Over the last decade, the idea of a static solar system history has dramatically shifted to one of dynamic change and mixing (DeMeo & Carry 2014). Dynamical models propose that the inner solar system was sculpted by the giant planets’ orbital migration (Grand Tack and Nice models) and in particular that today’s asteroid belt may not only hosts objects that formed in situ, typically between 2.2 and 3.3 AU, but also bodies that were formed beyond Jupiter and for some of them even beyond Neptune. In a broad stroke, the idea that the asteroid belt is a condensed version of the primordial solar system is progressively emerging.
Fig. 1: illustration of the dynamical history of small bodies in the Solar System based on the Grand Tack and Nice models, reproduced from DeMeo & Carry (2014). These models may not represent the actual history of the Solar System, but are possible scenarios. Nautilus will test them.
The major limitation to this predicted dynamical evolution is that the existing meteorite and asteroid data do not exclude it, but provide no proof for it either. The lack of evidence can be explained along two lines.
1) The objects that are predicted to be implanted bodies from the outer solar system (C- and D-types), and which therefore lie at the crux of current dynamical models, are not represented in our meteorite collections as indicated by laboratory spectroscopic measurements. This may result from the fact that being volatile-rich, these objects disintegrate in the atmosphere (as do comet fragments) also a consequence of their low density and their comet-like activity in some cases. Overall, these objects represent a significant fraction of main belt asteroids (>30%) and are mainly located in the outer part of the asteroid belt (between ~2.7 and ~3.3 AU).
2) We have no constraints on the composition of the volatiles that are present in those asteroids. This is perhaps the most important missing information as the nature of the volatiles as well as their isotopic composition (e.g., D/H ratio) provide far stronger constraints on the formation location of a body than the dust phase. Only water has been detected so far and its presence is not incompatible with in-situ formation. Detecting the presence of additional volatiles such as CO, HCN, CH4, etc… that are predicted to have condensed beyond 5 and even 10 AU among main belt asteroids in proportions similar to those observed in comets would definitively validate the Grand Tack and/or Nice models of the early dynamical evolution of the solar system.
Validating or invalidating these models via a mission to these objects would have far reaching implications. First, it would contribute to our understanding of the origin of Earth’s water and the epoch of its delivery, which is of prime importance when assessing how exceptional the appearance of life on a planetary object can be and thus whether our stellar system is original or not. Second, if it effectively turns out that the asteroid belt comprises small bodies that formed in the outer solar system, it will become an ideal and a much more accessible place than for instance the Kuiper Belt for future exploration of small bodies by interplanetary missions. In summary, such a mission would contribute to answering the following key questions about the early history of the Solar System:
– Did volatile-rich asteroids form near Jupiter’s and Saturn’s orbit and/or farther out in the Solar System or
simply at their current location?
– Are these bodies the source of Earth’s water?
– When did Earth acquire its water budget?
– Is there any evidence of life precursors in the form of simple organics present on or inside these bodies?
– What volatiles are present among main belt asteroids?
– How do they compare to primitive meteorites, comets and giant planet satellites?
The overarching science goal of the Nautilus mission, proposed by a team of European scientists with a strong participation of US scientists, is to explore a volatile-rich asteroid not represented in our meteorite collections (C- or D-type), and ultimately cast light on these aspects of the early history of the solar system. It directly addresses two of the four key scientific questions highlighted in ESA’s Cosmic Vision program:
- What are the conditions for planet formation and the emergence of life?
- How does the Solar System work?
This goal translates into a number of specific science objectives, which can only be reached by a rendezvous mission:
- Characterize the bulk composition (volatiles, organics, silicates) and variegation across the surface
- Identify active regions and the source of activity
- Characterize the chemical and isotopic composition of the volatiles
- Characterize the surface mineralogy and composition
- Characterize the thickness and structure of the regolith
- Characterize the astrobiological potential for future human and/or robotic exploration