Abstract
Discerning the total mass of Mars’ obliquity‐timescale (∼10
5
‐year) exchangeable CO
2
inventory has been elusive for decades due to the unknown adsorptive capacity of its regolith. Now, ...however, the stratigraphy of Mars’ recently discovered South Polar Massive CO
2
Ice Deposit (MCID) provides a record of orbit‐driven CO
2
exchange between its polar cap, atmosphere, and regolith with sufficient constraint to calculate the adsorptive capacity of its regolith and therefore the total mass of its exchangeable CO
2
inventory. We use a numerical climate model and Markov Chain Monte Carlo analysis to show that the observed MCID stratigraphic record is most consistent with a mobile CO
2
inventory of
kg (
mbar, 68% confidence interval) that exchanges on obliquity timescales. We find that adsorptive CO
2
exchange with the regolith on obliquity timescales likely occurs in the depth range of >∼200 m to <∼1 km, with the deeper bound set by thermal processes and adsorptive surface availability. Our best‐fit model yields a peak mean annual surface pressure 40% lower than CO
2
exchange models that neglect an adsorbing regolith. We provide machine‐readable text files of our results to aid future study of Mars’ climate.
Plain Language Summary
Mars’ atmosphere is primarily CO
2
. In addition to atmospheric CO
2
, Mars also has a polar CO
2
ice cap and a reservoir of CO
2
adsorbed in its regolith (i.e., CO
2
molecules forming a thin film on martian soil grains). Determining how much total CO
2
exchanges between the atmosphere, cap, and regolith over orbit‐driven climate cycles is important for understanding Mars’ climatic evolution. We constructed a climate model to calculate how CO
2
exchanges between the atmosphere, cap, and regolith over these ∼10
5
‐year cycles. The model outputs a predicted thickness of alternating layers of CO
2
and H
2
O ice in the CO
2
ice cap. Comparing our model results to the observed layering of the polar cap indicates that ∼100 millibars of CO
2
(∼17 × the current atmospheric mass) can exchange between the atmosphere, polar cap, and regolith during these cycles. Moreover, regolith adsorption likely decreases Mars’ peak atmospheric pressure by ∼40%, which implies that near‐surface habitable environments with liquid water are more difficult to create and sustain on Mars than previously thought.
Key Points
Mars’ mobile CO
2
inventory on obliquity timescales is ∼100 millibar, with adsorptive regolith exchange down to >∼200 m to <∼1 km depth
Mars’ peak surface pressure is ∼40% lower with best‐fit regolith adsorption parameters than when adsorption is ignored
The product of depth and specific surface area is most important regolith property influencing the polar CO
2
ice stratigraphy
This article discusses relevant physical properties of the regolith at the Mars InSight landing site as understood prior to landing of the spacecraft. InSight will land in the northern lowland plains ...of Mars, close to the equator, where the regolith is estimated to be
≥
3
–
5
m
thick. These investigations of physical properties have relied on data collected from Mars orbital measurements, previously collected lander and rover data, results of studies of data and samples from Apollo lunar missions, laboratory measurements on regolith simulants, and theoretical studies. The investigations include changes in properties with depth and temperature. Mechanical properties investigated include density, grain-size distribution, cohesion, and angle of internal friction. Thermophysical properties include thermal inertia, surface emissivity and albedo, thermal conductivity and diffusivity, and specific heat. Regolith elastic properties not only include parameters that control seismic wave velocities in the immediate vicinity of the Insight lander but also coupling of the lander and other potential noise sources to the InSight broadband seismometer. The related properties include Poisson’s ratio, P- and S-wave velocities, Young’s modulus, and seismic attenuation. Finally, mass diffusivity was investigated to estimate gas movements in the regolith driven by atmospheric pressure changes. Physical properties presented here are all to some degree speculative. However, they form a basis for interpretation of the early data to be returned from the InSight mission.
