The present‐day thermal state, interior structure, composition, and rheology of Mars can be constrained by comparing the results of thermal history calculations with geophysical, petrological, and ...geological observations. Using the largest‐to‐date set of 3‐D thermal evolution models, we find that a limited set of models can satisfy all available constraints simultaneously. These models require a core radius strictly larger than 1,800 km, a crust with an average thickness between 48.8 and 87.1 km containing more than half of the planet's bulk abundance of heat producing elements, and a dry mantle rheology. A strong pressure dependence of the viscosity leads to the formation of prominent mantle plumes producing melt underneath Tharsis up to the present time. Heat flow and core size estimates derived from the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission will increase the set of constraining data and help to confine the range of admissible models.
Plain Language Summary
We constrain the thermal state and interior structure of Mars by combining a large number of observations with thermal evolution models. Models that match the available observations require a core radius larger that half the planetary radius and a crust thicker than 48.8 km but thinner than 87.1 km on average. All best‐fit models suggest that more than half of the planet's bulk abundance of heat producing elements is located in the crust. Mantle plumes may still be active today in the interior of Mars and produce partial melt underneath the Tharsis volcanic province. Our results have important implications for the thermal evolution of Mars. Future data from the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission can be used to validate our models and further improve our understanding of the thermal evolution of Mars.
Key Points
We combine the largest‐to‐date set of 3‐D dynamical models with observations to constrain the thermal state and interior structure of Mars
Best‐fit models suggest a core radius strictly larger than 1,800 km and an average crustal thickness 48.8 km < dc < 87.1 km
Models suggest a large pressure dependence of the viscosity and a crust containing 65‐70% of the total amount of heat producing elements
The Heat Flow and Physical Properties Package HP
3
for the InSight mission will attempt the first measurement of the planetary heat flow of Mars. The data will be taken at the InSight landing site in ...Elysium planitia (136
∘
E, 5
∘
N) and the uncertainty of the measurement aimed for shall be better than ±5 mW m
−2
. The package consists of a mechanical hammering device called the “Mole” for penetrating into the regolith, an instrumented tether which the Mole pulls into the ground, a fixed radiometer to determine the surface brightness temperature and an electronic box. The Mole and the tether are housed in a support structure before being deployed. The tether is equipped with 14 platinum resistance temperature sensors to measure temperature differences with a 1-
σ
uncertainty of 6.5 mK. Depth is determined by a tether length measurement device that monitors the amount of tether extracted from the support structure and a tiltmeter that measures the angle of the Mole axis to the local gravity vector. The Mole includes temperature sensors and heaters to measure the regolith thermal conductivity to better than 3.5% (1-
σ
) using the Mole as a modified line heat source. The Mole is planned to advance at least 3 m—sufficiently deep to reduce errors from daily surface temperature forcings—and up to 5 m into the martian regolith. After landing, HP
3
will be deployed onto the martian surface by a robotic arm after choosing an instrument placement site that minimizes disturbances from shadows caused by the lander and the seismometer. The Mole will then execute hammering cycles, advancing 50 cm into the subsurface at a time, followed by a cooldown period of at least 48 h to allow heat built up during hammering to dissipate. After an equilibrated thermal state has been reached, a thermal conductivity measurement is executed for 24 h. This cycle is repeated until the final depth of 5 m is reached or further progress becomes impossible. The subsequent monitoring phase consists of hourly temperature measurements and lasts until the end of the mission. Model calculations show that the duration of temperature measurement required to sufficiently reduce the error introduced by annual surface temperature forcings is 0.6 martian years for a final depth of 3 m and 0.1 martian years for the target depth of 5 m.
