Venus Modeling Workshop (2017) 8030.pdf

VENUS GEOLOGICAL HISTORY: CURRENT PERSPECTIVES, UNKNOWNS, AND OPPORTUNITIES FOR THE MODELING COMMUNITY. James W. Head, Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912 USA ([email protected])

Introduction: Acquisition of Earth-based radar image Period), with rift zones often radiating from topographic observations, followed by regional image coverage rises; volcanism continues (perhaps to today [5]), but is (Venera 15/16) and finally global image coverage by primarily characterized by lobate lava flows associated Magellan, together with global altimetry, have provided with the rifts (the network rifting-volcanism regime). In the data necessary to analyze stratigraphic relationships summary, the geological record consists of the majority and produce a global geological map of Venus [1]. The of history that leaves no geological/geomorphological resulting stratigraphic column provides an outline of the record (Phase I), followed by Phase II, a period of in- major themes in the geological evolution of Venus in tense global tectonic deformation followed immediately terms of fundamental processes such as tectonism [2] by global shield plains and regional plains volcanically and volcanism [3]. The paucity of superposed and em- resurfacing over 60% of the planet, followed by Phase bayed impact craters and the impression that they are III, relative quiescence and development of a global rift- randomly distributed precluded the traditional counting ing system linking several broad rises. The last two of superposed craters on individual geological units to phases occurred in less than the last ~15-20% of the his- derive an impact crater size frequency distribution-based tory of Venus. absolute chronology. The recent utilization of techniques This scenario presents multiple major challenges to of buffered crater counting and related methods has re- various modeling communities: internal structure and cently provided a basis for linking the sequence of evolution, mantle convection, thermal evolution, geody- events in the geology/stratigraphy to an absolute chro- namic, geochemical, petrogenetic, atmospheric origin- nology [4]. We now have an interpretative framework dynamics-geochemistry-evolution, ionosphere, solar sys- for the geologic history of Venus that can be used as a tem formation and evolution. We outline these here. basis for identifying outstanding questions and applying Planetary Perspectives: What phases of typical ter- a wide range of modeling techniques to address these restrial planet evolution (e.g., accretion, satellite acquisi- questions. tion and loss, core formation, crustal segrega- The geological history of Venus can be characterized tion/growth/aftermath, magnetic field evolution, volatile by three basic consecutive phases (Fig. 1): Phase I rep- acquisition and degassing to form atmosphere/oceans, resents the period prior to the formation age of the geo- impact flux and basin formation, mantle and lithospheric morphological/geological units on the surface (the pre- evolution, ionospheric structure and evolution, influence Fortunian Period) and occupies the majority of the histo- of solar and interplanetary environment) can be estab- ry of Venus. Although some rocks comprising the oldest lished, modulated or ruled out from our knowledge of observed preserved unit, the tessera, could date from this Venus? If Venus transitioned from an Earth-like planet era, the observed geologic record starts with Phase II. to its current state, when, over what time period, and Phase II is comprised of two regimes, an initial global how did this take place? What is the cause of Venus’ tectonic regime which begins with the intense tectonic slow retrograde rotation? Could Venus have undergone deformation (the Fortunian Period) interpreted to have true polar wander? What is the explanation for the lack formed the globally distributed tesserae highlands of of a detectable magnetic field? What can evolutionary thickened crust that comprise about 7.3% of the planet, models say about the presence and fate of a moon(s)? followed by many tectonic structures in the surrounding What do solar system evolution models tell us about the highly deformed plains, including ridge belts, groove initial position and residence time of Venus relative to its belts and coronae. The second regime in Phase 2, the current position in the Solar System? What do spin-axis, global volcanic regime, starts with the emplacement of orbital parameter (e.g., obliquity, eccentricity) evolution volcanic plains dotted with thousands of small shield models tell us about the evolution of Venus? How can volcanoes, and is immediately followed by regional Venus’ geologic history models inform us about how plains interpreted to have been emplaced as flood basalts plate tectonics might have initiated Earth? How do Ve- in lows between the tesserae highlands, and then de- nus and Earth fit into the context of new models of ex- formed by wrinkle ridges. The shield and regional plains oplanetary system formation and evolution? comprise 61.3% of the surface of Venus. Thus, the vast Interior Evolution, Mantle Convection and Geody- majority of the observed surface geologic units on Venus namics: Venus appears to have undergone a relatively (80.7%) formed over a relatively short period of time recent distinctive global tectonic phase, followed by a (the Fortunian and Guineverian Periods), estimated to near global volcanic phase, a significant reduction in have lasted less than several hundreds of millions of volcanic flux, followed by an extended rift-dominated years. Phase III represents a distinctive change in style, phase of tectonism and volcanism. What is the relative an extended period of global network rifting (the Atlian role of Pratt, Airy and flexural isostasy in accounting for Venus Modeling Workshop (2017) 8030.pdf

