. Planetary growth slowed down significantly once a gap was cleared within its orbit. But even today planets continue to grow by small amounts as they sweep up micrometeor dust particles or are impacted every few million years by larger asteroids or comets such as the dramatic impact of comet Shoemaker-Levy with Jupiter.

Planetesimals that became modest in size but did not merge to form larger bodies became asteroids and comets. The asteroid belt may be result of fragmentation of planetisimals that were prevented from growing larger by the close proximity of Jupiter's gravitational pull. Other planetesimals were tossed about into random orbits from gravitational interaction with the larger planets. The Oort Cloud was formed early in the history of the solar system through gravitational interaction of planetisimals with Uranus and Neptune. (Close encounters with Saturn and Jupiter would have ejected the objects out of the solar system.) These planetisimals were thrown outwards close to the Solar System escape velocity. The Oort cloud consists of some 1,000,000,000,000 long-period comets that extends out to tens of thousands of AU, half way to our closest stellar neighbors. Comets found within Kuiper Belt may be early remnants of the Sun's protoplanetary disk. Additional debris left over from the earliest phases of solar system formation includes small grains of sand and small Meteorids.

Hubble Directly Observes Planet Orbiting Fomalhaut

HST Image of Fomalhaut and Fomalhaut b. Hubble Directly Observes Planet Orbiting Fomalhaut NASA's Hubble Space Telescope has taken the first visible-light snapshot of a planet circling another star. Estimated to be no more than three times Jupiter's mass, the planet, called Fomalhaut b, orbits the bright southern star Fomalhaut, located 25 light-years away in the constellation Piscis Australis (the Southern Fish). Fomalhaut has been a candidate for planet hunting ever since an excess of dust was discovered around the star in the early 1980s by NASA's Infrared Astronomy Satellite (IRAS). In 2004, the coronagraph in the High Resolution Camera on Hubble's Advanced Camera for Surveys produced the first-ever resolved visible-light image of a large dust belt surrounding Fomalhaut. It clearly showed that this structure is in fact a ring of protoplanetary debris approximately 21.5 billion miles across with a sharp inner edge. This large debris disk is similar to the Kuiper Belt, which encircles the solar system and contains a range of icy bodies from dust grains to objects the size of dwarf planets, such as Pluto. Hubble astronomer Paul Kalas, of the University of California at Berkeley, and team members proposed in 2005 that the ring was being gravitationally modified by a planet lying between the star and the ring's inner edge. Circumstantial evidence came from Hubble's confirmation that the ring is offset from the center of the star. The sharp inner edge of the ring is also consistent with the presence of a planet that gravitationally "shepherds" ring particles. Independent researchers have subsequently reached similar conclusions. Now, Hubble has actually photographed a point source of light lying 1.8 billion miles inside the ring's inner edge. The results are being reported in the November 13 issue of Science magazine. "Our Hubble observations were incredibly demanding. Fomalhaut b is 1 billion times fainter than the star. We began this program in 2001, and our persistence finally paid off," Kalas says. "Fomalhaut is the gift that keeps on giving. Following the unexpected discovery of its dust ring, we have now found an exoplanet at a location suggested by analysis of the dust ring's shape. The lesson for exoplanet hunters is 'follow the dust,'" says team member Mark Clampin of NASA's Goddard Space Flight Center. Observations taken 21 months apart by Hubble's Advanced Camera for Surveys' coronagraph show that the object is moving along a path around the star and therefore is gravitationally bound to it. The planet is 10.7 billion miles from the star, or about 10 times the distance of the planet Saturn from the sun. The planet's upper-mass limit is constrained by the appearance of the Fomalhaut ring. If the planet were much more massive, it would distort the ring, and the effect would be observable in the ring's structure. "It took the science team four months of analysis and theoretical modeling to determine that Fomalhaut b could not be more massive than three times the mass of Jupiter. Any more massive than that and its gravity would destroy the vast dust belt encircling the star," Kalas says. Numerous computer simulations show that circumstellar disks will be gravitationally modified by the tug of one or more unseen planets. The Fomalhaut ring has a sharp inner edge that is likely shaped by the gravitational influence of a planet. The inner edge of our solar system's Kuiper Belt is similarly shaped by the gravitational influence of Neptune. The planet is brighter than expected for an object of three Jupiter masses. One possibility is that it has a huge Saturn-like ring of ice and dust reflecting starlight. The ring might eventually coalesce to form moons. The ring's estimated size is comparable to the region around Jupiter that is filled with the orbits of the four largest satellites. Because the Fomalhaut system is only 200 million years old, the planet should be a bright infrared object. That's because it is still cooling through gravitational contraction. However, ground-based telescopic observations at infrared wavelengths have not yet detected the planet. This also sets an upper limit on its mass, because the bigger the planet, the hotter and brighter it would be. Kalas and his team first used Hubble to photograph Fomalhaut in 2004, and made the unexpected discovery of its debris disk, which scatters Fomalhaut's starlight. At the time they noted a few bright sources in the image as planet candidates. A follow-up image in 2006 showed that one of the objects is moving through space with Fomalhaut but changed position relative to the ring since the 2004 exposure. The amount of displacement between the two exposures corresponds to an 872-year-long orbit as calculated from Kepler's laws of planetary motion. Fomalhaut moves across the sky at 0.425 arcseconds per year, which is the apparent width of a penny seen from five miles away. The planet mysteriously dimmed by half a stellar magnitude between the 2004 and 2006 observations. This might mean that it has a hot outer atmosphere heated by bubbling convection cells on the young planet — sort of a Jupiter on steroids. Or, it might come from hot gas at the inner boundary of a ring around the planet. The planet may have formed at its location in a primordial circumstellar disk by gravitationally sweeping up remaining gas. Or, it may have migrated outward through a game of gravitational billiards where it exchanged momentum with smaller planetary bodies. It is commonly believed that the planets Uranus and Neptune migrated out to their present orbits after forming closer to the sun and then gravitationally interacted with smaller bodies. Fomalhaut is much hotter than our sun and is 16 times as bright. This means a planetary system could scale up in size with a proportionally larger Kuiper Belt feature and scaled-up planet orbits. For example, the "frost line" in our solar system — the distance where ices and other volatile elements will not evaporate — is roughly at 500 million miles from the sun. But for hotter Fomalhaut, the frost line is at roughly 1.9 billion miles from the star. Fomalhaut is burning hydrogen at such a furious rate through nuclear fusion that it will burn out in only 1 billion years, which is 1/10th the lifespan of our sun. This means there is little opportunity for advanced life to evolve on any habitable worlds the star might possess. Future observations will attempt to see the planet in infrared light and will look for evidence of water vapor clouds in the atmosphere. This would yield clues to the evolution of a comparatively newborn 100-million-year-old planet. Astrometric measurements of the planet's orbit will provide enough precision to yield an accurate mass. NASA's James Webb Space Telescope, scheduled to launch in 2013, will be able to make coronagraphic observations of Fomalhaut in the near- and mid-infrared. JWST will be able to hunt for other planets in the system and probe the region interior to the dust ring for structures such as an inner asteroid belt. The science team members are: P. Kalas, J. Graham, E. Chiang, and E. Kite (University of California, Berkeley), M. Clampin (NASA Goddard Space Flight Center, Greenbelt, Md.), M. Fitzgerald (Lawrence Livermore National Laboratory, Livermore, Calif.), and K. Stapelfeldt and J. Krist (NASA Jet Propulsion Laboratory, Pasadena, Calif.).

