Thursday, December 10, 2009

Iapetus' dichotomy explained

Iapetus is Saturn's icy yin-yang satellite. This Cassini image shows that the side of this satellite that leads in its orbit about Saturn is 10 times darker than its polar regions. This darkening was long thought to be due to contamination from Saturn's even more distant satellite Phoebe. Meteorites crashing into Phoebe will launch small debris that goes into orbit about Saturn, resulting in a vast dust ring that also contaminates Iapetus. This dust ring was only recently discovered by the Spitzer Space Telescope. Some of that dark dust gets deposited at Iapetus' leading face, which is then warmed by sunlight. Recent work by John Spencer and Tilmann Denk show that this can cause surface ice there to evaporate at the equator. That water vapor can migrate towards and then freeze out at the poles, brightening the satellite there, and giving it its yin-yang appearance. Additional images can be found at Cassini's CICLOPS website.

Saturday, November 21, 2009

Cassini's close look at Enceladus' tiger stripes

Cassini had a close flyby of Enceladus today. This Saturnian satellite is famous for its tiger stripes, which are warm crevasses in Enceladus' icy surface, seen above. Geysers within those cracks also jet water out into space; these are the faint emissions seen below, which looks obliquely towards the tiger stripes. Additional images from this flyby can be found at the CICLOPS website.

Friday, November 20, 2009

LCROSS: is Moon's water due to comet impacts?

This image shows the plume that was raised when the LCROSS booster rocket struck the Moon in a region that is permanently shadowed from the Sun. The mission's goal is to search for the water-ice that might be frozen in these shadowed regions, since such ice would be a valued resource for any astronauts that might return to the Moon.

There are two likely sources for this water-ice. One is the solar wind, which can implant hydrogen into the lunar soil, which would then combine with the oxygen in soil to make water. Another source is comet impacts, which can deposit water as well as other volatiles that can then freeze out in these permanently shadowed regions. Since the spectra collected by LCROSS also reveals other volatiles, such as methane, ethanol, ammonia and carbon dioxide, all of which are known to exist in comets, cometary impacts are now a favoured theory for depositing water on the Moon.

Tuesday, November 3, 2009

Late Heavy Bombardment
may be due to comet impacts

The Late Heavy Bombardment (LHB) was a period of intense bombardment of the inner Solar System that is thought to have occurred about 3.85 billion years ago, when the Solar System was only about 700 million years old. Most of the craters on the Moon, including the giant lava-filled basins (also called Mare), are thought to have formed during this brief but intense period of bombardment. Although the Earth would have suffered a similar bombardment, geologic processes have since erased any such craters that formed then.

To study this possible bombardment of the Earth, Jorgenson and colleagues studied ancient sedimentary samples they collected in Greenland that are 3.9 billion years old (see the abstract of his paper). These samples were probably deposited around the time of the LHB. They find that the iridium abundance in those samples are elevated by a factor of 7, which indicates that comets (and not asteroids) are the principal source for the LHB impactors. Evidently, the entire inner Solar System was bombarded by icy comets that likely originated in the outer Solar System. This cometary bombardment might also have been triggered by a sudden rearrangement of the outer planets orbits, Jupiter through Neptune.

Tuesday, October 13, 2009

Quaoar: a giant rock in the icy Kuiper Belt?

Wesley Fraser and Mike Brown (Caltech) used the Hubble Space Telescope to place an upper limit on the size of Quaoar, which is a giant Kuiper Belt Object that inhabits the outer part of the Solar System. The Kuiper Belt is the swarm of icy comets that orbit just beyond Neptune, and these bodies represent the debris that was left over when the giant planets formed. The new upper limit on Quaoar's diameter is D<1100km, which Fraser reported at the recent DPS planetary science conference; see his abstract for details. Combining this size limit with Quaoar's known mass (about one-fifth Pluto's) yields a density that is at least 3.5 gm/cm^3. This result is a bit of a surprise, since Quaoar's density is much greater than that of water-ice, while bodies in the outer Solar System are generally expected to be composed mostly of water ice. However, internal heating in a sufficiently large body can temporarily melt it soon after its formation, which would then cause its rocky component to settle to its center before its watery outer layer freezes out. Afterwards, an impact with another large Kuiper Belt may have stripped the young Quaoar of its icy mantle, which would leave its rocky core exposed, and may account for its high density. Although this scenario is rather speculative, it is also quite plausible.

