Land, Oceans, and Habitability of Exoplanets

Earth's balanced ratio of ocean to land is great for the biosphere: Land provides direct access to sunlight, while oceans ensure sufficient rainfall. How are the chances that Earth-sized exoplanets would show a similar surface? Here, we model fundamental feedback processes steering the geologic evolution. The focus is on uncertain parameters that likely vary among planets, such as the initial mantle temperature. We find that most planets should be quite different from Earth: Water worlds and dry land planets should be widespread. With a geologic carbon cycle model, we find the surface temperature of these planets to differ by 5°C, which would impact the expected biosphere.

Related publication: Höning & Spohn 2022, Astrobiology (in press)

Carbon Emissions, Climate Change and Sea Level Rise

Earth's climate is heating up - due to tons of carbon, humans are pumping into the atmosphere. As a consequence, ice sheets are melting, above all, Greenland and Antarctica. But the connection between carbon emissions, melting of ice sheets, and rise of the sea level is complex and non-linear. In this project, we explore tipping points of the Greenland ice sheet: Critical thresholds, above which the system changes so that sea level rise would speed up, without return.

Related publication: Höning et al. 2022, Geophysical Research Letters (under review)

Habitability and Early Evolution of Venus

Venus' blazing hot surface is a consequence of massive CO2 release into the atmosphere. But when and how did this happen? Was Venus once a habitable place with liquid water on its surface, similar to Earth today? Interior-atmosphere exchange processes that control Venus' habitability depend on the mode of convection, and in particular a stagnant-lid regime - as observed today - complicates the carbon cycle.

Related publication: Höning et al. 2021, Journal of Geophysical Research: Planets

Land Plants, Marine Organisms, and Earth's Climate

The long-term carbon cycle is an important feedback mechanism that keeps Earth's climate stable over millions of years. Land plants are an important component in this cycle, as they enhance the dissolution rate of rock. On shorter timescales, marine organisms play an important role, as they make their skeletons and shells out of calcium carbonate. If the atmospheric CO2 increases, biogenic calcium carbonate production slows down. Since calcium carbonate production leads to a release of CO2 into the atmosphere, these organisms help to stabilise the climate. This project addresses the question of how efficient life is in stabilising our climate over several thousand or million years.

Related publication: Höning 2020, Geochemistry, Geophysics, Geosystems
Related presentation: Climate oscillations damped by bioactivity, vPICO, EGU 2021

Planetary interior evolution and habitability

Billions of years ago, our Sun was much fainter, but still the Earth's surface was covered by oceans. Therefore, there must have been much more greenhouse gases in the atmosphere than today. In this project, we study the role of Earth's interior. We find that when the mantle was hotter than today, subduction of carbon into the mantle was rare, and CO2 could accumulate in the atmosphere. What is the implication for Earth-like exoplanets?

Related publication: Höning et al. 2019, Astronomy & Astrophysics
Related presentation: Climate evolution of rocky planets, EAI Seminar

Feedbacks in Continental Growth

Most of the Earth's continental volume was already present billions of years ago. Today, there is a continuous equilibrium between continental production and erosion. How can we explain that this equilibrium did not change much over the past billion years despite the cooling of Earth's mantle?

Related publication: Höning et al. 2019, Physics of the Earth and Planetary Interiors

Life, Feedback Loops, and the Interior Evolution of Earth

The Earth's system is complex machinery in which life plays a crucial role. Water in the Earth's mantle reduces its viscosity, which speeds up the oceanic plates. This affects the rate of volcanism and subduction, and thereby the flux of water between the mantle and the oceans. Life enters the system as it affects weathering and erosion rates of rocks, and thereby indirectly the subduction flux of sediments into the interior. How would the Earth system work without life?

Related publications: Höning & Spohn 2016, Physics of the Earth and Planetary Interiors
Höning et al. 2014, Planetary and Space Science