The following blog post has been written by Tim Gregory, who is undertaking a PhD at the University of Bristol. Tim is a recent graduate of our MSci Geology with Planetary Science degree programme, and his new paper, just published in the Meteoritics and Planetary Science research journal, was the outcome of his 4th year independent research project, supervised by Katherine Joy.
Meteorites are pieces of planetary bodies that land here on Earth. Knowing which planetary body a particular meteorite comes from is useful, because it allows planetary scientists to combine chemical data collected in Earth-bound labs with data collected from telescopes and spacecraft. However, linking meteorites to the particular planetary bodies from which they came is a huge challenge.
We know for sure that some meteorites come from the Moon and some come from Mars , but these types of meteorites are rare – there are less than 500 in total. The vast majority of meteorites (more than 50,000 of them!) come from asteroids, but planetary scientists aren’t always sure which asteroids they come from.
Uniquely, this is not the case the for the HED clan of meteorites. The HEDs most likely originate from the second largest asteroid in the asteroid belt, Vesta. There are more than 1700 known HED meteorites, making them widely available and, thus, very well studied. The clan takes its name from the three groups of meteorites which it contains: howardites, eucrites, and diogenites. Each of the three groups represents a different layer of Vesta.
Meteorites in the diogenites and eucrite groups are igneous in origin, meaning they formed from the cooling and crystallisation of once-molten rock. The diogenites formed when magma cooled deep within Vesta’s crust, and are made mostly of a mineral called orthopyroxene. The eucrites formed when magma cooled in the shallow crust of Vesta, and are made mostly of the minerals pyroxene and plagioclase. Some eucrites formed from ancient lava flows on the Vestan surface, and are similar to the rocks found in Hawaii and Iceland here on Earth.
The final group, the howardites, are not igneous rocks like the diogenites and eucrites. They are impactites, meaning they formed during impacts on Vesta’s surface. Over Solar System history, Vesta has been bombarded with countless impacts, leaving its surface peppered with craters. These impacts reach deep into Vesta’s crust, fragmenting and ejecting diogenitic and eucritic rocks as they form. This ejected material forms a layer of rock fragments on the surface of Vesta called a “regolith” (from the Latin “rego-” meaning “blanket” and “-lith” meaning rock). This regolith – made up of a mixture of fragments of diogenites and eucrites – forms the howardites.
Howardites are interesting because they can tell us about both the igneous history of Vesta, and its impact history.
The 1700 meteorites which originate from Vesta aren’t the only remarkable thing about this asteroid. Between 2011 and 2012, NASA’s Dawn spacecraft was in orbit around Vesta, collecting compositional information and imaging its surface in remarkable detail. The data collected by Dawn has allowed planetary scientists to piece together the history of Vesta’s surface, and understand how the composition of its surface varies from place to place.
Miller Range 11100 is a howardite that was recovered from Antarctica in 2011, and was studied here at the University of Manchester as part of an undergraduate Geology with Planetary Science MEarthSci project, in collaboration with researchers at the Natural History Museum (London). A combination of state of the art analytical techniques (including electron microprobe analysis and mass spectrometry) were used to analyse this piece of Vesta in intricate detail. The study was recently published in the journal Meteoritics and Planetary Science.
We found that as with all howardites, Miller Range 11100 is a mixture of fragments of diogenites and eucrites. Some of the fragments showed evidence that they were melted by the energy released during impact events on Vesta’s surface, but some were still fresh and had changed little since they cooled from their original magmas. We also found a mineral which is normally rare in howardites called olivine. The Earth’s upper mantle is predominantly made of olivine, and it is likely that Vesta’s mantle is similar in composition. It is, therefore, possible that olivine in MIL 11100 originated from Vesta’s mantle, which has implications for the thickness of Vesta’s crust, and also how deep impacts excavate down into the surface.
By combining the properties of this meteorite that we measured in the lab with the properties of different regions on Vestas surface measured by Dawn, we hypothesised where on Vesta’s surface Miller Range 11100 may have originated from. While this approach is approximate and first order, it demonstrates how sample analysis and remote sending data can be combined to pin down the source regions of HED meteorites.
The HED clan of meteorites allow planetary scientists to combine state of the art laboratory measurements on rocks here on Earth with remote sending data collected by NASA’s Dawn spacecraft. Along with Martian and Lunar meteorites, HEDs are the only meteorites where their planetary body of origin is known. Miller Range 11100 is one of many pieces of Vesta we have here on Earth, and is one piece of the puzzle that have helped us gain unique insights into Vesta.
The full paper can be read online at T. Gregory, K. H. Joy, S. Strekopytov, and N. M. Curran. (In Press). Geochemistry and Petrology of Howardite Miller Range 11100: A Lithologically Diverse Piece of the Vestan Regolith. Meteoritics and Planetary Science.