Mimas

Mimas (pronounced MY muss or MEE muss, adjective Mimantean) is an inner moon of Saturn (the innermost of the major moons) and looks somewhat like a bull's eye if viewed from a certain angle. The feature that causes this is the huge 140-kilometer-wide (88-mile) Herschel Crater, which is one-third the diameter of Mimas. If the object striking Mimas had been larger or been moving faster, Mimas would probably have been "disrupted" into pieces that might have collapsed back into a new moon or might have scattered into another ring of Saturn. The walls of Herschel Crater are approximately 5 kilometers (3 miles) high, parts of the floor are approximately 10 kilometers (6 miles) deep, and the central peak towers are almost 6 kilometers (4 miles) above the floor of the crater. A comparable crater on Earth would be 4,000 kilometers (2,500 miles) in diameter.

Mimas averages 396 kilometers (246 miles) in diameter. Shock waves from the Herschel impact may have caused the fractures, also called chasmata, on the opposite side of Mimas. Mimas is not quite big enough to hold a round shape; the shape is somewhat ovoid with dimensions of 209 x 196 x 191 kilometers (130 x 122 x 119 miles, respectively). Mimas orbits at a range of 185,520 kilometers (115,280 miles) from Saturn in a time of 22 hours and 37 minutes. This orbit makes Mimas the closest major moon of Saturn. Mimas is tidally locked to Saturn with one side always facing in toward its parent. Mimas' close orbit means that Mimas probably receives several times the rate of collisions as the other moons of Saturn.

Mimas and another Saturn moon, Rhea, have been called "the most heavily cratered body in the Solar System." Mimas would probably have been more heavily cratered except, being closer to Saturn, Mimas was warmer (and consequently softer) for a longer time so early features have faded away. However, with so many impacts the youngest craters have tended to obliterate the older ones, and these moons are cratered about as much as they can get.

Most of the Mimas surface is saturated with impact craters ranging in size up to greater than 40 kilometers (25 miles) in diameter, although none are anywhere near the size of Herschel. However, the craters in the South Pole region of Mimas are generally 20 kilometers (12.4 miles) in diameter or less. This suggests that some melting or other resurfacing processes occurred there later than on the rest of the moon. (Interestingly, the South Pole area of Enceladus appears to be the source of that moon's geysers.)

Mimas' low density (1.17 times the density of liquid water) indicates that it is composed mostly of water ice with only a small amount of rock. It seems to be solidly frozen at a temperature of 209 degrees Celsius (-344 degrees Fahrenheit). This is puzzling because Mimas is closer to Saturn than Enceladus, and the Mimantean orbit is much more eccentric (out of round) than the Enceladean orbit. Thus, Mimas should have much more tidal heating than Enceladus. Yet, Enceladus has geysers of water, while Mimas has one of the most heavily cratered surfaces in the Solar System. This suggests a frozen Mimas surface that has persisted for a very long time. This paradox has led astronomers to use the "Mimas test" by which a theory to explain the partially thawed water of Enceladus must also explain the entirely frozen water of Mimas.