NASA’s InSight mission to Mars will measure seismic signals to determine the planet’s interior structure. These highly sensitive seismometers are susceptible to corruption of their measurements by ...environmental changes. Magnetic fields, atmosphere pressure changes, and local winds can all induce apparent changes in the seismic records that are not due to propagating ground motions. Thus, InSight carries a set of sensors called the Auxiliary Payload Sensor Suite (APSS) which includes a magnetometer, an atmospheric pressure sensor, and a pair of wind and air temperature sensors. In the case of the magnetometer, knowledge of the amplitude of the fluctuating magnetic field at the InSight lander will allow the separation of seismic signals from potentially interfering magnetic signals of either natural or spacecraft origin. To acquire such data, a triaxial fluxgate magnetometer was installed on the deck of the lander to obtain magnetic records at the same cadence as the seismometer. Similarly, a highly sensitive pressure sensor is carried by InSight to enable the removal of local ground-surface tilts due to advecting pressure perturbations. Finally, the local winds (speed and direction) and air temperature are estimated using a hot-film wind sensor with heritage from REMS on the Curiosity rover. When winds are too high, seismic signals can be ignored or discounted. Herein we describe the APSS sensor suite, the test programs for its components, and the possible additional science investigations it enables.
We reexamine the Apollo Heat Flow Experiment in light of new orbital data. Using three‐dimensional thermal conduction models, we examine effects of crustal thickness, density, and radiogenic ...abundance on measured heat flow values at the Apollo 15 and 17 sites. These models show the importance of regional context on heat flux measurements. We find that measured heat flux can be greatly altered by deep subsurface radiogenic content and crustal density. However, total crustal thickness and the presence of a near‐surface radiogenic‐rich ejecta provide less leverage, representing only minor (<1.5 mW m−2) perturbations on surface heat flux. Using models of the crust implied by Gravity Recovery and Interior Laboratory results, we found that a roughly 9–13 mW m−2 mantle heat flux best approximate the observed heat flux. This equates to a total mantle heat production of 2.8–4.1 × 1011 W. These heat flow values could imply that the lunar interior is slightly less radiogenic than the Earth's mantle, perhaps implying that a considerable fraction of terrestrial mantle material was incorporated at the time of formation. These results may also imply that heat flux at the crust‐mantle boundary beneath the Procellarum potassium, rare earth element, and phosphorus (KREEP) Terrane (PKT) is anomalously elevated compared to the rest of the Moon. These results also suggest that a limited KREEP‐rich layer exists beneath the PKT crust. If a subcrustal KREEP‐rich layer extends below the Apollo 17 landing site, required mantle heat flux can drop to roughly 7 mW m−2, underlining the need for future heat flux measurements outside of the radiogenic‐rich PKT region.
Key Points
Crustal structure effects need to be accounted for in lunar heat flow
Lunar mantle heat flow suggests Earth‐like mantle composition
Heat flow measurement away from PKT region is needed
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.
The lack of magnetic anomalies within the major impact basins (Hellas, Argyre, and Isidis) has led many investigators to the conclusion that Mars' dynamo shut down prior to the time when these basins ...formed (∼4.0 Ga). We test this hypothesis by analyzing gravity and magnetic anomalies in the regions surrounding Tyrrhenus Mons and Syrtis Major, two volcanoes that were active during the late Noachian and Hesperian. We model magnetic anomalies that are associated with gravity anomalies and generally find that sources located below Noachian surface units tend to favor paleopoles near the equator and sources located below Hesperian surface features favor paleopoles near the geographical poles, suggesting polar wander during the Noachian‐Hesperian. Both paleopole clusters have positive and negative polarities, indicating reversals of the field during the Noachian and Hesperian. Magnetization of sources below Hesperian surfaces is evidence that the dynamo persisted beyond the formation of the major impact basins. The demagnetization associated with the volcanic construct of Syrtis Major implies dynamo cessation occurred while it was geologically active approximately 3.6 billion years ago. Timing of dynamo activity is fundamentally linked to Mars' climate via the stability of its atmosphere, and is coupled to the extent and duration of surface geologic activity. Thus, the dynamo history is key for understanding both when Mars was most geologically active and when it may have been most hospitable to life.
