Are HED meteorites from 4 Vesta?

“HED” meteorites include ~three different types of igneous rocks with chemical and isotopic similarities suggesting that they all formed on the same parent asteroid.

Those three types are:

1) Eucrites: basalts and cumulate igneous rocks that appear to have crystallized at shallow depths in the crust of a small planetesimal.  Eucrites are analogous to some crustal rocks on Earth.

2) Diogenites: iron and magnesium-rich (“mafic”) cumulate rocks made of varying proportions of pyroxene, olivine, and feldspar.  These meteorites are analogous to deep crustal and mantle rocks on Earth.

3) Howardites: regolith breccias that are a mixture of eucrite and diogenite material, from the impact-gardened surface of the HED parent body.

Together, these three classes make up the HED meteorites.  What do we know about the parent body these rocks came from?  Well…

We know the HED parent body is fairly old: its crustal and mantle rocks have crystallization ages dating to at least 4.47-4.48 billion years ago, based on many studies.   Some isotopic systems suggest that the HED parent body had fully differentiated and cooled / solidified within just 2-3 million years of CAI formation (Schiller et al., 2011).

There is also evidence for some chemical and isotopic variation within the HED parent body, but we don’t yet fully understand it (for example, see Utas and Warren, 2017).  The problem is that the meteorites classified as HEDs exhibit a range of composition and isotopic signatures, and they seem to gradate into igneous meteorites similar to HEDs, but which likely formed on different parent asteroids.  Some of these odd basaltic meteorites, like Ibitira, Pasamonte, and a few others, were historically classified as eucrites, and they are similar to eucrites in some respects (Greenwood et al., 2016).  However, they are different enough that they don’t seem to be from the HED parent body and should probably be reclassified as ungrouped achondrites (e.g. see Sanborn & Yin, 2014).

 

That’s the trouble with this topic.  I wanted to start with some broad strokes we know for sure about HED meteorites, and we’re already stumbling into unknowns.

And it gets so much messier than that.  There are chemical and isotopic similarities between HED meteorites and other large groups of meteorites like mesosiderites, main group pallasites, and IIIAB irons.  There’s a whole body of literature addressing possible relationships between these groups, dating back to at least the 1980s.  Some papers try to explain differences between the groups by claiming that they’re from similar, but different parent bodies, and others have argued that they’re all from the same parent planetesimal, and propose phenomena like collisions or isotope fractionation effects during differentiation to justify observed differences.

As of right now, we just don’t know for certain.  I’d say that current evidence suggests that mesosiderites and HEDs are not from the same parent body, but there’s a small population of researchers who still support that theory.

Understanding these relationships would tell us a lot about the HED parent body.  For example, if we knew that HED achondrites and IIIAB iron meteorites were from the same parent body, we could be pretty sure that the HED parent body is not 4 Vesta.  Why?  Thanks to their abundance on Earth, we know that fragments of the IIIAB core are scattered throughout the asteroid belt: the IIIAB parent asteroid was ~destroyed billions of years ago and scattered into interplanetary space.  But, 4 Vesta has never been disrupted.  Its core has been safely sitting under a hundred or so kilometers of solid rock for the past ~4.5 billion years.

I don’t want to get into mesosiderites and pallasites in detail here, but, again, it doesn’t look like either group is from 4 Vesta, for the same reason as IIIAB irons: 4 Vesta appears to be a relatively intact differentiated planetesimal.

 

Which brings us to HEDs and 4 Vesta.  What’s the solid evidence tying them together?  Well, we don’t have any samples returned from 4 Vesta, but the DAWN mission did collect a lot of information about the asteroid’s surface while orbiting it.

 

 

Long story short, an asteroid’s reflectance spectra (the wavelengths of light reflecting off its surface) act as a weak spectrometer and can tell us about surface composition.  From that, we can say for certain that Vesta’s surface is basaltic, just like eucrites.

 

Eucrites are the most common basaltic meteorites, and 4 Vesta is the biggest asteroid with a basaltic surface.  Those two facts were apparently enough for many collectors and scientists to start claiming that HED meteorites are from 4 Vesta.

How solid is that claim?

Well, as we saw above, there are a number of basaltic meteorites in collections on Earth that are similar to eucrites, but are not from the HED parent body, so sV or V-type asteroid spectra aren’t unique to HEDs and don’t prove a link between the two.  ~Any differentiated body with a basaltic crust should have a similar reflectance spectra.  For example, here’s a figure (from Beck et al., 2011) that includes spectra from some basaltic meteorites that probably aren’t from the HED parent body, like Pasamonte and Dhofar 007.  Needless to say, you wouldn’t be able to pick those two out of the literal lineup.

So, maybe 4 Vesta is the source asteroid for Pasamonte, and not typical HEDs.  The evidence for that rock being from 4 Vesta is just as good as the evidence for HED eucrites.  Or maybe a different group of basalts, like Bunburra Rockhole, are from 4 Vesta.  I haven’t seen any reflectance spectra for that meteorite in particular, but I’m sure it would be just as good of a match based on its composition.  It’s also very possible that we haven’t found any fragments of 4 Vesta on Earth yet.

Then there’s the iron problem.  We see very little evidence for mafic rocks analogous to diogenites on 4 Vesta’s surface.  It’s difficult to reconcile that with the number of diogenites and howardites found on Earth.  If mafic samples of 4 Vesta are regularly falling to Earth (and they are), those rocks should be exposed on the HED parent body.  But we don’t see ~any rocks like them exposed on the surface of 4 Vesta.  That detail suggests that HEDs in particular might not be from 4 Vesta.

Beyond that, there are many studies that have raised various issues with the theory that HED meteorites are from 4 Vesta.  For example, the HED parent body may have been much smaller than 4 Vesta.

To sum it up: the surface of Vesta is basaltic, like eucrites, but HEDs and 4 Vesta both exhibit traits which suggest that the two may not be related.  We also have examples in our collections of similar basaltic meteorites that we know are not from the HED parent body, and which would be equally good spectral matches.  Given the chemical and isotopic similarities between HED achondrites and other groups like IIIAB irons, main group pallasites, and mesosiderites, and the fact that we don’t know for certain if some or all of those groups are from the same parent body, assuming that one (or more) of them is from to 4 Vesta would be…a very big assumption, at this point.  HED meteorites could be from 4 Vesta: we just don’t know yet.

If you hear someone claiming that HEDs are from 4 Vesta, take what they’re saying with a big grain of salt.  I would be wary of anyone who says that as though it’s a fact.