The NASA InSight Lander on Mars includes the Heat Flow and Physical Properties Package HP
3
to measure the surface heat flow of the planet. The package uses temperature sensors that would have been ...brought to the target depth of 3–5 m by a small penetrator, nicknamed the mole. The mole requiring friction on its hull to balance remaining recoil from its hammer mechanism did not penetrate to the targeted depth. Instead, by precessing about a point midway along its hull, it carved a 7 cm deep and 5–6 cm wide pit and reached a depth of initially 31 cm. The root cause of the failure – as was determined through an extensive, almost two years long campaign – was a lack of friction in an unexpectedly thick cohesive duricrust. During the campaign – described in detail in this paper – the mole penetrated further aided by friction applied using the scoop at the end of the robotic Instrument Deployment Arm and by direct support by the latter. The mole tip finally reached a depth of about 37 cm, bringing the mole back-end 1–2 cm below the surface. It reversed its downward motion twice during attempts to provide friction through pressure on the regolith instead of directly with the scoop to the mole hull. The penetration record of the mole was used to infer mechanical soil parameters such as the penetration resistance of the duricrust of 0.3–0.7 MPa and a penetration resistance of a deeper layer (
>
30
cm
depth) of
4.9
±
0.4
MPa
. Using the mole’s thermal sensors, thermal conductivity and diffusivity were measured. Applying cone penetration theory, the resistance of the duricrust was used to estimate a cohesion of the latter of 2–15 kPa depending on the internal friction angle of the duricrust. Pushing the scoop with its blade into the surface and chopping off a piece of duricrust provided another estimate of the cohesion of 5.8 kPa. The hammerings of the mole were recorded by the seismometer SEIS and the signals were used to derive P-wave and S-wave velocities representative of the topmost tens of cm of the regolith. Together with the density provided by a thermal conductivity and diffusivity measurement using the mole’s thermal sensors, the elastic moduli were calculated from the seismic velocities. Using empirical correlations from terrestrial soil studies between the shear modulus and cohesion, the previous cohesion estimates were found to be consistent with the elastic moduli. The combined data were used to derive a model of the regolith that has an about 20 cm thick duricrust underneath a 1 cm thick unconsolidated layer of sand mixed with dust and above another 10 cm of unconsolidated sand. Underneath the latter, a layer more resistant to penetration and possibly containing debris from a small impact crater is inferred. The thermal conductivity increases from 14 mW/m K to 34 mW/m K through the 1 cm sand/dust layer, keeps the latter value in the duricrust and the sand layer underneath and then increases to 64 mW/m K in the sand/gravel layer below.
Assessment of InSight Landing Site Predictions Golombek, M.; Kass, D.; Williams, N. ...
Journal of geophysical research. Planets,
August 2020, 2020-Aug, 2020-08-00, 20200801, Letnik:
125, Številka:
8
Journal Article
Recenzirano
Odprti dostop
Comprehensive analysis of remote sensing data used to select the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) landing site correctly predicted the ...atmospheric temperature and pressure profile during entry and descent, the safe landing surface, and the geologic setting of the site. The smooth plains upon which the InSight landing site is located were accurately predicted to be generally similar to the Mars Exploration Rover Spirit landing site with relatively low rock abundance, low slopes, and a moderately dusty surface with a 3–10 m impact fragmented regolith over Hesperian to Early Amazonian basaltic lava flows. The deceleration profile and surface pressure encountered by the spacecraft during entry, descent, and landing compared well (within 1σ) of the envelope of modeled temperature profiles and the expected surface pressure. Orbital estimates of thermal inertia are similar to surface radiometer measurements, and materials at the surface are dominated by poorly consolidated sand as expected. Thin coatings of bright atmospheric dust on the surface were as indicated by orbital albedo and dust cover index measurements. Orbital estimates of rock abundance from shadow measurements in high‐resolution images and thermal differencing indicated very low rock abundance and surface counts show 1–4% area covered by rocks. Slopes at 100 to 5 m length scale measured from orbital topographic and radar data correctly indicated a surface comparably smooth and flat as the two smoothest landing sites (Opportunity and Phoenix). Thermal inertia and radar data indicated the surface would be load bearing as found.