Friday, September 25, 2009

Recent impacts on Mars exposes subsurface ice

The Mars Reconnaissance Orbiter (MRO) took these images of a 6m crater on Mars in October 2008 (left) and again in January 2009; see this press release for more details. This crater is absent from images acquired in 2007, so it must be due to a relatively recent impact. Note also the bright material in the crater that fades over time. This is to be expected if this were subsurface ice that was suddenly exposed to the surface. Water ice is not stable at the surface of Mars, and will sublimate (vaporize) over time. The MRO spacecraft has discovered several new craters where fresh ice appears to fade over time. This particular crater has a latitude of 43 degrees, which indicates that subsurface ice on Mars extends all the way from from the poles to Mars' mid-latitudes.

Saturday, September 5, 2009

Martian gullies at crater's edge

Gullies are often spotted in sloped terrain on Mars, like the ones seen here at the edge of Hale crater on Mars. This image was acquired by the Mars Reconnaissance Orbiter August 3, 2009; that spacecraft has been observing Mars since March 2006. Martian gullies are of great interest, since their dendritic appearance suggests that groundwater might be seeping out and flowing downhill. However, after many years of study, it is still unclear whether wet or dry processes are responsible for sculpting these gullies. Dry processes include boulders or avalanches that might carve out these gullies as rocks and gravel tumble downhill. Also keep in mind that the martian surface is too cold for liquid water to exist there. Nonetheless, any groundwater would absorb salts from the surrounding rock, which might lower its freezing point enough to exist in liquid form. And where there is liquid water, there is also the possibility for microbial life. For more details, see this press release.

Sunday, August 30, 2009

Circumstellar debris disk orbiting HD 32297

This is the circumstellar debris disk that orbits the star HD 32997, imaged with the Palomar 5m telescope by Dimitri Mawet and colleagues. The star lies at the cross, but its light has been blocked by a phase mask coronograph, which is a device that shifts the phase some of that starlight so that the star's light waves interfere with itself destructively, effectively making the very bright star dissapear from this image. This is very useful, since it also reveals the light from the much fainter circumstellar material.

The colored blobs indicate that there is a ring or perhaps a disk of dust in orbit about this star, with that disk/ring seen nearly edge on. The dust grains are visible because they are reflecting starlight, and the colors indicate the intensity of that reflected light. Of particular interest to me is the asymmetry seen in this disk, with one side being brighter than the other by ~50%.

These dusty disks usually have rather short lifetimes, since dust grains destroy each other when the collide with each other. Consequently, other unseen `planetesimals' are implicated here, since collisions by these asteroidal or cometary bodies are needed to continually resupply the disk with the dust seen here. And since comets or asteroids are evidently forming in this system, it seems plausible that larger planets might have formed here, too. Additional details are also available in the paper by Mawet et al.

Tuesday, August 18, 2009

Ingredients for prebiotic life found in comet

The Stardust spacecraft encountered comet Wild 2 on January 2, 2004, and collected samples of the comet's coma and tail. Those samples parachuted to Earth on January 15, 2006, and have been studied in labs ever since. From these samples, Jamie Elsila at NASA/Goddard reports the first ever detection of glycine in a comet. Glycine is a common amino acid, which when combined with others can build proteins, which are important building blocks in the chemistry of life. The detection of glycine in comet Wild 2 strengthens the argument that comet impacts on the early Earth provided the prebiotic chemistry that ultimately allowed life to begin on Earth. However, critics of this cometary-delivery theory would argue that comet impacts are far too fiery and energetic to deposit complex organic molecules on Earth whole and unscathed. Regardless, this interesting finding highlights the value of sample-return missions, which can provide valuable insight into composition of ancient and primitive bodies, like as comets and asteroids, that are the primordial building blocks of the planets. See this press release for more details.