the current topography of Venus? What are the more de- Ionosphere, Atmosphere, Climate and Hydro- tailed, testable predictions of models of the transition sphere: What are the most plausible current models for from mobile lithospheric lid to stagnant lid regimes? Can the history and evolution of the climate of Venus? What geodynamic models explain the observed near-global was the nature of the evolutionary transition to the cur- flood basalt phase following tessera formation? What rent atmosphere? Was it gradual, or did the apparently geodynamic and petrogentic models can account for the short-term global phase of tectonism and volcanism near-global distribution of small shield volcanoes? What mark an evolutionary step-function? How do variations coupled geodynamic/petrogenetic models can account in the solar wind over the short term and geologic time for the apparently very viscous magma represented by influence the atmosphere and atmospheric loss rates? the steep-sided domes and festoons? How can mantle What are the loss rates of water and other volatiles from convection and geodynamic models account for both the Venus upper atmosphere to space? What are the loss global small shield volcanoes (<~20 km) and global rates of volatiles to the surface through chemical reac- large shields (>~200 km)? What temperature-dependent tions and how did these change with time? Did Venus crust-mantle viscosity structure seems most consistent have an ocean and if so, what was its magnitude, dura- with the geological features and evolution? How can ge- tion and fate? How can impact crater ejecta patterns fur- odynamic models distinguish between episodic global ther inform us about atmospheric vertical structure, resurfacing and a one-time mobile-lid to stagnant lid global circulation, and evolution? What atmospheric transition? What does the global and temporal distribu- models best predict the unique surface properties of the tion of coronae and large shield volcanoes tell us about highest Venus elevations? How can the eolian alteration mantle convection patterns and the thermal evolution of of impact crater ejecta inform us about atmospheric evo- the lithosphere? Is Venus currently volcanically active? lution? What do atmosphere chemistry models predict Where and why? Can the current cratering record reveal about surface weathering and can this be recognized in information about changes in the evolution of CO2 at- the Venera panoramas or global surface properties? mospheric pressure? How can impact flux modeling and Conclusions: Observations from space mission to Ve- observations improve the chronology of Venus’ recent nus over the last 55 years have established a substantial geologic history? What does the configuration of the database of knowledge and raised significant new ques- late stage global rift systems tell us about recent mantle tions. Modeling from a wide range of communities to convection patterns? address a host of outstanding questions can lead to im- Surface Evolution and Relation to Atmosphere and portant new insights in the coming decades. Interior: What was the nature of the global event that References: [1] Ivanov & Head, PSS, 59, 1559, 2012; produced the tessera terrain? Was it truly global and [2] Ivanov & Head, PSS 113, 10, 2015; [3] Ivanov & what was the duration of this event? What do models of Head, PSS 84, 66, 2013; [4] Kreslavsky et al., Icarus atmospheric evolution and climate change predict about 250, 438, 2015; [5] Shalygin et al., GRL 42, 4762, 2015. the influence of the thermal structure of the crust and lithosphere how changes could be reflected and recognized in the style of tectonic deformation? What ex- plains the common correlation of coro- nae and rift zones? Are coronae causing rifting, or is rifting inducing upwelling? On the basis of comparative planetology modeling, what is the most plausible scenario for the nature and fate of water and oceans in earlier Venus history? How can impact cratering hydrocode models increase our understanding of crustal and mantle structure and evolu- tion? How can impact cratering hydro- code models inform us about the influ- ence of major impact events on the at- mosphere? How can physical volcanolo- gy models explain the apparent dearth of pyroclastic deposits? How can volcanic eruption and impact crater ejecta modeling link Venera Figure 1. Stratigraphic units, sequence and timing of the lander panoramas to global processes? geological history of Venus [1-3].