TP4

The NASA InSight Lander on Mars includes the Heat Flow and Physical Properties Package HP³ (see Spohn et al. (2018) for a description of the package) 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, it reached a depth of 40 cm, bringing the mole body 1–2 cm below the surface. A discussion of the lessons learned from the penetration failure and suggestions for an improved mole have been given by Spohn et al. (2022). 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. (compare Figure 1)

Figure 1. The HP³ mole before complete burial and the properties of the hole that the mole had punched in the duricrust

During the campaign the mole penetrated further aided by friction applied using the scoop at the end of the robotic Instrument Deployment Arm and direct support by the latter. The mole reversed its downward motion twice during attempts to provide friction through pressure on the regolith instead of directly with the scoop to the hull. The penetration record of the mole and its thermal sensors were used to measure thermal and mechanical soil parameters such as the penetration resistance of the duricrust. These parameter values are summarized in Table 1 below. The combined data suggest a model of the regolith that has an about 20 cm thick duricrust underneath a 1 cm thick sand layer and above another 10 cm of sand. Underneath the latter, a layer more resistant to penetration and possibly consisting of debris from a small impact crater was found. The thermal conductivity increases from 14 mW/m K in the 1 cm sand layer to 34 mW/m K in the duricrust and the sand layer underneath the duricrust to 64 mW/m K in the gravel layer below. Applying cone penetration theory, the resistance of the duricrust was used to estimate a cohesion of the latter of 4 - 25 kPa depending on the friction angle of the crust. Pushing the scoop with its blade into the surface and chopping of a piece of crust provided another estimate of the cohesion of 5.8 kPa.

The hammerings of the mole were recorded by the seismometer SEIS and the signals could be used to derive a P-wave velocity and an S-wave as listed (see also Brinkman et al., 2022) representative of the topmost tens of cm of the regolith. Together with a density provided by a thermal conductivity and diffusivity measurement using the mole thermal sensors of  about 1211 (Grott et al., 2021), the elastic moduli could be calculated from the seismic velocities.

Table 1. Model of the InSight landing site regolith

After burial, the mole was used to measure the thermal conductivity of the regolith as a function of the solar longitude (the seasons on Mars). The variations of the thermal conductivity are consequences of the variations in atmosphere pressure with the seasons and the contribution of atmosphere gas in the porous regolith contributing to the thermal conductivity.   

Figure 2 Thermal conductivity in the regolith top 40 cm as measured by sensors on the HP³ mole as a function of the solar longitude (seasons on Mars).

Brinkmann et al. (2022), submitted to J. Geophys. Res. Planets

Grott et al. (2021), Planet and Space Sci, DOI: 10.1029/2019EA000670

Spohn et al. (2018), Space Sci Rev, DOI:10.1007/s11214-018-0531-4   

Spohn et al. (2022), Advances in Space Research, DOI: 10.1016/j.asr.2022.02.009

How to cite: Spohn, T., Grott, M., Müller, N., and Hudson, T. and the HP-cubed team: Using the HP3 mole on InSight to probe the thermal and mechanical properties of the Martian regolith, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-408, https://doi.org/10.5194/epsc2022-408, 2022.

Analyses of the InSight seismic data indicate that there are three major seismic discontinuities beneath the lander where the seismic velocity abruptly increases. In addition to the crust-mantle interface at a depth of about 39 km, two major intracrustal discontinuities are observed at depths near 8 and 20 km (Lognonné et al. 2020, Knapmeyer-Endrun et al. 2021, Kim et al. 2021, Duran et al. 2022). There are many possible explanations for the existence of large-scale layering within the crust, and most of these involve a change in either chemical composition or porosity (see Figure 1). Here, we will assess these hypotheses using improved models of the crust that have been made possible by the InSight mission.

