Iapetus is the third largest moon of Saturn and this moon is best known for the remarkable two-tone colouration between its leading and trailing hemispheres, whereby the former is significantly darker than the latter. This enigmatic dichotomy has been debated for decades and radar and imaging observations by the Cassini spacecraft over the past few years has manage to paint a clearer picture of this two-faced moon. Iapetus has a mean diameter of 1470 kilometres and it orbits Saturn at a distance of 3.561 million kilometres, taking 79.32 Earth days to complete one orbit. Iapetus is the outermost of the regular satellites of Saturn and it is tidally locked such that one hemisphere permanently faces the direction of the moon’s motion around Saturn.
Credit: NASA/JPL/Space Science Institute
The difference in coloration between the two hemispheres of Iapetus is striking. The leading hemisphere of Iapetus appears dark with a slight reddish-brown tone, while the trailing hemisphere and the poles appear bright. Iapetus also has a massive equatorial ridge that runs precisely along the equator of the moon’s dark leading hemisphere and parts of the ridge tower more than 20 kilometres above the surrounding plains. This prominent equatorial ridge gives Iapetus a walnut-like appearance, as can be seen from images taken by the Cassini spacecraft. In this article, the focus will be on the remarkable two-tone colouration of Iapetus’ two hemispheres.
The leading hypothesis explaining the two-tone colouration of Iapetus is that the moon is continuously ploughing through a cloud of dark dust particles as it orbits around Saturn. Located far beyond the orbit of Iapetus is an irregular potato-shaped moon that is named Phoebe and these dust particles are believed to have originated from micrometeoroid impacts on Phoebe. In fact, all these dust particles from Phoebe form an enormous but extremely tenuous and virtually invisible ring of material around Saturn. The dust particles of this ring gradually migrate inwards towards Saturn. Phoebe has a retrograde orbit around Saturn and this means that it orbits Saturn in a direction that is opposite to that of Iapetus. Hence, the dust particles kicked off Phoebe are expected to collide with Iapetus head-on, at high velocities of approximately 7 kilometres per second.
The high velocity of the incoming dust particles explains why only the leading hemisphere of Iapetus gets coated by the dust particles as gravitational focusing by the gravity of Iapetus is insignificant. Furthermore, the dark region covering most of the leading hemisphere of Iapetus is centred precisely on the moon’s apex of motion. However, the result of dust deposition on the leading hemisphere of Iapetus cannot alone explain the extremely sharp boundaries between the regions of bright and dark material on the surface of Iapetus. Hence, a process of runaway ice sublimation has to occur for these sharp boundaries to exist. In such a process, regions darkened by dust absorbs more heat in the day, causing more ice to sublimate which in turn causes further surface darkening and heat absorption until no more surface ice is left.
The process of runaway ice sublimation removes ice from the darker regions and deposits them on the bright areas and at the frigid poles. Images of Iapetus taken by the Cassini spacecraft also show that ice removed from the darker regions can also be deposited on the cooler pole facing slopes of craters on the surface of Iapetus. This explains why the polar regions of Iapetus appear bright even though they extend into the leading hemisphere of Iapetus. The process of dust deposition and runaway ice sublimation both work together to give Iapetus its striking two-tone colouration.
Credit: NASA/JPL/Space Science Institute
Besides Iapetus, the moons Titan and Hyperion are also able to intercept the dust particles kicked off from Phoebe by micrometeoroid impacts. The orbits of Titan and Hyperion around Saturn are interior to the orbit of Iapetus. Hyperion is a small and irregularly shaped satellite of Saturn and it orbits Saturn between Titan and Iapetus. What is important about Hyperion is that it has a chaotic rotation whereby its orientation in space is unpredictable and changes all the time. Because of this, Hyperion does not have a two-tone colouration like Iapetus and its surface is instead more uniformly coated throughout by the dust particles.
A paper by Daniel Tamayo et al. 2011 entitled “Finding the Trigger to Iapetus' odd Global Albedo Pattern: Dynamics of Dust from Saturn's Irregular Satellites” investigates the capture of inward migrating dust particles by Iapetus, Hyperion and Titan. In this study, dust particles 10 micrometers or larger in size almost certainly strike Iapetus while a majority fraction of the dust particles ranging from 5 to 10 micrometers in size strike Titan. However, only a very small fraction of the inward migrating dust particles from Phoebe strike Hyperion due to the small physical size of Hyperion. Dust particles smaller than 5 micrometers in size migrate over a much shorter timescale as compared to the larger ones and most strike Saturn or completely escape the Saturn system.
Of the dust particles ranging from 5 to 10 micrometers in size, a majority fraction of them strike Titan as they migrate inward towards Saturn. Radiation pressure from sunlight significantly alters the trajectories of dust particles in this size regime. For these particles, the eccentricities of their orbits become large enough such that their orbits begin to cross the orbit of Titan before their probabilities of striking Iapetus approach certainty. There are two additional reasons that make Titan very efficient at intercepting dust particles. Firstly, Titan’s sheer size gives it a geometrical cross section that is over an order of magnitude larger than Iapetus’. Secondly, the relative velocities between the dust particles and Titan are substantially higher than for Iapetus, giving Titan a higher dust particle collision rate per unit frontal area. Like Iapetus, dust particles will strike Titan on its leading hemisphere. The thick atmosphere of Titan will fragment the incoming dust particles and globally distribute the materials that once make up the dust particles.
For dust particles larger than 10 micrometers in size, their slow inward migration gives Iapetus enough time to capture them before their orbits start to cross the orbit of Titan. Dust particles from other retrograde outer irregular satellites of Saturn can have comparable probabilities of striking Iapetus as the dust particles from Phoebe. A fraction of the surface material on Iapetus’ dark leading hemisphere may have originated from some of these retrograde outer irregular satellites of Saturn and this could explain the observed spectra differences between the surface material of Phoebe and Iapetus. Nevertheless, the amount of dust generated by Phoebe relative to the other retrograde outer irregular satellites of Saturn remains uncertain.