A numerical model of heat conduction through particulate media made of spherical grains cemented by various bonding agents is presented. The pore‐filling gas conductivity, volume fraction, and ...thermal conductivity of the cementing phase are tunable parameters. Cement fractions <0.001–0.01% in volume have small effects on the soil bulk thermal conductivity. A significant conductivity increase (factor 3–8) is observed for bond fractions of 0.01 to 1% in volume. In the 1 to 15% bond fraction domain, the conductivity increases continuously but less intensely (25–100% conductivity increase compared to a 1% bond system). Beyond 15% of cements, the conductivity increases vigorously and the bulk conductivity rapidly approaches that of bedrock. The composition of the cements (i.e. conductivity) has little influence on the bulk thermal inertia of the soil, especially if the volume of bond <10%. These results indicate that temperature measurements are sufficient to detect cemented soils and quantify the amount of cementing phase, but the mineralogical nature of the bonds and the typical grain size are unlikely to be determined from orbit. On Mars, a widespread surface unit characterized by a medium albedo (0.19–0.26) and medium/high thermal inertia (200–600 J s−0.5 m−2 K−1) has long been hypothesized to be associated with a duricrust. The fraction of cement required to fit the thermal data is less than ∼1–5% by volume. This small amount of material is consistent with orbital observations, confirming that soil cementation is an important factor controlling the thermal inertia of the Martian surface.
The Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) spacecraft landed successfully on Mars and imaged the surface to characterize the surficial geology. Here ...we report on the geology and subsurface structure of the landing site to aid in situ geophysical investigations. InSight landed in a degraded impact crater in Elysium Planitia on a smooth sandy, granule- and pebble-rich surface with few rocks. Superposed impact craters are common and eolian bedforms are sparse. During landing, pulsed retrorockets modified the surface to reveal a near surface stratigraphy of surficial dust, over thin unconsolidated sand, underlain by a variable thickness duricrust, with poorly sorted, unconsolidated sand with rocks beneath. Impact, eolian, and mass wasting processes have dominantly modified the surface. Surface observations are consistent with expectations made from remote sensing data prior to landing indicating a surface composed of an impact-fragmented regolith overlying basaltic lava flows.
We present a model of heat conduction for mono‐sized spherical particulate media under stagnant gases based on the kinetic theory of gases, numerical modeling of Fourier's law of heat conduction, ...theoretical constraints on the gas thermal conductivity at various Knudsen regimes, and laboratory measurements. Incorporating the effect of the temperature allows for the derivation of the pore‐filling gas conductivity and bulk thermal conductivity of samples using additional parameters (pressure, gas composition, grain size, and porosity). The radiative and solid‐to‐solid conductivities are also accounted for. Our thermal model reproduces the well‐established bulk thermal conductivity dependency of a sample with the grain size and pressure and also confirms laboratory measurements finding that higher porosities generally lead to lower conductivities. It predicts the existence of the plateau conductivity at high pressure, where the bulk conductivity does not depend on the grain size. The good agreement between the model predictions and published laboratory measurements under a variety of pressures, temperatures, gas compositions, and grain sizes provides additional confidence in our results. On Venus, Earth, and Titan, the pressure and temperature combinations are too high to observe a soil thermal conductivity dependency on the grain size, but each planet has a unique thermal inertia due to their different surface temperatures. On Mars, the temperature and pressure combination is ideal to observe the soil thermal conductivity dependency on the average grain size. Thermal conductivity models that do not take the temperature and the pore‐filling gas composition into account may yield significant errors.