Figure 1. Interpretations of crustal layering beneath the InSight landing site. In the first two schematics, the crustal layering is a result either of stepwise changes in porosity (left) or composition (center). Our preferred interpretation combines elements of both of these end-member models. The depth of each seismic interface below the surface is denoted on the left schematic.

As summarized in Wieczorek et al. (2022), there are a large number of hypotheses for the origin of the three-layered nature of the Martian crust. One or more layers could be composed of volcanic or sedimentary deposits. One of the discontinuities could represent the removal of pore space either by viscous deformation of the host rock or by the precipitation of cements in an ancient aquifer. One layer could represent thick impact basin ejecta deposits, whereas another could represent thick sequences of magmatic intrusions. One of the discontinuities could represent a change in crystallinity, such as is observed in the oceanic crust of Earth. Lastly, one or more discontinuities could be generated by the fractional crystallization of a giant impact melt pool associated with the Borealis impact. Importantly, each of these hypotheses has different implications for the expected thickness of the crustal layer, its composition, and its associated seismic velocity.

Based on the currently available information, we have constructed a plausible model for the observed crustal layering beneath the landing site. As shown in Figure 1, we interpret the uppermost layer as being a result of thick sequences of volcanic materials that were deposited in the early Hesperian, Noachian, and pre-Noachian periods. To account for the low seismic velocities of this layer, these materials would need to be heavily fractured, perhaps being similar in nature to the pyroclastic deposits that make up the nearby Medusae Fossae formation. Given the long duration over which these materials were emplaced, intercalated sedimentary deposits could be common and these materials might also have undergone substantial aqueous alteration at a later date. Ancient impact ejecta deposits from the Utopia basin are likely to be found below the layer of volcanic and sedimentary deposits, potentially in the middle layer between 8 and 20 km depth. The increase in velocity at 20 km depth is likely to be a consequence of the complete viscous closure of all remaining pore space at about 4 Ga when the crustal temperatures were elevated. The deepest layer, from depths of 20 to 39 km, likely corresponds to the initial crust that formed during the differentiation of the Borealis impact melt sheet.

The expected stratigraphy of the southern highlands is less certain. Nevertheless, we speculate that a 20 km seismic discontinuity would be found there as well that represents the transitions from porous to non-porous materials. Another major discontinuity that is likely to be present in the southern highlands would be the base of thick ejecta deposits derived from the ancient Borealis impact event.

It is important to recognize that the InSight landing site is located in the northern lowlands of Mars, and that the layering that is observed beneath the lander might only be indicative of local geologic structure. Future observations of surface waves, as well as analyses of crustal structure at the bounce point of PP waves, will allow us to assess how the thickness of the crustal layers varies across the planet.

References:

Durán, C. et al. (2022). Seismology on Mars: An analysis of direct, reflected, and converted seismic body waves with implications for interior structure. Phys. Earth Planet. Inter., 325, 106851, doi:10.1016/j.pepi.2022.106851

Kim, D. et al. (2021). Improving constraints on planetary interiors with PPs receiver functions. J. Geophys. Res. Planets, 126, e2021JE006983, doi:172810.1029/2021JE006983

Knapmeyer-Endrun, B. et al (2021). Thickness and structure of the martian crust from InSight seismic data. Science, 373, 438-443, doi:10.1126/science.abf8966

Lognonné, P., et al. (2020). Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data. Nature Geosci., 13, 213-220, doi:175910.1038/s41561-020-0536-y

Wieczorek, M. et al. (2022). InSight constraints on the global character of the Martian crust, J. Geophys. Res., doi:10.1029/2022JE007298

How to cite: Wieczorek, M. A. and the InSight Crust Working Group: Origins of crustal layering beneath the InSight landing site (and elsewhere), Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-400, https://doi.org/10.5194/epsc2022-400, 2022.

