Bang your HED — The Rock Stars of Celestial Bodies […]

Bang your HED — The Rock Stars of Celestial Bodies

A rare a beautiful peek into the minor-planet called “4 Vesta”. The story of how it got here is extraordinary, and I’m not talking about the trip that meteor hunter Michael Farmer had to take to Morocco to negotiate with the desert nomads.

4 Vesta accounts for 9% of the mass of the asteroid belt — big enough to provide enough gravity for a differentiated crust, mantle and core, like Earth and Mars, but unlike smaller moons and asteroids. The rocks from Vesta date back to the formation of the Solar System, 4.566 billion years ago (older than Earth itself and very close to the oldest thing you can touch). Vesta is the brightest asteroid in the night sky, occasionally visible from Earth with the naked eye.

Normally, the material deep beneath the surface of a planet stays there, and not even on Earth have we drilled deep enough to sample what is down there.

But Vesta had a colossal collision of planet-busting proportions. It left behind a mountainous crater twice as tall as Mt. Everest and 90% the diameter of Vesta itself. We know this from visual and spectroscopic analysis by the Hubble telescope and, more recently, the by the DAWN spacecraft that orbited Vesta for a year.

About 1 billion years ago, 1% of Vesta’s mass was violently ejected by a high-speed impactor, including several layers of crust (breccias, basalt and igneous rock) and perhaps as deep as the mantle. “Vesta likely came close to shattering,” said DAWN principal investigator Carol Raymond, noting that the blow left concentric sets of fracture lines around Vesta’s equator.

Some of the ejected rock was large enough to become “V-type asteroids” of their own right, and a few of those ended up in an unstable region of the asteroid belt in an orbital resonance with Jupiter, the big boy circling farther out. Every 100 million years, some of the asteroids in that region get ejected out, and a handful of those have been found to have randomly ended up in near-Earth orbits. Those V-type asteroids are subsequently struck by other impactors in multiple events over a subsequent time period of 6-73 million years, dislodging smaller rocks in random directions, a few of which hit Earth. A few of those are large enough to survive the burn of reentry but not so large as to burst into fragments from the impact of hitting the atmosphere at speeds exceeding Mach 32.

Where do they land? Two-thirds of all meteorites are lost in the ocean (due to the surface area ratios of ocean to land). And almost all of the stony landfalls will remain undiscovered because they do not stand out as special rocks from a distance if not witnessed as a fall. So these incredible cosmic voyagers are generally undiscovered except for witnessed falls, or lying on the ice sheets of Antarctica (where it is illegal to export meteors) and some states in Northwest Africa (where it is legal-ish) where the nomads know that if they find a rock in the desert, it’s a meteorite, and if it’s stony (less dense than the common meteorites of iron), then it might be rare and valuable (but that takes subsequent testing).

There are three broad classes of meteorites from 4 Vesta — the HED of Howardite, Eucrite and Diogenite — based on how deep the ejecta came from. This is the 5.6 kg main mass of NWA 8563, a rare monomict eucrite (one of 232 total) of cooled lava from various depths. I partnered with Michael Farmer to buy it from the nomads in Morocco. It’s somewhat like wildcatting for oil or minerals; you commit to buy it before you know what kind of stony meteorite it will turn out to be (Mars, Moon, or from places unknown). We had it sliced and analyzed and were delighted to learn of its origin. We were also delighted at how beautiful it looks inside, with a mysterious green and clearly differentiated clasts. Farmer said it is unlike anything he has seen before, and he has been a full-time meteorite hunter and trader for many decades. It is a melt breccia with mixed clasts of both shallow and deep material from Vesta’s crust. NWA 8563: https://www.lpi.usra.edu/meteor/metbull.php?sea=NWA%208563&sfor=names&ants&nwas&falls&valids&stype=contains&lrec=50&map=ge&browse&country=All&srt=name&categ=All&mblist=All&rect&phot&strewn&snew=0&pnt=Normal%20table&code=60976&fbclid=IwAR1xYoKrQeqbx48hksFT-yOeVW7RSyzni_MwKsUjim0A2yEYLa1tNeTiv8U

From the DAWN mission, we now know that Vesta is the only intact, layered planetary building block surviving from the very earliest days of the solar system, forming within the first 10 million years, long before Earth.