Are HED Meteorites Really from Vesta?

In science the humble working hypothesis can easily get presented and perpetuated as fact, usually with the best of intentions.

Meteorites come in many different flavors. They are typically grouped together based on some common feature or parent body. One such group is known as the HED meteorites, an acronym derived from the names of the three types of achondrite meteorites that compose the group; the howardite, the eucrite, and the diogenite. This is a very well studied clan of meteorites. They are thought by most researchers to originate from the asteroid Vesta or the associated V-type asteroids known as the Vestoids. The connection between the HED meteorites and Vesta is largely based on remote spectral analysis. We don't have any "ground truth" sample of Vesta or the V-type asteroids. While the data for Vesta and the Vestoids being the parent asteroid(s) for the HED meteorites is very compelling, It's not without some unanswered questions that at least some researchers think places legitimate doubt on the current consensus of Vesta and the Vestoids being the parent body for the HED meteorites.

With this in mind, let's step back and refresh ourselves on the basics of planetary differentiation. In the most simple terms, when asteroids exceed a certain size, the iron-nickel metal dispersed throughout the asteroid starts to sink to the center to form a metal core while the lighter silicates float above it to form a mantle.

So, if we find an iron meteorite the assumption is that we have found a liberated piece of the iron core of a large differentiated asteroid that has been cataclysmically destroyed. When an impact like this occurs with a differentiated asteroid, pieces of that iron-nickel core can eventually reach the surface of the Earth as an iron meteorite. With some serendipity, an observant earthling recognizes the sculpted hunk of iron as something extraordinary because hunks of solid metal like that are usually safely tucked away in the core of their thankfully intact planet.

Bear Creek IIIAB Iron Meteorite - Wikipedia Commons

Now that we have reviewed some of the basics about how an iron meteorite might have been formed and eventually made it to Earth, we can we get to the part where not every scientist who studies these processes is convinced the HED meteorites are from Vesta. Despite the spectrographic data linking HED meteorites to Vesta that we mentioned earlier, chemical and isotopic analysis of HED meteorites and the IIIAB class of iron meteorites seems to establish a difficult to reconcile relationship between the two. A paper published in Earth and Planetary Science Letters, by John Wasson of UCLA, shows a strong chemical and isotopic link between the IIIAB iron meteorite and the HED meteorites.

Unbrecciated Eucrite from Northwest Africa. Authentic HED meteorites - Top Meteorite

Vesta is roughly 500 kilometers in diameter, and is an intact asteroid with many other smaller associated "V type" asteroids known as the Vestoids having a similar spectral signature as Vesta proper. It is the "intact" part of that last descriptive sentence that calls current accepted theory into question. In short, the story that the data linking IIIAB irons to HED achondrites seems to tell us, is that it was a Vesta-like differentiated asteroid, but not Vesta itself (as it is still intact), that was completely obliterated in order to give us both IIIAB irons and HED achondrites from the same parent body. The hypothesis being that Vesta's core is still sequestered deep beneath the Vestan mantle and crust. If the isotopic and chemical data linking the IIIAB irons and HED clan together with the same parent body is correct, it is simply not possible that their common parent body is still intact or else we would not find the IIIAB iron meteorites here on Earth. They would necessarily still be part of the core of Vesta.

It seems to be an eloquent and quite compelling argument. Was it fashioned from more rigorously vetted data with greater accuracy? Does Dr. Wasson's research really establish an undeniable link between the IIIAB irons and the HED clan of meteorites? Do alternative interpretations exist that would allow for both an association between the IIIAB irons and the HED clan to be correct while still allowing the relationship between Vesta and the HED meteorites to remain valid? Perhaps the jury is still out on this one, despite popular sentiment and a preponderance of evidence supporting a link between the HED clan and Vesta. What do you think?