Mimas apparently cleared enough material to create the 4,800-kilometer (2,980-mile) wide gap (called the Cassini Division) between Saturn's two widest rings, the A and B rings. Observations from Cassini revealed that there is still some ring material in the Cassini Division, although it is sparse enough that the area appears dark from a distance.

Mimas is in resonance with two nearby moons, Dione and Enceladus. That is, these moons speed up slightly as they approach each other and slow down as they draw away, causing their orbits to vary slightly in a long series of complex changes, which help keep them locked in their positions.

Mimas strongly perturbs the tiny 3-kilometer (2-mile) diameter moon Methone, the 4-kilometer (3-mile) diameter moon Pallene, and the 2-kilometer (1-mile) diameter moon Anthe, all of which orbit between Mimas and the next major moon going out from Saturn, Enceladus. The vastly more massive Mimas causes the Methone orbit to vary by as much as 20 kilometers (12.4 miles). The perturbations are larger for tiny Anthe, and slightly smaller for Pallene.

Discovery

Ground-based astronomers could only see Mimas as little more than a dot until Voyagers I and II imaged it in 1980. The Cassini spacecraft has made several close approaches and provided detailed images of Mimas since Cassini achieved orbit around Saturn in 2004.

William [Friederick Wilhelm] Herschel discovered Mimas in 1789. His son, John Herschel suggested that the moons of Saturn be associated with Greek mythical brothers and sisters of Kronus, known to the Romans as Saturn.

The name Mimas comes from the god (or Titan) Mimas in Greek mythology who was slain by one of the gods of Olympus in the war between the Olympians and the titans. Different accounts have Mimas dispatched by Hercules, by Ares (the god of war), or by Zeus himself using a thunderbolt. Legend has it that the island of Prochyte near Sicily rests on his body.

Astronomers also refer to Mimas as Saturn I based on its distance being the closest to Saturn. The International Astronomical Union now controls the official naming of astronomical bodies.

Mimas Statistics
Discovered by	William Herschel
Date of discovery	1789
Mass (kg)	3.80e+19
Mass (Earth = 1)	6.3588e-06
Equatorial radius (km)	196
Equatorial radius (Earth = 1)	3.0731e-02
Mean density (gm/cm^3)	1.17
Mean distance from Saturn (km)	185,520
Rotational period (days)	0.942422
Orbital period (days)	0.942422
Mean orbital velocity (km/sec)	14.32
Orbital eccentricity	0.0202
Orbital inclination (degrees)	1.53
Escape velocity (km/sec)	0.161
Visual geometric albedo	0.5
Magnitude (Vo)	12.9
Mean surface temperature	-200°C

Views of Mimas

Views all images of Mimas

Flying by the "Death Star" Moon

In this view captured by NASA's Cassini spacecraft on its closest-ever flyby of Saturn's moon Mimas, large Herschel Crater dominates Mimas, making the moon look like the Death Star in the movie "Star Wars."

Herschel Crater is 130 kilometers, or 80 miles, wide and covers most of the right of this image.

Cassini came within about 9,500 kilometers (5,900 miles) of Mimas on Feb. 13, 2010.This mosaic was created from six images taken that day in visible light with Cassini's narrow-angle camera on Feb. 13, 2010. The images were re-projected into an orthographic map projection. This view looks toward the area between the region that leads on Mimas' orbit around Saturn and the region of the moon facing away from Saturn. Mimas is 396 kilometers (246 miles) across. This view is centered on terrain at 11 degrees south latitude, 158 degrees west longitude. North is up. This view was obtained at a distance of approximately 50,000 kilometers (31,000 miles) from Mimas and at a sun-Mimas-spacecraft, or phase, angle of 17 degrees. Image scale is 240 meters (790 feet) per pixel. (Courtesy NASA/JPL/Space Science Institute)

Streaked Craters in False-Color
This false-color view of Saturn's moon Mimas from NASA's Cassini spacecraft accentuates terrain-dependent color differences and shows dark streaks running down the sides of some of the craters on the region of the moon that leads in its orbit around Saturn. The image was taken during Cassini's closest-ever flyby of the moon.

The false-color image shows how colors vary across the moon's surface, particularly the contrast between the bluish terrain on the right side of this view near Herschel Crater and greenish terrain elsewhere. The origin of the color differences (exaggerated by computer enhancement) is not yet understood, but may be caused by subtle differences in the surface composition between the two terrains.

This image also shows dark streaks trailing down the sides of some craters (marked red in the annotated version), often originating from pockets of dark contaminants embedded just below the rim of the crater wall. The pockets likely represent small, pre-existing, dark-floored craters that were buried by the blanket of material thrown out from the newer impact that created the crater rim. The material from a newly exposed dark layer eventually moves downslope and forms a streak. Streaks sometimes are seen originating from the floors of smaller dark-floored craters perched along rims of larger craters.

