The geology of planetary atmospheres

Some years ago, I began giving lectures with both "Geology" and "Atmospheres" in the title. In part, this was to emphasize that atmospheres are dynamic entities, evolving in response to volatile cycling between the outer envelope of a planet and planetary interiors. Understanding atmospheres, whether of Solar System planets or exoplanets, requires an intimate understanding of the geochemistry of the interior, and of the physical processes mediating exchange between the interior and envelope.  Another "geological" aspect of exoplanet atmospheres is that many exoplanets are hot enough that substances ordinarily thought of as rocks or minerals exist as condensible vapours in the envelope, leading to a manifestation of mineralogical processes in situ in the envelope itself.  Unprecedented atmospheric characterizations from the James Webb Space Telescope (JWST) have accelerated the realization that addressing the grand challenge problems of planetary structure and evolution must erase the traditional boundaries between atmospheric physics and Earth science disciplines dealing with geodynamics and mineral physics. The demands of these problems call for an integrated approach to training the next generation of researchers to meet the emerging challenges.

In this lecture, I will highlight some examples of the interplay between planetary envelopes and planetary interiors, focusing on lava planets, "hot rocks" (rocky planets too hot to support surface liquid water but not hot enough to have molten surfaces), the deep carbon cycle on habitable rocky worlds, and subNeptunes. Recent JWST data driving these inquiries will be surveyed. The general programme is to determine the extent to which astronomical observations -- which probe only the outer skin of a planet's volatile envelope (if present)-- together with mass, radius and age data can constrain the composition and structure of the interior, which cannot be directly observed.  subNeptunes present an especially interesting case, because mony currently accessible targets have a predominantly rocky composition (by mass), surrounded by a lower molecular weight envelope which interacts physically and chemically with a permanent magma ocean at the silicate/envelope interface.  For subNeptunes with a sufficiently massive envelope, the interface with the silicate mantle can be hot enough to drive the silicate itself supercritical, blurring the distinction between mantle and envelope.  Lack of experimental data on equations of state, geochemical reaction constants and opacities currently constitutes a serious impediment to progress in modelling subNeptune thermochemical structure and evolution.