Space instrumentation

Abstract:

The 31 km diameter and 7.5 km deep de Gerlache crater, located 30 km from the southern pole of the Moon was surveyed. At its bottom a 15 km diameter younger crater can be also found beside many smaller overprinting craters. At moderately sloping terrains a few m high, 100–200 m wide, curving quasi-parallel, km long set of ridges could be identified, which seem to be widespread on the surface, and might cover the half or even more of the crater. We named these “girland like features” in this work, which seem to be produced by mass movements on slopes (however differ from most of the already identified slope features, which show downslope elongated lineaments or fallen/redeposited debris on the Moon). At all locations they are superposed by recently formed 10–50 m diameter craters, thus might be older than the equilibrium crater population shown age of about 100 Ma old. This is the first identification of these features at the polar terrains, where they might contribute both in the shielding or exposing of subsurface ice. In de Gerlache crater ice occurrences have previously been located on moderately steep slopes, indicating they might be exposed by mass movement processes, where active movements might have happened in the last some 10 Ma using crater statistics based age of the shallow regolith layer. Only half of them were located at areas with modelled surface temperatures below 110 K, where temperature might be not enough to keep most of the deposited H2O there on Ga time scale. However the real values are probably more diverse because of the limited spatial resolution of available temperature data. Target areas are indicated for possible future missions, where periodic solar illumination, and subsurface ice at 0.5 m depth could be also present.

Abstract:

On 2010 May 18 Cassini’s Composite Infrared Spectrometer (CIRS) observed Dione’s leading hemisphere as its surface went into solar eclipse. Surface temperatures derived from each of CIRS’ focal plane 3 (FP3, 600−1100 cm−1) show a rapid decrease in Dione’s surface temperature upon eclipse ingress. This change was compared to the model surface emission to constrain bolometric Bond albedo and thermal inertia. Seven FP3 detectors were able to constrain the observed surface’s thermophysical properties. The bolometric Bond albedo derived from these detectors are consistent with one another (0.54 ± 0.05 to 0.62 ± 0.03) and that of diurnal studies (e.g., 0.49 ± 0.11, Howett et al. 2014). This indicates that Dione’s albedo is uniform to within the uncertainties across the observed region of its leading hemisphere. The derived thermal inertias are consistent across detectors, 9 ± 4 J m−2 K−1 s−1/2 (MKS) to 16 ± 8 MKS, and with previous diurnal studies (e.g., 8 to 12 MKS, Howett et al. 2014). The skin depth probed by the eclipse thermal wave is ∼0.6–1 mm, which is much shallower than that probed by diurnal cycles (∼50 mm). Thus, the agreement in thermal inertia between the eclipse and diurnal studies indicates that Dione’s subsurface structure is uniform from submillimeter to subcentimeter depths. This is different from the Jovian system, where eclipse-derived thermal inertias are much lower than those derived from diurnal studies. The cause of this difference is not known, but one possibility is that the E-ring grains that bombard Dione’s leading hemisphere overturn it, causing uniformity to centimeter depths.

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.