InSight landed on Mars in November 2018¹ and carries the InSight FluxGate Magnetometer, IFG² which has provided the first surface magnetic field measurements on Mars^1,3. Previous magnetic field measurements taken at orbital altitudes have provided global coverage, with limited spatial resolution. Laboratory analyses of meteorites provide information on magnetic properties of martian rocks, but without detailed local context for their provenances⁴. Advances from the IFG are thus unique and complementary science, specifically characterizing the crustal ambient static and external time-varying fields at a single location on Mars. External fields provide information on the planet’s interaction with the interplanetary magnetic field and the ionosphere. Crustal magnetic fields carry information about the ancient dynamo and crustal conditions at the time at which magnetization was acquired, and on how the crust has been modified by subsequent exogenic and endogenic processes⁴. IFG data for sols 14-736 were collected almost continuously, with some data gaps from electronics anomalies. After sol 736, the magnetometer was operational for shorter periods due to power constraints (Fig. 1) ⁵. A range of studies have been enabled by IFG data, supported by results from other instruments such as the seismometer, and we summarize those.

Strong crustal fields provide evidence for an ancient dynamo. The IFG measured a surface magnetic field strength of ~2000 nT, ~10x stronger than predicted from satellite data^3,6 and consistent with an ancient Earth-like dynamo³. The strong surface field indicates that magnetization at wavelengths shorter than those resolvable from current satellite data (~150 km) contribute substantially to the overall magnetic field. Characterization of the crust through seismic measurements⁷ and geologic inferences³ of subsurface layering allow assessment of magnetization of the crust (Fig. 2). Depending on the depth at which the magnetization is carried, specifically whether it is in the seismically-determined deep layer of Noachian origin or also in the shallow Hesperian-aged crust, the minimum magnetization required to explain the surface field is ~2 A/m or ~0.4 A/m. Seismic characterization of crustal structure⁸ indicates a deep subsurface layer (> 20 km, Fig. 2) of no porosity, while the upper crust (<20 km, Fig. 2) is less porous⁸. Magnetization of these layers require an early active dynamo (>~4 Ga).  Fractured, less porous material could have provided pathways for hydrothermal circulation and chemical remanent magnetization^4,8,10. Magnetization of the most surficial layer of Hesperian age would be consistent with a long-lasting (up to ~3.7 Ga) dynamo⁹.  

IFG data also reveal time-varying fields at the planetary surface that include contributions with different periods and origins. External fields have been observed and characterized from orbit^11–13. However, the degree to which external fields penetrate to and interact with the surface could not be studied prior to the InSight landing. Static and long-duration observations from a surface magnetometer are advantageous because, unlike satellite measurements, temporal variability in the field is not mixed with spatial variability. Here, we summarize different external magnetic field phenomena, transient and periodic that have been observed in IFG data (Fig. 3). Periodic variations include short period waves (100s-1000s^3,14), diurnal variations¹⁵, the ~26 sol Carrington period associated with solar rotation¹⁶,  and seasonal^15,17 fluctuations. Transient events are observed in response to space weather¹⁸ and dust movement^19,20.

The inclusion of the magnetometer on InSight has provided unique and substantial scientific contributions to the overall mission results, as well as a starting point for future planetary surface magnetic field investigations. To overcome limitations of current data sets, we look forward to Mars sample return, as well as possible near-surface investigations. Including magnetometers on future missions at a variety of surface locations for long duration observations will be of great value in understanding a range of external field phenomena, including the influence of crustal magnetic fields on ionospheric currents and the effects of space weather during different phases of a solar cycle. We further advocate for regional investigations for example via a helicopter²⁰ that can provide local magnetic field measurements at a spatial scale commensurate with detailed geological knowledge, to further constrain evolution of Mars’ ancient dynamo and explore the magnetic properties of the crust.

Figure 1: a) Martian years 1 (blue) and 2 (red) of the magnetic field amplitude, B, versus solar longitude (l_s). All data up to sol 1106 of InSight operations are included (PDS release 13). The blue vertical dashed line marks the beginning of the mission. (b) Corresponding sol numbers.

