January 17, 2018

22.04.10-The Exotic World of Prometheus.

The moons of Saturn continue to astound. The count now stands at 61, and one by one, NASA’s Cassini orbiter is giving us a close up look at these unique worlds, some for the first time. Last year, Cassini passed within 36,000 miles of Prometheus just the day after Christmas. Discovered by Voyager 1 in 1980, this shepherd moon dips within the F-ring once every 15 hour orbit. This fact is apparent as the oblong cratered surface on the 74 mile long moon is coated with a fine layer of dust, giving it a smooth appearance. The constant “plowing” of these moons through Saturn’s rings cause the grooves that we see, and also confines the F-ring. These images are especially satisfying to Carolyn Porco, lead scientist of the Cassini research team who was also on hand for the tiny moon’s initial discovery by Voyager in 1980. It’s likely that we won’t get another look at this bizarre shepherd moon for some years to come!

03.04.10- Messenger and the Mysteries of Mercury.

The history of the inner most planet is an enduring puzzle to planetary scientists. On September 29th of last year, NASA’s Messenger spacecraft passed within 142 miles of Mercury’s night side in an orbital “tweak” on its way to eventual orbital insertion on March 18th, 2011. During that pass, the spacecraft once again measured the trailing exo-sphere, a thin trailing wind made of sodium, potassium, calcium, and magnesium. This “mercurial wind” is replenished either by solar radiation pressure, micro-meteoroid impact, or a combination of the two. The mystery is the ratios of calcium and magnesium observed that is significantly different than predicted. Mercury is a rocky iron world that is over half core and believed to have only a thin mantle and crust. Either Mercury formed that way early in its history, a young Sun boiled away a majority of silicates, or Mercury suffered a major crust stripping impact. Further evidence for the impact scenario comes from Messenger’s neutron spectrometer, which registered a conspicuous lack of low-energy neutrons emanating from the surface of the planet itself. This is highly suggestive of an iron and titanium rich surface similar to what’s found in basaltic rock on the lunar near side. Whatever the case, plenty of surprises await us as Messenger takes up permanent residence around Mercury next year!