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.