Abstract
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 ...CO
2
frost, yet visible wavelength imagery does not display any ice signature. We interpret this systematic absence as an indication of CO
2
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 CO
2
‐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 CO
2
ice forms at all latitudes on Mars with a strong seasonal control. In this study, we show that diurnal CO
2
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 CO
2
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 CO
2
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
Limb sounding of thermal emission in the infrared wavelength range is a powerful technique for measuring temperature and aerosols in the martian atmosphere. However, the long optical path may provide ...challenges to limb retrievals in high aerosol conditions. These can be mitigated by considering limb measurements in the far infrared, where opacities of most aerosols are lower than in the mid-infrared. We present analyses of radiative properties of Mars dust and water ice aerosols at far infrared wavelengths based on measurements by the Mars Climate Sounder (MCS) in limb geometry at mid- and far infrared wavelengths. For dust aerosols, derived far infrared radiative properties show a homogeneous behavior that is consistent with particle sizes in the order of 1 μm effective radius. Far infrared radiative properties for water ice aerosols exhibit a larger variability in local time and region, leading to significant differences between the aphelion cloud belt and the north polar hood, with the resulting parameters suggesting particle sizes around 3 μm or larger. Using the derived parameters, we develop a method for retrieving aerosol profiles from MCS limb measurements that combines information from mid- and far infrared spectroscopic channels. The use of far infrared channels enables aerosol profile retrievals from limb measurements that typically reach about a scale height deeper into the atmosphere than would be possible using mid-infrared channels only. The extended vertical range of the aerosol profiles allows the derivation of aerosol column optical depths through vertical integration, with dust column derivations in global or large-scale regional dust storms also being available by extrapolating dust profiles below the lowest retrievable altitude of a limb measurement. The quantification of aerosol columns allows us to retrieve surface brightness temperatures from MCS on-planet viewing measurements that are corrected for atmospheric contributions. We show that differences between surface brightness and top-of-the-atmosphere temperatures are typically within 20 K, with surface brightness temperatures generally being warmer (colder) than top-of-the-atmosphere temperatures at daytime (nighttime), except at high latitudes.
Display omitted
•We derive radiative properties of Mars dust and water ice in the far infrared from Mars Climate Sounder limb measurements.•Use of combined mid- and far infrared measurements enables aerosol profile retrievals of extended vertical range.•The extended vertical range of aerosol profiles allows the derivation of aerosol columns through vertical integration.
We use the Martian surface temperature response to Phobos transits observed next to the InSight lander in Elysium Planitia to constrain the thermal properties of the uppermost layer of regolith. ...Modeled transit lightcurves validated by solar panel current measurements are used to modify the boundary conditions of a 1D heat conduction model. We test several model parameter sets, varying the thickness and thermal conductivity of the top layer to explore the range of parameters that match the observed temperature response within its uncertainty both during the eclipse as well as the full diurnal cycle. The measurements indicate a thermal inertia (TI) of 103−16+22Jm−2K−1s−1/2 in the uppermost layer of 0.2–4 mm, significantly smaller than the TI of 200Jm−2K−1s−1/2 derived from the diurnal temperature curve. This could be explained by larger particles, higher density, or some or slightly higher amount of cementation in the lower layers.
Plain Language Summary
The Mars moon Phobos passed in front of the Sun from the perspective of the InSight lander on several occasions. The Mars surface temperatures measured by the lander became slightly colder during these transits due to the lower amount of sunlight the surface received at this time. The transits only last 20–35 s and therefore only the very top layer, about 0.3–0.8 mm, of the ground has time to cool significantly. The top layer cools and heats up faster than we expected based on the temperature changes of the day‐night cycle, which affects about 4 cm of the ground. Based on this observation we conclude that the material in the top mm of the ground is different from that below. A possible explanation would be an increase of density with depth, a larger fraction of smaller particles such as dust at the top, or a layer where particles are slightly cemented together beginning at 0.2–4 mm below the surface.
Key Points
The Martian surface temperature response to Phobos transits at the InSight landing site is interpreted
The thermal inertia of the uppermost layer of soil is 103−16+22Jm−2K−1s−1/2
The thermal conductivity or density of the top 0.2–4 mm is significantly less than that of the top 4 cm
Carbon dioxide is Mars' most active volatile. The seasonal and diurnal processes of when and where it condenses and sublimates are determined by energy balance between the atmosphere and surface ice ...in Mars' vapor pressure equilibrium climate. Mars' current obliquity ensures that the polar caps are stable locations for seasonal condensation. The eccentricity of Mars' orbit is the major driver of differences in seasonal behavior of CO2 between the northern vs southern hemisphere. In particular, the current positions of perihelion and aphelion, in addition to the large elevation difference between the poles, dominate the ways seasonal processes transpire in the two hemispheres. We summarize and discuss the unprecedented observations of these processes that have been collected by the Mars Reconnaissance Orbiter over the last 8.5 Mars Years. The longer southern fall and winter allows more time for CO2 ice to accumulate and densify in the southern hemisphere. Northern winter coincides with the perihelion dust storm season, thus the north polar seasonal ice deposits are expected to contain a greater concentration of dust in relation to CO2 and H2O ices. With less time for densification and more contaminants the northern seasonal layer of CO2 ice is likely weaker than the southern layer.
