Astronomy Picture of the Day - powered by LUNEXIT.IT

There is no doubt that Rhea's Surface Features strongly resemble those of Dione, thus suggesting similar composition and histories. The average temperature on Rhea is about 99 Kelvin (such as −174° Celsius) in Direct Sunlight and between 73 K (−200° C) and 53 K (−220° C) in the shade.

Rhea has a rather typical heavily Cratered Surface,with the exceptions of a few large Dione-type Chasmata - or deep Fractures, also known as "Wispy Terrain") on its Trailing Hemisphere (such as the side of the Celestial Body facing away from the direction of motion along Rhea's orbit)and a very faint "line" of material at its Equator that does not have a completely satisfying explanation about its true nature yet - even though it was thought, some time ago, that such a Line could have been created by deorbiting (---> falling down) material coming from Rhea's alleged Ring System (see, about this specific sub-topic, the last four paragraphs of this article).

Rhea has two very large Impact Basins, which are about 400 and 500 Km across. The more Northerly and less degraded of the two, called Tirawa, is roughly comparable to the Basin Odysseus (located on the Saturnian moon Tethys). There is also a 48 Km-diameter Impact Crater at about 112° West Long. that is prominent because of an extended system of Bright Rays. This Impact Crater, called Inktomi (but also nicknamed "The Splat"), may be one of the youngest Craters located on the Inner Moons of Saturn. No evidence of any endogenic activity has been discovered. Its Surface can be divided into two geologically different areas based on Crater Density: the first area contains Craters which are larger than 40 Km, where the second area, covering parts of the Polar and Equatorial Regions of Rhea, has only Impact Craters under that aforementioned size. This fact suggests that a Major Resurfacing Event must have occurred in a far distant past of this moon.

The Leading Hemisphere of Rhea is heavily cratered and uniformly bright. As on the Jovian moon Callistus, the Craters of Rhea lack the High Relief Features (such as, among others, Central Peaks/Uplifts) which are seen, instead, both on the Moon and Mercury. On the Trailing Hemisphere there is a network of bright swaths on a dark background and few visible Impact Craters. It had been thought that these bright areas might be material ejected from Ice Volcanoes early in Rhea's history when its interior was still liquid. However, observations of Dione, which has an even darker Trailing Hemisphere and similar, but more prominent, bright streaks, show that the streaks are actually Ice Cliffs resulting from extensive fracturing of the moon's Surface. The January 17, 2006 distant Fly-by of Rhea carried out by the NASA - Cassini Spacecraft yielded images of the Wispy (Trailing) Hemisphere at better resolution and a lower Sun Angles than previous observations. Images from this and subsequent Fly-Bys showed that Rhea's Streaks are, in fact, tectonically formed Ice Cliffs (also known as Chasmata), very similar to those that can be found on the other Saturnian moon, Dione.

On November 27, 2010, NASA announced the discovery of a tenuous Atmosphere on Rhea, such as an Exosphere, that mainly consists of Oxygen and Carbon Dioxide in proportion of roughly 5 to 2. The Surface Density of the Exosphere is from 10⁵ to 10⁶ molecules per cubic centimeter, depending (also) on the Local Temperature. The main source of the Oxygen is thought to be the Radiolysis of Water Ice at the Surface by Ions supplied by the Magnetosphere of Saturn. The source of the Carbon Dioxide, instead, is less clear, but it may be related to oxidation of the organics present in the Ice or, maybe, it could even be due to Outgassing Phenomoena.

On March 6, 2008, NASA announced that Rhea may have a tenuous Ring System. This discovery, if confirmed, would have marked the first discovery of Rings around a moon. The Rings' existence was inferred by observed changes in the flow of electrons trapped by Saturn's Magnetic Field as Cassini passed by Rhea. Dust and Debris could extend out to Rhea's Hillsphere, but were thought to be denser nearer the moon, with three narrow Rings of higher density. The case for a Ring was strengthened by the subsequent finding of the presence of a set of small UltraViolet-bright spots distributed along Rhea's Equator (interpreted - we repeat - as the Impact Points of Deorbiting Ring Material).

However, when the NASA - Cassini Orbiter made targeted observations of the putative Ring-Plane from several angles, NO EVIDENCE of Ring Material was found, suggesting that another explanation for the earlier observations was needed. In relation to this point, we wish to highlight that this specific - even though not final, at least for the time being - conclusion, had been quickly hypothesized by Dr Paolo C. Fienga, in a small Abstract entitled "Rings of Fire", and dated March, 11, 2008.

In this Abstract, Dr Paolo C. Fienga hypothesized that the alleged Rings of Rhea could just have been the result of a Transient Phenomenon, such as the crumbling of a small Asteroid which DID NOT successfully passed the Roche Limit and whose rocky remains would have LIKELY stayed - only for a relatively short time period - near/in the proximities of Rhea, or maybe even around the moon itself, BUT ON VERY UNSTABLE orbits, and then, eventually, they would have either fallen down on Rhea's Surface (like it seems that it actually happened), or got dispersed in the Saturnian Space.

Apparently, this hypothesis - not very much appreciated at the time - seems to have found, only four years after being conceived, the substance that it needed to be proven as - very likely - the correct one.

This frame has been colorized in Natural Colors (such as the colors that a perfect human eye would actually perceive if someone were onboard the NASA - Cassini Spacecraft and then looked outside, towards the Saturnian moon Rhea), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team.