Plain Language Summary
Orbital remote sensing data were used to select the landing site for InSight and to confirm that the surface met the engineering constraints required for a safe landing and successful instrument deployment. By relating remote sensing signatures to surface characteristics at landing sites, these sites can be used as ground truth for orbital data and are essential for selecting and validating landing sites for future missions. Models of the atmosphere that included monitoring during approach by orbiting spacecraft compared well with the deceleration profile and surface pressure encountered by the spacecraft during entry, descent, and landing, and the spacecraft landed successfully. The smooth plains on which InSight landed are consistent with expectations made prior to landing. Remote sensing data indicated that the InSight landing site would be similar to the Mars Exploration Rover Spirit landing site. Both sites have relatively low rock abundance, low slopes, and a moderately dusty surface, with a 3–10 m impact fragmented regolith over Hesperian to Early Amazonian basaltic lava flows.
Key Points
The atmosphere, safe surface, and geologic setting of the landing site were correctly predicted by remote sensing data before landing
The modeled atmospheric temperature profiles and surface pressure were within 1 sigma of the measured deceleration profile and surface pressure
InSight’s surface is similar to Spirit’s with low rock abundance, low slopes, moderate dust, and is composed of impact regolith over basalt
Modeling suggests that thermal circulations over Mars's highest volcanoes transport water vapor and dust from the surface into the middle atmosphere, forming detached layers in these constituents. ...Intense vertical mixing also takes place in regional and global dust storms, which can generate detached layers that are extreme in both altitude and magnitude. Here we employ observations by the Mars Climate Sounder (MCS) on board Mars Reconnaissance Orbiter, taking advantage of improved vertical coverage in MCS's aerosol retrievals, to discover a new class of extreme detached dust layers (EDDLs). Observed during minimal dust storm activity and furthermore distinguished by their potentially large and measurable horizontal extent (>1000 km), these EDDLs cluster near Olympus Mons and the Tharsis Montes, from which they likely originate. The existence of these EDDLs suggests that vertical mixing by topographic circulations can be much stronger than previously modeled and more frequent than previously observed.
Key Points
Distinctive detached dust layers cluster near Olympus Mons, Tharsis Montes
Layers imply occasional, intense dust transport by topographic circulations
Vertical extent of Arsia Mons spiral cloud observed for the first time
The InSight lander rests on a regolith‐covered, Hesperian to Early Amazonian lava plain in Elysium Planitia within a ∼27‐m‐diameter, degraded impact crater called Homestead hollow. The km to cm‐scale ...stratigraphy beneath the lander is relevant to the mission's geophysical investigations. Geologic mapping and crater statistics indicate that ∼170 m of mostly Hesperian to Early Amazonian basaltic lavas are underlain by Noachian to Early Hesperian (∼3.6 Ga) materials of possible sedimentary origin. Up to ∼140 m of this volcanic resurfacing occurred in the Early Amazonian at 1.7 Ga, accounting for removal of craters ≤700 m in diameter. Seismic data however, suggest a clastic horizon that interrupts the volcanic sequence between depths of ∼30 and ∼75 m. Meter‐scale stratigraphy beneath the lander is constrained by local and regional regolith thickness estimates that indicate up to 10–30 m of coarse‐grained, brecciated regolith that fines upwards to a ∼3 m thick loosely‐consolidated, sand‐dominated unit. The maximum depth of Homestead hollow, at ∼3 m, indicates that the crater is entirely embedded in regolith. The hollow is filled by sand‐size eolian sediments, with contributions from sand to cobble‐size slope debris, and sand to cobble‐size ejecta. Lander‐based observations indicate that the fill at Homestead hollow contains a cohesive layer down to ∼10–20 cm depth that is visible in lander rocket‐excavated pits and the HP3 mole hole. The surface of the landing site is capped by a ∼1 to 2 cm‐thick loosely granular, sand‐sized layer with a microns‐thick surficial dust horizon.
