A close encounter with the mid-sized, icy kind

Cassini looks over the heavily cratered surface of Rhea during the spacecraft's flyby of the moon on March 10, 2012, from a distance of about 43,000 km.
Cassini looks over the heavily cratered surface of Rhea during the spacecraft’s flyby of the moon on March 10, 2012, from a distance of about 43,000 km. Image: NASA/JPL-Caltech/Space Science Institute

In three days, NASA’s Cassini mission will fly by Saturn’s second-largest moon Rhea. While interest in the Saturnian moons has been hogged by the largest – Titan – Cassini‘s images of Rhea could provide important new information about a class of natural satellites that it exemplifies: the so-called ‘mid-sized’ moons. While Titan is big enough to be a planet, it is also an exception. Only 13 of Saturn’s 62 confirmed moons are bigger than 50 km in diameter. Among them, Rhea is the largest, and its diameter is still less than a third of Titan’s.

As the artist Michael Carroll wrote,

With the exception of planet-sized Titan, these moons make up a mid-sized family of bodies that are more poorly understood than the larger Galilean satellites.

Cassini will take two sets of images of Rhea during its fly-by scheduled for February 10. The first set, an 11-frame mosaic of its northern hemisphere facing away from Saturn, will be taken from a distance of 46,943 km. The second set will be a 16-frame mosaic of the southern hemisphere shot from a distance of 53,700 km. The images will have a peak resolution of 305 m/pixel. On both occasions, the probe will use the Imaging Science Subsystem (ISS), whose cameras are specifically designed to be able to study the moons’ surfaces, as well as the planet’s glorious rings.

Including Rhea, Saturn has seven known sizeable moons with icy surfaces; the other six are Dione, Enceladus, Hyperion, Iapetus, Phoebe and Tethys). Of them, Dione and Enceladus are thought to be significant contributors to the halo of charged particles that also orbits Saturn, around its giant magnetosphere. Enceladus is especially known to eject fountains of water vapor from near its south pole, whose constituent droplets become ionized by the magnetosphere. On the other hand, Rhea does the opposite of contribute to the halo: it absorbs charged particles, as a result of which its surface is charged. In fact, its surface has the strongest negative potential among the mid-sized icy moons.

Extensive studies of Earth’s moon have revealed that such an electrostatic potential (caused due to charged particles streaming in from the Sun) has accelerated dust particles into space. Astronomers have reason to believe a similar mechanism at play on some of Saturn’s moons – but with more intensity – could be moving dust particles between the moons. And Rhea could be at the center of this dusty relay.

Moreover, its tendency to absorb charged particles promotes a feeble radiolysis of its surface ice, feeding a thin atmosphere of ozone, hydrogen peroxide and molecular oxygen around it. Similar mechanisms have been used to explain molecular oxygen in the atmospheres of Dione and Jupiter’s moons Europa and Ganymede, and possibly on other Saturnian and Jovians moons, and on exoplanets, as well.

These finds could be further augmented by continued observations of Rhea. The last of them happened in 2013, during Cassini‘s last targeted mission for the moon when it came within 1,000 km. Then, it had found a surface studded with more craters than were seen on the other icy moons. Astronomers think this implies Rhea was battered with comets that could have been the source of a whiff of carbon dioxide that has been detected emanating from it. At the same time, Rhea alone might not have been battered. It is farther outward than Dione, Enceladus and Tethys, whose innards are warmed by tidal forces generated by Saturn’s gravitational pull. The warmth melts some of the surface ice and could cover up possibly hidden craters.

These features – mid-range mass, a highly charged surface with an unusual number of craters, oxygen in its atmosphere, and a depravity of tidal warming – all together keep Rhea in the spotlight, if only a fluctuating one. Even now, as Cassini embarks on the first of its 20 flybys of Saturn’s icy satellites in 2015, the ISS’s cameras will be turned toward Rhea only on February 10. On February 12, it will image Titan during the second of seven flybys, all slated for this year. In fact, at this moment, Europa remains the most talked-about icy moon in the Solar System.