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.

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.

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.

Late Heavy Bombardmentmay 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.

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.

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.

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.