Tuesday, August 11, 2009

Planetary-sized impacts around HD 172555?

Casey Lisse (JHU/APL) and colleagues recently used the Spitzer Space Telescope to collect infrared spectra of the dust that orbits the relatively young 12 million year-old star HD 172555. Their observations are described in this preprint. Their spectra shows that this star's circumsteller dust is, as expected, rich in silicate, which is the principle ingredient in circumstellar dust. What is surprising here is that this dust is a glassy silicate, like tektite or obsidian, which tends to form when rocky bodies collide at high speeds of ~10km/sec. These spectra also indicate the presence of ample amounts of SiO gas, which is vaporized rock. From these spectra, Lisse and colleagues infer that this system suffered a recent giant impact via the collision of two large ~1000km bodies (the size of Ceres, the largest asteroid in our Solar System). They estimate that this giant impact occurred within the past ~100 thousand years. There are two possible interpretations of these observations. (1) Collisions among ~1000km-sized protoplanets at HD 172555 indicate that this system is currently undergoing planet formation. This is an important step in the planet-formation process, and is necessary if one wishes to ultimately produce a system of ~10,000km-sized terrestrial planets. (2) Alternatively, giant impacts are instead destroying the protoplanets that orbit HD 172555, and that astronomers are witnessing the collisional destruction of a young planetary system. Which outcome is more likely is presently unclear. See this Spitzer page for more details, as well as the above artist's rendition of a giant impact.

Saturday, August 8, 2009

Small moonlet discovered in Saturn's B ring

The Cassini spacecraft spotted this tiny moonlet as it orbits within Saturn's vast and dense B ring. Again, the rings are observed very near equinox, so the Sun's illumination streams nearly along the ring plane, and small objects can cast long shadows here. From the 40km length of the moonlet's shadow, Cassini scientists can infer its diameter of 0.4km. The view here is of the outer part of the B ring. Saturn is far off to the left, and the dark region on the right is the Cassini Division, in which the Huygen's ringlet (grey ribbon) also resides. Check the CICLOPS website for more details and other great images of Saturn's ring/satellite system.

New radar image of rare triple asteroid system

A new triple asteroid system was discovered by Marina Brozovic and Lance Benner (JPL) via radar imaging; see this press release for details. Shown here are two small ~50m satellites orbiting a larger 700m asteroid named 1994 CC. Although other triple asteroid systems are known, most are in the main asteroid belt that lies between Mars and Jupiter. However, this is only the second triple asteroid system to have been detected among the near-Earth asteroid population.

Thursday, August 6, 2009

Topography at outer edge of Saturn's B ring?

This very interesting Cassini image shows a close-up of the outer edge of Saturn's B ring. Saturn is far to the left, so the ring's orbital motion here is either up or down (and my best guess says down). The Cassini Division is to the right, just beyond the B ring. The Huygen's ringlet is the prominent gray band that orbits 300km beyond the B ring's edge.

Recall that Saturn is almost at equinox (Tuesday August 11!), so the Sun is just above Saturn's equator. Consequently, sunlight is streaming almost parallel to the ring plane, which allows even very modest vertical structures in the ring to cast long shadows across the ring plane. Evidently, the B ring's outer edge has topography, since it cast shadows that are hundreds of kilometers long! Judging by how ragged the shadows are, this ring-edge seems to resemble a mountain range, which is quite a surprise since the rest of the ring-plane is extremely flat. Note also the bright diagonals, one of which is clearly casting a shadow. These streaks might be due to ring material moving radially, perhaps due to avalanches of ring-matter tumbling down the supposed mountainside? Such radial motion would then get dragged along a diagonal due to the ring's faster orbital speed in regions closer to Saturn. But at this stage, this is all just speculation...

This image was acquired on July 26, 2009, and can be found at the Cassini Equinox Mission's raw image archive. A followup comment will describe how to use this archive effectively to search for other interesting Cassini images.