Figure 2: The minimum magnetization required by B=2013 nT (within its 99% confidence intervals)²¹ for the crust below InSight⁸. Burial depth describes the depth extent of the unmagnetized layers above the top of the magnetized layer. A burial depth of 200 m (blue), corresponds to burial beneath the young (H: Hesperian, HNt: Hesperian-Noachian transition) near-surface lava flow³ and magnetizations are at least ~0.4 A/m if the entire underlying crust is magnetized. A burial depth of 10 km (blue) requires magnetizations >1 A/m, hosted by Noachian units. The velocity profiles show the seismically-determined interface depths⁷.

Figure 3: A composite power spectral density (PSD) plot for the surface magnetic field strength at the InSight landing site. Estimates for longer periods are derived using a Lomb-Scargle algorithm (black), shorter periods (purple) show a Welch spectrum.

[1] Banerdt, W. et al. Nat. Geosci. (2020).[2] Banfield, D. et al. SSR (2019). [3] Johnson, C. L. et al.  Nat. Geosci.(2020). [4]Mitteholz, A. & Johnson, C. L. Frontiers (2022). [5] Joy, S. et. al. (2019). [6] Smrekar, S. et al. SSR (2018). [7] Knapmeyer-Endrun, B. et al. Science (2021). [8] Wieczorek, M. et al. JGR (2022). [9] Mittelholz, A. et al. Sci. Adv. (2020). [10] Gyalay, S. et al. GRL (2020). [11] Mittelholz, A. et al. JGR (2017). [12] Ramstad, R. et al. Nat. Astron. (2020).  [13] Brain, D. et al. JGR (2003). [14] Chi, P. et al. LPSC (2019). [15] Mittelholz, A. et al. JGR (2020). [16] Luo, H. et al. JGR (2022). [17] Mittelholz, A. et al. LPSC (2021). [18] Mittelholz, A. et al. GRL (2021). [19] Thorne, S. et al. PSS (2022).  [20] Bapst, J. et al. AAS (2021). [21] Parker, R. JGR (2003). 

How to cite: Mittelholz, A., Johnson, C. L., Fillingim, M. O., Joy, S., Langlais, B., Thorne, S. N., Wieczorek, M., Smrekar, S., and Banerdt, W. B.: The surface magnetic field environment from InSight, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-988, https://doi.org/10.5194/epsc2022-988, 2022.

The Martian dichotomy is the most conspicuous feature of the surface of the planet. The difference in elevation between the Northern and Southern hemispheres of Mars likely originates from a difference in crustal thickness. Inversion of topography and gravity data constrained by seismic data from the InSight NASA mission suggests that the southern crust is on average thicker by 18 to 28 km than the northern one if one assumes a uniform crustal density of 2900 kg m^-3 (Knapmeyer-Endrun et al., 2021 - Wieczorek et al, 2022).

Several explanations have been proposed for the origin of this crustal dichotomy, involving external processes, such as a large impact (Marinova et al., 2008), or internal ones, such as a degree-one mantle convection (Yoshida and Kageyama, 2006). Here we show that a positive feedback mechanism between crustal growth and partial melting in the mantle could have created this dichotomy. Indeed, because the crust is enriched in heat-producing elements (HPE), the lithosphere of a one-plate planet is thinner where the crust is thicker, inducing a lower pressure at the base of the lithosphere. Because of the pressure-dependence of the mantle solidus, partial melting is more important below a thinner lithosphere, causing a larger rate of melt extraction and crustal growth where the crust is thicker. Larger wavelength perturbations in crustal thickness and extraction, and thus hemispherical perturbations, grow faster because thermal diffusion dampens smaller wavelengths faster.

