On March 9, Cassini made a close approach of Rhea at an altitude of 997 kilometers (620 miles).
Cassini’s encounter with Rhea was the final close encounter with the moon, and the fourth flyby overall. The encounter was primarily designed to measure changes in Rhea’s gravity field, which will give clues as to the moon’s interior structure. The main question is whether or not the moon has differentiated – whether the original mix of ice and rock has separated into a rocky core and an icy mantle.
Originally, Rhea was believed to have been big enough that gravity had allowed the denser rock to separate from the surrounding ice and sink to its core. However, data from previous Cassini flybys showed that might not be the case. The problem was that a measure of the moon’s gravity distribution, its moment of inertia, was too high. The value calculated by Cassini flybys would mean that the moon hadn’t differentiated at all. That number has been refined somewhat lower, suggesting that Rhea has at least partially differentiated, but the exact value of the moment of inertia has yet to be determined.
Previously, the moment of inertia had been calculated by simply measuring the change in Cassini’s orbit caused by the flyby. However, the large spread in the moment of inertia derived from previous encounters meant that the more sensitive Radio Science Subsystem (RSS) needed to be used. The RSS is designed to transmit radio waves of known frequency back towards Earth. Because the original frequency is known to scientists here on Earth, it allows for very precise determinations of velocity changes caused by slight changes in gravitational fields as the probe moves around a body. These precise measurements allow for a more refined measure of the moment of inertia, which will allow us to finally settle the question of whether or not the moon has differentiated.
Since the Rhea flyby was primarily designed to allow the RSS to measure the moon’s gravity field, the probe was unable to slew to take images of the moon. As a result only a handful of images were returned from the moon. Soon after closest approach, Cassini snapped a few images of the surface in the direction it was pointed, along with a series of photos further out to get a global view.
The close-up images reveal a long, narrow fracture where a portion the surface has faulted and sank relative to its surroundings. The fracture is similar to scarps found on the Moon and Mercury, where cooling has caused the crust to contract. The scarp imaged by Cassini appears to be fairly young, as it offsets fresher-looking craters. The exact age of the scarp will probably never be determined, it does indicate that Rhea is still contracting as it cools.
The flyby marks the last extremely close approach (less than 10,000 km) to Rhea for the Cassini mission. The next close encounter will not be until 2015, at a planned distance of approximately 47,000 km (29,000 miles). That flyby will be a nice photo-op to get some nice global pictures of the planet, but will be nowhere near the detail seen by previous close-up encounters.