Saturday, August 1, 2009

Vertical ripples in Daphnis' edge-waves

The small speck casting a shadow here is Daphnis, which is an 8km satellite that orbits within the narrow Keeler gap that lies near the outer edge of Saturn's rings. Saturn is also very near equinox, so the sunlight that is illuminating this scene travels nearly parallel to the ring-plane. Consequently, this small satellite casts a rather prominant shadow across the ring. Daphnis also has a small inclination, so its motion carries it a bit above and then below the ring plane during each orbit of Saturn. Daphnis' gravity then tugs the nearby ring material up and down, too, which results in the vertical ripples that are seen at the Keeler gap's inner and outer edges. Note that these bright ripples also cast shadows as well. This image was acquired by the Cassini spacecraft on June 8, 2009, and many more such images can also be found at the CICLOPS website.

Rover spots meteorite on Mars?

The Opportunity rover recently spotted this rock on Mars, whose texture resembles a meteorite. The rover is seen using its X-ray spectrometer, which should reveal its composition, and will hopefully determine whether this rock is indeed a meteorite. See this press release for more pictures and details.

Friday, July 24, 2009

New Hubble image of impact site on Jupiter

This optical HST image of the impact site on Jupiter was collected by Heidi Hammel (SSI) and others using HST's Wide Field Camera 3. This camera is still new and not fully calibrated yet, since it was installed only two months ago by Shuttle astronauts. Nonetheless, it is still able to produce a magnificent image. The current estimate for the impactor diameter is about a third of a kilometer. See this press release for more details.

Thursday, July 23, 2009

Great eclipse pictures

Some wonderful pictures of yesterday's total solar eclipse over Asia are available at

Monday, July 20, 2009

Keck image shows likely impact on Jupiter

Above is an infrared image of Jupiter, as observed by Paul Kalas, Michael Fitzgerald and Franck Marchis at the Keck telescope. Note the bright spot, which overlaps the mysterious black spot that recently appeared on Jupiter (see earlier post, below), indicating that this spot is indeed hot. This supports the notion that Jupiter was indeed hit by an impacting comet or asteroid. See New Scientist for more.

New dark spot on Jupiter:
due to asteroid or comet impact?

Anthony Wesley from Murrumbateman Australia reports seeing a black spot in Jupiter's atmosphere. Black spots on Jupiter are reported from time to time, the most famous example of which was due to the impact of comet Shoemaker-Levy 9. That comet orbited Jupiter unseen until 1992, when it was disrupted by that planet's gravitational tide into about 20 fragments during a particularly close approach to the planet. Those fragments then struck Jupiter in 1994, resulting in similarly black (but much larger) bullseyes. Followup observations of this new spot will hopefully reveal whether it might be due to an impact by a comet or asteroid, which is a very rare astronomical event. Stay tuned.

Wednesday, June 24, 2009

Enceladus might have a saltwater ocean

Frank Postberg (Max Planck Institute, Germany) has a Nature letter on Cassini's detection of sodium salts in Saturn's E ring; see also this press release. This is a very interesting result, because it implies that Saturn's satellite Enceladus might have a liquid water ocean beneath its icy surface. Recall that in 2005, the Cassini spacecraft spotted geysers shooting tiny ice crystal from cracks in Enceladus' surface (pictured). Those ice grains go into orbit about Saturn and form that planet's tenuous E ring. During subsequent passages through the E ring, Cassini's dust detector was then used to determine the composition of those ice grains, and found them to contain salt at the 1% level. Because those ice grains originated inside Enceladus, Postberg and co-authors argue that these grain's high salinity is possible if, under Enceladus' ice, there is also a liquid ocean there that lies on top of a rocky core that is the source of the salt. Note that there is also an astrobiology angle here, too, since if Enceladus is warm enough to maintain a liquid water ocean, then there is also the possibility for ocean life there, too.