Plain Language Summary
The InSight lander has geophysical instruments that are designed to determine the interior structure of Mars. Understanding the results from these instruments requires a geological analysis of materials beneath the landing site at Elysium Planitia. This study presents data that describe the vertical sequence of rocks and soils beneath the lander, as well as the geologic history. The results indicate that InSight rests on a 1.7‐billion‐year‐old lava plain that is covered in a 10–30 m thick regolith that was produced by impact cratering and modified by wind. The uppermost portion of the regolith is a ∼3 m thick horizon of sand. InSight rests on sand within a degraded impact crater. The sandy material contains a slightly cohesive horizon that is only ∼1–2 cm beneath the lander and is up to 10–20 cm thick. The sandy horizon overlies rock fragments that get progressively larger with depth. Bedrock of basaltic lava exists beneath the regolith down to a depth of ∼170 m. The bedrock is interrupted by weaker materials between depths of ∼30 and 75 m. Beneath ∼170 m, the sequence is dominated by ancient (3.7–4.1 billion years old), possibly sedimentary materials.
Key Points
InSight rests on Early Amazonian basaltic lava with an up to 10–30 m thick regolith. The upper 3 m of the regolith is sand dominated
The regolith contains a 10–20 cm thick cohesive horizon or duricrust. This horizon rests 1–2 cm beneath the lander
The upper 1–2 cm of the regolith comprises loosely‐consolidated sand to pebbles. Sand is rarely mobilized under current wind conditions
The present climate of Mars is punctuated by recurring dust storm events, where dust is lifted from the surface and is transported by the atmosphere. Dust addition or removal can brighten or darken ...the surface, as well as affect thermal insulating properties because of its low thermal conductivity. Of particular interest is the recurrence of global dust storms (GDSs) and whether their frequency is controlled by the replenishment or depletion of finite surface reservoirs between events. Global climate models predict changes in dust coverage before and after global storms, but output varies substantially regarding the amounts and locations of transported dust. The analysis of global, multiyear observations of surface temperature and albedo from orbit can constrain changes in dust coverage and/or thickness. We calculate and map interannual differences in surface temperature from Mars Year (MY) 24 through MY35 to identify regions of dust redistribution. Regional temperature changes across the MY25 GDS can be explained by changes in albedo and do not require changes in surface thermal properties, supporting extremely small dust thickness changes. Across the MY34 GDS, we find less extensive changes in surface temperature, indicating a reduced impact on dust redistribution compared to MY25. However, we identify a region between Acidalia and Arabia Terra that experienced substantial dust removal and positively correlates with visible high‐resolution orbital images. Our work supports minuscule changes in dust thickness from the observed GDSs and is consistent with effectively infinite dust reservoirs on timescales of at least 103 years.
Plain Language Summary
Storm activity on Mars can mobilize surface deposits of fine dust and redistribute that dust across the surface. It is not known whether surface dust must be replenished or removed in specific geographic regions as a condition for the largest, global‐scale storms to initiate. Surface dust deposition and removal can affect both the reflectivity and the physical properties of the surface. Fine‐grained dust is more reflective than the typical Martian surface and large quantities of dust can thermally insulate the surface like a blanket. Both effects will impact the surface temperature. By mapping year‐to‐year changes in surface temperature from orbital observations, we identify regions where dust has been deposited or removed. The largest changes result from global‐scale dust storms that can last weeks. The observations support very small changes in dust thickness across the surface (less than the width of a human hair), where dust is only affecting the reflectivity of the surface. Analysis of observations spanning 10 Mars years (almost 20 Earth years) captures two global storms with vastly different propensities for dust redistribution. We argue that dust reservoirs (sources) can supply a global dust storm in any given Mars year.
