The Solar System As an Exoplanet Guide: Findings, Surprises and Caveats from the First Phase of Human and Robotic Exploration

Exoplanets in our Backyard 2020 (LPI Contrib. No. 2195) 3054.pdf

THE SOLAR SYSTEM AS AN EXOPLANET GUIDE: FINDINGS, SURPRISES AND CAVEATS FROM THE FIRST PHASE OF HUMAN AND ROBOTIC EXPLORATION. James W. Head1, 1Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI USA ([email protected]).

Introduction: Sixty years of space exploration has is unlikely (Earth, Venus, Mercury). Tectonic systems provided unprecedented knowledge of, and perspec- and heat-loss mechanisms: The terrestrial planets, tives on, the origin and evolution of our Solar System. including “Earth-like” Venus, do not have multiple This initial phase of human and robotic exploration lithospheric plates and plate tectonics, instead being has both confirmed and rejected previous theories, characterized by single global lithospheric plates revealed completely new information, and unveiled (“one-plate planets”) and losing heat by conduction sobering new insights into the reality of the multiple (Fig. 3). The role of size in planetary evolution: Small paths of planetary formation and evolution, all of planets (Mercury, Mars) and the Moon lost heat effi- which can be applied to the exploration of exoplanets ciently due to the high surface area to volume ratio, and other planetary systems. We are in the early stag- stabilizing lithospheres that thickened with time. In- es of an unprecedented period of mutual, bilateral ternal structure and mantle convection: The first-order education and learning. internal structure of the terrestrial planets is known Opportunities for the Solar System Community: but the details of mantle composition, convection Other planetary systems offer untold numbers of indi- scale-lengths, and history, are not uniformly known or vidual examples of planets, systems of planets, and understood. Planetary scale and crustal and magnetic stars. Exploration of this huge parameter space is yet fields have been detected but the cause of their gener- another framework for increased understanding of the ation, and the nature and timing of their evolution origin and evolution of our Solar System. It can also and/or demise is not well known. Spin-axis/orbital assist in the development of future Solar System ex- parameters: Variations in planetary obliquity, eccen- ploration strategies. tricity and precession have been shown to have a sig- Opportunities for the Exoplanet Community: nificant influence on climate and the migration of The Solar System provides a rich and accessible rec- volatile species; deposits from huge tropical mountain ord of the origin, evolution and fate of a small number water-ice glaciers and smaller polar CO2 glaciers have of planets and satellites. The lessons learned from been observed on Mars. The huge Tharsis rise on initial assumptions and evolving outcomes is both Mars is likely to have caused True Polar Wander instructive and sobering, and provides a template for (TPW), but the timing and magnitude is debated. exploring and understanding other planets and plane- Petrogenetic evolution: The compositions, including tary systems. volatile content, and percent partial melting, of man- Findings and Surprises from Terrestrial Planet tles producing secondary crusts are likely to have Exploration: Planetary formation and early evolu- been quite different from the Earth, and to have var- tion: The original Laplacian theory of nebular con- ied over the course of geologic time. Details of terres- densation and planetary formation predicted a rela- trial planet mantle evolution are largely unknown. tively orderly trend in temperature and pressure decay Planetary processes: Impact cratering has clearly from the interior of the solar nebula, consistent with played a major role in all aspects of early planetary the first-order observations of the high-density of history, including primary crust formation. The vol- Mercury, the water-rich habitable zone, and the dis- umes, styles and flux of planetary volcanism varied tinctive differences between terrestrial planets and gas significantly throughout history, related to initial giants (Fig. 1). More recent studies of the order of volatile content, lithospheric heat loss mechanisms, planet formation and its effects on planetary migra- and planetary compositional layering. Tectonic style tion as well as the role of giant impacts in stripping and importance varied substantially with one-plate planetary crusts have significantly modified this sim- planets dominated by vertical processes (uplift, load- ple picture. Problems have also arisen in accounting ing, flexure) in contrast to the lateral plate tectonics for the small size and volatile abundances on Mars. of Earth. Planetary atmospheres: First order observa- The Venus D/H ratio suggests significantly more wa- tions have been made on current atmospheres of ter on Venus than Earth. Formation and evolution of Earth, Venus and Mars, but great uncertainty exists planetary crusts: Planetary crusts turned out to be dis- about their formation and evolution. How do we dis- tinctly un-Earth-like (Fig. 2) with primary crust gen- tinguish primary and secondary atmospheres, what are erated by accretionary energy, producing magma the major processes and rates of acquisition and loss oceans and a plagioclase flotation crust on the Moon, to space, how did Venus acquire and retain its ‘runa- and secondary crusts by partial melting of the mantle. way greenhouse’ atmosphere, was early Mars ‘warm Not fully understood is the transition from primary to and wet’ or ‘cold and icy’ and how did it evolve to its secondary crusts on bodies where plagioclase flotation current state, what role did the evolving Sun have on Exoplanets in our Backyard 2020 (LPI Contrib. No. 2195) 3054.pdf

