Planetary surfaces

Abstract:

Thermal emission spectra can provide key insights into the composition and thermophysical properties of the regolith on the Moon and other airless bodies. However, under lunar surface environmental conditions, the uppermost millimeters of the regolith (from which thermal emission originates) cannot be characterized by a single temperature, leading to changes in spectral characteristics that should be accounted for in interpreting thermal emission measurements. Here, we develop and apply a Monte Carlo radiative transfer method to model thermal emission from particulate media with varying, nonisothermal subsurface temperature profiles. We model emission spectra for three major lunar mineral phases (pyroxene, olivine, and plagioclase), and investigate the effects of particle size and packing density. Modeled spectra are compared to lab measurements acquired under both ambient and simulated lunar conditions. We find that in some cases, the model provides useful constraints on the magnitude of the temperature profile established in a lab sample under lunar-like conditions, whereas in other cases, lab spectra are not well represented by the linear temperature profiles considered in this work. The model is generally successful at predicting changes in spectral contrast under lunar-like conditions, but less successful in accurately predicting shifts in the position of the Christiansen feature emissivity maximum; we illustrate and discuss the validity of the modeling approach for a range of different cases. Model results can also be used to quantify the depth within which observed thermal emission originates; this depth depends on composition and grain size, and ranges from ∼100 to 1000 μm for representative packing densities.

Abstract:

We combine novel laboratory experiments and exospheric modeling to reveal that "dynamic" Ly-α photolysis of Plutonian methane generates a photolytic refractory distribution on Charon that increases with latitude, consistent with poleward darkening observed in the New Horizons images. The flux ratio of the condensing methane to the interplanetary medium Ly-α photons, φ, controls the distribution and composition of Charon's photoproducts. Mid-latitude regions are likely to host complex refractories emerging from low-φ photolysis, while high-φ photolysis at the polar zones primarily generate ethane. However, ethane being colorless does not contribute to the reddish polar hue. Solar wind radiolysis of Ly-α-cooked polar frost past spring sunrise may synthesize increasingly complex, redder refractories responsible for the unique albedo on this enigmatic moon.

Abstract:

A near-surface thermal model for Arrokoth is developed based on the recently released 105 facet model of the body. This thermal solution takes into account Arrokoth's surface reradiation back onto itself. The solution method exploits Arrokoth's periodic orbital character to develop a thermal response using a time-asymptotic solution method, which involves a Fourier transform solution of the heat equation, an approach recently used by others. We display detailed thermal solutions assuming that Arrokoth's near-surface material's thermal inertia I = 2.5 W/m−2 K−1 s1/2. We predict that at New Horizons' encounter with Arrokoth, its encounter hemisphere surface temperatures were ∼57-59 K in its polar regions, 30-40 K in its equatorial zones, and 11-13 K for its winter hemisphere. Arrokoth's orbitally averaged temperatures are around 30-35 K in its polar regions and closer to 40 K near its equatorial zones. Thermal reradiation from the surrounding surface amounts to less than 5% of the total energy budget, while the total energy ensconced into and exhumed out of Arrokoth's interior via thermal conduction over one orbit is about 0.5% of the total energy budget. As a generalized application of this thermal modeling together with other Kuiper Belt object origins considerations, we favor the interpretation that New Horizons' REX instrument's 29 ± 5 K brightness temperature measurement is consistent with Arrokoth's near-surface material being made of sub-to-few-millimeter-size tholin-coated amorphous H2O ice grains with 1 W/m−2 K−1 s1/2 < I < 10-20 W/m−2 K−1 s1/2 and which are characterized by an X-band emissivity in the range 0.9 and 1.