Land Fraction Diversity on Earth-like Planets and Implications for their Habitability

A balanced ratio of ocean to land is believed to be essential for an Earth-like biosphere and one may conjecture that plate-tectonics planets should be similar in geological properties. After all, the volume of continental crust evolves towards an equilibrium between production and erosion. If the interior thermal states of Earth-sized exoplanets are similar to the Earth's, one might expect a similar equilibrium between continental production and erosion to establish and, hence, a similar land fraction. We will show that this conjecture is not likely to be true. Positive feedback associated with the coupled mantle water - continental crust cycle may rather lead to a manifold of three possible planets, depending on their early history: a land planet, an ocean planet and a balanced Earth-like planet. In addition, thermal blanketing of the interior by the continents enhances the sensitivity of continental growth to its history and, eventually, to initial conditions. Much of the blanketing effect is however compensated by mantle depletion in radioactive elements. A model of the long-term carbonate-silicate cycle shows the land and the ocean planet to differ by about 5 K in average surface temperature. A larger continental surface fraction results both in higher weathering rates and enhanced outgassing, partly compensating each other. Still, the land planet is expected to have a substantially dryer, colder and harsher climate possibly with extended cold deserts in comparison with the ocean planet and with the present-day Earth. Using a model of balancing water availability and nutrients from continental crust weathering, we find the bioproductivity and the biomass of both the land and ocean planet to be reduced by a third to half of Earth's. The biosphere on these planets might not be substantial enough to produce a supply of free oxygen

Bifurcation in the growth of continental crust

Is the present-day water-land ratio a necessary outcome of the evolution of plate tectonic planets with a similar age, volume, mass, and total water inventory as the Earth? This would be the case - largely independent of initial conditions - if Earth's present-day continental volume were at a stable unique equilibrium with strong self-regulating mechanisms of continental growth steering the evolution to this state. In this paper, we question this conjecture. Instead we suggest that positive feedbacks in the plate tectonics model of continental production and erosion may dominate and show that such a model can explain the history of continental growth. We investigate the main mechanisms that contribute to the growth of the volume of the continental crust. In particular, we analyze the effect of the oceanic plate speed, depending on the area and thickness of thermally insulating continents, on production and erosion mechanisms. Effects that cause larger continental production rates for larger values of continental volume are positive feedbacks. In contrast, negative feedbacks act to stabilize the continental volume. They are provided by the increase of the rate of surface erosion, subduction erosion, and crustal delamination with the continental volume. We systematically analyze the strengths of positive and negative feedback contributions to the growth of the continental crust. Although the strengths of some feedbacks depend on poorly known parameters, we conclude that a net predominance of positive feedbacks is plausible. We explore the effect of the combined feedback strength on the feasibility of modeling the observed small positive net continental growth rate over the past 2-3 billion years

The Long-Term Evolution of the Atmosphere of Venus: Processes and Feedback Mechanisms: Interior-Exterior Exchanges

This work reviews the long-term evolution of the atmosphere of Venus, and modulation of its composition by interior/exterior cycling. The formation and evolution of Venus’s atmosphere, leading to contemporary surface conditions, remain hotly debated topics, and involve questions that tie into many disciplines. We explore these various inter-related mechanisms which shaped the evolution of the atmosphere, starting with the volatile sources and sinks. Going from the deep interior to the top of the atmosphere, we describe volcanic outgassing, surface-atmosphere interactions, and atmosphere escape. Furthermore, we address more complex aspects of the history of Venus, including the role of Late Accretion impacts, how magnetic field generation is tied into long-term evolution, and the implications of geochemical and geodynamical feedback cycles for atmospheric evolution. We highlight plausible end-member evolutionary pathways that Venus could have followed, from accretion to its present-day state, based on modeling and observations. In a first scenario, the planet was desiccated by atmospheric escape during the magma ocean phase. In a second scenario, Venus could have harbored surface liquid water for long periods of time, until its temperate climate was destabilized and it entered a runaway greenhouse phase. In a third scenario, Venus’s inefficient outgassing could have kept water inside the planet, where hydrogen was trapped in the core and the mantle was oxidized. We discuss existing evidence and future observations/missions required to refine our understanding of the planet’s history and of the complex feedback cycles between the interior, surface, and atmosphere that have been operating in the past, present or future of Venus

Planetary Exploration Horizon 2061 Report, Chapter 3: From science questions to Solar System exploration

This chapter of the Planetary Exploration Horizon 2061 Report reviews the way the six key questions about planetary systems, from their origins to the way they work and their habitability, identified in chapter 1, can be addressed by means of solar system exploration, and how one can find partial answers to these six questions by flying to the different provinces to the solar system: terrestrial planets, giant planets, small bodies, and up to its interface with the local interstellar medium. It derives from this analysis a synthetic description of the most important space observations to be performed at the different solar system objects by future planetary exploration missions. These observation requirements illustrate the diversity of measurement techniques to be used as well as the diversity of destinations where these observations must be made. They constitute the base for the identification of the future planetary missions we need to fly by 2061, which are described in chapter 4. Q1- How well do we understand the diversity of planetary systems objects? Q2- How well do we understand the diversity of planetary system architectures? Q3- What are the origins and formation scenarios for planetary systems? Q4- How do planetary systems work? Q5- Do planetary systems host potential habitats? Q6- Where and how to search for life?Comment: 107 pages, 37 figures, Horizon 2061 is a science-driven, foresight exercise, for future scientific investigation

Venus Evolution Through Time: Key Science Questions, Selected Mission Concepts and Future Investigations

In this work we discuss various selected mission concepts addressing Venus evolution through time. More specifically, we address investigations and payload instrument concepts supporting scientific goals and open questions presented in the companion articles of this volume. Also included are their related investigations (observations & modeling) and discussion of which measurements and future data products are needed to better constrain Venus’ atmosphere, climate, surface, interior and habitability evolution through time. A new fleet of Venus missions has been selected, and new mission concepts will continue to be considered for future selections. Missions under development include radar-equipped ESA-led EnVision M5 orbiter mission (European Space Agency 2021), NASA-JPL’s VERITAS orbiter mission (Smrekar et al. 2022a), NASA-GSFC’s DAVINCI entry probe/flyby mission (Garvin et al. 2022a). The data acquired with the VERITAS, DAVINCI, and EnVision from the end of this decade will fundamentally improve our understanding of the planet’s long term history, current activity and evolutionary path. We further describe future mission concepts and measurements beyond the current framework of selected missions, as well as the synergies between these mission concepts, ground-based and space-based observatories and facilities, laboratory measurements, and future algorithmic or modeling activities that pave the way for the development of a Venus program that extends into the 2040s (Wilson et al. 2022)

The Impact of Life on Climate Stabilization Over Different Timescales

Surface life has been argued to be crucial in keeping a planet habitable in the long term. Biologically enhanced weathering compensates for increasing solar luminosity, and temperature-dependent plant productivity weakens climate perturbations. Furthermore, a reduced calcification rate of marine organisms provides a negative feedback to rising atmospheric CO2. Here, I present a model of the long-term carbon cycle including biological enhancement of weathering and marine calcification. Climate oscillations of periods from thousands to millions of years arise from a simple model of mountain uplift and erosion. I systematically study the influence of the biologically driven feedbacks on damping these oscillations. For oscillations of periods 0.5 Myr. These findings are sensitive to the ratio of land to oceans, however. Furthermore, the mantle carbon degassing rate plays a role, since the temperature dependence of biological primary productivity may be smaller at higher temperatures. Altogether, life can be argued to stabilize the climate on timescales longer than some 100 kyr, while details depend on the geological state of the planet