Friday, June 12, 2009

Shadows along the Keeler Gap

This Cassini image is looking towards the outer edge of Saturn's main A ring. The dark band near the ring's outer edge is the Keeler gap, which is maintained by the small 8km satellite Daphnis, which is the white speck there that casts a shadow across the ring plane. Daphnis' orbit is also inclined slightly relative to the ring plane, which carries it above/below the ring plane with each orbit about Saturn. Due to this up/down motion, Daphnis' gravity on the ring also pulls the nearby ring material at the gap's edge up/down by about 1 km. And because Saturn is near its equinox, the Sun's illumination here is almost horizontal across the ring plane, causing these km-high piles of ring particles to appear brightly lit on their sunward side, which also casts shadows across the ring plane. Check the Cassini/CICLOPS website for more details, or the recent paper in AJ by Weiss et al (subscription required).

Wednesday, June 10, 2009

Shadows cast by disk-embedded planets

A recent theory paper by Hannah Jang-Condell (U. Maryland) examines the shadows that might be cast by recently-formed planets as they orbit within the circumstellar disk in which they formed. Her numerical models show that the planet's gravity will 'depress' the disk there. If that disk were then viewed by an astronomer at optical wavelengths, then that depressed spot would resemble a dark pothole, since that depression is not illuminated by the central star (see Figure). The exception is at the pothole's far side, which would instead appear as an illuminated bright spot. Note that extra-solar planets are difficult to see via direct imaging. However this work suggests a new technique that might be used to discover unseen planets indirectly---by searching for these planet's darkened potholes and dimples that they create in a planet-forming disk.

Tuesday, May 26, 2009

Mars landers:
destroying the organics they were sent to find?

Even if there is no life present on Mars, that planet should have organic molecules on its surface, due to impact delivery by asteroids and comets, which are known to contain organic carbon-bearing molecules. However, organics have not been detected by any landers sent to look for these materials---not by the Viking landers, nor by the more recent Phoenix lander. Organics are detected by heating soil samples in an onboard oven, and looking for their signature in the gases released during heating.

However, Douglas Ming (Johnson Space Center) and colleagues note that the Martian surface contains perchlorate salt, which releases oxygen when heated. Ming's experiments also show that this released oxygen might then burn up any organics present in the soil samples. So if Ming's thinking is correct, this might explain why the Martian landers have not detected organics on Mars---their detection methods might have been burning up the organics that they seek. For more information, see this New Scientist article, as well as Ming's LPSC abstract.

Friday, May 22, 2009

Split Comet Schwassmann-Wachmann 3

Comet Schwassmann-Wachmann 3 broke up into several fragments in 1995. Its orbit period is 5.4 years, so came closest to the Sun again in 2001 and 2006. Bill Reach and colleagues used the Spitzer Space Telescope to observe this comet in infrared wavelengths in May 2006. The faint band connecting the fragments is a trail of debris that traces their orbit about the Sun. These astronomers detected 55 fragments along this comet's orbit, several of which are seen above. The color image of two brighter components show their dusty tails in red with a hint of green to show that CO2 gas is emitted from the sunward-facing parts of the comet nuclei. These fragments comae and tails are generated as the icy cometary nuclei warm and sublimate (evaporate) in the sunlight, whose weak pressure also sweeps the dust away in the anti-sunward direction. A preprint by Reach et al on these Spitzer observations is also available.

Friday, May 15, 2009

Shuttle transits the Sun

Space Shuttle Atlantis was photographed in silhouette against the Sun on May 13th by French astrophotographer Thierry Legault, while in Florida. The shuttle's transit across the Sun lasted only 0.8 seconds, and this picture was taken before the shuttle caught up with Hubble. See Legault's website for other other very nice astronomy pictures.

Wednesday, May 13, 2009

Girl who named Pluto dies at 90

Venetia Phair, UK, was 11 when Pluto was discovered by Clyde Tombaugh at the Lowell Observatory in 1930. Her grandfather, Falconer Madan, told her of the discovery, and discussed possible names for the new planet. ‘Why not call it Pluto?’, she suggested. Madan passed the suggestion on to friend Herbert Hall Turner, professor of astronomy at Oxford, who in turn telegraphed Lowell Observatory, which endorsed the suggestion. Venetia's grandfather gave her a five-pound note for her idea. Venetia died April 30 in her home in Banstead, England.

