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The BepiColombo Laser Altimeter (BELA) is one of ten payloads on-board the Mercury Planetary Orbiter (MPO) on cruise to Mercury as part of the ESA and JAXA mission BepiColombo. BELA is a single beam laser altimeter operating at 1064 nm. The nominal pulse repetition is 10 Hz, while the pulse energy of each transmitted pulse is 50 mJ. The detector features a silicon avalanche photo diode (APD) with enhanced quantum efficiency for infrared radiation. The detected return pulse signal can be amplified using 16 predefined gain settings within the Analogue Electronics Unit (AEU). Finally, the amplified signal is digitized with a sampling frequency of 80 MHz, i.e. in samples of 12.5 ns, and is analysed by an on-board signal processing unit, the Range Finder Module (RFM). Within this pulse processing, nominal values for time of flight are determined for up to four alternative return pulse candidates. Additional parameters such as pulse width and pulse energy are obtained. Since launch in 2018 the receiver, power converter, and data processing modules of BELA were checked regularly twice a year. The instrument showed excellent performance and all tested parts are operating nominaly. However, the transmitter was not operated because the line-of-sight of the lasers is right in the direction of the BepiColombo transfer module. Hence, the instrument’s data acquisition and processing were tested with dark noise measurements. The expected power of the return pulse from Mercury’s surface will vary roughly between 1 and few hundred nW, depending on altitude and surface slope. In order to face this large variability, 16 gain settings with signal amplifications between 4 and 44 dB will be used for different phases within the orbit. The measurements during the cruise checkouts were used to characterize the noise of the receiver as a function of the commanded gain. By simulating realistic return pulse signals over one revolution about Mercury the commanded gain setting for each spacecraft altitude will be optimized with the ultimate goal of reaching the highest accuracy in the range measurement. Here we will report on the results of the first 5 BELA cruise checkouts, performed between 2019 and June 2021. BELA is a joint German–Swiss project with the participation of Spain. LML acknowledges financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award to the Instituto de Astrofısica de Andalucia (SEV-2017-0709) and from project PGC2018-099425-B-I00 (MCI/AEI/FEDER, UE).

In mantle convection studies, two-dimensional geometry calculations are predominantly used, due to their reduced computational costs when compared to full 3-D spherical shell models. Although various 3-D grid formulations [e.g. 1, 2] have been employed in studies using complex scenarios of thermal evolution [e.g., 3, 4], simulations with these geometries remain highly expensive in terms of computational power and thus 2-D geometries are still preferred in most of the exploratory studies involving broader ranges of parameters. However, these 2-D geometries still present drawbacks for modeling thermal convection. Although scaling and approximations can be applied to better match the average quantities obtained with 3D models [5], in particular, the convection pattern that in turn is critical to estimate melt production and distribution during the thermal evolution is difficult to reproduce with a 2D cylindrical geometry. In this scope, another 2D geometry called “spherical annulus” has been proposed by Hernlund and Tackley, 2008 [6]. Although steady state comparison between 2D cylindrical, spherical annulus and 3D geometry exist [6], so far no systematic study of the effects of these geometries in a thermal evolution scenario is available. In this study we implemented a 2-D spherical annulus geometry in the mantle convection code GAIA [7] and used it along 2-D cylindrical and 3-D geometries to model the thermal evolution of 3 terrestrial bodies, respectively Mercury, the Moon and Mars. First, we have performed steady state calculations in various geometries and compared the results obtained with GAIA with results from other mantle convection codes [6,8,9]. For this comparison we used several scenarios with increasing complexity in the Boussinesq approximation (BA).In a second step we run thermal evolution simulations for Mars, Mercury, and the Moon using GAIA with 2D scaled cylinder, spherical annulus and 3D spherical shell geometries.In this case we considered the extended Boussinesq approximation (EBA), an Arrhenius law for the viscosity, a variable thermal conductivity between the crust and the mantle, while taking into account the heat source decay and the cooling of the core, as appropriate for modeling the thermal evolution. A detailed comparison between all geometries and planets will be presented focussing on the convection pattern and melt production. In particular, we aim to determine which 2D geometry reproduces most accurately the results obtained in a 3D spherical shell model. Aknowledgments: The authors gratefully acknowledges the financial support and endorsement from the DLR Management Board Young Research Group Leader Program and the Executive Board Member for Space Research and Technology.References: [1] Kageyama and Sato, G3, 2004; [2] Hüttig and Stemmer, G3, 2008; [3] Crameri & Tackley, Progress Planet. Sci., 2016; [4] Plesa et al., GRL (2018); [5] Van Keken, PEPI, 2001; [6] Hernlund and Tackley, PEPI, 2008; [7] Hüttig et al, PEPI 2013; [8] Kronbichler et al., GJI, 2012; [9] Noack et al., INFOCOMP 2015.