To model this effect, we use a parametric bi-hemispherical thermal evolution model where a well-mixed convective mantle is topped by two types of lithospheres (North and South) characterized by two potentially different thermal structures (Thiriet et al. 2018). The enrichment in HPE of the crust evolves during crust extraction as the enrichment of the newly formed crustal material depends on mantle melt fraction below the lithosphere, mantle enrichment and partition coefficient. In order to study the growth of a hemispherical perturbation, we impose a small initial difference in lithosphere or crust thickness in between the North and South. We then follow the thermal evolution, mantle melting, crustal growth and crustal enrichment in HPE in both hemispheres over 4.5 Gyr (Fig.1). Our model mainly depends on the mantle reference viscosity, that controls the cooling rate of the convective mantle, on and mantle permeability, that controls crustal extraction from the mantle.

Our results show that this positive feedback mechanism can indeed create a significant crustal dichotomy. The range of North-South crustal thickness differences that we obtain by varying the different model parameters largely encompasses that predicted by inversion of topography and gravity data, assuming different crustal densities. In particular, two types of thermal history allow to reproduce the crustal thickness difference predicted by InSight. The first one is obtained for a rather low viscosity and high mantle permeability; it shows a rapid and early extraction of the crust (Fig1. Solid line) and results in a cold potential temperature at the present-day. The second one is for a higher viscosity and lower mantle permeability; it leads to a late and prolonged extraction of the crust (Fig1. dashed line) and results in a warmer mantle potential temperature and a thicker lid at the present-day. In both cases, the crust is extracted during the first Gyr. The enrichment in HPE of the crust predicted by our model is in agreement with GRS data. 

Figure 1 : Evolution of the (a) lid thickness (b) Crust thickness, (c) average melt fraction in the partially melted zone below the lid. (d) Crustal HPE enrichment relatively to the Bulk Silicate Mars as a function of time for the Northern (blue lines) and Southern (orange lines) hemispheres for 2 different simulations that allow to reproduce the difference in crustal thickness deduced from the InSight mission. One evolution (shown in dashed lines) is for a rather high permeability k₀ = 9.1.10⁻¹⁰ m² and low reference viscosity η₀ = 4.5x10²⁰ Pa.s, while the second evolution (shown in solid lines) has a lower permeability k₀ = 3.72.10⁻¹¹ m² and a higher viscosity η₀ = 2.02x10²¹ Pa.s.

Acknowledgment :

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 101001689) and from the ANR (grant MAGIS, ANR-19-CE31-0008-08).

Bibliography :

Knapmeyer-Endrun, B., Panning, M. P., Bissig, F., Joshi, R., Khan, A., Kim, D., Leki ́c, V.,
Tauzin, B., Tharimena, S., Plasman, M., et al. (2021). Thickness and structure of the martian crust
from insight seismic data. Science, 373(6553):438–443.

Wieczorek, M. A., Broquet, A., McLennan, S. M., Rivoldini, A., Golombek, M.,
Antonangeli, D., Beghein, C., Giardini, D., Gudkova, T., Gyalay, S., et al. Insight constraints
on the global character of the martian crust. Journal of Geophysical Research: Planets, page
e2022JE007298.

Marinova, M. M., Aharonson, O., and Asphaug, E. (2008). Mega-impact formation of the mars
hemispheric dichotomy. Nature, 453(7199):1216–1219.

Yoshida, M. and Kageyama, A. (2006). Low-degree mantle convection with strongly temperature-
and depth-dependent viscosity in a three-dimensional spherical shell. Journal of Geophysical Re-
search: Solid Earth, 111(B3).

Thiriet, M., Michaut, C., Breuer, D., and Plesa, A.-C. (2018). Hemispheric dichotomy in litho-
sphere thickness on mars caused by differences in crustal structure and composition. Journal of
Geophysical Research: Planets, 123(4):823–848.

How to cite: Bonnet Gibet, V., Michaut, C., and Wieczorek, M.: A new mechanism for the formation of the Martian dichotomy, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-255, https://doi.org/10.5194/epsc2022-255, 2022.

