Matthias Grott

A spherical harmonic model of the lithospheric magnetic field of Mars

We present a model of the lithospheric magnetic field of Mars which is based on Mars Global Surveyor orbiting satellite data and represented by an expansion of spherical harmonic (SH) functions up to degree and order 110. Several techniques were applied in order to obtain a reliable and well-resolved model of the Martian lithospheric magnetic field: A modified Huber-Norm was used to properly treat data outliers, the mapping phase orbit data was weighted based on an a priori analysis of the data, and static external fields were treated by a joint inversion of external and internal fields. Further, temporal variabilities in the data which lead to unrealistically strong anomalies were considered as noise and handled by additionally minimizing a measure of the horizontal gradient of the vertically down internal field component at surface altitude. Here we use an iteratively reweighted least squares algorithm to approach an absolute measure (L1 norm), allowing for a better representation of strong localized magnetic anomalies as compared to the conventional least squares measure (L2 norm). The resulting model reproduces all known characteristics of the Martian lithospheric field and shows a rich level of detail. It is characterized by a low level of noise and robust when downward continued to the surface. We show how these properties can help to improve the knowledge of the Martian past and present magnetic field by investigating magnetic signatures associated with impacts and volcanoes. Additionally, we present some previously undescribed isolated anomalies, which can be used to determine paleopole positions and magnetization strengths.

Thermal and mechanical properties of the near-surface layers of comet 67P/Churyumov-Gerasimenko

Thermal and mechanical material properties determine comet evolution and even solar system formation because comets are considered remnant volatile-rich planetesimals. Using data from the Multipurpose Sensors for Surface and Sub-Surface Science (MUPUS) instrument package gathered at the Philae landing site Abydos on comet 67P/Churyumov-Gerasimenko, we found the diurnal temperature to vary between 90 and 130 K. The surface emissivity was 0.97, and the local thermal inertia was 85 ± 35 J m−2 K−1s-1/2. The MUPUS thermal probe did not fully penetrate the near-surface layers, suggesting a local resistance of the ground to penetration of >4 megapascals, equivalent to >2 megapascal uniaxial compressive strength. A sintered near-surface microporous dust-ice layer with a porosity of 30 to 65% is consistent with the data.

Thermal evolution and Urey ratio of Mars

The upcoming InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission, to be launched in 2016, will carry out the first in situ Martian heat flux measurement, thereby providing an important baseline to constrain the present-day heat budget of the planet and, in turn, the thermal and chemical evolution of its interior. The surface heat flux can be used to constrain the amount of heat-producing elements present in the interior if the Urey ratio (Ur)—the planets heat production rate divided by heat loss—is known. We used numerical simulations of mantle convection to model the thermal evolution of Mars and determine the present-day Urey ratio for a variety of models and parameters. We found that Ur is mainly sensitive to the efficiency of mantle cooling, which is associated with the temperature dependence of the viscosity (thermostat effect), and to the abundance of long-lived radiogenic isotopes. If the thermostat effect is efficient, as expected for the Martian mantle, assuming typical solar system values for the thorium-uranium ratio and a bulk thorium concentration, simulations show that the present-day Urey ratio is approximately constant, independent of model parameters. Together with an estimate of the average surface heat flux as determined by InSight, models of the amount of heat-producing elements present in the primitive mantle can be constrained.

Paleopole Reconstruction of Martian Magnetic Field Anomalies

Introduction: Investigating a planets magnetic paleopole position can reveal important information on events like polar reversals or true polar wander (TPW). A variety of investigations have been performed [1,2,3,4,5] usually reporting the best fitting, or a cluster of paleopole positions. These investigations indicate that analyzing the same anomaly using different assumptions can lead to different conclusions for the paleopole positions associated with the underlying sources [5]. To address this issue we applied the method developed by [6] which has the benefit that no assumptions concerning the geometry of the magnetic source are necessary. In addition, this method provides a measure of misfit for the calculated paleopole position and a confidence limit can be defined to determine an area of admissible paleopole locations. Five crustal magnetic field anomalies will be discussed here. One is the Australe Montes anomaly which has been investigated by [4], four of them are isolated anomalies identified by [7]. They will be denoted as follows: The four anomalies from the publication of [7] will be denoted A1, A2, A3, and A4. They are located at 52°S / 2.5°W, 64°S / 28°E, 57°N / 167°E, and 49.5°N / 169°E, respectively. The Australe Montes anomaly is located at 81°S / 23.4°E and will be denoted A. Montes. Method: To apply the method of [6], isolated crustal magnetic field anomalies are chosen. Here an isolated anomaly is defined by the absence of a surrounding magnetic field from sources outside the anomaly itself. Further, it is assumed that the anomalys magnetization has been acquired during a geologically short period within a constant main magnetic field, leading to an anomaly with uniform magnetic orientation [6]. To calculate a paleopole position, a number of N equally spaced dipoles with uniform orientation are distributed within the radius R0 (Fig. 1 / red circle) [6] on the Martian surface. In the same way a distribution of N observation points inside the radius R1 (Fig.1 / black circle), with R0 < R1, is generated and the downward component of the magnetic field is determined from a magnetic field model at 120 km altitude. Here we use the spherical harmonic model up to degree and order 110 by [7] calculated from the entire Mars Global Surveyor (MGS) data set. Because the magnetic orientation is set a priori, the remaining unknowns are the N magnetization strengths Mi of the N dipoles. Since it is assumed that Mi ≥ 0, Mi is calculated using a non negative least square fit algorithm [8], taking only Bz into account. From Mi, a forward model of the magnetic model field can be calculated and the residuals and standard deviation between the model and the spherical harmonic magnetic field can be determined (Fig. 1). The repetition of this calculation for all possible magnetic orientations in steps of 1° in inclination and 2° in declination leads to a distribution of standard deviations for the different magnetic orientations. The paleopole position of every forward model can then be calculated from the magnetization orientation unit vectors using standard coordinate transformations [9] that take the location of the anomaly into account. Here we adopt the convention that the paleopole location is defined as the south magnetic pole [9].

