Icarus

Magnetic investigations of icy moons have provided some of the most compelling evidence available confirming the presence of subsurface, liquid water oceans. In the exploration of ocean moons, especially Europa, there is a need for mathematical models capable of predicting the magnetic fields induced under a variety of conditions, including in the case of asymmetric oceans. Existing models are limited to either spherical symmetry or assume an ocean with infinite conductivity. In this work, we use a perturbation method to derive a semi-analytic result capable of determining the induced magnetic moments for an arbitrary layered body, provided each layer is nearly spherical...

The deep convective cloud-environment feedback loop is likely important to Titan's global methane, energy, and momentum cycles, just as it is for Earth's global water, energy, and momentum budgets. General circulation models of Titan's atmosphere are unable to explicitly simulate deep convection and must instead parameterize the impact of this important subgrid-scale phenomenon on the model-resolved atmospheric state. The goal of this study is to better quantify through cloud resolving modeling the effects of deep convective methane storms on their environment and to feed that information forward to improve parameterizations in global models...

The impact of methane convection on the circulation of Titan is investigated in the Titan Atmospheric Model (TAM), using a simplified Betts-Miller (SBM) moist convection parameterization scheme. We vary the reference relative humidity ( RHSBM ) and relaxation timescale of convection ( τ ) parameters of the SBM scheme. Titan's atmosphere is mostly insensitive to changes in τ , but convective instability and precipitation are highly impacted by changes in RHSBM . Convection changes behavior from occurring in infrequent (<1 per Titan year), intense events at summer solstice that quickly encompass the entire globe at low RHSBM to near-continuous precipitation at the poles during summer at high RHSBM (>85%)...

Daily, global wide angle imaging of Mars clouds in MARCI (MARs Color Imager, (Malin et al., 2008)) ultraviolet and visible bands reveals the spatial/seasonal distributions and physical characteristics of perihelion cloud trails (PCT); a class of high altitude (40-50 km), horizontally extended (200-1000 km, trending W to WSW) water ice clouds formed over specific southern low-to-mid latitude (5S-40S), mesoscale (~50 km) locations during the Mars perihelion, southern summer season. PCT were first reported in association with rim regions of Valles Marineris (Clancy et al...

Soil on the Moon is darkened by space weathering, a process generally assumed to be dominated by the solar wind and/or micrometeoroid impacts. Recent work, however, predicts that another process darkens the soil: large solar energetic particle events may cause dielectric breakdown (or "sparking"), melting and vaporizing soil at a rate comparable to that of micrometeoroids. Unlike the solar wind and/or micrometeoroids, a combination of dielectric breakdown and micrometeoroid weathering can explain how the reflectance of the lunar maria varies with latitude at 750 and 1064 nm, and this combination provides a reasonable mechanism to explain how magnetic anomalies form prominent swirls in the maria...

The climatology of baroclinic waves in the northern hemisphere of Mars is investigated through analysis of observations by the infrared sounders on Mars Reconnaissance Orbiter (MRO) and Mars Global Surveyor (MGS). We focus on the lowest scale height above the surface, where the waves have a large impact on the Martian dust cycle. Profiles retrieved by the MRO Mars Climate Sounder (MCS) rarely reach the lower atmosphere at the season and location of interest. To fill this gap, we turn to observations in the MCS B1 channel (32 microns) when the instrument is viewing the surface...

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Numerous studies show that major Venus highlands display anomalously high radar reflectivity and low radar emissivity relative to the planetary average. This is thought to be the result of the formation of minerals having high dielectric constants via weathering reactions occurring between the surface and the deep atmosphere in these elevated terrains, where temperatures are lower. These reactions are a function of rock composition, atmospheric composition, and degree of weathering, or age. Here, we examine the Magellan radar emissivity, altimetry and backscatter data for all mapped tesserae and mountain belts on Venus...

The five largest planets all have strong intrinsic magnetic fields that interact with their satellites, many of which contain electrically conducting materials on global scales. Conducting bodies exposed to time-varying magnetic fields induce secondary magnetic fields from movement of eddy currents. In the case of spherically symmetric conducting bodies, matching magnetic solutions at the boundary results in relatively simple relations between the excitation field and the induced field. In this work, we determine the more complicated induced magnetic field from a near-spherical conductor, where the outer boundary is expanded in spherical harmonics...

Spin off events and impacts can eject boulders from an asteroid surface and rubble pile asteroids can accumulate from debris following a collision between large asteroids. These processes produce a population of gravitational bound objects in orbit that can impact an asteroid surface at low velocity and with a distribution of impact angles. We present laboratory experiments of low velocity spherical projectiles into a fine granular medium, sand. We delineate velocity and impact angles giving ricochets, those giving projectiles that roll-out from the impact crater and those that stop within their impact crater...

