Some asteroids are active- shedding mass, like a comet. Bennu is one of them, observed by OSIRIS-REx to be ejecting pebbles to cobbles into space. Now, from the former Soviet, now Russian journal Solar System Research, for October 2024, vol. 58:
Busarev, V. V. Petrova, E. V. Shcherbina, M. P. et al. Sublimation-Driven Dust Activity of Primitive-Type Asteroids as a Sign of the Presence of H20 Ice p. 715 S0038094624700503
If the name Busarev doesn’t sound familiar, well, okay. Very few people, outside of small-body research, read a former Soviet astronomy journal, and of them, how many read Busarev’s works. Works like the previous:
Busarev, V. V. Makalkin, A. B. et al. 2018, New candidates for active asteroids: Main belt (145) Adeona, (704) Interamnia, (779) Nina, (1474) Beira, and near-Earth (162173) Ryugu, Icarus vol. 304 p. 83
Now, I’ll grant you: Busarev’s record isn’t 100%. Ryugu was found to be hydrated, with liquid water in inclusions only, and the others are still TBC. That hardly qualifies as active, since the Hayabusa2 probe never firmly detected shedding of dust in the region about Ryugu. There’s no chance Busarev, coauthors, or anyone else could have found this water using ground telescopes.
Still, let the discussion begin. The presence of ice, (see also), bound water, and even liquid water in asteroids is not ruled out; one sign of that asteroidal water would be mass shedding, due to volatilization and activity. In Busarev et al.’s cases, ground telescopes did not actually resolve asteroids as anything but dots. They turn to indirect signs, like spectra and varying albedo. A change of albedo at perihelion (the point in a body’s orbit when it’s closest to the Sun, and therefore warmest) is pretty suspicious; if a body will activate, it should activate at perihelion, like comets do. For “primitive” (undifferentiated, chondritic) asteroids, water is present as bound (mineral) hydration, even if in trace levels. The temperature needed to release this water is higher, hence the term “bound.” Busarev et al. 2024 are claiming detection of such signs in asteroids 164 Eva, 360 Carlova, 750 Oskar, and 629 Bernardina. However, water is also present (if just not high) in rocky, “warm” asteroids- the Dawn mission detected a water presence at Vesta. This water is assumed to be a coating of primitive, carbonaceous chondritic material, from accretion of small asteroids/dust in the Main Belt. That is, no asteroid is an island, certainly in the Belt, and these bodies are cross-contaminating each other. Busarev et al. 2024 also claim signs of activity at 757 Portlandia, 1121 Natascha, and 1687 Glarona- not carbonaceous chondritic material, and not at 2-12% water.
The Solar System is a very interesting place, and the human mind is fallible, looking for patterns that we’re already familiar and comfortable with. If it’s wrong to say ‘all asteroids are bone dry,’ it would be equally wrong to cry water at the slightest hint. But you can read Busarev et al. all you want, as well as colleagues, including meteoriticists. Some asteroids have water, and we have more than slight hints. Let the discussion continue on which ones.