Constraining the date of the Martian dynamo shutdown by means of crater magnetization signatures

Mars is characterized by a strong crustal magnetic field, particularly over its southern hemisphere, which is believed to be the remnant of an ancient core dynamo. The dynamo ceased operating approximately 4 Ga ago, although the exact time is still under debate. The scope of this study is to introduce constraints on the possible timing of its cessation by studying the magnetization signatures over some craters which have reliable crater retention ages and are large enough for the impact to have reset the crustal magnetization within.

Mars’ Crustal Magnetic Field

Fossil magnetic fields within the Martian crust record the history of the planet’s ancient dynamo and hence retain valuable information on the thermal and chemical evolution of Mars. In order to decode this information, we have derived a spherical harmonic model of the crustal magnetic field. This model was derived from satellite vector magnetometer data, and allows to study the crustal magnetic field at high resolution down to surface altitudes. Based on this model, we calculate the required magnetization of the Martian crust, and discuss how the resulting strong magnetization might be explained. Further, we study the magnetization of impact craters and volcanoes, and conclude that the Martian core dynamo shut down most probably in the Noachian, at about 4.1 Gyr ago. Finally, we address the derivation of magnetic paleopole positions. In a first step, we use synthetic tests in order to outline under which constraints paleopole positions can be determined from satellite measurements. In a second step, we use these insights to present a scheme to estimate paleopole positions including an assessment of their underlying uncertainties.

Present‐Day Mars' Seismicity Predicted From 3‐D Thermal Evolution Models of Interior Dynamics

The Interior Exploration using Seismic Investigations, Geodesy and Heat Transport mission, to be launched in 2018, will perform a comprehensive geophysical investigation of Mars in situ. The Seismic Experiment for Interior Structure package aims to detect global and regional seismic events and in turn offer constraints on core size, crustal thickness, and core, mantle, and crustal composition. In this study, we estimate the present-day amount and distribution of seismicity using 3-D numerical thermal evolution models of Mars, taking into account contributions from convective stresses as well as from stresses associated with cooling and planetary contraction. Defining the seismogenic lithosphere by an isotherm and assuming two end-member cases of 573 K and the 1073 K, we determine the seismogenic lithosphere thickness. Assuming a seismic efficiency between 0.025 and 1, this thickness is used to estimate the total annual seismic moment budget, and our models show values between 5.7 × 1016 and 3.9 × 1019 Nm.

HP3-RAD: A compact radiometer design with on-site calibration for in-situ exploration

Many processes on planetary bodies are driven by their respective surface energy balance, and while planetary climate is influenced by the dynamics of the atmospheric boundary layer, surface radiation drives the Yarkovksy and YORB effects on small airless bodies. In addition, insolation governs cometary activity and drives the dust cycle on Mars. The radiative flux received and emitted at the surface of solar system bodies is thus a fundamental quantity, which is driven by the reception of solar radiation in the visible wavelength band, while re-radiation primarily occurs in the thermal infrared. Knowledge of the relevant radiative fluxes enables studies of thermo-physical surface properties, and radiometers to measure surface brightness temperatures have been payloads on many missions. The HP3-RAD is part of the Heat Flow and Physical Properties Package (HP3) on the InSight mission to Mars. It is a light-weight thermal infrared radiometer with compact design. HP3-RAD measures radiative flux in 3 spectral bands using thermopile detectors. The 120 g device includes integrated front-end electronics as well as a deployable cover that protects the sensors from dust contamination during landing. In addition, the cover is simultaneously used as a calibration target. The instrument concept as well as its implementation will be described, and special emphasis will be put on technological challenges encountered during instrument development. Potential future improvements of the design will be discussed.

Interior structure models of terrestrial exoplanets and application to CoRoT-7 b

In this study, we model the internal structure of CoRoT-7b as a type example for a terrestrial extrasolar planet using mass and energy balance constraints. Our results suggest that the deep interior is predominantly composed of dry silicate rock, similar to the Earth’s Moon. A central iron core, if present, would be relatively small and less massive (< 15 wt.% of the planet’s total mass) as compared to the Earth’s (core mass fraction 32.6 wt.%). Furthermore, a partly molten near-surface magma ocean could be maintained, provided surface temperatures were sufficiently high and the rock component was mainly composed of Earth-like mineral phase assemblages.

Mercury's low-degree geoid and topography controlled by insolation-driven elastic deformation

Mercury experiences an uneven insolation that leads to significant latitudinal and longitudinal variations of its surface temperature. These variations, which are predominantly of spherical harmonic degrees 2 and 4, propagate to depth, imposing a long-wavelength thermal perturbation throughout the mantle. We computed the accompanying density distribution and used it to calculate the mechanical and gravitational response of a spherical elastic shell overlying a quasi-hydrostatic mantle. We then compared the resulting geoid and surface deformation at degrees 2 and 4 with Mercurys geoid and topography derived from the MErcury, Surface, Space ENvironment, GEochemistry, and Ranging spacecraft. More than 95% of the data can be accounted for if the thickness of the elastic lithosphere were between 110 and 180 km when the thermal anomaly was imposed. The obtained elastic thickness implies that Mercury became locked into its present 3:2 spin orbit resonance later than about 1 Gyr after planetary formation.