Although not the prime focus of the InSight mission, the near-surface geology and physical properties investigations provide critical information for both placing the instruments (seismometer and ...heat flow probe with mole) on the surface and for understanding the nature of the shallow subsurface and its effect on recorded seismic waves. Two color cameras on the lander will obtain multiple stereo images of the surface and its interaction with the spacecraft. Images will be used to identify the geologic materials and features present, quantify their areal coverage, help determine the basic geologic evolution of the area, and provide ground truth for orbital remote sensing data. A radiometer will measure the hourly temperature of the surface in two spots, which will determine the thermal inertia of the surface materials present and their particle size and/or cohesion. Continuous measurements of wind speed and direction offer a unique opportunity to correlate dust devils and high winds with eolian changes imaged at the surface and to determine the threshold friction wind stress for grain motion on Mars. During the first two weeks after landing, these investigations will support the selection of instrument placement locations that are relatively smooth, flat, free of small rocks and load bearing. Soil mechanics parameters and elastic properties of near surface materials will be determined from mole penetration and thermal conductivity measurements from the surface to 3–5 m depth, the measurement of seismic waves during mole hammering, passive monitoring of seismic waves, and experiments with the arm and scoop of the lander (indentations, scraping and trenching). These investigations will determine and test the presence and mechanical properties of the expected 3–17 m thick fragmented regolith (and underlying fractured material) built up by impact and eolian processes on top of Hesperian lava flows and determine its seismic properties for the seismic investigation of Mars’ interior.
A vigorous regional dust storm substantially altered both the global atmospheric thermal structure and the magnitude and spatial distribution of dust loading within the Mars atmosphere between 1 and ...9 June 2018. We examine the development and decay of this storm in latitude, longitude, altitude, and time, employing observations by the Mars Climate Sounder on board the Mars Reconnaissance Orbiter. Dust layer top altitudes rose from seasonal‐normal values of ~40 to ~70 km. Dust lofting to high altitudes was localized between 0° and ~60°W longitude, and between 60°N and 60°S latitude. Intensification of paired meridional overturning circulation cells within the study area is confirmed by strong nighttime dynamical heating in higher latitudes of both hemispheres. By the end of this episode, significant dust loading was present at altitudes greater than 50 km above all longitudes on Mars, and other dust lifting centers had been activated.
Plain Language Summary
Why do some Martian dust storms, in some Mars years, expand to become global in scale, while the vast majority do not? We use infrared observations of the Mars atmosphere by the Mars Climate Sounder, on board the Mars Reconnaissance Orbiter spacecraft, to study this question. We examine the earliest days of the global dust storm of 2018, from 29 May through 9 June 2018. One key difference between regional and global dust storms is the altitude, within the atmosphere, to which dust is lofted during the storms. We show that dust was carried to much higher altitudes than normal during the first week of June, and that this major pulse of high‐altitude dust lofting was localized between about 0° and 60°W longitude. These longitudes include the “Acidalia storm track.” The circulation pattern, within this corridor, looks very much like a textbook Hadley circulation, where warm air rises rapidly above the equatorial latitudes, flowing both northward and southward, to later sink back toward the surface in middle latitudes. This circulation strengthened considerably during the earliest days of the global storm. These observations have important implications bearing on the mechanisms of Martian global dust storm occurrence.
Key Points
We examine the development and decay of a vigorous regional dust storm during the initial growth phase of the 2018 global dust storm
Rapid vertical expansion of dust clouds (to ~70‐km altitudes) was localized above the Acidalia storm track during this episode
Strong localized descending branch adiabatic heating confirms strengthening of a regional‐scale Hadley circulation within the study area
The heat flow and physical properties package measured soil thermal conductivity at the landing site in the 0.03–0.37 m depth range. Six measurements spanning solar longitudes from 8.0° to 210.0° ...were made and atmospheric pressure at the site was simultaneously measured using InSight's Pressure Sensor. We find that soil thermal conductivity strongly correlates with atmospheric pressure. This trend is compatible with predictions of the pressure dependence of thermal conductivity for unconsolidated soils under martian atmospheric conditions, indicating that heat transport through the pore filling gas is a major contributor to the total heat transport. Therefore, any cementation or induration of the soil sampled by the experiments must be minimal and soil surrounding the mole at depths below the duricrust is likely unconsolidated. Thermal conductivity data presented here are the first direct evidence that the atmosphere interacts with the top most meter of material on Mars.
