Molten exoplanets as a window into the earliest Earth
Our own world started out as a literal hell — at the onset of the ‘Hadean’ eon the Moon-forming impact melted and vaporized large parts of the adolescent Earth. The resulting ‘magma ocean’ cooled down over a few million years, eventually allowing the water vapor in the atmosphere to rain out and form the earliest oceans not too long after the last giant collision — or so the story goes, according to the combined picture from geochemical, astronomical, and planetary science studies. But does this story hold equally well for other planets, such as Venus, Mars, or even rocky planets in other, extrasolar planetary systems?
This blog article summarises the research published in Lichtenberg et al., Journal of Geophysical Research: Planets (2021), openly accessible at arXiv:2101.10991.
The question of how this magma ocean epoch evolves is of foremost importance for better understanding the planetary surface conditions on young rocky planets, and in turn our own origins. Latest estimates for the emergence of the earliest life on Earth push the timeline further and further back toward the formation of the planet itself. We have evidence for liquid water on the surface as early as ~4.4 billion years ago (the Solar System formed ~4.567 billion years ago), and there is (contentious) evidence for life-like chemistry as far back as 4.1 billion years. This means if we want to understand how susceptible rocky worlds are to developing life, we must better grasp their earliest conditions — we need to find out how young planets evolve. Even though geochemical proxies push these boundaries ever-farther back, there are fundamental limits to what we can achieve on our own planet — 4.5 billions years of geological activity, and perhaps about 3 billion years of plate tectonics, have erased many telltale signs of the earliest surface environments.
Did life originate on land? Under what kind of climate? Was there even land or was the planet covered in global oceans? Apart from some very broad bounds, we don’t know. And so, from the Earth Science perspective alone, we are limited in looking back at the eons to try and decipher our own origins. The emerging study of extrasolar planets in the last ~25 years, however, is a game changer: the latest exoplanet surveys offer an inkling of the climatic and surface conditions of exoplanets in our cosmic backyard. Most of these are hot planets, very close to their star. They are often molten from intense stellar radiation and may have had their atmosphere stripped away.
However, upcoming surveys that are currently in the construction or planning phase — in particular large direct imaging surveys that aim to detect and characterize planets with Earth-like insolation — will be able to image young, Hadean-Earth-analogue planets, and thus provide us with more information to understand how these exoplanets differ or are similar to Earth. In an ironic twist of fate, we stand to learn more about ourselves by looking away from Earth, out in the vast void that is the galaxy!
To be able to do this, we need to develop some baseline understanding of how planets with different inventories of volatiles — the molecules that make up the atmospheres of planets — evolve and how they determine the climatic state of planets. This is what we did in this study. We took a step back from the standard picture gained from studying Earth and asked the general question of how a rocky planet with a completely different volatile inventory may evolve. In particular, we aimed to simulate the thermal and climatic evolution of young planets that start their lives molten — this can happen due to the primordial heat from accretion (like the Moon-forming impact), decay of heat producing elements, iron core formation, or the intense insolation of exoplanets on short orbits.
Different histories of accretion can lead to substantially different initial inventories of volatiles, but the internal evolution of planets can also lead to differences in the amount and species of volatiles that can cycle between the mantle and atmosphere. While the planet is still molten, this cycling between mantle and atmosphere is the most rapid. For instance, water is highly soluble in silicate mantles of molten planets, and so the pressure and temperature in the atmosphere are directly connected with the cooling and crystallization of the mantle during the magma ocean phase.
When a molecule like water is in the atmosphere it delays the cooling of the planet due to the greenhouse effect. This is similar to adding additional carbon dioxide in the present-day Earth atmosphere: the more greenhouse-inducing volatiles are in the atmosphere, the less efficiently the planet can radiate its heat to space. This means the planet takes longer to cool down from the ancient hellish conditions. We tested seven molecules, ones that have either specific affinity to outgas early on and are poorly dissolved in magmas — such as H2 or CO2 — and strongly greenhouse-inducing and soluble ones like H2O.