Seismic measurements of the InSight lander confirm tectonic activity in an extraterrestrial geological system for the first time: the large graben system Cerberus Fossae (Giardini et al., 2020). In-depth analysis of available marsquakes thus allows unprecedented geophysical characterization of an active extensional structure on Mars, using the epicenter locations, depths, magnitudes, focal mechanisms and spectral character from marsquake data. In summary, InSight seismic data show:

• Both major families of marsquakes, characterized by low and high frequency content, LF and HF events respectively, can be located on central and eastern parts of the graben system (Zenhäusern et al., 2022). This is in agreement with the decrease in structural maturity towards the East as inferred from orbital images (Perrin et al., 2022). Specifically, we find that the distance distribution of the larger LF marsquakes peaks near Zunil crater and the Cerberus Mantling Unit, which has been hypothesized to be of volcanic origin (Horvath et al., 2021).
• The two event families correspond to two depth regimes: LF marsquake hypocenters are located at about 15-50 km, based on identification of depth phases (Durán et al., 2022; Stähler et al., 2021), while the HF marsquakes are likely much shallower and at 0-5 km depth (van Driel et al., 2021).
• Estimated magnitudes are between 2.8 and 3.8 (Böse et al., 2021; Clinton et al., 2021), resulting in a total seismic moment release within Cerberus Fossae of 1.4-5.6×10¹⁵ Nm/yr, or at least half of the observed seismic moment release of the entire planet.
• Estimated focal mechanisms of deep marsquakes (Brinkman et al., 2021; Jacob et al., 2022) show primarily extensional normal faulting, compatible with the image-based interpretation as a graben system.
• The deeper LF marsquakes are “slow” compared to terrestrial quakes, i.e. lack high frequency energy in the seismic body waves. This can be explained by low stress drop and a weak, potentially warm source region.

We propose a geological model that integrates these observations: The deep LF quakes are caused by the large-scale extensional stress pattern, while fractures occur in this specific location only due to the presence of a dike from Elysium Mons. The shallow seismicity is caused in a brittle region near the surface, potentially on the subsurface continuation of the graben flanks. This could potentially explain the seasonality of the HF event rate, which peaks at the times of maximum solar illumination of the bottom in the Cerberus Fossae (Knapmeyer et al., 2021). 

While a small number of large endogenic marsquakes have been observed in other regions on Mars, specifically Southern Tharsis (Horleston et al., 2022), Cerberus Fossae represents a uniquely active seismic setting. Current day tectonic activity seems to be driven by volcanic processes, and furthermore, we find no trace of seismic activity on compressional thrust faults on Mars, as opposed to the models of seismicity driven by secular cooling and lithospheric contraction.

References:

Böse, M., et al., 2021. Magnitude Scales for Marsquakes Calibrated from InSight Data. Bull. Seismol. Soc. Am. https://doi.org/10.1785/0120210045
Brinkman, N., et al., 2021. First focal mechanisms of marsquakes. J. Geophys. Res. Planets. https://doi.org/10.1029/2020je006546
Clinton, J.F., et al., 2021. The Marsquake catalogue from InSight, sols 0–478. Phys. Earth Planet. Inter. 310. https://doi.org/10.1016/j.pepi.2020.106595
Durán, C., et al., 2022. Seismology on Mars: An analysis of direct, reflected, and converted seismic body waves with implications for interior structure. Phys. Earth Planet. Inter. 325, 106851. https://doi.org/10.1016/j.pepi.2022.106851
Giardini, D., et al., 2020. The seismicity of Mars. Nat. Geosci. 13, 205–212. https://doi.org/10.1038/s41561-020-0539-8
Horleston, A., et al., 2022. The far side of Mars - two distant marsquakes detected by InSight. Seism. Rec. accepted.
Horvath, D.G., et al., 2021. Evidence for geologically recent explosive volcanism in Elysium Planitia, Mars. Icarus 365, 114499. https://doi.org/10.1016/j.icarus.2021.114499
Jacob, A., et al., 2022. Seismic sources of InSight marsquakes and seismotectonic context of Elysium Planitia, Mars. Tectonophysics in revision.
Knapmeyer, M., et al., 2021. Seasonal seismic activity on Mars. Earth Planet. Sci. Lett. 576, 117171. https://doi.org/10.1016/j.epsl.2021.117171
Perrin, C., et al., 2022. Geometry and Segmentation of Cerberus Fossae, Mars: Implications for Marsquake Properties. J. Geophys. Res. Planets 127, e2021JE007118. https://doi.org/10.1029/2021JE007118
Stähler, S.C., et al., 2021. Seismic detection of the martian core. Science 373, 443–448. https://doi.org/10.1126/science.abi7730
van Driel, M., et al., 2021. High-Frequency Seismic Events on Mars Observed by InSight. J. Geophys. Res. Planets 126, e2020JE006670. https://doi.org/10.1029/2020JE006670
Zenhäusern, G., et al., 2022. Low Frequency Marsquakes and Where to Find Them: Back Azimuth Determination Using a Polarization Analysis Approach. ArXiv220412959 Phys.