Plain Language Summary
A soil's ability to transport heat is a fundamental parameter that holds information on quantities like soil bulk porosity, composition, grain size, and the state of cementation or induration. In the soil, heat is transported through grain‐to‐grain contacts as well as through the pore filling CO2 gas. The heat flow and physical properties package (HP3) of the InSight Mars mission measured soil thermal conductivity at the landing site repeatedly over the course of a martian year. As atmospheric pressure changes between seasons due to the redistribution of CO2 across the planet, we found that soil thermal conductivity also changes. Thermal conductivity increased for increased atmospheric pressure, a behavior typical for unconsolidated material. This implies that the amount of cement or induration of the sampled soil must be minimal.
Key Points
We measured thermal conductivity of the martian soil and found that its conductivity strongly correlates with atmospheric pressure
We conclude that heat conduction through the pore‐filling gas is significant and that cementation of the soil must be minimal
Our data show that the atmosphere directly interacts with the top most meter of material on Mars
Selection of the InSight Landing Site Golombek, M.; Kipp, D.; Warner, N. ...
Space science reviews,
10/2017, Letnik:
211, Številka:
1-4
Journal Article
Recenzirano
The selection of the Discovery Program InSight landing site took over four years from initial identification of possible areas that met engineering constraints, to downselection via targeted data ...from orbiters (especially Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) and High-Resolution Imaging Science Experiment (HiRISE) images), to selection and certification via sophisticated entry, descent and landing (EDL) simulations. Constraints on elevation (
≤
−
2.5
km
for sufficient atmosphere to slow the lander), latitude (initially 15°S–5°N and later 3°N–5°N for solar power and thermal management of the spacecraft), ellipse size (130 km by 27 km from ballistic entry and descent), and a load bearing surface without thick deposits of dust, severely limited acceptable areas to western Elysium Planitia. Within this area, 16 prospective ellipses were identified, which lie ∼600 km north of the Mars Science Laboratory (MSL) rover. Mapping of terrains in rapidly acquired CTX images identified especially benign smooth terrain and led to the downselection to four northern ellipses. Acquisition of nearly continuous HiRISE, additional Thermal Emission Imaging System (THEMIS), and High Resolution Stereo Camera (HRSC) images, along with radar data confirmed that ellipse E9 met all landing site constraints: with slopes <15° at 84 m and 2 m length scales for radar tracking and touchdown stability, low rock abundance (<10 %) to avoid impact and spacecraft tip over, instrument deployment constraints, which included identical slope and rock abundance constraints, a radar reflective and load bearing surface, and a fragmented regolith ∼5 m thick for full penetration of the heat flow probe. Unlike other Mars landers, science objectives did not directly influence landing site selection.
•The PacMan thermal anomalies on the icy Saturnian moons have been explained through radiation-induced sintering of grains.•The differing strength of the anomalies on each of the moons is related to ...the varying importance of the resurfacing mechanisms.•Changes caused by radiation processing can be used to constrain grain sizes and porosites of the icy regoliths.
The so-called ‘PacMan’ features on the leading hemispheres of the icy Saturnian moons of Mimas, Tethys and Dione were initially identified as anomalous optical discolorations and subsequently shown to have greater thermal inertia than the surrounding regions. The shape of these regions matches calculated deposition contours of high energy plasma electrons moving opposite to the moon’s orbital direction, thus suggesting that electron interactions with the grains produce the observed anomalies. Here, descriptions of radiation-induced diffusion processes are given, and various sintering models are considered to calculate the rate of increase in the contact volume between grains in an icy regolith. Estimates of the characteristic sintering timescale, i.e. the time necessary for the thermal inertia to increase from that measured outside the anomalous regions to that within, are given for each of the moons. Since interplanetary dust particle (IDP) impact gardening and E-ring grain infall would be expected to mix the regolith and obscure the effects of high energy electrons, sintering rates are compared to rough estimates of the impact-induced resurfacing rates. Estimates of the sintering timescale determined by extrapolating laboratory measurements are below ∼0.03 Myr, while the regolith renewal timescales are larger than ∼0.1 Myr, thus indicating that irradiation by the high energy electrons should be sufficient to form stable thermal anomalies. More detailed models developed for sintering of spherical grains are able to account for the radiation-induced anomalies on Mimas and Tethys only if the regoliths on those bodies are relatively compact and composed of small (≲ 5 µm) grains or grain aggregates, and/or the grains are highly non-spherical with surface defect densities in the inter-grain contact regions that are much higher than expected for crystalline water ice grains at thermal equilibrium. These results are consistent with regolith thermal conductivity models which can only be reconciled with spacecraft observations if the contacts between grains are assumed to have much lower thermal conductance than predicted for idealized grains. The strength of the anomalies on Tethys and Dione appear to be limited by E-ring grain infall, while on Mimas IDP gardening limits the strength of the anomaly. The smaller flux of more deeply penetrating high energy (>1 MeV) electrons on Dione can account for the small thermal inertia differences measured there. Determining regolith sintering rates and the corresponding effect on thermal conductivity can, in principle, provide an independent constraint on the regolith grain geometries and exposure timescales for icy bodies.