•Carbon dioxide forms a seasonal polar cap every winter on Mars.•In the spring the sublimation of CO2 is different in the two hemispheres.•These differences are due to Mars' elliptical orbit and different elevations of the hemispheres.•Perihelion occurs in late southern spring, allowing more time for ice to densify.•Aphelion in northern spring allows the summer dust to contaminate the cap.
In this paper we define and describe morphological features that have colloquially been termed “spiders” and map their distribution in the south polar region of Mars. We show that these features go ...through a distinct seasonal evolution, exhibiting dark plumes and associated fan‐shaped deposits during the local defrosting of the seasonal cap. We have documented the seasonal evolution of the cryptic region and have found that spiders only occur within this terrain. These observations are consistent with a geyser‐like model for spider formation. Association with the transparent (cryptic) portion of the seasonal cap is consistent with basal sublimation and the resulting venting of CO
2
gas. Also consistent with such venting is the observation of dark fan‐shaped deposits apparently emanating from spider centers. Spiders are additionally confined to the polar layered deposits presumably due to the poorly consolidated and easily eroded nature of their upper surface.
Abstract
We use the Martian surface temperature response to Phobos transits observed next to the InSight lander in Elysium Planitia to constrain the thermal properties of the uppermost layer of ...regolith. Modeled transit lightcurves validated by solar panel current measurements are used to modify the boundary conditions of a 1D heat conduction model. We test several model parameter sets, varying the thickness and thermal conductivity of the top layer to explore the range of parameters that match the observed temperature response within its uncertainty both during the eclipse as well as the full diurnal cycle. The measurements indicate a thermal inertia (TI) of
in the uppermost layer of 0.2–4 mm, significantly smaller than the TI of
derived from the diurnal temperature curve. This could be explained by larger particles, higher density, or some or slightly higher amount of cementation in the lower layers.
Plain Language Summary
The Mars moon Phobos passed in front of the Sun from the perspective of the InSight lander on several occasions. The Mars surface temperatures measured by the lander became slightly colder during these transits due to the lower amount of sunlight the surface received at this time. The transits only last 20–35 s and therefore only the very top layer, about 0.3–0.8 mm, of the ground has time to cool significantly. The top layer cools and heats up faster than we expected based on the temperature changes of the day‐night cycle, which affects about 4 cm of the ground. Based on this observation we conclude that the material in the top mm of the ground is different from that below. A possible explanation would be an increase of density with depth, a larger fraction of smaller particles such as dust at the top, or a layer where particles are slightly cemented together beginning at 0.2–4 mm below the surface.
Key Points
The Martian surface temperature response to Phobos transits at the InSight landing site is interpreted
The thermal inertia of the uppermost layer of soil is
The thermal conductivity or density of the top 0.2–4 mm is significantly less than that of the top 4 cm
In this paper we define and describe morphological features that have colloquially been termed “spiders” and map their distribution in the south polar region of Mars. We show that these features go ...through a distinct seasonal evolution, exhibiting dark plumes and associated fan‐shaped deposits during the local defrosting of the seasonal cap. We have documented the seasonal evolution of the cryptic region and have found that spiders only occur within this terrain. These observations are consistent with a geyser‐like model for spider formation. Association with the transparent (cryptic) portion of the seasonal cap is consistent with basal sublimation and the resulting venting of CO2 gas. Also consistent with such venting is the observation of dark fan‐shaped deposits apparently emanating from spider centers. Spiders are additionally confined to the polar layered deposits presumably due to the poorly consolidated and easily eroded nature of their upper surface.
The NASA InSight Lander on Mars includes the Heat Flow and Physical Properties Package HP
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 (
depth) of
. 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.
The online version contains supplementary material available at 10.1007/s11214-022-00941-z.
PDF