The craters seen in this mosaic also show relatively dark markings along the lower portion of their crater walls (marked in green in the annotated version of the image). Cassini scientists interpret this darkening as evidence for the gradual concentration of impurities from evaporating icy materials in areas where the dark impurities slide slowly down the crater wall and the bright ice is baked away by the sun and the vacuum of space.

To create this false-color view, ultraviolet, green and infrared images were combined into a single picture that exaggerates the color differences of terrain on the moon. These data were combined with a high-resolution image taken in visible light to provide the high-resolution information from the clear filter image and the color information from the ultraviolet, green, and infrared filter images. The natural color of Mimas visible to the human eye may be a uniform gray or yellow color.

This view looks toward the area between the region that leads in Mimas' orbit around Saturn and the region of Mimas that faces away from Saturn. Mimas is 396 kilometers (246 miles) across. This view is centered on terrain at 11 degrees north latitude, 156 degrees west longitude. This mosaic combines images taken with the Cassini spacecraft narrow-angle camera on Feb. 13, 2010. The view was acquired at a distance of approximately 34,000 kilometers (21,000 miles) from Mimas. Image scale is 180 meters (590 feet) per pixel. (Courtesy NASA/JPL/Space Science Institute)

Bizarre Temperatures on Mimas
The image shows NASA's Cassini spacecraft imaging science subsystem visible-light mosaic of Mimas from previous flybys on the left. The right-hand image shows the new CIRS temperature data mapped on top of the visible-light image.

The annotated version illustrates the unexpected and bizarre pattern of daytime temperatures found on Saturn's small inner moon Mimas (396 kilometers, or 246 miles, in diameter). The data were obtained by the composite infrared spectrometer (CIRS) on the Cassini spacecraft during the spacecraft's closest-ever look at Mimas on Feb. 13, 2010.

In the annotated version, the upper left image shows the expected distribution of temperatures. The white sun symbol shows the point where the sun is directly overhead, which is at midday close to the equator. Just as on Earth, the highest temperatures (shown in yellow) were expected to occur after midday, in the early afternoon.

The upper right image in the annotated version shows the completely different pattern that Cassini actually saw. Instead of the expected smoothly varying temperatures, this side of Mimas is divided into a warm part (on the left) and a cold part (on the right) with a sharp, v-shaped boundary between them. The warm part has typical temperatures near 92 Kelvin (minus 294 Fahrenheit), while typical temperatures on the cold part are about 77 Kelvin (minus 320 Fahrenheit). The cold part is probably colder because surface materials there have a greater thermal conductivity, so the sun's energy soaks into the subsurface instead of warming the surface itself. But why conductivity should vary so dramatically across the surface of Mimas is a mystery.

The lower two panels in the annotated version compare the temperature map to Mimas' appearance in ordinary visible light at the time of the observations. The map used to create this image is a mosaic of images taken by Cassini's imaging science subsystem cameras on previous flybys of Mimas. The cold side includes the giant Herschel Crater, which is a few degrees warmer than its surroundings. It's not yet known whether Herschel is responsible in some way for the larger region of cold temperatures that surrounds it.

The green grid shows latitudes and longitudes on Mimas at 30-degree intervals.

Cassini took 85 minutes to make the temperature map, as the spacecraft receded from Mimas. During that time, the distance to Mimas increased from 38,000 to 67,000 kilometers (24,000 to 42,000 miles) and the longitude of the center of Mimas' disk increased from 128 degrees west to 161 degrees west, due to the moon's rotation.

Because of this changing geometry, the alignment of the temperatures relative to specific features or coordinates on Mimas is shown only approximately. The temperatures were calculated from the brightness of the moon's infrared heat radiation, measured by CIRS at a wavelength of 12 to 16 microns, and are color coded according to the scale in the lower right of the annotated figure. (Courtesy NASA/JPL/GSFC/SWRI/SSI)

Color Near Herschel Crater

Subtle color differences on Saturn's moon Mimas are apparent in this false-color view of Herschel Crater captured by NASA's Cassini spacecraft during its closest-ever flyby of that moon.

The image shows terrain-dependent color variations, particularly the contrast between the bluish materials in and around Herschel Crater (130 kilometers, or 80 miles, wide) and the greenish cast on older, more heavily cratered terrain elsewhere. The origin of the color differences is not yet understood, but may be caused by subtle differences in the surface composition between the two terrains. False color images from Cassini's previous closest encounter, in 2005, also showed such variations.

Herschel Crater covers most of the bottom of this image. To create this false-color view, ultraviolet, green and infrared images were combined into a single picture that exaggerates the color differences of terrain on the moon. These data were combined with a high-resolution image taken in visible light to provide the high-resolution information from the clear-filter image and the color information from the ultraviolet, green and infrared filter images.

The natural color of Mimas visible to the human eye may be a uniform gray or yellow color, but this mosaic has been contrast-enhanced and shows differences at other wavelengths of light.

During its closest-ever flyby on Feb. 13, 2010, Cassini came within about 9,500 kilometers (5,900 miles) of Mimas. This view looks toward the northern part of the hemisphere of Mimas that leads in the moon's orbit around Saturn. Mimas is 396 kilometers (246 miles) across. North on Mimas is up and rotated 12 degrees to the left.

