How common are water-rich asteroids?
If humans are ever to live in space, they are going to need to find water, for drinking, for making breathable air, and even for fuel. Water could be extracted from water-rich asteroids known to exist in the asteroid belt and near-Earth space.
One such asteroid is 101955 Bennu, an asteroid about 500 m across that was visited by the OSIRIS-REx spacecraft between 2018 and 2021. Bennu is considered a likely fragment of a large 100-km scale protoplanet. In visiting the tiny asteroid, OSIRIS-Rex revealed information which will keep planetary scientists busy for decades. One particularly interesting recent discovery is evidence of flowing water on the primordial protoplanet or parent body. That is a lot of water.
This is based on recent analysis of boulders on the surface of Bennu that reveal geological evidence that the material forming the boulder was originally deposited as accumulating sediment. This implies that there was running water within the parent body of Bennu.
This is particularly interesting because of its connection to (1) Ceres. The dwarf planet (1) Ceres was visited by the Dawn spacecraft between 2016 and 2018, just before OSIRIS REx reached Bennu.
Evidence from images and spectral data, i.e., what wavelengths of light are absorbed or reflected by the surface, suggests that liquid water is abundant in the subsurface of Ceres or has been in the geologically recent past (last ~100 Myr), resulting in mud volcanoes and mineral deposits from hot spring-like fountains within giant impact craters.
Could the parent body have Bennu have been another Ceres? It is not known how large the parent body was, but it is predicted to have been on the scale of 100 km. This is also about the scale of several Ceres-like asteroids, such as 52 Europa and 704 Interamnia. Could the Bennu parent body have been another Ceres-like object with features like the fountains of Occator Crater or even mud volcanoes like Ahuna Mons on Ceres?
Since we do not have high resolution of the surface geology of the Ceres-like asteroids and sending a spacecraft to them is unlikely to happen any time soon, another way of investigating this question is through a combination of modeling and sample analysis.
The OSIRIS-REx mission did not just take images of the surface of Bennu, it also collected samples for return to Earth. Furthermore, based on spectral data from Ceres, we know what minerals to expect from its water-rich subsurface conditions. One way to test whether or not Ceres-like conditions could have prevailed on the surface of the Bennu parent body would be to compare the mineralogy of the samples from Bennu with computer models that simulate what minerals would be stable at the pressure and temperature conditions inside a 100 km scale protoplanet.
A match between the mineralogy of the OSIRIS-REx samples, the computer models predicting the geochemistry of the Bennu parent body, and the mineralogy of Occator Crater based on spectral data would strongly suggest something like Occator Crater may have existed on the surface of the Bennu parent body. This would add to the number of known Ceres-like asteroids and suggest that they are common in the solar system, important for water-resources to support a future space-faring civilization. Lack of such a match would suggest that the Bennu parent body was distinct from Ceres, but still water-rich, which would make the solar system even more interesting.
In addition to being important from the perspective of space resources, the abundance of water-rich asteroids is also important for the search for life. Wherever we find liquid water on Earth, we find life. An abundance of Ceres-like worlds also means an abundance of worlds which could reveal more of
the origin and nature of life in the universe.
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