Scientists have looked particularly closely at the eucrite flavor of HED meteorites. Eucrites are basaltic, a volcanic rock composition that is extremely common across the solar system, found on the surfaces of Earth, Moon, and Mars. The eucrites' isotopic and trace element signatures suggest that they formed on a small planetesimal that has lost some component of its lighter elements, which agrees with the observations and measurements of Vesta.

The first part of this LPSC session focused on ways that scientists are trying to unravel and interpret the chemical signatures seen in eucrites. The discovery of more meteorite samples and advances in geochemical analysis have complicated the eucrites’ story and meant that scientists have had to update the system of meteorite classification.

Chemical isotopes are a useful tool for investigating planetary processes. Some isotopes can be used as a proxy for planetary parentage. The updated classification suggests that not all of the eucrites originate from one body. David Mittlefehldt and Matthew Sanborn both discussed this during their talks, with Mittlefehldt looking at variations in oxygen isotopes, and Sanborn looking at chromium isotopes. They both came to the same conclusion: there must have been more than one Vesta-sized body in the asteroid belt. From a geological point of view this isn't surprising; basaltic melts are what you make when you partially melt a rocky body, so if many planetesimals experienced partial melting, then there should be basalts all over the place. However, the observations of over half a million asteroids in the asteroid belt suggest that Vesta is unique in showing a basaltic crust. All of the other bodies must have been incorporated into the larger bodies when they formed, or were completely smashed apart during collisions.

Isotopes can also tell us about planetary degassing and the role of volatiles during volcanic eruptions, and Thomas Barrett discussed chlorine isotope signatures and their trends seen across the different volcanic eucrite groups. Work like this can help us answer the question about how much volatile material (e.g. water) there might be on a planet when it forms, or what brought the water to the planet.

I take a slightly different approach to understanding planetary differentiation. Instead of trying to infer the processes which have occurred from the chemical signatures recorded in meteorites, I use a forward modeling approach. I try to synthetically produce the minerals and chemistries, a discipline called Experimental Petrology. The idea is very simple – I mix together the chemical elements that we think the rock of interest is made from, and then (for want of a better word) bake it at high temperature and pressure. I then look at the sample and compare it to the meteorites. The reasoning behind this approach is that if we can recreate the chemistry we see, then we know what conditions the rock formed under, and what the composition is. This approach has been used extensively to understand the diversity of rocks seen on Earth, the Moon, Mars and Mercury, with many of the LPSC sessions including talks from other experimentalists. In this session I was talking about how the chemistry of different minerals changes over the course of crust formation of Vesta, and in particular how the trace elements (like the Rare Earth Elements) divide among mineral phases. Understanding this kind of behavior is essential for understanding the observed range in chemical signatures of the HEDs.

Jasmeet Dahliwal concluded the discussion of the HED meteorites by talking about her work which looks at the siderophile elements in eucrites and diogenites. Siderophile elements are ‘iron-loving’: during the planetary differentiation process, they are sequestered into the metal core, and therefore are not found in crustal rocks. Indeed, the siderophile elements (such as osmium and platinum) are present in much lower abundances in the differentiated meteorites than the undifferentiated ones, and are used as a tool to quantify core formation on planets. Dahliwal spoke about the analytical difficulties involved with measuring such small concentrations of these elements, inkeeping with the observations from the moment of inertia and gravity field that Vesta has a metal core, and how these elements may be used to measure the crystallization processes of the silicate portion of a planet.

So to conclude what we learnt from this session is that progress is being made into understanding how Vesta formed, using detailed chemical measurements. There is still a lot of work to be done in the field of meteoritics though, particularly with new samples being discovered and collected every year.

We’re lucky with the HEDs, as they can be put into geological context with their parent body. The results and implications of the chemical work can be compared with the observations of the planet, such as by trying to match Dawn observations of specific surface areas of Vesta with actual meteorite compositions.

Vesta is unique, as it is the only surviving baby planet, and this work is essential to our understanding of the early stages of formation of the larger planets. This work will help us understand how the Earth, Mars, Venus and Mercury have formed and evolved to be so different in terms of their size, and characteristics. It will also help us to understand how the Moon formed. Understanding how our own planets have formed will also allow us to understand how exoplanets form, and their potential to harbor life.