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Vesta | is it really the parent body of the HED Achondrite meteorites?

Posted by Dustin Dickens on

 

Even in science, and especially on the fringes of the more sexy disciplines, the humble working hypothesis and often downright fictions can easily get presented and perpetuated as fact with the best of intentions. It's never fun to learn that despite one's best efforts, they have been doing just this.

Meteorites come in many different flavors, and sometimes those flavors are grouped together based on some common feature. One such group is known as the HED meteorites, an acronym derived from the names of the three types of achondritic meteorites that compose the group; the howardite, the eucrite, and the diogenite. This is a very well studied group of stones. They are thought by most researchers to originate from the minor-planet 4 Vesta. The connection between the HED meteorites and Vesta is largely based on remote spectral analysis. While the data for a connection is compelling, the story begins to change when more specific chemical and isotopic data is brought to bare.

At this point we need to step back and refresh ourselves on the basics of planetary differentiation. In the most simple terms, when asteroids exceed about 25 kilometers in diameter, the roughly 20% iron-nickel metal dispersed throughout the asteroid starts to sink to the center to form a metal core while the lighter silicates float above to form a mantle.

So, most of the time, if we find an iron meteorite the common wisdom goes that we have found a piece of the iron core of a large differentiated body with a diameter greater than 25 kilometers that has been cataclysmically destroyed. Pieces of the inner most iron-nickel core are flung across the solar system. Eventually a bipedal sentient creature walking in the desert of a rocky planet recognizes the sculpted hunk of iron as extraterrestrial, because their hunks of solid native iron are safely tucked away at the center of their thankfully intact planet.

Bear Creek IIIAB Iron Meteorite - Wikipedia Commons

Now we get to the part where meteoritics, and cosmochemistry in particular, shine a bright light onto the otherwise dim remote investigations previously undertaken. Despite the spectrographic data linking HED meteorites to Vesta itself, recent isotopic work on HED and the IIIAB class of iron meteorites tell a different story. A recent paper published in Earth and Planetary Science Letters, by John Wasson - UCLA shows a strong chemical and isotopic link between the IIIAB iron meteorite and the HED meteorites.

HED Achondrite | Eucrite Umb. NWA 10962 - Top Meteorite

Vesta is technically a minor-planet roughly 500 kilometers in diameter, with lots of other smaller "V type" asteroids with a similar spectral signature roaming around throughout its local area. What the data linking IIIAB irons to HED achondrites seems to tell us, is that a Vestan like differentiated parent body, but not Vesta itself (as it is still intact), was completely obliterated in order to give us both IIIAB irons and HED achondrites from the same parent body.

A simple, but quite compelling deduction fashioned from more rigorously vetted data with greater accuracy. As is the case with most changing tides in science, the jury is still out. However, the HED link to Vesta that has become somewhat of a mantra of mine when doing outreach will now, in all good conscience, need to be updated to include this data and hypothesis.


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