How To Identify Surface Of Exoplanets? (Planetary Science)

Friends, the Kepler mission has detected a wealth of exoplanets that do not resemble any planetary bodies in the Solar System, with sizes in between Earth and Neptune, 1.0 REarth < Rp < 3.9 REarth, we call them “subneptune planets”. For these intermediate-sized exoplanets, it is difficult to tell whether these exoplanets have surfaces or where their surfaces are located.

Now, Yu and colleagues proposed that the abundances of trace species such as ammonia, methane or others in the visible atmospheres of these sub-Neptunes or exoplanets can be used as proxies for determining the existence of surfaces and approximate surface conditions. As an example, they used a state-of-the-art photochemical model to simulate the atmospheric evolution of K2-18b and investigated its final steady-state composition with surfaces located at different pressures levels (Psurf).

Schematic diagram describing the main chemical pathways for the net production and loss of important observable species in the deep-surface or no-surface case versus the shallow-surface case for K2-18b. Arrows of different thicknesses indicate levels of importance for each chemical pathway, from high to low importance: thick solid arrows, thin solid arrows, dashed arrows. The upper atmosphere at pressures less than a few bars is dominated by photochemistry and vertical diffusion/transport, the troposphere from a few bars to the deep quench points (pressure depending on species involved) is dominated by vertical diffusion and transport-induced quenching, and the deep atmosphere below the quench points (provided that the atmosphere is hot enough) is dominated by thermochemical equilibrium. For the shallow, cool surface case, temperatures never get high enough for thermochemistry to be effective. The dashed arrow from CxHy to the surface indicates potential condensation/sedimentation of refractory hydrocarbons on the surface of the exoplanet, which is a process that could happen if the surface temperatures are cold enough, but is not currently occurring in their model because none of the hydrocarbons considered are refractory enough to condense for the atmospheric and surface conditions of the planets considered here. The colored species indicate abundance fluctuations compared to the deep/no surface case, red means an increase in abundance and blue means a decrease in abundance. © Yu et al.

They demonstrated that the presence of a surface can significantly alter the atmospheric abundances in a hydrogen-dominated sub-Neptune atmosphere and identified a few potentially observable trace species that are affected by the inclusion of a cool, shallow surface: NH3, HCN, CH4, C2H2, H2O, CO, and CO2. The change in abundance of these species with an inclusion of a surface is due to the fact that thermochemical kinetics, which occurs efficiently in the deep, hot part of the atmosphere, is inhibited at lower temperatures and pressures. The presence of a cool surface at low atmospheric pressures can shut down or significantly impede thermochemical recycling. Thus, photochemically-fragile species (NH3, HCN, CH4, C2H2, H2O) are destroyed over time, with no recycling from the deep atmosphere, and photochemically-stable species (CO, CO2) survive and build up in the atmosphere.

Figure 2. A flowchart of possible steps to identify the existence of surface and the surface level for a hydrogen-dominated exoplanet with properties similar to K2-18b. The process starts with the calculation of the abundances of species for the no-surface case, and then using a combination of the ratios between the observed abundances ([X]) and the no-surface abundances ([X]no-surface), f[X] = [X]/[X]no-surface, and the ratios between two species, f [X] = [A]/[B], we can predict whether this exoplanet has a deep surface (surface pressure, Psurf > 100 bar), an intermediate surface (10 bar < Psurf < 100 bar), or a cold and shallow surface (Psurf < 10 bar). © Yu et al.

The atmospheric abundances of these species are also affected differently by different surface conditions, because each species can kinetically approach thermochemical equilibrium at different pressure/temperature points. Among all the species, the abundances of CH4, C2H2, H2O, CO, and CO are only affected by cool, low-pressure surfaces, as their equilibrium point is relatively shallow (∼30 bar, ∼1100 K for expected K2-18b thermal conditions). The abundances of NH3 and HCN are affected by deeper surfaces because they have deeper equilibrium points (∼90 bar, ∼1300 K for K2-18b thermal conditions)

“We found that the surface location has a significant impact on the atmospheric abundances of trace species, making them deviate significantly from their thermochemical equilibrium and “no-surface” conditions. This result arises primarily because the pressure-temperature conditions at the surface determine whether photochemically-produced species can be recycled back to their favored thermochemical-equilibrium forms and transported back to the upper atmosphere. For an assumed H2-rich atmosphere for K2-18b, we identify seven chemical species that are most sensitive to the existence of surfaces: ammonia (NH3), methane (CH4), hydrogen cyanide (HCN), acetylene (C2H2), ethane (C2H6), carbon monoxide (CO), and carbon dioxide (CO2). The ratio between the observed and the no-surface abundances of these species, can help distinguish the existence of a shallow surface (Psurf < 10 bar), an intermediate surface (10 bar < Psurf < 100 bar), and a deep surface (Psurf > 100 bar).

They also identified combinations of these species that can serve as proxies for identifying a surface and evaluating the approximate surface conditions of K2-18b or other similar sub-Neptunes. They expect this framework to be applied to other small observable exoplanets in the future.

Like, the upcoming exoplanet spectroscopy missions such as the James Webb Telescope (JWST) and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) would be excellent in characterizing atmospheric composition of exoplanets with near- to mid-infrared spectra, but these wavelengths are generally not sensitive to the surfaces of exoplanets. Thus, identifying potential observable atmospheric species that could be used to point to the existence of exoplanet surfaces would be an intriguing science angle for the two missions.

Reference: Xinting Yu, Julianne I. Moses, Jonathan J. Fortney, Xi Zhang, “How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres”, Arxiv, pp. 1-30, 2021.

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