Solar system

Abstract:

The pattern of airy and airless rocky planets presently being uncovered by JWST is a record of what happens when ionospheres are pushed to their limits by their host stars. Orbiting as close to a red dwarf host as the Parker Probe is to the Sun, a massive rocky planet could harbour liquid water oceans beneath an ionosphere several times hotter than its star’s effective temperature, exhibiting spectacular airglow and aurora. Not only is this a distinct and observable possibility, but planets of this kind may make up a significant fraction of habitable worlds.What maximum temperature can a tightly bound ionosphere, composed primarily of carbon, nitrogen, and oxygen atoms, reach before escaping into space as a hydrodynamic wind? This question lies at the crux of the 500-hour Rocky Worlds DDT Program and the guiding hypothesis of a universal cosmic shoreline.Locally, the terminal temperatures of these extreme ionospheres are determined by heating from XUV photons emitted by the star’s corona and cooling through collisional excitation of atoms that emit visible and infrared photons. Globally, the thermal structure is determined by photochemistry, fluid dynamics, and electromagnetic interactions. Additionally, stellar cycle variation of ionospheric conditions is likely key to atmospheric evolution. In this talk, we will discuss the key knowns and unknowns in predicting the “edge of airlessness” for the population of rocky exoplanets within the observational reach of the James Webb Space Telescope.

Abstract:

Iron is a powerful element shaping rocky planets. The bulk iron content of a planet exerts a first-order control on its interior structure, of fundamental importance to geodynamic processes. Across the rocky planets and dwarf planets in the solar system, bulk iron contents vary considerably, appearing to correlate with orbital distance, and possibly the Sun’s magnetic field strength (McDonough & Yoshizaki, 2021). Potentially-rocky exoplanets show an even greater spread in bulk density and hence inferred bulk iron content. Such exoplanet censuses have begun to give us access to cosmic-scale statistics. We build on McDonough & Yoshizaki (2021) to present a tentative, positive trend between rocky exoplanets’ iron contents and the energy they receive from their host star (instellation). Previous studies have searched for such a trend in iron content with other factors; in particular, with host star iron abundance, as such a link would be evidence for a planet-star compositional connection. If planet bulk iron content is also affected by disk processes, then any other trends would become more complicated to interpret. We use our results to address exoplanet bulk compositional diversity, including the formation of super-Mercuries, and discuss potential implications of high iron contents on broader planet evolution.McDonough, W. F., & Yoshizaki, T. (2021). Terrestrial planet compositions controlled by accretion disk magnetic field. Progress in Earth and Planetary Science, 8, 39.

Abstract:

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. 

Abstract:

The InSight lander represents a unique opportunity to correlate seismic data with impact events identified in orbital images, enabling the characterization of the physical properties of the martian crust and mantle. Here, we present the first comprehensive catalog of impacts that occurred during the InSight mission within a 50° radius around the lander. We use a machine learning-enabled approach to identify 123 date-constrained impacts with diameters between ∼1 and 22.5 m. We estimate an impact rate of 2.7 × 10−6/km2/year for >3.9 m effective diameter, which is ∼1.6–∼2.5 times higher than previously derived for Mars. We identify 49 seismic events with one or several potential impact match(es) including a 21.5 m crater located near Cerberus Fossae. Our catalog will enable a more accurate characterization of the propagation of seismic body waves at intermediate distances to InSight (5–50°), with major implications for estimates of other seismic event distances.

Abstract:

To date, eight meteoroid impacts have been identified in the seismic record of NASA's InSight mission on Mars, occurring either within 300 km or beyond 3,500 km. We report the association of a high-frequency marsquake, S0794a, with a new 21.5-m-diameter impact crater discovered at an intermediate distance of 1,640 km in the tectonically active Cerberus-Fossae graben system. This impact enables the direct comparison between surface and subsurface sources, as well as providing the first data point in the critical gap between previous impacts, both in distance and crater size. Additionally, the location of this event necessitates a reassessment of assumed seismic raypaths that were thought to propagate along a slow crustal waveguide. We find that the raypaths instead penetrate and travel through the faster mantle, implying numerous identified marsquake epicenters should be relocated up to two times farther from InSight, with implications for seismically derived impact rates and regional seismicity.