climate and atmospheric retention, how do stochastic ties will provide an improved understanding of both processes such as impact cratering influence atmos- our Solar System and exoplanets and exoplanetary pheric formation, evolution and loss. Geological his- systems. tory: The early-stabilized lithospheres of the small planetary bodies Moon, Mercury and Mars have provided an invaluable record of the impact, volcanic and tectonic processes that may have operated on Earth and Venus in their earlier, largely missing history (Fig. 4). Habitability: Exploration of Mars and the outer planet satellites has led to new perspectives on possible environments conducive to the formation and evolution of life. Planetary Perspectives and Questions: Mercury: Did a major impact strip the early crust, producing the observed high Fe/Si ratio? How does small mantle convective scale-length influence the generation, as- cent and eruption of secondary crust? Venus: What plausible evolutionary pathways can lead to the loss of the majority of its history by recent crustal resur- facing and to the currently observed state of the atmosphere? Earth: What are the effects of the formation of the Moon on the internal structure and sub- Fig. 1. Variations in planetary position, other factors. sequent evolution of Earth, and can this account for observed Earth-Venus differences? Moon: How can a body accreting from the aftermath of a Mars-sized impact into proto-Earth retain volatiles? Mars: What is the nature and evolution of the early atmosphere and the climate, and what accounts for the transition from primary to secondary atmosphere, and to that of today? Some Perspectives and Caveats: Terracentrism: We know the (recent) Earth so well that we view eve- rything through this lens; it took decades to appreciate Fig. 2. Origin and evoution of planetary crusts. the role of impacts in Earth history. As Rodney Brooks has said, the retreat from specialness has be- come a route! Uniformitarianism: This concept has served geologists well in the last centuries, weaning us from Usher-ian and biblical deluge interpretations, but has built in a distaste for the inevitable stochastic processes. The Role of Stochastic Processes: Punctu- ated, stochastic events are a fundamental part of planetary evolution. The Promise and Pitfalls of Para- digms: Paradigms are essential in making sense out of a complex Universe, but Kuhn-ian revolutions are Figure 3. Mechanisms of planetary heat loss. The po- inevitable and necessary; keep an eye out for those sition of ancient Venus is unknown. data that don’t fit the paradigm. The Space-Time Continuum: We spend virtually all of our time in the lower left-hand corner; spend time in deep space and deep time. Don’t forget option d): When considering multiple interpretations, it could be: “None of the above”. Venus (Fig. 4) turned out to be that way! Summary: The lessons learned from initial assumptions and evolving outcomes in the exploration of the Solar System provide a template for exploring Fig. 4. Geological evolution of the terrestrial plan- and understanding other planets and planetary sys- ets; width of line represents estimated amount pre- tems. Mutual interactions between the two communi- served today dating from that time.