How Do You Find the Surface of an Exoplanet? Ask Its Atmosphere!

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Title: How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres
Authors: Xinting Yu (余馨婷) et al.
First Author’s Institution: University of California Santa Cruz
Status: Published in ApJ

Of the 4,400 (and counting!) exoplanets, the population of intermediate-sized planets is one of the most interesting. With sizes between Earth and Neptune not seen in our solar system, the most commonly occurring type of planet can be a confusing one. A planet in this category could be a giant terrestrial planet, with a solid surface and thin atmosphere (a “super-Earth”), or it may be more like a shrunken down version of the solar system’s ice giants (a “sub-Neptune”), with a surface located deeper within the planet at high-pressure levels, if there is one at all. Even though many intermediate-sized exoplanets have been discovered, the internal structure of any one planet isn’t always clear. Large uncertainties in the masses and radii of these planets, and hence in their densities, can make understanding their precise compositions a challenge, and even the most sensitive upcoming telescopes like JWST and ARIEL cannot directly probe surfaces, leaving many exoplanets in composition limbo.

However, JWST and ARIEL will be capable of precisely measuring the atmospheres of such planets — so what if there was a way to find out how deep the surface of an exoplanet lies by studying its atmosphere? The authors of today’s paper investigate whether there is a relation between the abundances of species found in an exoplanet’s atmosphere and the location of the exoplanet’s surface.

Under Pressure

As it turns out, the existence of a solid surface plays a key role in the makeup of the atmospheres within our own solar system. Both Jupiter and Saturn’s moon Titan contain very little ammonia (NH3) within their upper atmospheres, as it gets destroyed by photochemical reactions that occur there. But while Jupiter contains significant amounts of NH3 deep within its atmosphere, Titan does not. The difference here is Titan’s surface. In Jupiter, the lack of a solid surface means the constituent parts of NH3 are transported into the hot, high-pressure lower atmosphere where they can reform into ammonia via thermochemical reactions, whereas Titan’s surface prevents its atmosphere from reaching high enough temperatures and pressures for the recycling reactions to occur. Titan instead has a larger abundance of nitrogen, left over from the destroyed NH3.

The authors propose that a similar situation could occur with other species within the atmospheres of exoplanets. To test this theory, they modelled the atmospheric evolution of sub-Neptune K2-18b under varying surface assumptions: first with no surface, and then with a surface at one of three different pressure levels.

two diagrams illustrate chemical pathways in the atmosphere of an exoplanet for different surface conditions.

Figure 1: Diagrams describing the main chemical pathways within the atmosphere of K2-18b for a deep surface or no surface (left) and a shallow surface (right). Arrow thickness indicates the importance of each pathway, with dashed arrows being the least important. In both cases, UV photons impacting the upper atmosphere cause photochemical reactions that break down sensitive molecules such as NH3, HCN, H2O, and CH4. In the deep/no surface model, thermochemistry in the deep, hot atmosphere recreates the molecules lost to photochemistry. In the shallow surface case, atmospheric temperatures are never hot enough for thermochemistry to be effective, causing a decrease in abundances of the species in blue compared to the no surface case, and an increase for the red species. [Yu et al. 2021]

Much like within the solar system, if K2-18b has a shallow surface, the atmosphere is never hot enough for thermochemical reactions to take place, meaning the abundances of photochemically fragile species such as ammonia decrease compared to when the surface is much deeper or non-existent, as shown in Figure 1. For each version of the model, the changes in the volume mixing ratios of key chemical species within the observable atmosphere demonstrate the impacts of surfaces at different pressure levels.

four plots showing volume mixing ratios under four different surface conditions

Figure 2: Plots showing how the volume mixing ratios of key chemical species change with pressure through the atmosphere of K2-18b with different surfaces. Higher pressures indicate deeper into the atmosphere. The pale blue shaded region indicates the observable part of the atmosphere. [Yu et al. 2021]

When the planet has no surface or a very deep surface, large amounts of hydrocarbons and nitrites such as hydrogen cyanide (HCN) are produced, while significant quantities of ammonia are found deep in the atmosphere just like in the case of Jupiter. When a surface exists at 10 bars, key nitrogen species can no longer be replenished and produced as easily, leading to decreasing volume mixing ratios for HCN and NH3. For the shallowest, Earth-like surfaces, thermochemistry is prevented for the majority of species, and the atmosphere is now also depleted in water (H2O) and methane (CH4). As Figure 2 shows, changing the presence or depth of a planet’s surface will change the abundances of a whole host of species — but are these changes significant enough to distinguish between surfaces?

A New Tool For Observers?

Using the finding that a variety of species are uniquely sensitive to the presence of different surfaces, the authors are able to use the abundance ratios between a species when a surface is and isn’t present, and between different pairs of species to tentatively outline a way to distinguish where a surface could be.

flowchart diagramming possibilities to identify the pressure depth of the planet surface from species abundance ratios

Figure 3: Flowchart to aid in the possible determination of the pressure level of a surface within an exoplanet similar to K2-18b using the observed abundance ([X]) ratios of different species. [Adapted from Yu et al. 2021]

With the flowchart shown in Figure 3 at hand, it could be possible to determine where surfaces lie within exoplanets, provided accurate abundance measurements are available. Unfortunately, current measurements of K2-18b’s Neptunian atmosphere aren’t precise enough to make a prediction about any potential surface, but upcoming JWST observations of this planet could provide further information.

So, does this mean the mystery of intermediate planet surfaces can finally be resolved? Not completely. More modelling is needed to extend the range of planetary parameters and scenarios. In the future, the flowchart could be expanded to include exciting but less well-studied species such as phosphine (PH3). The current study also does not consider the potential impacts of processes that occur on the surface, such as volcanic activity and reactions with oceans or rocks, or the potential escape of gases from the top of the atmosphere — all processes that could change the observed abundance ratios in an exoplanet. Nevertheless, today’s paper outlines an exciting new concept that extends our toolkit as we continue to try to understand the growing number of strange new worlds waiting to be explored.

Original astrobite edited by Alice Curtin.

About the author, Lili Alderson:

Lili Alderson is a first year 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.