Energy sources involved in the early stages of planetary formation can cause partial or even complete melting of the mantle of terrestrial bodies leading to the formation of magma oceans. Upon planetary cooling, solidification is expected to take place from the bottom upwards because of the steeper slope of the liquid adiabat with respect to the liquidus (Elkins-Tanton, 2012; Solomatov, 2015). Fractional solidification, in particular, can lead to the formation of a compositional layering that can play a fundamental role for the subsequent long-term dynamics and evolution of the interior (Tosi et al., 2013; Plesa et al., 2014). In order to assess to what extent primordial compositional heterogeneities generated upon magma ocean solidification can be preserved, we investigate the cooling and solidification of a whole-mantle magma ocean along with the conditions that allow solid-state convection to start mixing the mantle before solidification has completed. To this end, we run 2-D numerical simulations in cylindrical geometry using the finite-volume code GAIA (Huttig et al., 2013). We treat the liquid magma ocean in a parametrized fashion while we self-consistently solve the conservation equations of thermochemical convection in the growing solid mantle accounting for pressure-, temperature- and melt-dependent rheology. We consider two end-member cases: fractional crystallization, where melt is instantaneously extracted into the overlying liquid leaving beneath a differentiated mantle, and batch crystallization where melt remains in contact with the silicate matrix throughout solidification causing no differentiation. By testing the effects of different cooling rates and Rayleigh numbers, we show that for a lifetime of the liquid magma ocean between 1 and 10 Myr (Lebrun et al., 2013), the onset of solid state convection prior to complete mantle crystallization is possible and that part or even all of the compositional heterogeneities generated upon fractionation can be erased by efficient mantle stirring (Figure 1). We discuss the consequences of our findings in relation to the early and long-term evolution of compositional heterogeneities generated via fractional crystallization of magma oceans in terrestrial bodies with emphasis on Mars' thermochemical history.

The Ganymede Laser Altimeter (GALA) is one of the ten scientific instruments selected for the Jupiter Icy Moons Explorer (JUICE) mission currently implemented under responsibility of the European Space Agency (ESA). JUICE is scheduled for launch in mid 2022; arrival at Jupiter will be by end of 2029 with the nominal science mission—including close flybys at Ganymede, Europa, and Callisto and a Ganymede orbit phase—ending by mid 2033. GALA’s main objective is to obtain topographic data of the icy satellites of Jupiter: Europa, Ganymede, and Callisto. By measuring the diurnal tidal deformation of Ganymede, which crucially depends on the decoupling of the surface ice layer from the deep interior by a liquid water ocean, GALA will obtain evidence for (or against) a subsurface ocean in a 500 km orbit around the satellite and will provide constraints on Ganymede’s ice shell thickness. In combination with other instruments, it will characterize the morphology of surface units on Ganymede, Europa, and Callisto providing not only topography but also surface roughness and albedo (at 1064 nm) measurements. GALA is a single-beam laser altimeter operating with up to 50 Hz (nominal 30 Hz) shot frequency at a wavelength of 1064 nm and pulse lengths of 5.5 ± 2.5 ns using a Nd:YAG laser. The return pulse is detected by an Avalanche Photo Diode (APD) with 100 MHz bandwidth and is digitized at a sampling rate of 200 MHz providing range measurements with a subsample resolution of 0.1 m and surface roughness measurements from pulse-shape analysis on the scale of the footprint size of about 50 m at 500 km altitude. The instrument is developed in collaboration of institutes and industry from Germany, Japan, Switzerland, and Spain. © 2019, CEAS., Financial support was provided under grant 50 QJ 1401 on behalf of the DLR Space Administration by the German Bundesministerium fur Wirtschaft und Energie. This research has been supported by the Spanish Ministerio de Economia y Competitividad under Contract ESP 2016-76076-R.