Key Points
We have developed a new method to model magnetic anomalies on Mars
We present evidence that the dynamo was active during the Hesperian epoch
We present evidence of magnetic reversals and true polar wander
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
We use a scaling relationship between fault length and scalar moment to predict seismicity from the Cerberus Fossae graben using shear modulus and fault lengths. Cerberus Fossae is 20–40° ...(1,200–2,300 km) from the InSight lander and matches the location of 21 recorded seismic events with Mw ≥ 3. These unique seismic observations make an ideal laboratory to test our method of predicting seismicity on another planet using surface fault observations. Terrestrial faults have observed patterns of rupture depth distributions and segmentation. We use these patterns and magnitudes of detected marsquakes to predict moment release of events in Cerberus Fossae over a range of plausible Martian rupture vertical slip extents (VSEs) of 2, 20, and 40 km, which are rooted in estimates of crustal thickness, elastic thickness, and seismicity depth. We sum individual events for each case to determine cumulative moment release and use deformation duration to determine annual moment release. Predicted seismicity rates are dependent on the duration of deformation, which in this case is well constrained to 2–10 Ma. We compare our results to events recorded at Cerberus Fossae by InSight and find that seismicity rates for the cases with a 40 km maximum VSE or no limit on VSE are within an order of magnitude of observed seismicity. Faults with maximum VSEs of 2 km best match observed magnitudes. Our approach, using only observed fault lengths, segmentation, seismogenic thickness estimates, and deformation duration, produces seismicity estimates that are as accurate as methods that take rupture offset into account.
Plain Language Summary
We develop a method to estimate the magnitudes and frequencies of quakes on Mars in the Cerberus Fossae region using a scaling relationship between fault length and quake magnitude. Our method estimates quake magnitude for each fault length, and ultimately estimates cumulative seismic moment release, or total energy released by the formation of the geologic structure. We calculate seismicity rate by dividing the cumulative moment release by the time of deformation for the geological structure. Our method to estimate seismicity requires fewer assumptions and assumed geophysical constants than previously proposed methods. We use Leonard’s (2010, https://doi.org/10.1785/0120090189) aspect ratio of fault length and width. Thus, a maximum faulting depth informs the maximum vertical slip extent (VSE), which in turn limits the maximum fault length and the maximum allowable quake magnitude. Segmentation patterns of the faults inform the magnitude‐frequency distribution of marsquakes but do not account for a high enough occurrence of small quakes. Resulting seismicity rates are within an order of magnitude of what has been measured by the InSight seismometer with a VSE of 40 km or no VSE, and marsquake magnitudes are best represented by a VSE of 2 km.
Key Points
We estimate seismicity using a fault length‐moment release scaling relationship. Our results align with InSight seismometer observations
Marsquake magnitudes are limited by vertical slip extents, which are constrained by likely faulting depths
Marsquake magnitude distribution is constrained by segmentation patterns of the graben. Seismicity rate is constrained by deformation time
Venus' young surface age implies significant geologic activity. Several lines of evidence suggest current volcanism, but there is no data to directly constrain current tectonism. Wrinkle ridges are ...an abundant, globally distributed fault type, and may have formed relatively recently. We estimate seismicity using a scaling relationship between fault length and moment release. We obtain fault lengths from previously mapped wrinkle ridges and limit their lengths in two ways: (1) with realistic vertical slip extents (VSEs) of 50, 30, and 10 km, and (2) with four tiers of segmentation. A VSE limits fault dimensions and is constrained by faulting depth, and segmentation is important to produce a realistic magnitude-frequency distribution. We use 100 million years as a deformation time to calculate annual seismicity for the wrinkle ridges. Resulting moment release rates are 5.1×1017 N⋅m/yr, 3.7×1017 N⋅m/yr, and 9.1×1016 N⋅m/yr for fault VSEs of 50 km, 30 km, and 10 km, respectively. We predict seismicity roughly 1 order of magnitude more than the extrapolated Mars global seismicity from measurements at a single station, and ∼5 orders of magnitude less seismicity than is observed on Earth. This estimate is conservative since we assume each segment breaks once and the faults are mapped using low resolution global data. Moreover, Venus exhibits many other likely sources of seismicity, including volcanoes, ridge belts, graben, and other faults. Wrinkle ridges and other features likely produce quake magnitudes detectable through infrasound using a balloon-based barometer.
•We estimate seismicity from wrinkle ridges with a fault scaling relationship.•Plausible faulting depths limit slip extents and the range of quake magnitudes.•Segmenting faults yields smaller quakes and b-values akin to terrestrial regimes.•Seismicity rate is constrained by deformation time.•More seismicity may occur at wrinkle ridges than is observed on Mars.
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
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