How to cite: Stähler, S. C., Mittelholz, A., Perrin, C., Kawamura, T., Kim, D., Knapmeyer, M., Zenhäusern, G., Clinton, J., Giardini, D., Logonné, P., and Banerdt, W. B.: Seismicity unveils tectonics in Cerberus Fossae, Mars, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1126, https://doi.org/10.5194/epsc2022-1126, 2022.

In August 2021, the first seismological determination of the core radius of Mars was published by the InSight team (Stähler et al., 2021). We take this opportunity to take a mental step backwards and assume a historical perspective on the scientific investigation of planetary cores, and how our knowledge about them, especially in terms of their size, evolved.

The first thoughts about the Earth's interior that we would place into the history of science or rather into that of religion originate in the 17th century. Descartes suggested that the Earth is a former star which produced so many sunspots that it became encrusted in them, and that later processing of sunspot-material resulted in the surface we have today. The innermost part of the Earth, however, is still unaltered solar matter.

Isaac Newton, in the posthumously published "System of the World", suggested that the gravity of a single, isolated mountain could be used to determine the density ratio between the surface and the interior of the Earth. Respective experiments were conducted by Bouguer and, later, by Maskelyne and Hutton - the latter concluded from the result, that the presence of a heavy, metallic core could explain the overall mass of the Earth as well as the density contrast resulting from Newton's experiment. A metallic core was also suggested by Wiechert, by the end of the 19th century. When Oldham demonstrated the S wave core shadow in 1906, he did not make any suggestions about the nature of the central region.

In the early 20th century, it was however doubted that any material could withstand the conditions of the deep interior, or that a segregation of metals could take place. One alternative approach was indeed solar matter, another one a metallic high pressure state of silicate rock: The possibility that the core mantle boundary is a phase boundary like the 410 and 660 km discontinuities was long supported by some.

The consequence of the phase boundary model was that neither the Moon nor Mars could have cores, for the simple reasons that they are too small to provide the necessary internal pressures. This claim fit well with the moment of inertia factors as they were observed back then: Until the mid-1960s it was assumed that the MoIF of the Moon exceeds 0.4, and that of Mars is too close to 0.4 to indicate much differentiation.

In both cases, the space age led to a revision: Having spacecraft near or at the respective bodies turned out to be crucial for a sufficiently precise determination of the MoIF.

