Evolving Terrestrial Atmospheres: Can Fire and Air Make A Watery Earth?

Title: Water On Hot Rocky Exoplanets

Authors: Edwin S. Kite, Laura Schaefer

First Author’s Institution: University of Chicago

Status: Published in ApJ Letters, available on arXiv

Most Sun-like stars are thought to be home to a hot rocky exoplanet – that could mean that there are more than 300 million potentially habitable planets in our galaxy! However, whether any of these planets have atmospheres remains unknown. Unlike the Blue Marble we call home, the way that many of these planets form leaves them as dead rocks.

Most terrestrial planets larger than Earth (known as super-Earths) are thought to form as sub-Neptunes, consisting of a silicate magma ball surrounded by a thick atmosphere accreted from the planetary disc during formation. Because this atmosphere is dominated by light hydrogen molecules, it has a low mean molecular weight ( $\mu$ , the average weight of each molecule in the atmosphere) and is later lost to space via atmospheric escape, leaving behind the bare super-Earth. While it’s possible for planets to regain an atmosphere via volcanic activity or impacts from comets, what if there was a way for super-Earths to develop atmospheres while they evolve from sub-Neptunes?

Today’s paper explores a potential pathway that can not only generate super-Earth atmospheres, but could allow them to be retained for billions of years.

When Magma Meets Air

The authors consider what happens to the products that form when a sub-Neptune’s magma reacts with its atmosphere. Iron oxides in the magma react with the atmospheric hydrogen, producing water, and iron, which sinks to the planet’s core. While some of this steam escapes into the atmosphere and mixes with the hydrogen, most of it dissolves and remains trapped in the magma, creating a planet made up of a slightly watery magma ball surrounded by a slightly higher mean molecular weight atmosphere. But as the atmosphere begins to escape, what happens to the water?

A cartoon representation of the steps a Sub-Neptune can take while evolving into a Super-Earth. The image has the title "Pathways to a high molecular-weight atmosphere" at the top, and is divided into three horizontal colour strips. A large, pale blue strip is at the top labelled "SUB-NEPTUNES". Directly underneath that is a smaller pale yellow strip labelled "RADIUS VALLEY", followed by a pale red strip the same size as the blue strip labelled "SUPER-EARTHS". At the far left of the image an arrow points upwards and is labelled "planet radius". The image is divided into a left and right half by a thin black line. The left side is labelled "Exogenic high-molecular-weight atmospheres". In the blue strip a large blue circle containing a smaller orange circle represents a sub-Neptune and is labelled as mu approximately = 2. A large arrow points directly down from the sub-Neptune passing through the yellow radius valley into the red super-Earth region and is labelled "Atmosphere lost to space". The arrow leads to a smaller red circle representing an atmosphere-less super-Earth labelled "bare rock". An arrow points to the right from the bare rock to another circle of the same size surrounded by a very thin green atmosphere layer, representing a super-Earth with a water-dominated atmosphere. This super-Earth is labelled mu > 10, and the arrow leading to it is labelled "Regain atmosphere e.g. from comet impacts". The right side of the cartoon is labelled "THIS STUDY: Endogenous high-molecular-weight atmospheres". In the blue strip the same mu = 2 sub-Neptune represented by a large blue circle containing a smaller orange circle is seen as on the left, however a smaller arrow points to where the blue Sub-Neptune region and yellow Radius Valley strips meet. The arrow is labelled "Magma reacts with atmosphere". Here, the orange circle representing the magma core of the Sub-Neptune is surrounded by a slightly larger circle shaded with a green and blue gradient and is labelled as mu = 2 to 7. From this planet, an arrow labelled "Atmospheric loss + H2O from magma" points down across the yellow Radius Valley to the red Super-Earth region to a Super-Earth with a water-dominated atmosphere. It looks the same as the water-dominated Super-Earth on the left handside of the image, and is also labelled mu > 10

Figure 1: A graphical representation of the potential pathways a sub-Neptune can take to become a super-Earth with a high mean molecular weight ( $\mu_\mathrm{atm}$ ) atmosphere. Blue atmospheres are hydrogen dominated, while green atmospheres are water dominated. The left-hand side shows a sub-Neptune losing its atmosphere to space, becoming a bare rock, and later regaining a high $\mu_\mathrm{atm}$ atmosphere. The right-hand side shows the pathway outlined in today’s paper, with a sub-Neptune evolving to a super-Earth with a water dominated atmosphere via atmosphere-magma interactions. In each pathway, the sub-Neptune moves across the Radius Valley, decreasing in radius as it goes. Adapted from Figure 4 in the paper.