The images were obtained with Cassini's narrow-angle camera on that day at a distance of approximately 16,000 kilometers (10,000 miles) from Mimas. The images were re-projected into an orthographic map projection. A black and white image, taken in visible light with the wide-angle camera, is used to fill in parts of the mosaic. Image scale is 90 meters (195 feet) per pixel. (Courtesy NASA/JPL/Space Science Institute)

Mimas Showing False Colors
False color images of Saturn's moon, Mimas, reveal variation in either the composition or texture across its surface.

During its approach to Mimas on Aug. 2, 2005, the Cassini spacecraft narrow-angle camera obtained multi-spectral views of the moon from a range of 228,000 kilometers (142,500 miles).

The image at the left is a narrow angle clear-filter image, which was separately processed to enhance the contrast in brightness and sharpness of visible features. The image at the right is a color composite of narrow-angle ultraviolet, green, infrared and clear filter images, which have been specially processed to accentuate subtle changes in the spectral properties of Mimas' surface materials. To create this view, three color images (ultraviolet, green and infrared) were combined into a single black and white picture that isolates and maps regional color differences. This "color map" was then superimposed over the clear-filter image at the left.

The combination of color map and brightness image shows how the color differences across the Mimas surface materials are tied to geological features. Shades of blue and violet in the image at the right are used to identify surface materials that are bluer in color and have a weaker infrared brightness than average Mimas materials, which are represented by green.

Herschel crater, a 140-kilometer-wide (88-mile) impact feature with a prominent central peak, is visible in the upper right of each image. The unusual bluer materials are seen to broadly surround Herschel crater. However, the bluer material is not uniformly distributed in and around the crater. Instead, it appears to be concentrated on the outside of the crater and more to the west than to the north or south. The origin of the color differences is not yet understood. It may represent ejecta material that was excavated from inside Mimas when the Herschel impact occurred. The bluer color of these materials may be caused by subtle differences in the surface composition or the sizes of grains making up the icy soil. (Courtesy NASA/JPL/Space Science Institute)

Mimas in 3D
This 3D picture of Saturn's moon Mimas, shows it to be one of the most heavily cratered Saturnian moons, with little if any evidence for internal activity. Mimas has been so heavily cratered that new impacts can only overprint or even completely obliterate other older craters. Mimas is 397 kilometers or (247 miles) across.

The moon displays an unexpected array of crater shapes. The highest crater walls tower 6 kilometers (4 miles) above the floors and show signs of material sliding down slope. Indeed, many of the large craters -- more than 15 kilometers (10 miles) in diameter -- appear to be filled in with rough-surfaced material, likely the result of landslides triggered by subsequent impacts elsewhere on Mimas' surface. Some of these deposits have craters superimposed on them, demonstrating that the landslides themselves may be quite old.

Grooves, some of which are over a kilometer deep, cut across the surface for more than 100 kilometers (63 miles). These are some of the only indications that there might have once been internal activity under this ancient, battered surface. (Copyright Calvin J. Hamilton)

A World of Hurt
The most detailed images ever taken of Saturn's moon Mimas show it to be one of the most heavily cratered Saturnian moons, with little if any evidence for internal activity. Mimas has been so heavily cratered that new impacts can only overprint or even completely obliterate other older craters. Mimas is 397 kilometers or (247 miles) across.

The moon displays an unexpected array of crater shapes. The highest crater walls tower 6 kilometers (4 miles) above the floors and show signs of material sliding down slope. Indeed, many of the large craters -- more than 15 kilometers (10 miles) in diameter -- appear to be filled in with rough-surfaced material, likely the result of landslides triggered by subsequent impacts elsewhere on Mimas' surface. Some of these deposits have craters superimposed on them, demonstrating that the landslides themselves may be quite old.

Grooves, some of which are over a kilometer deep, cut across the surface for more than 100 kilometers (63 miles). These are some of the only indications that there might have once been internal activity under this ancient, battered surface. (Courtesy NASA/JPL/Space Science Institute)

Mimas & Herschel Crater
This image of Mimas was acquired by the Voyager 1 spacecraft on November 11, 1980 from a range of 425,000 kilometers (264,000 miles). The large crater on the right limb is named Herschel. It is 130 kilometers (80 miles) wide and one-third the diameter of Mimas. Herschel is 10 kilometers (6 miles) deep, with a central mountain almost as high as Mount Everest on Earth. This impact probably came close to disintegrating the moon. (Copyright Calvin J. Hamilton)

Amazing Icy Moons
A scene straight out of science fiction, this fantastic view shows, from left to right, Saturn's moon's Mimas, Dione and Rhea, on the far side of Saturn's nearly edge-on rings.

The trailing hemispheres of all three moons are sunlit here, and wispy markings can be seen on the limbs of both Dione and Rhea. The diameter of Mimas is 397 kilometers (247 miles), Dione is 1,118 kilometers (695 miles) and Rhea is 1,528 kilometers (949 miles). (Courtesy NASA/JPL/Space Science Institute)

Mimas Blues
Mimas drifts along in its orbit against the azure backdrop of Saturn's northern latitudes in this true color view. The long, dark lines on the atmosphere are shadows cast by the planet's rings.

