Onset of solid-state mantle convection and mixing during magma ocean solidification

©2017. American Geophysical UnionThe energy sources involved in the early stages of the formation of terrestrial bodies can induce partial or even complete melting of the mantle, leading to the emergence of magma oceans. The fractional crystallization of a magma ocean can cause the formation of a compositional layering that can play a fundamental role for the subsequent long‐term dynamics of the interior and for the evolution of geochemical reservoirs. In order to assess to what extent primordial compositional heterogeneities generated by magma ocean solidification can be preserved, we investigate the solidification of a whole‐mantle Martian magma ocean, and in particular the conditions that allow solid‐state convection to start mixing the mantle before solidification is completed. To this end, we performed 2‐D numerical simulations in a cylindrical geometry. We treat the liquid magma ocean in a parameterized way while we self‐consistently solve the conservation equations of thermochemical convection in the growing solid cumulates accounting for pressure‐, temperature‐, and, where it applies, melt‐dependent viscosity. By testing the effects of different cooling rates and convective vigor, we show that for a lifetime of the liquid magma ocean of 1 Myr or longer, the onset of solid‐state convection prior to complete mantle crystallization is likely and that a significant part of the compositional heterogeneities generated by fractionation can be erased by efficient mantle mixing. We discuss the consequences of our findings in relation to the formation and evolution of compositional reservoirs on Mars and on the other terrestrial bodies of the solar system.DFG, 276817549, Kristallisation des irdischen Magmaozeans: Thermo- und Geodynami

Scaling laws for internally heated mantle convection

Diese Arbeit stellt eine neue Methode vor, um Mantelkonvektion terrestrischer Planeten in einer 3D sphärischen Kugelschale zu simulieren. Die erforderlichen Differentialgleichungen werden dabei erstmalig mittels der Finiten -Volumen-Methode für irreguläre Voronoi-Gitter diskretisiert. Diese Diskretisierung (D) ist zweite Ordnung und voll implizit. Für mehr als 1000 CPUs ist die Simulation effizient parallelisiert. Damit wurde das Spiralgitter, eine neuartige Gitterstruktur mit lateral variierenden Auflösungen, untersucht. Weiterhin wurde ein bikonjugiertes Gradientenverfahren angewandt um das resultierende Gleichungssystem zu lösen. Die D. des Spannungstensors erlaubt eine Zell-Zell Variation der Viskosität von 8 und systemweit von 45 Größenordnungen. Um die Anwendbarkeit dieser neuen Methode zu demonstrieren, wurde eine Parameterstudie mit 88 Fällen gewählt, wobei Skalierungsgesetze für Wärmetransport, Grenzschichtendicken und Strukturkomplexität für planetare Mäntel ermittelt wurden. This work presents a new method to simulate mantle convection in a 3D spherical shell with fully spatially varying viscosities. The formulation of the governing equations is based on the finite-volume (FV) method for fully irregular grids using Voronoi-cells. The simulation code is efficiently parallelized for more than 1000 CPUs. A new irregular grid with varying lateral resolution, the spiral grid, was investigated. The discretization method is second-order accurate in space and time. The Krylov-subspace solver BiCGS with a Jacobi preconditioner is employed to solve the resulting system of equations. The discretization of the stress tensor can handle viscosity variations of up to 8 orders of magnitude from cell-to-cell and up to 45 orders of magnitude system wide. As an application to purely internally heated mantle convection in a spherical shell, a parameter study of 88 cases is carried out to derive scaling laws for heat transport, stagnant-lid thickness and structural complexity

Noise Performance of the Ganymed Laser Altimeter Receiver

We report here on qualification and calibration test results demonstrating the science performance of the Avalanche Photodiode (APD) detector of the Ganymede Laser Altimeter. Most importantly, the measures for noise reduction of the analogue receiver electronics are described. This includes proper grounding of the APD module (Excelitas C30654) and filtering in its vicinity and the additional measures to isolate switching noise of the analogue-to-digital convertors (ADCs) from the sensitive single-ended analogue sensor pre-amplifier electronics of the APD

