A Messenger from Mercury? The Wad Alhath Aubrite Pegmatite

What we know for sure is that this is a very usual meteorite, made of creamy crystals, it’s an extraterrestrial pegmatite.

A recent paper proposes that aubrites (a class of similar meteorites like this one), are samples of ancient Mercury, parts of its mantle that were blasted off in a massive collision, removing a third of its mass. Some of the material ended up in the asteroid belt, pushed by the intense solar winds or that epoch, forming the E-type asteroids. And like many other asteroids, various gravitational resonances with Jupiter eventually flung this material to Earth, a time capsule from the formative years of our solar system.

OK, so what supports this hypothesis? First, isotope analysis shows the aubrites left their parent body 4.563 billion years ago (20 million years before Earth formed). Our solar system is 4.567 billion years old.

We also know it was a crazy violent time back then. Entire planets were flung out of our solar system to drift away in cold, dead space. A planet the size of Mars smashed into Earth and ejected enough material to form our moon. Mercury came out of this rock fight looking very strange; it is very dense with a meager mantle compared to its heavy iron core. The aubrites might shed light on the formative years.

“They seem to match scientific models of conditions on the planet Mercury in earlier days of the solar system. We have often said that aubrites are very good analogues for Mercury.” — Dr. Camille Cartier, a planetary scientist at the University of Lorraine in France in the NYT. “Data from NASA’s Messenger spacecraft that orbited Mercury from 2011 to 2015 supports similarities between Mercury’s composition and aubrites. ‘I think aubrites are the shallowest portions of the mantle of a large proto-Mercury,’ Dr. Cartier said. ‘This could resolve the origin of Mercury.’”

The BepiColombo mission is on its way to Mercury now and will try to test the hypothesis by measuring the planet’s nickel at the surface. If confirmatory, a meteoriticist colleague on FB summarized: “aubrites may suddenly be promoted from an oddity into some of the most remarkable meteorites ever collected — pieces of the solar system’s innermost world.”

Now, of the aubrites, this 2.4kg aubrite pegmatite is quite unique. It was found this year in Mali, and will be called Wad Alhath. It does not look like any other meteorite I have seen. It is almost entirely enstatite — a mineral common to the early stages of crystalline silicate formation in space. It’s one of the few silicate minerals observed outside the Solar System, particularly around evolved stars and planetary nebulae such as NGC 6302. Pegmatite is the crystal form, an orthorhombic and centrosymmtric cluster of crystals.

This aubrite has the highest concentration of enstatite (98%) and lowest iron (undetectable) of any aubrite. It makes for a beautiful enigma.

Here are some more technical details on the Mercury hypothesis from the scientific paper:

“Large proto-Mercury models match AuPB’s [Aubrite Parent Body, the place it came from] inferred characteristics: Aubrites share similar exotic mineralogies with Mercury’s lavas and are therefore regarded as potential analogues to Mercury’s crust.

A long-standing idea holds that proto-Mercury once possessed a larger silicate mantle that was removed by an early giant impact(s). N-body numerical simulations of solar system formation systematically predict bigger Mercury analogues, with 0.2 to 0.6 Earth masses. All these models are consistent with P recorded by Ni and Co Dmetal/silicate in the AuPB. Ni and Co abundances in aubrites support an AuPB with a mass of 0.3 to 0.8 Earth mass”

•E-type asteroids as the secondary aubrite parent body: “E-type asteroids are rubble pile asteroids with reflectance spectra and low densities consistent with an aubritic composition. They are located in the innermost belt, forming a large proportion of the Hungaria population, and encountered among the Apollo near-Earth asteroid group. Their orbits are consistent with the fall dates and the long cosmic ray exposure ages of aubrites, supporting the idea that they are the immediate source body of these meteorites. E-type asteroids represent a total mass of ~1.46*1018kg, which represents only a few ppm of the material that would be stripped out by a giant impact on a large proto-Mercury. The age of aubrites coincides with an early epoch during which the Sun’s wind, magnetic field strength and rotation rate each greatly exceeded their present-day value. We propose that following a giant impact, this early wind would have provided sufficient drag upon ejected debris to remove them from Mercury-crossing trajectories and generated a tailwind upon debris, propelling them to greater orbital radii”

• Implications for inner solar system early history: “In the scenario of a giant impact occurring onto a large proto-Mercury and sending some small debris up to the Hungaria region, it is likely that large amounts of ejected debris are gravitationally captured by the inner planets during their outward course. Up to 20% of escaped particles could collide with Venus, and about 5% with Earth. If proto-Mercury was 0.3 to 0.8 Earth masses and lost most of its mantle, that would potentially represent ~ 1% to 2.5% Earth mass of aubritic material accreting to the Earth.”

If so, we are not just “made of stars” but a bit of Mercury too.