Our computer simulations of the coupled mantle-atmosphere system reveal that the cooling timescale and thermal evolution of young planets is very sensitive to the molecule in question. For instance, planets that are covered by large H2 atmospheres — these can for instance be inherited from the protoplanetary disk in which the planet formed — can massively delay cooling. Planets that start out with a different volatile setting may experience magma ocean episodes that last hundreds of millions or even billions of years! A novel aspect of our study is that we find that differences in processes that govern mantle crystallization can lead to observable differences in the atmospheric spectrum. This is what exoplanet surveys can pick up in the years to come, enabling us to test our predictions. And molten planets glow more strongly than cooler planets. This means, if exoplanet surveys specifically target young stars they may be able to catch the glow from these planets, and thereby infer the climatic and interior environment of the young molten state, which is mostly lost to the past on our own planet.
This is particularly interesting for the question of the origin of life on our own planet. Since the late 1950s prebiotic chemists in principle favor so-called reducing climatic conditions, which are dominated by molecules such as CH4 and may feature HCN and derivatives, as being more amenable for creating the chemical networks that started life. Especially in the last ~10–15 years, concerted efforts to simulate these earliest chemical networks in laboratories provided more and more evidence that mildly reducing conditions on the planetary surface with a restricted amount of UV influx can favor an Earth-like origin of life. However, all geochemical proxies on Earth seem to tell us that the early planetary crust was oxidizing — more prone to create CO2 or H2O dominated atmospheres. This conundrum is unresolved but can be better understood by inferring the earliest planetary conditions on rocky exoplanets. By probing the atmospheric and mantle conditions directly during — and shortly after — the magma ocean epoch, we will be able to tell how the climatic and chemical conditions on rocky planets evolve.
From a planetary physics perspective, observing exoplanetary magma oceans will help us to understand the difference between Venus and Earth in our own Solar System. One theory for why Venus features hellish conditions at present-day says that Earth and Venus diverged on their evolutionary paths extremely early during the magma ocean phase. Earth was able to cool down to clement surface conditions. But Venus was closer to the Sun and therefore remained locked in the magma ocean phase for far longer. During this phase the water in the atmosphere broke down and the resulting hydrogen was lost to space over a long time. This way the planet was never able to cling onto enough water to eventually form oceans and evolve to ‘habitable’ conditions.
To date, most models of magma ocean cooling have only focussed on the insulating effects of water vapor, and sometimes included carbon dioxide. Our new simulations, however, indicate that the cooling paths of magma ocean planets depend critically on the specific amount and combination of volatiles in the planet. For example, CO2, H2O, and CH4-dominated atmospheres behave similarly and enable cooling down to a solid planet on a timescale of about 2 million years. On the other hand, H2-dominated atmospheres show very extended cooling sequences that can reach hundreds of millions of years, and perhaps even billions of years if the planet cannot lose this hydrogen efficiently to space. Future exoplanet observations will directly test this hypothesis: if we observe numerous molten planets within the so-called runaway greenhouse limit — which depends on insolation and inventory of volatiles — this means the magma ocean epoch was the defining phase that eventually enabled Earth to give rise to life and left Venus as a hothouse.
Ultimately, we will be able to use observations of far-away worlds to gaze into the deep past of our own world. Before we can do so, however, we will need to continue to develop the theory to robustly identify the diverse climatic settings that can arise depending on the available volatiles. Supplementing novel observations of these distant worlds with the theory to accurately describe their composition, atmosphere, and interior will provide crucial insights into the nature of rocky planets. It will guide us ever closer to deciphering the earliest surface environment on Earth and how this facilitated the rise of life. Ultimately, we would all like to know — how many ‘Earth-like’ worlds are really out there?
This research was supported by funding from the Simons Collaboration on the Origins of Life, the Swiss National Science Foundation, and the European Research Council.
A video summary of the research can be found here:
Read the full paper: ‘Vertically resolved magma ocean–protoatmosphere evolution: H2, H2O, CO2, CH4, CO, O2, and N2 as primary absorbers’, Journal of Geophysical Research: Planets (2021), T. Lichtenberg, Dan J. Bower, Mark Hammond, Ryan Boukrouche, Patrick Sanan, Shang-Min Tsai, Raymond T. Pierrehumbert. doi: 10.1029/2020JE006711. arXiv:2101.10991.