Key Points
Interannual surface temperature changes observed about the Mars Year (MY) 25 global dust storm (GDS) are explained fully by surface albedo changes due to dust redistribution
Small dust thickness changes about GDS and meter‐thick reservoirs imply quasi‐infinite dust availability on the order of ∼1,000 years or more
Interannual temperature changes observed about the MY34 GDS are corroborated by visible images showing clear evidence of dust removal
The heat flow and physical properties package (HP3) of the InSight Mars mission is an instrument package designed to determine the martian planetary heat flow. To this end, the package was designed ...to emplace sensors into the martian subsurface and measure the thermal conductivity as well as the geothermal gradient in the 0–5 m depth range. After emplacing the probe to a tip depth of 0.37 m, a first reliable measurement of the average soil thermal conductivity in the 0.03–0.37 m depth range was performed. Using the HP3 mole as a modified line heat source, we determined a soil thermal conductivity of 0.039 ± 0.002 W m−1 K−1, consistent with the results of orbital and in‐situ thermal inertia estimates. This low thermal conductivity implies that 85%–95% of all particles are smaller than 104–173 μm and suggests that soil cementation is minimal, contrary to the considerable degree of cementation suggested by image data. Rather, cementing agents like salts could be distributed in the form of grain coatings instead. Soil densities compatible with the measurements are 1211−113+149 kg m−3, indicating soil porosities of 63−9+4%.
Plain Language Summary
The heat flow and physical properties package (HP3) of the InSight Mars mission is an instrument package that was designed to measure soil temperature as well as the soil's ability to transport heat, the so called thermal conductivity. After the probe was inserted to a depth of 0.37 m a first measurement of the soil's thermal conductivity was performed. The soil was found to be a poor thermal conductor with average conductivity close to 0.039 W m−1 K−1. As thermal transport properties in sands are related to grain size, the latter can be estimated based on the performed measurement. We find that particles must be smaller than about 150 μm, corresponding to fine sand that may be intermixed with dust. Further, salts in the soil can act as cementing agents, which connect individual sand grains and thus increase the strength of grain‐to‐grain contacts and therefore thermal conductivity. However, given the low thermal conductivity determined here, the amount of such cement must be minimal, contrary to what is suggested by image data. Finally, we find that the soil must have significant porosity of about 60% to be compatible with our measurements.
Key Points
The Heat Flow and Physical Properties Package (HP3) measured the average thermal conductivity of the martian soil
Average soil thermal conductivity in the 0.03–0.37 m depth range is 0.039 ± 0.002 W m−1 K−1
This implies that 85%–95% of all particles are smaller than 104–173 μm
Observations of the South Polar Residual Cap suggest a possible erosion of the cap, leading to an increase of the global mass of the atmosphere. We test this assumption by making the first comparison ...between Viking 1 and InSight surface pressure data, which were recorded 40 years apart. Such a comparison also allows us to determine changes in the dynamics of the seasonal ice caps between these two periods. To do so, we first had to recalibrate the InSight pressure data because of their unexpected sensitivity to the sensor temperature. Then, we had to design a procedure to compare distant pressure measurements. We propose two surface pressure interpolation methods at the local and global scale to do the comparison. The comparison of Viking and InSight seasonal surface pressure variations does not show changes larger than ±8 Pa in the CO2 cycle. Such conclusions are supported by an analysis of Mars Science Laboratory (MSL) pressure data. Further comparisons with images of the south seasonal cap taken by the Viking 2 orbiter and MARCI camera do not display significant changes in the dynamics of this cap over a 40 year period. Only a possible larger extension of the North Cap after the global storm of MY 34 is observed, but the physical mechanisms behind this anomaly are not well determined. Finally, the first comparison of MSL and InSight pressure data suggests a pressure deficit at Gale crater during southern summer, possibly resulting from a large presence of dust suspended within the crater.