Miniaturized Laser Altimeter for Small Satellite Applications

Laser Altimetry is a powerful tool to create absolutely calibrated digital terrain maps of planetary surfaces, to analyze their surface geology, and to get insight into the interior structure of planetary bodies by measuring tidal elevations and libration amplitudes and frequencies. The recent ESA missions BepiColombo and the Jupiter Icy Moons Explorer (JUICE) carry the first European laser altimeter instruments, i.e., the BepiColombo Laser Altimeter (BELA) and the Ganymede Laser Altimeter (GALA), the latter of which has a strong contribution from JAXA teams. The measurement principle of a laser altimeter is very simple. It is based on the time-of-flight measurement of an optical pulse. BELA, which is now on the way to Mercury orbit, applies a diode-laser pumped Nd:YAG laser sending pulses with an energy of 50 mJ, a width of about 5 ns, and a repetition rate of 10 Hz. Over typical ranging distances of 400 km to more than 1000 km, the BELA telescope collects pulses with a few hundred photons and a width of about 25 ns where the time of arrival gives the mean topographic altitude of the area illuminated by the 5 to 40 m diameter laser beam. The return pulse width further gives information on slope and roughness within this area. GALA is a similar instrument with 17 mJ pulse energy but 30 Hz repetition rate and was launched in April 2023 to enter the Jovian system after a eight-year cruise to fly-by at Europa and Callisto and finally orbit the Jovian moon Ganymede at an altitude of about 500 km above its icy surface. BELA and GALA are instruments that consume about 50 W and have a mass of close to 15 kg and 25 kg, respectively. The instrument dimensions are largely determined by the telescope diameter of about 30 cm. In order to enable the use of these instruments on small satellites the size, weight and power (SWaP) budgets need to be drastically reduced. This can be achieved by deriving the time-of-flight information from just a single return photon. The reduction factor of about 100 in the detected photon number can be shared by a reduction in laser energy and a reduction of telescope aperture diameter. Our aim is to reduce laser pulse energy from 17 mJ to 1 mJ and telescope diameter from 22 cm (for GALA) to 8 cm which implies in total a reduction factor about 130. GALA typically detects 700 photons per pulse at an altitude of 500 km which leads to about 5 photons to be analyzed per event by a single photon detection laser altimeter. The major challenges for a single photon detection laser altimeter are the reduction of the background photon rate by reducing the field-of-view of the telescope as well as better spectral filtering. We present first results from a conceptual experimental study of such a system designed for use on small satellites applying a newly developed detection scheme using a Single Photon Avalanche Diode (SPAD) and a diode-laser pumped microchip Nd:YAG laser emitting 1 mJ pulses with a pulse width of 1 ns. The reductions in dimension, mass, and power consumption of this instrument are discussed, and the scientific performance is simulated based on first experimental results. The feasibility of accommodating the instrument on the modular TUBiX20 microsatellite platform developed by Technische Universität Berlin is explored and the necessary requirements for attitude and orbit determination and control as well as SWaP budgets are derived

The BepiColombo Laser Altimeter (BELA) during Near-Earth Commissioning Phase (NECP)

The ESA/JAXA joint mission BepiColombo to Mercury was launched successfully on October 20, 2018 (UTC) from Kourou, French Guiana. Currently BepiColombo is on its nominal 7-years cruise to the innermost planet. BepiColombo consists of two spacecraft, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO), both targeted for different orbits around Mercury after arrival in December 2025. The BepiColombo Laser Altimeter (BELA) is one of ten payloads on the MPO. After launch the spacecraft and the instruments entered the Near-Earth Commissioning Phase (NECP), including a first switch-on of BELA on November24, 2018. Here we report on the status of the instrument based on the analysis of NECP data and on data from a second switch-on, planned for June 2019

The Ganymede Laser Altimeter (GALA) for the Jupiter Icy Moons Explorer (JUICE): Mission, science, and instrumentation of its receiver modules

The Jupiter Icy Moons Explorer (JUICE) is a science mission led by the European Space Agency, being developed for launch in 2023. The Ganymede Laser Altimeter (GALA) is an instrument onboard JUICE, whose main scientific goals are to understand ice tectonics based on topographic data, the subsurface structure by measuring tidal response, and small-scale roughness and albedo of the surface. In addition, from the perspective of astrobiology, it is imperative to study the subsurface ocean scientifically. The development of GALA has proceeded through an international collaboration between Germany (the lead), Japan, Switzerland, and Spain. Within this framework, the Japanese team (GALA-J) is responsible for developing three receiver modules: the Backend Optics (BEO), the Focal Plane Assembly (FPA), and the Analog Electronics Module (AEM). Like the German team, GALA-J also developed software to simulate the performance of the entire GALA system (performance model). In July 2020, the Proto-Flight Models of BEO, FPA, and AEM were delivered from Japan to Germany. This paper presents an overview of JUICE/GALA and its scientific objectives and describes the instrumentation, mainly focusing on Japan’s contribution