Centauri Dreams recently discussed the discovery of so-called ‘main-belt comets’ — icy objects found in asteroid-like orbits that apparently formed in the inner Solar System rather than on its outer edges. The work, performed by Henry Hsieh and David Jewitt (University of Hawaii) raises questions about the origins of Earth’s water supply, which had been thought to have been delivered by cometary impacts on the primordial Earth. Could this water have, in fact, been delivered by main-belt comets, and could a mission to one of them yield the answer?
A sharp-eyed reader wanted to know more: assuming we flew such a mission, how could we pin down the main-belt comets as the source, as opposed to the huge population of long-period comets with their highly elliptical orbits? Henry Hsieh was kind enough to respond:
In recent years, the debate over the origin of the Earth’s water has focused on the so-called D/H (deuterium to hydrogen) ratio of ocean water, comet water, and meteorite water (in the form of hydrated minerals; used as a proxy for “asteroid” water). The two following links provide technical and layman’s discussions, respectively, on this issue:
Jewitt, Chizmadia, Grimm et al., Water in the Small Bodies of the Solar System (see Section 4.1)
Essentially, measurements of long-period comets seem to show that they are overabundant in deuterium as compared to terrestrial water. Therefore, there must be another source of water, perhaps the dominant one, that has a D/H ratio closer to that of “standard ocean water”. The asteroid belt is a possibility (recent orbital models also show that asteroids from the main belt were more likely than comets from the outer solar system to strike the Earth in sufficient quantities to supply the volume of water we see here today), but until now, the only D/H measurements we have of asteroids were derived indirectly from hydrated minerals (which are not quite the same as actual water ice) in meteorites.
Now that the MBC [main-belt comet] class has been identified, we would obviously very much like to know what the D/H ratio in MBC ice is. We surmise that it may be different from that measured for other comets since these objects formed at a different location (and therefore at a different temperature, which may affect deuterium abundance/retention in forming planetesimals) in the protoplanetary disk that was the early solar system, and might be closer to the D/H ratio measured for Earth water. This of course needs to be confirmed or refuted by observational evidence yet to be obtained.
Unfortunately, the activity of the MBCs discovered thus far is too faint to hope to make a meaningful D/H measurement from the ground. The close proximity of the MBCs to the Earth, their stable, predictable orbits, and the fact that they possess surface or near-surface ice that even occasionally sublimates, releasing water vapor into space, makes them attractive spacecraft targets, however. A visit to one or more MBCs by a spacecraft equipped to make D/H measurements would certainly shed more light on this issue.
Centauri Dreams’ note: A mission to measure deuterium/hydrogen ratios in the main-belt comets so far identified seems well within the reach of current technology, and as we noted yesterday, NASA has just reversed its cancellation of the DAWN mission to the two largest main-belt asteroids. The measurements that could result from a main-belt comet mission would provide information that is more than purely historical. We’ve seen that at least one nearby star of high astrobiological interest, Tau Ceti, seems to be surrounded by a vast cometary cloud (see The Comets of Tau Ceti). Our understanding of what it takes to create terrestrial water worlds could be materially advanced by further information on the delivery mechanism of water to the habitable zone surrounding such stars.