George D. McDonald’s research while affiliated with University of Oregon and other places

Haumea, thought to be the Kuiper Belt's 3rd most massive object, has a fast 3.92 hr rotational period, resulting in its shape as a triaxial ellipsoid. Here, we make the first detailed predictions of Haumea's surface morphology, considering in particular effects stemming from its unique shape. Given observations have indicated Haumea's surface to be predominantly inert water ice, we predict crater characteristics, with craters likely to be the predominant surface feature on Haumea. In calculating Haumea's surface gravity, we find that g varies by almost two orders of magnitude, from a minimum of 0.0126 m/s^2 at the location of the equatorial major axis, to 1.076 m/s^2 at the pole. We also find a non-monotonic decrease in g with latitude. The simple to complex crater transition diameter varies from 36.2 km at Haumea's location of minimum surface gravity to 6.1 km at the poles. Equatorial craters are expected to skew to larger volumes, have depths greater by a factor of > 2, and have thicker ejecta when compared with craters at high latitudes. Considering implications for escape of crater ejecta, we calculate that Haumea's escape velocity varies by 62% from equator to pole. Despite higher escape velocities at the poles, impacts there are expected to have a higher mass fraction of ejecta escape from Haumea's gravitational well. Haumea may be unique among planet-sized objects in the solar system in possessing dramatic variations in crater morphology across its surface, stemming solely from changes in the magnitude of its surface gravity.

Liquid water is a critical component of habitability. However, the production and stability of surficial liquid water can be challenging on planets outside the Habitable Zone and devoid of adequate greenhouse warming. On such cold, icy exoEarths, basal melting of regional, global ice sheets by geothermal heat provides an alternative means of forming liquid water. Here, we model the thermophysical evolution of ice sheets to ascertain the geophysical conditions that allow liquid water to be produced and maintained at temperatures above the pressure controlled freezing point of water ice on exoEarths. We show that even with a modest, Moon like geothermal heat flow, subglacial oceans of liquid water can form at the base of and within the ice sheets on exoEarths. Furthermore, subglacial oceans may persist on exoEarths for a prolonged period due to the billion year half lives of heat producing elements responsible for geothermal heat. These subglacial oceans, often in contact with the planets crust and shielded from the high energy radiation of their parent star by thick ice layers, may provide habitable conditions for an extended period.

Liquid water is a critical component of habitability. However, the production and stability of surficial liquid water can be challenging on planets outside the Habitable Zone and devoid of adequate greenhouse warming. On such cold, icy exo-Earths, basal melting of regional/global ice sheets by geothermal heat provides an alternative means of forming liquid water. Here, we model the thermophysical evolution of ice sheets to ascertain the geophysical conditions that allow liquid water to be produced and maintained at temperatures above the pressure-controlled freezing point of water ice on exo-Earths. We show that even with a modest, Moon-like geothermal heat flow, subglacial oceans of liquid water can form at the base of and within the ice sheets on exo-Earths. Furthermore, subglacial oceans may persist on exo-Earths for a prolonged period due to the billion-year half-lives of heat-producing elements responsible for geothermal heat. These subglacial oceans, often in contact with the planet’s crust and shielded from the high energy radiation of their parent star by thick ice layers, may provide habitable conditions for an extended period.

Liquid water is a critical component of habitability. However, the production and stability of surficial liquid water can be challenging on planets devoid of adequate greenhouse warming. On such cold, icy exo-Earths, basal melting of regional/global ice sheets by geothermal heat provides an alternative means of forming liquid water. Here, we model the thermophysical evolution of ice sheets to ascertain the geophysical conditions that allow liquid water to be produced and maintained at temperatures above freezing on exo-Earths. We show that even with modest, Moon-like geothermal heat, subglacial oceans of liquid water can form at the base of and within the ice sheets on exo-Earths. Furthermore, subglacial oceans may persist on exo-Earths for a prolonged period due to the billion-year half-lives of the heat-producing elements responsible for geothermal heat. These subglacial oceans, often in contact with the planet’s crust and shielded from the high energy radiation of their parent star by thick ice layers, may provide habitable conditions for an extended period.

Surface modification on Jupiter’s volcanically active moon, Io, has to date been attributed almost exclusively to lava emplacement and volcanic plume deposits. Here we demonstrate that wind-blown transport of sediment may also be altering the Ionian surface. Specifically, shallow subsurface interactions between lava and Io’s widespread sulfur dioxide (SO2) frost can produce localized sublimation vapor flows with sufficient gas densities to enable particle saltation. We calculate anticipated outgassing velocities from lava–SO2 frost interactions, and compare these to the saltation thresholds predicted when accounting for the tenuous nature of the sublimated vapor. We find that saltation may occur if frost temperatures surpass 155 K. Finally we make the first measurements of the dimensions of linear features in images from the Galileo probe, previously termed “ridges”, which demonstrate certain similarities to dunes on other planetary bodies. Io joins a growing list of bodies with tenuous and transient atmospheres where aeolian sediment transport may be an important control on the landscape. Dunes may form on Jupiter’s moon Io. Despite a tenuous atmosphere, interactions between widespread lava and sulfur dioxide frost may produce vapor flows dense enough to mobilize sand grains. Ridge-like features may be evidence of this phenomenon.

