Black rain: The burial of the Galilean satellites in irregular satellite debris

Irregular satellites are dormant comet-like bodies that reside on distant prograde and retrograde orbits around the giant planets. They are likely to be captured objects. Dynamical modeling work indicates they may have been caught during a violent reshuffling of the giant planets ∼4 Gy ago (Ga) as described by the so-called Nice model. According to this scenario, giant planet migration scattered tens of Earth masses of comet-like bodies throughout the Solar System, with some comets finding themselves near giant planets experiencing mutual encounters. In these cases, gravitational perturbations between the giant planets were often sufficient to capture the comet-like bodies onto irregular satellite-like orbits via three-body reactions. Modeling work suggests these events led to the capture of on the order of ∼0.001 lunar masses of comet-like objects on isotropic orbits around the giant planets. Roughly half of the population was readily lost by interactions with the Kozai resonance. The remaining half found themselves on orbits consistent with the known irregular satellites. From there, the bodies experienced substantial collisional evolution, enough to grind themselves down to their current low-mass states.Here we explore the fate of the putative irregular satellite debris in the Jupiter system. Pulverized by collisions, we hypothesize that the carbonaceous chondrite-like material was beaten into small enough particles that it could be driven toward Jupiter by Poynting–Robertson (P–R) drag forces. Assuming its mass distribution was dominated by D > 50 μm particles, we find that >40% ended up striking the Galilean satellites. The majority were swept up by Callisto, with a factor of 3–4 and 20–30 fewer particles reaching Ganymede and Europa/Io, respectively. Collision evolution models indicate most of this material arrived about 4 Ga, but some is still arriving today. We predict that Callisto, Ganymede, Europa, and Io were buried about 4 Ga by ∼120–140 m, 25–30 m, 7–15 m, and 7–8 m of dark debris, respectively. The first two values are consistent with observations of the deepest dark lag deposits found on the most ancient terrains of Callisto and Ganymede. The rest of the debris was likely worked into the crusts of these worlds by geologic and impact processes. This suggests the debris is a plausible source of the dark lag material found in Europa’s low-lying crevices. More speculatively, it is conceivable that the accreted dark particles were a significant source of organic material to Europa’s subsurface ocean.

► Collisions among ancient irregular satellite population made lots of dark particles.
► About 0.04% lunar masses driven onto Galilean satellites by non-gravitational forces.
► Callisto, Ganymede, Europa once buried by 120–130, 25–30, and 15 m of dark debris.
► Ancient surfaces still buried; younger surfaces have dark lag and fresh debris.
► Ancient debris possibly provided organic material to Europa’s subsurface ocean.