In Situ Science and Instrumentation for Primitive Bodies

Abstract

Our study began with the goal of developing new methods to test the radically new understanding of solar system formation that has recently emerged, and to identify innovative instrumentation targeted to this purpose. In particular, we were seeking to test predictions of dynamical models such as the Nice model (after the founding research group in Nice, France), and to do so through interdisciplinary collaboration between the planetary dynamics communities that have formulated (and largely dominated discussion of) these new ideas, and the meteoritics and cosmochemistry communities who will be most involved in any in situ mission to an outer solar system body. Our study was principally focused on coming up with explicit tests of the predictions of these new dynamical models of solar system evolution. The key outcome of our first workshop was the realization that fundamental work is needed before these two communities—dynamics and meteoritics/cosmochemistry—are really ready to come to a collective understanding of early solar system evolution. Planetary dynamics examines solar system history through the orbital properties of large populations of bodies, but says little specific about any one of them. In fact, at present it appears that there is nothing you could learn about any one body that this community would consider to be a concrete test of the Nice model (or another similarly broad model of solar system evolution). On the other hand, people who study planetary materials through meteoritics and in situ missions are strongly focused on the idiosyncratic properties of individual bodies but don't actually know how to identify the properties of a primitive body that depend upon its orbital evolution. Without such tools, it isn't clear how this community can turn insights regarding one body into statements about broad classes of related bodies. This is a frustrating moment in the study of solar system evolution—both the dynamics and meteoritics/cosmochemistry communities have well developed and consequential hypotheses about solar system evolution, but it isn't obvious that either knows how to make a concrete statement that is testable by the other. Our reaction to this impasse was to step back from the narrow problem of testing the Nice model as a whole (or similar specific dynamical models) and ask whether there might be specific instances—particular bodies or groups of bodies—where we could forge a link between the dynamical and meteoritic/cosmochemical approaches. If so, this could serve as a foundation that will eventually lead to a synthesis of the dynamical and cosmochemical understanding of solar system evolution. The key, we imagine, is to find a case where dynamical approaches lead to clear predictions about mineralogical or chemical properties of individual bodies, so that mineralogical or cosmochemical approaches could test those predictions through in situ or remote observations. There was consensus amongst our team that we should be able to use dynamics to predict the chemistry of a primitive body based on knowledge of where the body originated in the solar nebula and the thermal history it has undergone. We are in a unique position to make this new type of connection between dynamical models and chemistry because of the diverse backgrounds represented in our group, which includes dynamicists, astronomers, geochemists, cosmochemists, spectroscopists, mineralogists, and instrument developers. For our second workshop, we further expanded our team to address new directions, specifically drawing on expertise in geochemistry of returned samples and meteorites. Throughout our study, we had extensive discussions about the composition of primitive bodies, where in many cases little is known from telescopic observations. Moreover, there is no known meteorite collection of materials from the most relevant group of parent bodies (e.g., D-types – Trojan asteroids, irregular satellites, Phobos and Deimos, and some outer main belt asteroids). Trojan asteroids were identified as the most interesting target because they represent a large reservoir of D-types that can potentially be linked to origins in the outer solar system (primitive Kuiper belt). Dynamical histories have not yet made specific predictions about the chemistry of these bodies because the field is still in its infancy and there has been little interaction between dynamicists and chemists. We concluded that we need to develop our own theoretical framework starting from the beginning—what are the starting materials? How were they processed during and after migration? Then, we need to actually do the lab work to simulate these materials and look for markers. A search for these markers would be the basis of the science motivation for future missions to these bodies. Because of the current lack of knowledge about the compositions of these bodies, we found that choosing a specific suite of in situ instruments to develop for such a mission would be premature at this point. (For a primer on in situ instruments for planetary surface exploration, see Appendix B). It is understood that any mission to the Trojans would operate under extreme constraints of mass and power so that it would not be possible to send all possible instrumentation to characterize the surface. Hence, we must develop the theoretical and laboratory framework first so that we can tailor the instruments to the most important measurements. The expected significance of the identification of these markers (the topic of our follow-on proposal) is that it would have implications for all future missions to small bodies (not just the Trojans). It is understood that in order to gain the most detailed knowledge of both chemical and isotopic compositions of small bodies, sample return would be preferred. However, if we can identify one or several very specific markers, it will become feasible to search for these with a small suite of in situ instruments at a number of target bodies. Or, even better, it may be possible for us to identify spectral properties that can be observed remotely. Our goal is to work our way to an understanding of these sorts of dynamically important signatures.

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