Spatial and temporal scales of aqueous alteration in icy planetesimals

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This aim of this research programme is to understand better the evolution of the small bodies of rock and ice that lie in the outer parts of the solar system. These bodies are collectively called 'planetesimals', and are of great interest to planetary scientists because they have remained unchanged for most of the 4,500 million year history of the solar system. Thus, they can tell us much about how it formed and developed. This research project is concerned with planetesimals that contained liquid water, probably for a brief period soon after their formation. Pieces of these planetesimals have fallen to Earth as meteorites called 'carbonaceous chondrites', which are highly valued by scientists because their chemical compositions indicate that they are least altered rocks available for study. However, despite their very primitive chemistry, the carbonaceous chondrites are made mainly of minerals that were formed by water reacting with their parent planetesimal, and this process of aqueous alteration would be expected to have also modified the chemical composition of the rock. This contradiction between a primitive chemistry and secondary mineralogy can only be explained if water within the planetesimal was static. However, recent computer simulations of planetesimal evolution consistently predict that only the smallest bodies could have contained static water and in most it must have flowed through the rocks, modifying their chemical compositions along its path. In this research programme we will test the assumptions and predictions of these models by obtaining new information on the behavior and history of water within planetesimals using one group of carbonaceous chondrites called the CMs. These meteorites contain small crystals of minerals called carbonates that crystallized from the water. By examining the compositions, internal structures and distributions of carbonate crystals using a range of microscope-based techniques, we will address the following questions: Was the water stationary or did it flow in the same way that hot water moves through rocks on Earth? Did the water exist for only a brief period in a small body or was it present for millions of years within a larger planetesimal? Did planetesimal interiors contain water or was it present only close to their surface? Results of this research will increase our understanding of how planetesimals formed and evolved and will enable us and other scientists to assess and potentially modify the computer models of planetesimal interiors. Ultimately this work is significant for our understanding of the early history of the solar system but also of the present-day composition and internal structure of comets and asteroids. These bodies are currently the focus of a great deal of international research activity, having been visited recently by several space probes, and are targets for future unmanned and possibly manned exploration.