Plain Language Summary
Observations of the permanent CO2 ice cap at the south pole of Mars in the 2000s suggested that the cap was eroding, possibly releasing a significant amount of CO2 into the atmosphere. To test this hypothesis, we compare surface pressures recorded by Viking in the 1970s and those recorded by InSight in 2018–2021 to confirm or refute the suspected increase of the atmospheric mass. After establishing our comparison method, we correct the influence of the sensor temperature on the InSight pressure data, which was discovered during our investigation. Comparison of the pressure data, as well as images of the seasonal caps taken by orbiters, do not reveal any change in the atmospheric mass or the dynamics of the seasonal ice caps that develop during the martian year. These conclusions are reinforced by reanalyzing the pressure data recorded by the Curiosity rover. Only small interannual changes are observed, potentially related to the effect of the dust storms that happened on Mars between 2016 and 2018. Finally, we report a possible pressure deficit at MSL's location during southern hemisphere summer, potentially explained by the unusual presence of dust in the crater air.
Key Points
We propose a recalibration of InSight pressure data to correct an unexpected sensitivity to the sensor temperature
A comparison between the InSight and Viking 1 pressure data does not show variations larger than ±8 Pa in the global atmospheric mass
This comparison also supports the absence of long‐term variability in the dynamics of seasonal cap formation and sublimation
New Craters on Mars: An Updated Catalog Daubar, I. J.; Dundas, C. M.; McEwen, A. S. ...
Journal of geophysical research. Planets,
July 2022, 2022-07-00, 20220701, Letnik:
127, Številka:
7
Journal Article
Recenzirano
Odprti dostop
We present a catalog of new impacts on Mars. These craters formed in the last few decades, constrained with repeat orbital imaging. Crater diameters range from 58 m down to <1 m. For each impact, we ...report whether it formed a single crater or a cluster (58% clusters); albedo features of the blast zone (88% halos; 64% linear rays; 10% arcuate rays; majority dark‐toned; 4% light‐toned; 14% dual‐toned); and exposures of ice (4% definite; 2% possible). We find no trends in the occurrences of clusters with latitude, elevation, or impact size. Albedo features do not depend on atmospheric fragmentation. Halos are more prevalent at lower elevations, indicating an atmospheric pressure dependence; and around smaller impacts, which could be an observational bias. Linear rays are more likely to form from larger impacts into more consolidated material and may be enhanced by lower atmospheric pressure at higher elevations. Light‐ and dual‐toned blast zones occur in specific regions and more commonly around larger impacts, indicating excavation of compositionally distinct material. Surfaces covered with bright dust lacking cohesion are favored to form detectable surface features. The slope of the cumulative size frequency distribution for this data set is 2.2 for diameters >8 m (differential slope 2.9), significantly shallower than the slope of new lunar craters. We believe that no systematic biases exist in the Martian data set sufficient to explain the discrepancy. This catalog is complete at the time of writing, although observational biases exist, and new discoveries continue.
Plain Language Summary
We present a list of all the new impact craters on Mars. These craters formed in the last few decades, constrained with before and after images taken from orbiting spacecraft. Craters range in size from 58 m across down to smaller than can be measured. For each impact, we report whether it formed a single crater or a cluster of craters that were created by one impact event; whether the area around the impact is relatively bright or dark; what types of features are around the impact site, like rays or halos; and whether the impact excavated any water ice. We look at how these aspects vary with location on Mars and how that relates to how they might have formed. We find that fewer small craters compared to larger craters are currently forming on Mars than on the Moon. We examine several possible explanations, including known biases in the datasets, impacting populations, target material properties, and atmospheric effects, but none of these can explain the discrepancy.