Monday, May 11, 2009

Shuttle mission to Hubble

Space shuttle Atlantis launched today, to service the Hubble Space Telescope for the forth and final time. Astronauts will replace several new cameras and
spectrographs (which will be used to measure the composition of stars, planets, galaxies, etc), and repair others. Astronauts will also replace one of two redundant devices that handle data, will replace Hubble's batteries, and will replace all of the HST's six gyroscopes (which orient the telescope), of which only half now work. One of three Fine Guidance Sensors--which also help point the telescope and are used to hunt for extra-solar planets--will also be replaced. A ring will also be attached to the telescope's backend, so that a rocket can at a later date be attached and used to deorbit Hubble at the end of its life. Hubble has been operating for 18 years, and has been one of the most productive telescopes ever. This servicing mission should extend its life for another 5+ years.

Thursday, May 7, 2009

Shadows at the edge of Saturn's B ring

This fascinating image is from the April 15 Astronomy Picture of the Day. The image was acquired by the Cassini spacecraft, which is in orbit about Saturn. The brighter part of this image is a close-up of the outer edge of Saturn's B ring, while the lower darker part shows the fainter ring material that orbits in the Cassini Division. Saturn is approaching equinox, which means that the Sun is near the ring plane, which also allows the satellite Mimas to cast its shadow on the ring plane (dark vertical streak).

Note also the dark `cookie bites' missing from the outer edge of the B ring. These seem to be shadows cast by something that lies right at the ring edge, possibly very large ring particles orbiting there. But note the bright ringlet that also appears at the B ring's outer edge; if that ringlet is puffy, or otherwise kinky in the vertical direction, then that ringlet might be casting these shadows. Also keep in mind that Mimas has a 2:1 resonance at the B ring's edge, which is where a ring particle orbits twice for every orbit of Mimas. So it is conceivable that resonance might be 'snowplowing' the B ring edge, with ring material also piling up in a vertically above and below the ring-plane as Mimas also shoves it radially inwards. If so, then this snowpiling might instead be responsible for these shadows.

NASA's 2010 budget

NASA announces its budget for fiscal year 2010. The table is from the budget summary, which shows how NASA spends its money. The 2010 budget is about $18 billion, with more than half spent on Exploration/Space Ops (eg, manned spaceflight), and almost a third on Science, with the rest spent on activities at various NASA centers (Johnson, Ames, etc). Of interest to me is the Planetary Science budget line, which stays flat until year 2011.

Wednesday, May 6, 2009

Image of planet orbiting Fomalhaut

This remarkable image shows the motion of the Jupiter-mass planet as it orbits the star Fomalhaut (see inset). This optical image was acquired by Paul Kalas (UC Berkeley) and colleagues using the Hubble Space Telescope. Interestingly, this star also harbors a dusty circumstellar debris disk. The planet orbits just inside a gap within this disk, and its gravity is responsible for keeping that gap clear of dust. Note also that the debris disk appears to resemble a ring, due to the disk's inner edge being illuminated by the central star. The dust in these debris disks is thought to be generated by collisions among unseen planetesimals (eg, asteroids or comets) that also probably orbit within this disk. The ellipse in this graphic has a radius of 30 AU (1 AU = Sun-Earth distance), which is the radius of Neptune's orbit. The radius of the disk's inner edge is about 140 AU, which is about 3 times larger than the size of our Solar System's Kuiper Belt, which is also a belt of comets orbiting just beyond Neptune.

Welcome to Solar System Watch!

First post...welcome! This blog is devoted to the latest astronomy news, with a special emphasis on planetary science, which is also my field of study. And if you are wondering who is doing the blogging here, please have a look at my profile.

The blog's title graphic is extracted from this image of Saturn that was acquired by the Cassini spacecraft on May 4, 2005. Here we are looking obliquely onto the dark side of the ring plane. Saturn was in winter when this image was acquired, so the Sun is south of the rings, which also cast shadows onto Saturn's northern hemisphere.