Moat Paper: Vesta’s Water

A new paper in Meteoritics & Planetary Science brings up a key issue:

Michalik, T. Maturilli, A. Cloutis, E. A. et al. 05 April 2024 Laboratory VIS-NIR reflectance measurements of heated Vesta regolith… 14156

Water on Vesta. Water on the asteroid (4) Vesta. The Dawn probe was planned to visit two significant bodies in the asteroid belt, as a means to study the early relicts from the formation of the Solar System. (Hence, the name Dawn- not an acronym, but a metaphorical and historical target.) One of those bodies, Ceres, was anticipated to be water-rich, due to remnant ices surviving in the outer (more or less) Solar System where it’s cold. The other, Vesta, was anticipated to be dry. Vesta shows the signatures of olivine and pyroxene in our spectroscopes (which I’ve mentioned from other papers). Olivine and pyroxene are minerals rendered from other minerals by magmatism, and then form the mantles of Earth, Mars, etc. Vesta was, thus, once a ball of magma; these balls are a thousand degrees or more, and can’t possibly retain ice, right? Right?

Wrong. The Dawn mission found water on both asteroids. The key difference is that Vesta, while differentiating into a crust-mantle-core like Earth and Mars, was then ‘painted’ by the infall of carbonaceous chondrite impactors. Carbonaceous chondrites, like…Ceres! Since Vesta is in the Main Asteroid Belt, and the Belt is bringing (on geologic timescales) its asteroids into crossings and collisions, the Belt is populated with shards and dust and other ejecta from all these collisions. And since, unlike comets, all these Main Belt objects are co-orbiting in somewhat similar circles about the Sun, they cross each other with (relatively) low energies. Belt-to-Belt collisions are slower and milder that collisions by comets. This means impactors (for lack of a better word) are closer to landers than artillery, and their compositions are less-affected and closer to those of their parent bodies than steeper impactors from faster-crossing threats. In the extreme, dust particles have their orbits driven more circular, due to the Poynting-Robertson effect, Yarkovsky effect, etc. (Technically, P-R and Y effects happen on everything in orbit around a star. But a body’s mass, and therefore inertia, resists change; heavy objects don’t want their orbits modified. Dust particles are light enough that P-R and Y effects actually do something, on some relevant timescale.) Dust particles then land on asteroids, from other asteroids, at gentler, survivable speeds.

Since the Main Belt has fragments and dust drifting about, asteroids here ‘cross-contaminate’ each other. Vesta, as a large object (hundreds of kilometers across) has the gravity to pull in nontrivial impactors and dust. Since Vesta pulls in dust and rocks, and some amount of that material came from carbonaceous chondrite objects, Vesta’s surface is painted with carbonaceous-chondrite material. This includes the water and organic compounds contained in carbonaceous chondrites. The amount of water (plus other volatiles like dry ice, light organics, ammoniated and nitrogenated compounds, etc.) is high enough that Dawn spotted the geologic signs of water: pit craters and other forms of mass wasting.

When a layer of dirt (here, “regolith”- not the same as Earth dirt) contains ice, volatiles, etc., disturbances can release those volatiles. The ‘dirt’ appears to have disappeared a bit- the volatiles escape to the vacuum of space, leaving behind less dirt. We recognize such disturbed dirt as pitted terrain, on Earth, on Mars, on Mercury (though volatiles are sulfur, sodium, etc. at Mercurian temperatures), Ceres, and yes, on Vesta:

Denevi, B. Blewitt, D. et al. 12 Oct 2012 Pitted Terrain on Vesta and Implications for the Presence of Volatiles, Science 1225374
Scully, J. E. C. Russell, C. T. Yin, A. et al. 2015 Geomorphological evidence for transiernt water flow on Vesta, Earth and Planetary Science Letters, vol. 411 p. 151 .12.004
Sizemore, H. Platz, T. Schorghofer, N. et al. 2017 Pitted terrains on (1) Ceres and implications for shallow subsurface volatile distribution, Geophys. Res. Letts. vol. 44 p. 13 0
Scully, J. E. C. Poston, M. J. Carey, E. M. et al. 2022 Curvilinear Gullies, Lobate Deposits and Pitted Terrain on Vesta and Ceres Formed by Transient, Brine-Mobilized Flows, AGU Meeting 380942

Any individual author might be a speculator, a crank, or simply a person to brush off, rhetorically and pedagogically. Three separate lead authors, spanning a decade, with coauthors? Brush not, water doubter. The Michalik et al. 2024 at hand is itself a follow-up to:

Michalik, T. Matz, K.-D. Schröder, S. et al. 2021 The unique spectral and geomorphological characeristics of pitted impact deposits associated with Marcia crater on Vesta, Icarus vol. 369 article 114633.

That’s four lead authors, spanning twelve years, and multiple areas of Vesta. With coauthors. Oh, and we have meteorites from Vesta: the HED (Howardite-Eucrite-Diogenite) meteorites. These samples show the olivine-pyroxene blend, plus contamination from carbonaceous chondrites, and water.

As Vesta has water and organics, and is (relatively) inwards as far as the Main Belt goes, humanity will reach Vesta, Justitia, etc. before Ceres (in the middle of the Belt) and the Main Belt Comets (generally, in the outer Belt, with exceptions). In turn, Vesta and Justitia are closer to the Sun, with more solar power potential (including less dust, obscuring the light). Consider our descendants: they will, at some point, reach the Belt, and colonize Vesta and/or Justitia due to their resources. Surfaces bearing carbonaceous-chondrite material, with water and organics for self-sustaining colonies.

Vesta is the one asteroid visible (barely) to the naked eye. (It just looks like any other star- you wouldn’t notice if it were not pointed out to you, same as Uranus is technically a naked-eye object.) Justitia is not visible without a telescope. If you go outside at certain months of the year, in a nice, dark place, you can tell yourself that Vesta is one of those ‘stars’ up there, and Justitia is somewhere, too. You can tell yourself that you can see the future of humanity.