Key Points
We present 1203 new impacts on Mars 1–58 m in diameter, 58% of which are clusters, with date constraints from orbital images
Dark and light albedo features around new craters include halos and rays, some of which depend on location, surface properties, or size
The slope of the size frequency distribution of new Martian craters is shallower than lunar craters, even considering systematic biases
The stability of the residual carbon dioxide cap near the south pole of Mars is currently not well understood. The cap's survival depends on its radiation budget, controlled by the visible albedo and ...infrared emissivity. We investigated the role of CO2 snowfall in altering the albedo and emissivity, leading to the observed asymmetry in the net CO2 accumulation at the two poles. Uncontaminated snowfall increases albedo, and lowers emissivity, due to scattering by optically thick clouds and granular surface deposits. Data from the Mars Climate Sounder (MCS) show that fall and winter snowfall is correlated with higher springtime albedo at both poles. For the seasonal CO2 deposits in each polar region >60° latitude, we find mean albedo values of 0.39 in the north and 0.51 in the south, and winter 32‐μm emissivity values of 0.84 in the north and 0.87 in the south. Using a radiative transfer model and the MCS data, we find that the north polar deposits have ∼10× higher dust content than those in the south, explaining the ∼31% lower albedo of the north seasonal cap during spring. Our model shows that greater amounts of snowfall can explain the ∼4% lower emissivity of the north polar seasonal cap. These findings demonstrate that winter snowfall and dust transport affect the composition of Mars' seasonal ice caps and polar energy balance. Snowfall and dust loading are therefore important in modeling the CO2 cycle on Mars, as well as the planet's long‐term climate variations.
Plain Language Summary
The permanent carbon dioxide (CO2, also known as dry ice) deposits on Mars control the planet's global atmospheric pressure. Seasonal fluctuations of the CO2 cycle drive pressure variations measured at the surface anywhere on the planet. These variations are buffered by the stable permanent CO2 deposit at the south pole. The north pole has the more favorable altitude and pressure to be in equilibrium with the atmosphere, but satellite observations showed that the residual cap at the south pole was in equilibrium and not the north. Other studies found evidence of carbon dioxide snowfall in both polar regions. Using data from Mars Climate Sounder on board the Mars Reconnaissance Orbiter, we found that there is an asymmetry between the optical properties of the northern and the southern permanent caps. We attribute this asymmetry to dust and snowfall quantities, which promote more CO2 accumulation in the south, relative to the north.
Key Points
Infrared emissivity and visible albedo of Mars' seasonal ice caps are correlated
The north seasonal cap has greater snowfall, lower emissivity, and lower albedo
Higher dust loading in the northern fall and winter seasons causes lower ice cap albedo
The InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission will perform the first Martian in situ heat flow measurement by deploying the Heat Flow and ...Physical Properties Package (HP3) onto the Martian surface. In order to estimate the heat flow coming from the planetary interior, HP3 will measure the local subsurface thermal gradient as well as the local thermal conductivity to a depth of up to 5 m. From these measurements, local heat flow can be determined, but this will in general differ from the heat flow emanating from the planetary interior due to atmosphere‐induced perturbations. Here we quantify heat flow perturbation induced by dust loading of the Martian atmosphere using dust opacity data obtained by the Mars Exploration Rover Opportunity. Dust opacity data span the time period between Mars year (MY) 27 and MY 32, thus incorporating the global dust storm event of MY 28 as a signal. We consider two end‐member cases for the regolith thermal conductivity and find that the background planetary heat flow is superposed by atmosphere‐induced perturbations of less than 1.5 mW m−2 at depths below 2 m if regolith thermal conductivity is low and around 0.025 W m−1 K−1 on average. If thermal conductivity is high and around 0.05 W m−1 K−1 on average, perturbations are less than 2.5 mW m−2 at depths below 3 m. Overall, the influence of interannual variability on subsurface heat flow is found to be moderate following a global dust storm. Considerably smaller perturbations are introduced by regional dust storms, which are of shorter duration and smaller magnitude.
Key Points
We use a 5 Mars years time series of the Martian atmospheric dust opacity data to quantify the magnitude of the heat flow perturbations
Numerical simulations show minor and moderate pertubation of the heat flow caused by local and global dust storm, respectively
The heat pulse introduced by global dust storms travels up to a depth of 3 m and can only be observed in the Mars year following the storm
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