Asteroid of the Half-Month: 4 Vesta

We’ll start with an easy one: Vesta, the second biggest asteroid in the Main Belt (after 1 Ceres).

Vesta is big enough that it produces other asteroids; impactors do not destroy it, but in some cases spall off debris. These are the Vestoid family asteroids, and the HED meteorites (for Howardite-Eucrite-Diogenite). Between HED samples recovered on Earth, and the Dawn spacecraft that orbited Vesta and Ceres, astronomer A. Rivkin states that the body is as well-characterized as our Moon and Mars. And what do we now know?

  • Vesta is so big it was once molten rock- not too surprising. The magma separated, like Earth did, into core, mantle, and crust- “differentiation.”
  • Vesta took two massive impacts, spraying forth the Vestoids. The Main Belt was a violent place, and that wasn’t 4.5 billion years ago, but “only” one billion or so.
  • Yes, the HEDs are chips off the old space spud.
  • Vesta was damp; one intriguing observation is that not only is there residual moisture today, but possibly “mudslides.”

All in all, the body is a “planetesimal”- the embryos, very common in the early Solar System, that came at each other until only planets stood; everything smaller got railroaded. Vesta’s composition is surprisingly like Earth, Mars, the Moon, etc. which is part of the attraction.

I’ve seen some astronomers go so far as “the last planetesimal”, but that seems a bit flowery. We haven’t ‘probed’ 2 Pallas or 3 Juno (sorry for verbing there). Nor do we have a good handle on what ice bodies did/do in the outer Solar System, versus accretion of rock/metal planetesimals here where it’s warmer.

Also premature: a dry Vesta. Telescopes on or near Earth saw no water signature; given that The Big V is in the inner Main Belt (orbital radius: 2.5 AU), it then seemed logical that frosts could not survive. The Dawn mission would then contrast one dry planetesimal, and Ceres, further out and presumed colder and frosty. In fact, a water vapor spectrum was observed at Ceres before Dawn arrived.

Dawn, instead, got an H2O two-fer. Vesta’s surface is contaminated by infalling asteroids, specifically, carbonaceous chondrites, which are not merely moist. It is common for carbonaceous chondrites to have water percentages in the double digits; the record was one found to be over a third water. Newer, darker terrains on Vesta (usually craters and their ejecta) are damp impact splotches; older, brighter areas are also drier. The planetesimal, then, acts as a dipstick or sight glass in the tank that is small-body hydration.

More dubious are claims (Scully et al. 2014) that crater walls show flow features, indicating water today. Plenty of other scientists have plenty of other explanations, like dustslides, possibly levitated by static forces. It’s a bit presumptuous to cry ‘water’ when we’re grappling with dust and statics, on very-low-gravity worlds, in the solar wind and hard UV. Also claimed: crystals found in one HED indicate they were formed in water, not magma. Well, meteoriticists find lots of things, I’d like to see if this report replicates.

Most of Big V formed from magma, so asteroid mining isn’t any better than Earth mining- that core would have sucked up valuable metals, just like Earth’s core depleted the other parts of their metals. But as general manufacturing or habitation, the presence of measurable water makes the place that much more attractive. As a target of study, it’s at least as important as we presumed. Elements similar to Earth- including water but others too- indicate accretion of the planetesimals was, in fact, important in the history of the Solar System… our history. I’ll get into more detail on that in future posts.