Saturn's northern hemisphere is presently relatively cloud-free, and rays of sunlight take a long path through the atmosphere. This results in sunlight being scattered at shorter (bluer) wavelengths, thus giving the northernmost latitudes their bluish appearance at visible wavelengths.

At the bottom, craters on icy Mimas (398 kilometers, or 247 miles across) give the moon a dimpled appearance.

Images taken using infrared (930 nanometers), green (568 nanometers) and ultraviolet (338 nanometers) spectral filters were combined. The colors have been adjusted to match closely what the scene would look like in natural color. See PIA06142 for a similar view in natural color.

The images were obtained using the Cassini spacecraft narrow angle camera on Jan. 18, 2005, at a distance of approximately 1.4 million kilometers (870,000 miles) from Saturn. Resolution in the image is 8.5 kilometers (5.3 miles) per pixel on Saturn and 7.5 kilometers (4.7 miles) per pixel on Mimas. The image has been contrast-enhanced to aid visibility. (Courtesy NASA/JPL/Space Science Institute)

Nature's Canvas
In a splendid portrait created by light and gravity, Saturn's lonely moon Mimas is seen against the cool, blue-streaked backdrop of Saturn's northern hemisphere. Delicate shadows cast by the rings arc gracefully across the planet, fading into darkness on Saturn's night side.

The part of the atmosphere seen here appears darker and more bluish than the warm brown and gold hues seen in Cassini images of the southern hemisphere, due to preferential scattering of blue wavelengths by the cloud-free upper atmosphere.

The bright blue swath near Mimas (398 kilometers, or 247 miles across) is created by sunlight passing through the Cassini division (4,800 kilometers, or 2,980 miles wide). The rightmost part of this distinctive feature is slightly overexposed and therefore bright white in this image. Shadows of several thin ringlets within the division can be seen here as well. The dark band that stretches across the center of the image is the shadow of Saturn's B ring, the densest of the main rings. Part of the actual Cassini division appears at the bottom, along with the A ring and the narrow, outer F ring. The A ring is transparent enough that, from this viewing angle, the atmosphere and threadlike shadows cast by the inner C ring are visible through it. (Courtesy NASA/JPL/Space Science Institute)

Mimas
This is another image of Mimas acquired by the Voyager 1 spacecraft on November 13, 1980. The surface is heavily cratered indicating an ancient surface age. (Copyright Calvin J. Hamilton)

Janus prorile

Views of Janus

Profile of Janus
This shadowy scene is one of the Cassini spacecraft's closest views of Saturn's moon Janus.

The slopes of some craters here display hints of the darker material better seen on Epimetheus in PIA09813. A bright linear feature runs up the wall of the large crater at bottom center.

The view looks toward southern latitudes on Janus (179 kilometers, or 111 miles across). North is toward the top of the image and rotated 58 degrees to the right.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on June 30, 2008. The view was obtained at a distance of approximately 33,000 kilometers (21,000 miles) from Janus and at a Sun-Janus-spacecraft, or phase, angle of 120 degrees. Image scale is 200 meters (656 feet) per pixel. (Courtesy NASA/JPL/Space Science Institute)

Janus Up Close
From just beneath the ringplane, Cassini stares at Janus (181 kilometers, or 113 miles across).

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on April 29, 2006 at a distance of approximately 218,000 kilometers (135,000 miles) from Janus. Image scale is about 1 kilometer (0.6 mile) per pixel. (Courtesy NASA/JPL/Space Science Institute)

Crater View
The Cassini spacecraft eyes a prominent crater on the moon Janus.

The south pole lies on the terminator at the bottom left of the image. This view is centered on terrain at 16 degrees south latitude, 64 degrees west longitude. This view looks toward the leading hemisphere of Janus (179 kilometers, or 111 miles across). North on Janus is up and rotated 31 degrees to the right.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on July 26, 2009. The view was acquired at a distance of approximately 98,000 kilometers (61,000 miles) from Janus and at a Sun-Janus-spacecraft, or phase, angle of 58 degrees. Image scale is 586 meters (1,922 feet) per pixel (Courtesy NASA/JPL/Space Science Institute)

Janus' Cratered South
The Cassini spacecraft looks toward the south pole and cratered surface of Saturn's moon Janus.

The pole of Janus lies on the terminator about one-third of the way inward from the bottom of the image. This view is centered on terrain at 42 degrees south latitude, 32 degrees west longitude. Lit terrain seen here is on the Saturn-facing side of Janus (179 kilometers, or 111 miles across).

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on July 26, 2009. The view was acquired at a distance of approximately 100,000 kilometers (62,000 miles) from Janus and at a Sun-Janus-spacecraft, or phase, angle of 63 degrees. Image scale is 600 meters (1,968 feet) per pixel. (Courtesy NASA/JPL/Space Science Institute)

Spots on Janus
This close-up look at Saturn's moon Janus reveals spots on the moon's surface which may be dark material exposed by impacts. If the dark markings within bright terrain are indeed impact features, then Janus' surface represents a contrast with that of Saturn's moon Phoebe, where impacts have uncovered bright material beneath a darker overlying layer. Janus is 181 kilometers (113 miles) across.