Before dawn on the dustiest regions of Mars, surfaces measured at or below ∼148 K are common. Thermodynamics principles indicate that these terrains must be associated with the presence of CO2 frost, ...yet visible wavelength imagery does not display any ice signature. We interpret this systematic absence as an indication of CO2 crystal growth within the surficial regolith, not on top of it, forming hard‐to‐distinguish intimate mixtures of frost and dust, that is, dirty frost. This particular ice/regolith relationship unique to the low thermal inertia regions is enabled by the large difference in size between individual dust grains and the peak thermal emission wavelength of any material nearing 148 K (1–2 μm vs. 18 μm), allowing radiative loss (and therefore ice formation) to occur deep within the pores of the ground, below several layers of grains. After sunrise, sublimation‐driven winds promoted by direct insolation and conduction create an upward drag within the surficial regolith that can be comparable in strength to gravity and friction forces combined. This drag displaces individual grains, possibly preventing their agglomeration, induration, and compaction, and can potentially initiate or sustain downslope mass movement, such as slope streaks. If confirmed, this hypothesis introduces a new form of CO2‐driven geomorphological activity occurring near the equator on Mars and explains how large units of mobile dust are currently maintained at the surface in an otherwise soil‐encrusting world.
Plain Language Summary
Surface CO2 ice forms at all latitudes on Mars with a strong seasonal control. In this study, we show that diurnal CO2 ice is not observed in visible wavelength imagery in dusty terrains, where diurnal frost preferentially forms. We interpret this situation as the indication of the presence of hard‐to‐distinguish dirty frost, where ice crystals grow within the surficial regolith, not on top of it, resulting in apparent soil‐like dark ice. At sunrise, sublimation‐driven winds within the regolith are occasionally strong enough to displace individual dust grains, initiating and sustaining dust avalanches on steep slopes, forming ground features known as slope streaks. This model suggests that the CO2 frost cycle is an active geomorphological agent at all latitudes and not just at high or polar latitudes, and possibly a key factor maintaining mobile dust reservoirs at the surface.
Key Points
Near dawn, diurnal frost is not apparent on cold, dusty, low thermal inertia terrains
These observations are consistent with a model of dirty diurnal CO2 frost, fluffing up the surface layer when it sublimes
This mechanism could trigger dynamic phenomena on the Martian surface and lead to the formation of slope streaks
Geological investigations planned for the Europa Clipper mission will examine the formation, evolution, and expression of geomorphic structures found on the surface. Understanding geologic features, ...their formation, and any recent activity are key inputs in constraining Europa’s potential for habitability. In addition to providing information about the moon’s habitability, the geologic study of Europa is compelling in and of itself. Here we provide a high-level, cross-instrument, and cross-discipline overview of the geologic investigations planned within the Europa Clipper mission. Europa’s fascinating collection of ice-focused geology provides an unparalleled opportunity to investigate the dynamics of icy shells, ice-ocean exchange processes, and global-scale tectonic and tidal stresses. We present an overview of what is currently known about the geology of Europa, from global to local scales, highlighting outstanding issues and open questions, and detailing how the Europa Clipper mission will address them. We describe the mission’s strategy for searching for and characterizing current activity in the form of possible active plumes, thermal anomalies, evidence for surface changes, and extremely fresh surface exposures. The complementary and synergistic nature of the data sets from the various instruments and their integration will be key to significantly advancing our understanding of Europa’s geology.
PDF