Surface modification on Jupiter's volcanically active moon, Io, has to date been attributed almost exclusively to lava emplacement and volcanic plume deposits. Here we demonstrate that wind-blown transport of sediment may also be altering the Ionian surface. Specifically, shallow subsurface interactions between lava and Io's widespread sulfur dioxide (SO$_2$) frost can produce localized sublimation vapor flows with sufficient gas densities to enable particle saltation. We calculate anticipated outgassing velocities from lava-SO$_2$ frost interactions, and compare these to the saltation thresholds predicted when accounting for the tenuous nature of the sublimated vapor. We find that saltation may occur if frost temperatures surpass 155 K. Finally we make the first measurements of the dimensions of linear features in images from the Galileo probe, previously termed "ridges", which demonstrate certain similarities to dunes on other planetary bodies. Io joins a growing list of bodies with tenuous and transient atmospheres where aeolian sediment transport may be an important control on the landscape.

From orbit, the visibility of Titan’s surface is limited to a handful of narrow spectral windows in the near-infrared (near-IR), primarily from the absorption of methane gas. This has limited the ability to identify specific compounds on the surface—to date Titan’s bulk surface composition remains unknown. Further, understanding of the surface composition would provide insight into geologic processes, photochemical production and evolution, and the biological potential of Titan’s surface. One approach to obtain wider spectral coverage with which to study Titan’s surface is by decreasing the integrated column of absorbers (primarily methane) and scatterers between the observer and the surface. This is only possible if future missions operate at lower altitudes in Titan’s atmosphere. Herein, we use a radiative transfer model to measure in detail the absorption through Titan’s atmosphere from different mission altitudes, and consider the impacts this would have for interpreting reflectance measurements of Titan’s surface. Over our modeled spectral range of 0.4–10 μm, we find that increases in the width of the transmission windows as large as 317% can be obtained for missions performing remote observations at the surface. However, any appreciable widening of the windows requires onboard illumination. Further, we make note of possible surface compounds that are not currently observable from orbit, but could be identified using the wider windows at low altitudes. These range from simple nitriles such as cyanoacetylene, to building blocks of amino acids such as urea. Finally, we discuss the implications that the identifications of these compounds would have for Titan science.

Numerous laboratory experiments, starting in the Viking Lander era, have reported that frictional interactions between Martian analog dust grains can catalyze electrostatic processes (i.e. triboelectrification). Such findings have been cited to suggest that Martian dust devils and dust storms may sustain lightning storms, glow discharges, and other complex electrostatic phenomena. However, in many cases (if not most), these experiments allowed Martian dust simulant grains to contact foreign surfaces (for instance, the wall of an environmental chamber or other chemically-dissimilar particles). A number of authors have noted that such interactions could produce charging that is not representative of processes occurring near the surface of Mars. More recently, experiments that have identified and corrected for collisions between dust simulants and chemically dissimilar laboratory materials have either failed to replicate near-surface Martian conditions or directly measure discharging. In this work, we experimentally characterize the triboelectrification of a Martian dust simulant resulting from both isolated particle–particle collisions and particle–wall collisions under a simulated Martian environment. We report the direct detection of spark discharges in a flow composed solely of natural basalt grains and isolated from artificial surfaces. The charge densities acquired by the fluidized grains are found to be of order 10⁻⁶ Cm⁻² (in excess of the theoretical maximum charge density for the near-surface Martian environment). Additionally, we demonstrate that the interaction of simulant particles with experimental walls can modulate the polarity of spark discharges. Our work supports the idea that small-scale spark discharges may indeed be present in Martian granular flows and may be qualitatively similar to small-scale discharges in terrestrial volcanic vents.

From orbit, the visibility of Titan's surface is limited to a handful of narrow spectral windows in the near-infrared (near-IR), primarily from the absorption of methane gas. This has limited the ability to identify specific compounds on the surface -- to date Titan's bulk surface composition remains unknown. Further, understanding of the surface composition would provide insight into geologic processes, photochemical production and evolution, and the biological potential of Titan's surface. One approach to obtain wider spectral coverage with which to study Titan's surface is by decreasing the integrated column of absorbers (primarily methane) and scatterers between the observer and the surface. This is only possible if future missions operate at lower altitudes in Titan's atmosphere. Herein, we use a radiative transfer model to measure in detail the absorption through Titan's atmosphere from different mission altitudes, and consider the impacts this would have for interpreting reflectance measurements of Titan's surface. Over our modeled spectral range of 0.4 - 10 micron, we find that increases in the width of the transmission windows as large as 317% can be obtained for missions performing remote observations at the surface. However, any appreciable widening of the windows requires onboard illumination. Further, we make note of possible surface compounds that are not currently observable from orbit, but could be identified using the wider windows at low altitudes. These range from simple nitriles such as cyanoacetylene, to building blocks of amino acids such as urea. Finally, we discuss the implications that the identifications of these compounds would have for Titan science.