Janus may be a porous body, composed mostly of water ice.

This image was taken in visible light with the Cassini spacecraft narrow-angle camera on May 20, 2005, at a distance of approximately 357,000 kilometers (222,000 miles) from Janus and at a Sun-Janus-spacecraft, or phase, angle of 6 degrees. Resolution in the original image was 2 kilometers (1 mile) per pixel. The view was magnified by a factor of two and contrast-enhanced to aid visibility of the moon's surface. (Courtesy NASA/JPL/Space Science Institute)

So Close
Saturn's moons Janus and Prometheus look close enough to touch in this stunningly detailed view.

From just beneath the ringplane, Cassini stares at Janus (181 kilometers, or 113 miles across - on top) on the near side of the rings and Prometheus (102 kilometers, or 63 miles across - on bottom) on the far side. The image shows that Prometheus is more elongated than Janus.

The view takes in the Cassini Division (4,800 kilometers, or 2,980 miles wide), from its outer edge to about halfway across its width. (Courtesy NASA/JPL/Space Science Institute)

Janus Viewed from Voyager 2
This image of Janus was acquired by the Voyager 2 spacecraft on August 25, 1981. It is the highest resolution image available. (Copyright Calvin J. Hamilton)

Pseudo Color Image of Janus
This is a colorized version of the high resolution Voyager 2 spacecraft image of Janus that was taken on August 25, 1981. (Copyright Calvin J. Hamilton)

Janus With Saturn's Rings in the Background
This Voyager 1 picture was taken of Janus in front of Saturn's rings. It was acquired on November 12, 1981. (Copyright Calvin J. Hamilton)

Janus
This image of Janus was acquired by the Voyager 1 spacecraft on November 12, 1981. (Copyright Calvin J. Hamilton)

Simple Cylindrical Map of Janus
This image is a Simple Cylindrical map of Janus centered at 180 degrees longitude. It was constructed from Voyager 1 and Voyager 2 pictures of Janus using a shape model of Peter Thomas. (Courtesy A. Tayfun Oner)

Topographic Map of Janus
This is a topographic map of Janus. It is based upon the shape model by Phil Stooke. As with all maps, it is the cartographer's interpretation; not all features are necessarily certain given the limited data available. This interpretation stretches the data as far as possible. (Courtesy A. Tayfun Oner)

Conformal Projection of Janus
This image is a shaded relief map of Janus, the larger co-orbital satellite of Saturn. As with all maps, it is the cartographer's interpretation and not all features are necessarily certain given the limited data available. This interpretation stretches the data as far as is feasible. The image shows two different views of Janus in a Morphographic Conformal Projection. One view shows the leading side and the other the trailing side. (Courtesy Phil Stooke/NSSDC/NASA)

Janus

Janus [JAY-nus] is the sixth satellite of Saturn. It was discovered by Audouin Dollfus in 1966 and was named after the god of gates and doorways. It is depicted with two faces looking in opposite directions. Janus has an irregular shape with a size of 196x192x150 kilometers (122x119x93 miles) in diameter. It is heavily cratered with several craters 30 kilometers (19 miles) in diameter. The pervasive cratering indicates that its surface must be several billion years old. Prometheus appears to have fewer craters indicating a younger surface while Pandora appears to have an older surface. Janus has few linear features.

Janus and Epimetheus share the same orbit of 151,472 kilometers (94,125 miles) from Saturn's center or 91,000 kilometers (56,547 miles) above the cloud tops. They are only separated by about 50 kilometers (31 miles). As these two satellites approach each other they exchange a little momentum and trade orbits; the inner satellite becomes the outer and the outer moves to the inner position. This exchange happens about once every four years. Janus and Epimetheus may have formed from a disruption of a single parent to form co-orbital satellites. If this is the case, the disruption must have happened early in the history of the satellite system.

Janus Statistics
Discovered by	Audouin Dollfus
Date of discovery	1966
Mass (kg)	2.01e+18*
Mass (Earth = 1)	3.3635e-07
Radius (km)	98x96x75
Radius (Earth = 1)	1.5365e-02
Mean density (gm/cm^3)	0.67*
Mean distance from Saturn (km)	151,472
Rotational period (days)	0.6945
Orbital period (days)	0.6945
Mean orbital velocity (km/sec)	15.87
Orbital eccentricity	0.007
Orbital inclination (degrees)	0.14
Escape velocity (km/sec)	0.0523
Visual geometric albedo	0.8
Magnitude (Vo)	14.5

What is Consuming Hydrogen and Acetylene on Titan?

Two new papers based on data from NASA's Cassini spacecraft scrutinize the complex chemical activity on the surface of Saturn's moon Titan. While non-biological chemistry offers one possible explanation, some scientists believe these chemical signatures bolster the argument for a primitive, exotic form of life or precursor to life on Titan's surface. According to one theory put forth by astrobiologists, the signatures fulfill two important conditions necessary for a hypothesized "methane-based life."