Heterolithic, boulder-containing, pebble-strewn surfaces occur along the lower slopes of Aeolis Mons ("Mt. Sharp") in Gale crater, Mars. They were observed in HiRISE images acquired from orbit prior to the landing of the Curiosity rover. The rover was used to investigate three of these units named Blackfoot, Brandberg, and Bimbe between sols 1099 and 1410. These unconsolidated units overlie the lower Murray formation that forms the base of Mt. Sharp, and consist of pebbles, cobbles and boulders. Blackfoot also overlies portions of the Stimson formation, which consists of eolian sandstone that is understood to significantly postdate the dominantly lacustrine deposition of the Murray formation...

Thermal tides, stationary waves, and general circulation are investigated using a T63 Venus general circulation model (GCM) with solar and thermal radiative transfer in the presence of high-resolution surface topography, based on time average analysis. The simulated wind and static stability are very similar to the observed ones (e.g., Horinouchi et al., 2018; Ando et al., 2020). The simulated thermal tides accelerate an equatorial superrotational flow with a speed of ~90 m s-1 around the cloud-heating maximum (~65 km)...

In 2011 a thermally anomalous region was discovered on Mimas, Saturn's innermost major icy satellite (Howett et al., 2011). The anomalous region is a lens-like shape located at low latitudes on Mimas' leading hemisphere. It manifests as a region with warmer nighttime temperatures, and cooler daytime ones than its surroundings. The thermally anomalous region is spatially correlated with a darkening in Mimas' IR/UV surface color (Schenk et al. 2011) and the region preferentially bombarded by high-energy electrons (Paranicas et al...

Recurring Slope Lineae (RSL) on Mars have been enigmatic since their discovery; their behavior resembles a seeping liquid but sources of water remain puzzling. This work demonstrates that the properties of RSL are consistent with observed behaviors of Martian and terrestrial aeolian processes. Specifically, RSL are well-explained as flows of sand that remove a thin coating of dust. Observed RSL properties are supportive of or consistent with this model, which requires no liquid water or other exotic processes, but rather indicates seasonal aeolian behavior...

Multiple studies reveal that most of Venus highlands exhibit anomalously high radar reflectivity and low radar emissivity relative to the lowlands. This phenomenon is thought to be the result of atmosphere-surface interactions in the highlands, due to lower temperatures. These reactions are a function of rock composition, atmospheric composition, and degree of weathering. We examine the Magellan radar emissivity, altimetry and SAR data for all major volcanoes and coronae on Venus. We characterize and classify edifices according to the pattern of the variation of radar emissivity with altitude...

Refined calibrations of CRISM images are enabling identification of smaller deposits of unique aqueous materials on Mars that reveal changing environmental conditions at the region surrounding Mawrth Vallis. Through characterization of these clay-sulfate assemblages and their association with the layered, phyllosilicate units of this region, more details of the aqueous geochemical history can be gleaned. A stratigraphy including five distinct mineral horizons is mapped using compositional data from CRISM over CTX and HRSC imagery across 100s of km and from CRISM over HiRISE imagery across 100s of meters...

Gravity waves in Mars's atmosphere strongly affect the general circulation as well as middle atmospheric cloud formation, but the climatology and sources of gravity waves in the lower atmosphere remain poorly understood. At Earth, the statistical variance in satellite observations of thermal emission above the instrumental noise floor has been used to enable measurement of gravity wave activity at a global scale. Here is presented an analysis of variance in calibrated radiance at 15.4 μm (635-665 cm-1 ) from off-nadir and nadir observations by the Mars Climate Sounder (MCS) on board Mars Reconnaissance Orbiter (MRO); a major expansion in the observational data available for validating models of Martian gravity wave activity...

Mass-spring model simulations are used to investigate past spin states of a viscoelastic Phobos and Deimos. From an initially tidally locked state, we find crossing of a spin-orbit resonance with Mars or a mean motion resonance with each other does not excite tumbling in Phobos or Deimos. However, once tumbling our simulations show that these moons can remain so for an extended period and during this time their orbital eccentricity can be substantially reduced. We attribute the tendency for simulations of an initially tumbling viscoelastic body to drop into spin-synchronous state at very low eccentricity to the insensitivity of the tumbling chaotic zone volume to eccentricity...

The influence of dust devils on the martian atmosphere depends on their capacity to loft dust, which depends on their wind profiles and footprint on the martian surface, i.e., on their radii, R . Previous work suggests the wind profile depends on a devil's thermodynamic efficiency, which scales with its height, h . However, the precise mechanisms that set a dust devil's radius have remained unclear. Combining previous work with simple assumptions about angular momentum conservation in dust devils predicts that R ∝ h 1/2 , and a model fit to observed radii and heights from a survey of martian dust devils using the Mars Express High Resolution Stereo Camera agrees reasonably well with this prediction...

The Japan Aerospace Exploration Agency's (JAXA) Kaguya spacecraft carried a suite of instruments to map the Moon and its environment globally. During its extended mission, the average altitude was 50 km or lower, and Kaguya science products using these data hence have an increased spatial resolution. However, the geodetic position quality of these products is much worse than that of those acquired during the primary mission (at an altitude of 100 km) because of reduced radiometric tracking and frequent thrusting to maintain spacecraft attitude after the loss of momentum wheels...