Over the past years, large scale numerical simulations of planetary interiors have become an important tool to understand physical processes responsible for the surface features observed by various space missions visiting the terrestrial planets of our Solar System. Such large scale applications need to show good scalability on thousands of computational cores while handling a considerable amount of data that needs to be read from and stored to a file system. To this end, we analyzed numerous approaches to write files on the Cray XC40 Hazel Hen supercomputer. Our study shows that HPC applications parallelized using MPI highly benefit from utilizing the MPI I/O facilities. By implementing MPI I/O in Gaia, we improved the I/O performance up to a factor of 100. Additionally, in this study we present applications of the fluid flow solver Gaia using high resolution regional spherical shell grids to study the interior dynamics and thermal evolution of terrestrial bodies of our Solar System.

Thermo-chemical convection is the primary process that controls the large-scale dynamics of the mantle of the Earth and terrestrial planets. Its numerical simulation is one the principal tools for exploiting the constraints posed by geological and geochemical surface observations performed by planetary spacecrafts. In the present work, the authors discuss the modeling of active compositional fields in the framework of solid-state mantle convection using the cylindrical/spherical code Gaia. They compare an Eulerian method based on double-diffusive convection against a Lagrangian, particle-based method. Through a series of increasingly complex benchmark tests, the authors show the superiority of the particle method when it comes to model the advection of compositional interfaces with sharp density and viscosity contrasts. They finally apply this technique to simulate the Rayleigh-Taylor overturn of the Mars’ and Mercury’s primordial magma oceans.

Internally heated 3-D mantle convection models in a spherical shell with temperature and pressure dependent viscosity have been performed to provide new insights into the various convection regimes, the transition from steady state convection to time-dependent convection and the associated convection pattern. The analysis of a total of 91 simulations reveals four regime types, i.e., a mobile-lid regime, a sluggish regime, a low-degree regime, and a stagnant-lid regime. The occurrence of these regimes depends on the viscosity contrast and the internal Rayleigh number. The low-degree regime occurs close to the boundary of the stagnant-lid regime in case of temperature dependent viscosity. In case of additional pressure dependence, the range of the low-degree regime is smaller and a narrow range of the sluggish-lid regime exists in the weakly convecting part. Furthermore, the transition to the stagnant-lid regime occurs at a lower viscosity contrast. For the stagnant-lid regime we have derived a scaling law describing the heat transport. Similar scalings could not be obtained for the other regimes as this seems to require also a correlation of the convective pattern with the internal Rayleigh number. Such a relation is only given for the stagnant-lid regime in case of 3D spherical geometry. The stagnant-lid cases in steady state show a minimal possible degree of the convective pattern that is independent on the pressure dependence of viscosity and remains constant until time-dependent convection sets in with increasing Ra. At this stage, the dominant degree of the convective pattern increases with increasing internal Ra but the slope varies with the pressure dependence of the viscosity. Highlights: 1. Internally heated 3-D mantle convection models in a spherical shell with temperature and pressure dependent viscosity have been performed to provide new insights into the various convection regimes. 2. We were able to predict the pattern of convection (dominant degree) for high Rayleigh numbers in the stagnant-lid regime. 3. A case study of 91 3D simulations helped to identify a low-degree regime close to the border to the stagnant-lid regime. 4. We were able to determine the rheological constant through full inversion.