In the case of Mars, the Mariner IV mission greatly improved the knowledge of radius, mass, and moment of inertia factor of the planet (because of the atmosphere, even the radius was rather uncertain and observational results depended on the optical wavelength used in photography). After Mariner IV, an iron core suddenly became feasible, if not necessary, again. Mariner VI and VII showed that Mars is neither a small Earth nor a big Moon, but something different - and the global photographic map resulting from the Mariner IX mission showed all the now familiar surface structures for the first time. With Mariner IX it also became possible to map the surface gravity, and the gravity anomaly of Tharsis was discovered - which is so enormous that it biases the J₂ gravity coefficient, and invalidates the previously used hypothesis of hydrostatic equilibrium. Several methods to compensate for this were suggested in the following. A replacement for the hydrostatic assumption became available with precession measurements using Viking radio signals, later augmented by Pathfinder and other missions. It became finally possible to determine the MoIF, which turned out to be significantly below that of the homogeneous sphere. The most significant progress in terms of the estimation of the core radius was however Mariner IX: After this mission, core radii below 1000 km were no longer discussed.

The Viking missions produced important clues for the identification of the SNC meteorites as of martian origin, and thus for improved models of the chemical composition of Mars. This provided better contraints for the densities of core and mantle. A comparison of the core radii discussed in the literature after Viking however shows that none of these models could constrain the core radius with a sufficient precision. Different models were developed, but in the long run, the range of uncertainty of the core radius proved rather stable for more than 40 years.

The results obtained by InSight still build upon the knowledge of geodetic and gravity measurements as well as on geochemistry, but they add seismic data as constraints that are more sensitive to the sought-after structural parameters than to density.

References

Listing all references relevant for the above text would require much more space than is available here. The discussion of the abstract is a condensate from Knapmeyer & Walterová (2022), where all the references can be found.

Knapmeyer, M., Walterová, M. (2022), Planetary Core Radii: From Plato towards PLATO, under review at Advances in Geophysics.

Stähler et al., (2021). Seismic detection of the martian core, Science, vol. 373, 443-448, DOI: 10.1126/science.abi7730

How to cite: Knapmeyer, M. and Walterová, M.: The core radius of Mars: a historical perspective, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-803, https://doi.org/10.5194/epsc2022-803, 2022.

Introduction

Wrinkle ridges are significant landforms on planetary bodies, and most of them occur in flood basalt units of large igneous provinces, [1]-[2]. On Mars, the circum-Tharsis wrinkle ridge system developed under compressional stresses associated with the response of the lithosphere due to the Tharsis volcanic load [1]. The morphology of ridges shows large variations and may reflect subsurface fault patterns [3]–[5]. Numerous studies on their physical dimensions [6]–[10], their accommodated horizontal strain (e.g., [11]–[12]), as well as a variety of conceptual formation models (e.g., [13]–[17]) have been performed to better understand the morphologies and geodynamic significance of wrinkle ridges. A variety of tectonic models including buckling, thrust/reverse faulting, fault-bend folding,  and fault propagation folding have been proposed to explain the formation of wrinkle ridges(e.g., [9]–[19]).

Even though there are many studies on wrinkle ridges, it is still uncertain what the subsurface of these structures looks like. To get insights into the subsurface we selected sites, where deep morphological incisions provide such exposures. Hence, we used steep escarpments formed by impact craters, collapse pits, and valleys. A prerequisite for this study is the availability of high-resolution remote sensing data and digital elevation models to investigate the fault patterns that exist in the subsurface of wrinkle ridges.  

Methodology

We used High-Resolution Imaging Science Experiment (HiRISE) (~0.25 m/px) [20], and Context Camera (CTX) (~6–7 m/px) [21] satellite imageries to generate high-resolution digital elevation models (DEMs) by using the Ames Stereo Pipeline [22] in combination with the Integrated System for Imagers and Spectrometers (ISIS) software [23]. CTX and HiRISE DEMs with the digital raster graphic (DRG) files were used to analyze and measure topographic offsets. We have selected twelve different study areas (with multiple outcrops from A to D) that all belong to the system of circum-Tharsis wrinkle ridges. Our area of interest includes regions at Solis Planum, at the borders of Nilus Dorsa, at the Coprates Chasma, at the south of Lunae Planum, and the Thaumasia Planum that shares significantly akin structures with a south of the Mela's Fossae (Fig. 1).