One key finding comes from a paper online now in the journal Icarus that shows hydrogen molecules flowing down through Titan's atmosphere and disappearing at the surface. Another paper online now in the Journal of Geophysical Research maps hydrocarbons on the Titan surface and finds a lack of acetylene.

This lack of acetylene is important because that chemical would likely be the best energy source for a methane-based life on Titan, said Chris McKay, an astrobiologist at NASA Ames Research Center, Moffett Field, Calif., who proposed a set of conditions necessary for this kind of methane-based life on Titan in 2005. One interpretation of the acetylene data is that the hydrocarbon is being consumed as food. But McKay said the flow of hydrogen is even more critical because all of their proposed mechanisms involved the consumption of hydrogen.

"We suggested hydrogen consumption because it's the obvious gas for life to consume on Titan, similar to the way we consume oxygen on Earth," McKay said. "If these signs do turn out to be a sign of life, it would be doubly exciting because it would represent a second form of life independent from water-based life on Earth."

To date, methane-based life forms are only hypothetical. Scientists have not yet detected this form of life anywhere, though there are liquid-water-based microbes on Earth that thrive on methane or produce it as a waste product. On Titan, where temperatures are around 90 Kelvin (minus 29 degrees Fahrenheit), a methane-based organism would have to use a substance that is liquid as its medium for living processes, but not water itself. Water is frozen solid on Titan's surface and much too cold to support life as we know it.

The list of liquid candidates is very short: liquid methane and related molecules like ethane. While liquid water is widely regarded as necessary for life, there has been extensive speculation published in the scientific literature that this is not a strict requirement.

The new hydrogen findings are consistent with conditions that could produce an exotic, methane-based life form, but do not definitively prove its existence, said Darrell Strobel, a Cassini interdisciplinary scientist based at Johns Hopkins University in Baltimore, Md., who authored the paper on hydrogen.

Strobel, who studies the upper atmospheres of Saturn and Titan, analyzed data from Cassini's composite infrared spectrometer and ion and neutral mass spectrometer in his new paper. The paper describes densities of hydrogen in different parts of the atmosphere and the surface. Previous models had predicted that hydrogen molecules, a byproduct of ultraviolet sunlight breaking apart acetylene and methane molecules in the upper atmosphere, should be distributed fairly evenly throughout the atmospheric layers.

Strobel found a disparity in the hydrogen densities that lead to a flow down to the surface at a rate of about 10,000 trillion trillion hydrogen molecules per second. This is about the same rate at which the molecules escape out of the upper atmosphere.

"It's as if you have a hose and you're squirting hydrogen onto the ground, but it's disappearing," Strobel said. "I didn't expect this result, because molecular hydrogen is extremely chemically inert in the atmosphere, very light and buoyant. It should 'float' to the top of the atmosphere and escape."

Strobel said it is not likely that hydrogen is being stored in a cave or underground space on Titan. The Titan surface is also so cold that a chemical process that involved a catalyst would be needed to convert hydrogen molecules and acetylene back to methane, even though overall there would be a net release of energy. The energy barrier could be overcome if there were an unknown mineral acting as the catalyst on Titan's surface.

The hydrocarbon mapping research, led by Roger Clark, a Cassini team scientist based at the U.S. Geological Survey in Denver, examines data from Cassini's visual and infrared mapping spectrometer. Scientists had expected the sun's interactions with chemicals in the atmosphere to produce acetylene that falls down to coat the Titan surface. But Cassini detected no acetylene on the surface.

In addition Cassini's spectrometer detected an absence of water ice on the Titan surface, but loads of benzene and another material, which appears to be an organic compound that scientists have not yet been able to identify. The findings lead scientists to believe that the organic compounds are shellacking over the water ice that makes up Titan's bedrock with a film of hydrocarbons at least a few millimeters to centimeters thick, but possibly much deeper in some places. The ice remains covered up even as liquid methane and ethane flow all over Titan's surface and fill up lakes and seas much as liquid water does on Earth.

"Titan's atmospheric chemistry is cranking out organic compounds that rain down on the surface so fast that even as streams of liquid methane and ethane at the surface wash the organics off, the ice gets quickly covered again," Clark said. "All that implies Titan is a dynamic place where organic chemistry is happening now."

The absence of detectable acetylene on the Titan surface can very well have a non-biological explanation, said Mark Allen, principal investigator with the NASA Astrobiology Institute Titan team. Allen is based at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Allen said one possibility is that sunlight or cosmic rays are transforming the acetylene in icy aerosols in the atmosphere into more complex molecules that would fall to the ground with no acetylene signature.

"Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed," Allen said. "We have a lot of work to do to rule out possible non-biological explanations. It is more likely that a chemical process, without biology, can explain these results - for example, reactions involving mineral catalysts."

Is there life out there?

Researchers have calculated that up to 37,964 worlds in our galaxy are hospitable enough to be home to creatures at least as intelligent as ourselves.

If there are 37,964 planets that could be home to aliens in our galaxy, the Milky Way, I wonder how many there are in the entire universe?

Well, apparently there are about 125 billion galaxies, meaning that there are probably about 4,745,500 billion planets which are able to sustain life. That’s a lot, and I betcha Earth is not the only one with life on it! Read more here.

What do you think, is there life on another planet?