Using models of planets, the atmospheres of each planet are removed in small steps, reassessing the equilibrium between the magma and the atmosphere each time. With each step, the pressure at the surface of the magma decreases, allowing some of the gases trapped there to escape. As the atmospheric loss continues, the model atmosphere gets thinner and thinner, while the large reservoir of H₂O dissolved in the magma continues to be released. As outlined in Figure 1, over time the hydrogen will be completely lost, leaving behind a 150-500 km thick atmosphere and a water dominated world! This kind of watery atmosphere can be referred to as being endogenic as it originates from within the planet, as opposed to the exogenic atmospheres created by external processes, like being hit by an icy comet.

Water, Water, Everywhere?

The length of time for which a planet has a water dominated atmosphere depends on how aggressive the atmospheric loss is. While smaller planets very close to their stars are at higher risk from atmospheric loss, planets at greater distances from their stars are safer and may never endure the process. Planets in between these extremes are able to keep hold of their newly acquired H₂O dominated atmospheres for varying lengths of time, but could potentially retain them for billions of years. So which planets can we expect to have watery envelopes?

When plotted on a graph of planetary radius v.s. orbital period, the larger radius sub-Neptunes and smaller radius super-Earths are separated by a lack of planets often known as the Radius Valley. As a sub-Neptune loses its atmosphere its radius decreases, moving it down through the Radius Valley. The authors predict that provided the planet has a long enough period, and the interactions between magma and atmospheres are sufficiently efficient, the evolving planets that are able to retain H₂O-dominated atmospheres should be found lining the Radius Valley in a “water-belt”, as seen in Figure 2.

A plot of radius (y axis) vs period (x axis) showing planets near the Radius Valley. The x axis is logarithmic, and shows period varying between 1 and 100 days, while the y axis is linear, and shows radius varying between 1 and 2.5 Earth radii. The top right corner of the plot is shaded blue, with a boundary crossing diagonally from 2.3 Earth radii at 1 days to 1.65 Earth radii at 100 days, demonstrating the area occupied by sub-Neptunes. The bottom left corner of the plot is shaded red, with a boundary crossing diagonally from 1.75 Earth radii at 1 days to 1.3 Earth radii at 100 days, demonstrating the area occupied by super-Earths. Both regions appear to be equally populated by black data points, representing planets. In between red and blue sections is a yellow section, which stretches diagonally from upper left to lower right across the plot, representing the Radius Valley. Here, there are noticeably fewer planets than in the rest of the plot. A green shaded region sits on the lower edge of the Radius Valley in the Super-Earth space, labelled "water-belt". This water belt begins at 8 days, where it covers planets with radii of 1.35 to 1.6 Earth radii, and extends to 100 days, where it covers planets with radii of 1.3 to 1.1 Earth radii. Dashed lines sit on the lower edge of the water belt, and directly through the middle.

Figure 2: The “water-belt” of super-Earths, shown in period-radius space for planets orbiting stars less than 3 Gyrs old. The blue region shows the area occupied by sub-Neptunes, while the red region shows the area occupied by super-Earths. The yellow region in between is known as the Radius Valley. The water-belt, where super-Earths with water dominated atmospheres may exist is shown in green. The upper and lower dashed lines give the water-belt predictions for magmas with lower and higher amounts of iron oxides present. Figure 3 in the paper.

Testing whether such planets exist could be relatively straight forward. Directly detecting the atmosphere of this kind of planet may be possible using a phase curve – a measurement of the light reflected and blocked by a tidally locked planet as it passes behind and in front of its host star. If the planet has retained the watery atmosphere, then heat can be more effectively distributed from the permanently illuminated dayside to the cold, dark nightside, leading to a smaller temperature difference between the two faces than would be the case for a bare, atmosphere-free rock. As endogenic atmospheres are likely to have smaller carbon to oxygen ratios than those on other super-Earths, observing the spectroscopic features of these atmospheres with the upcoming James Webb Space Telescope could also help distinguish between the two!

Astrobite edited by Huei Sears

Featured image credit: NASA/JPL-Caltech

About the Author

About Lili Alderson

Lili Alderson is a PhD student at the University of Bristol studying exoplanet atmospheres with space-based telescopes. She spent her undergrad at the University of Southampton with a year in research at the Center for Astrophysics | Harvard-Smithsonian. When not thinking about exoplanets, Lili enjoys ballet, film and baking.

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