Alexander Stark

German Aerospace Center (DLR) | DLR · Department of Planetary Geodesy

We investigate the impact of viscoelastic tidal deformation of the Moon on the motion of a polar orbiter. The dissipative effects in the Moon’s interior, i.e., tidal phase lags, are modeled as Fourier series sampled at given frequencies associated with linear combinations of Delaunay arguments, the fundamental parameters describing the lunar motion...

We use the tidal deformations of the Moon induced by the Earth and the Sun as a tool for studying the inner structure of our satellite. Based on measurements of the degree-two tidal Love numbers k2 and h2 and dissipation coefficients from the GRAIL mission, Lunar Laser Ranging and Laser Altimetry on board of the LRO spacecraft, we perform Monte Car...

Plain Language Summary Due to its axial tilt, seasons also exist on Mars. Up to one third of the atmosphere's CO2 is in annual exchange with the polar regions through seasonal deposition/sublimation processes. Here, we make use of previously proposed approaches of analyzing the Mars Orbiter Laser Altimeter profiles and obtain spatio‐temporal level...

Plain Language Summary Each Martian year, up to one third of the atmosphere's CO2 is transported from pole to pole, being deposited and sublimated depending on the season. Accurate measurements of snow level and volume variations of the resulting seasonal polar caps can serve as crucial constraints on the Martian volatile cycles. In this study, we...

We search for stable orbits in the vicinity of the triple asteroid system, 2001-SN263, which will be target of the deep space mission ASTER, currently under study by the Brazilian space agency (AEB). Our numerical simulations include gravitational forces caused by the three bodies of 2001-SN263, higher-order terms of the primary’s gravity field, pl...

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...

Measurement of tidal deformation through self-registration of laser profiles: Application to Earth’s Moon

The Mars Orbiter Laser Altimeter (MOLA) Precision Experiment Data Records (PEDR) serve as the geodetic reference of Mars. However, these MOLA footprints were geolocated using outdated auxiliary information that dates back to 2003. In this study, we recompute the MOLA PEDR footprint locations and investigate the impact of the updated spacecraft orbi...

We investigate the feasibility and demonstrate the merits of using Mars Orbiter Laser Altimeter (MOLA) profiles to retrieve seasonal height variations of CO2 snow/ice cap in Mars’ polar areas by applying a co-registration strategy. We present a prototype analysis on the research region of [85.75°S, 86.25°S, 300°E, 330°E] that is located on the resi...

We propose the exploration of polar areas on Mars by a next-generation orbiter mission. In particular, we aim at studying the seasonal and regional variations in snow-deposits, which – in combination with measurements of temporal variations in rotation and gravity field – will improve models of the global planetary CO2 cycle. A monitoring of polar...

Low frequency radar sounders have the potential to generate altimetric profiles, but the feasibility of utilizing planetary radar sounding data as an alternative to laser altimetry has not been assessed using existing data to date. Therefore, we have developed, implemented, and evaluated an algorithm to process SHAllow RADar sounder (SHARAD) data o...

The Jupiter Icy Moons Explorer (JUICE) is a science mission led by the European Space Agency, being developed for launch in 2022. 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-sc...

Spaceborne synthetic aperture radar interferometry and tomography are well-established techniques for the exploration of terrestrial ice sheets. In the frame of DLR’s mission concept Enceladus Explorer (EnEx), these techniques are considered for providing topography, deformation, and composition measurements, as well as 3-D metric-resolution imagin...

In this White Paper, submitted to ESA in response to the Voyage 2050 Call, we advocate for establishing key scientific priorities for the future of Mercury exploration, including the development of specific science goals for a landed mission. We support the Mercury science community in fostering closer collaboration with ongoing and planned exoplan...

We investigate the feasibility and demonstrate the merits of using Mars Orbiter Laser Altimeter (MOLA) profiles to retrieve seasonal height variations of CO2 snow/ice cap in Mars' polar areas by applying a co-registration strategy. We present a prototype analysis on the research region of [85.75°S, 86.25°S, 300°E, 330°E] that is located on the resi...

Previously, we have proposed and validated the co-registration of local dynamic Mars Orbiter Laser Altimeter (MOLA) profile segments to static Digital Terrain Models (DTMs) as an approach for obtaining seasonal CO2 ice cover height variations on Mars. Building upon this, we introduce a post-correction procedure called "bi-RPCA" to further lower res...

Proposed method for an Io-Jupiter Passive radar deployment.

We assessed the impact of the uncertainties in the gravity field harmonic coefficients of Mercury on the orbital evolution of the Mercury Planetary Orbiter (MPO), part of the European-Japanese BepiColombo mission. We used in our simulations the most recent estimation of the gravity field model of Mercury determined from radio tracking data of the N...

In preparation for the ESA/JAXA BepiColombo mission to Mercury, thematic working groups had been established for coordinating the activities within the BepiColombo Science Working Team in specific fields. Here we describe the scientific goals of the Geodesy and Geophysics Working Group (GGWG) that aims at addressing fundamental questions regarding...

Laser altimeters are commonly used in planetary research for their high geodetic accuracy. A key procedure in processing of laser altimeter data is the geolocation. In this process, the time-of-flight measurements are converted to coordinates of laser pulse footprints on the surface of the target body. Here, we present a consistent and systematic f...

This work evaluates the added benefit of using laser altimeter measurements for orbit reconstruction. As a spacecraft orbits a celestial body, its altimetry swaths progressively cross previous swaths. These locations, known as crossover points, yield valuable information about the orbited body and the spacecraft trajectory. The mathematical express...

The BepiColombo Laser Altimeter (BELA) is the first European laser altimeter constructed for interplanetary flight. BELA uses a 50 mJ pulsed Nd:YAG laser operating at 10 Hz with a 20 cm aperture receiver to perform the ranging. The instrument also uses a digital approach for range detection and pulse analysis. The ranging accuracy is expected to be...

In cooperation with Russia, the Brazilian deep space mission ASTER plans to send a small spacecraft to investigate the triple asteroid 2001-SN263. The nearest launch opportunities for this project include June 2022 and June 2025. One main exploration campaign is being planned with focus on the largest asteroid (Alpha). Among the instruments under d...

We use global data from the Lunar Orbiter Laser Altimeter (LOLA) to retrieve the lunar tidal Love number h2 and find h2=0.0387±0.0025. This result is in agreement with previous estimates from laser altimetry using crossover points of LOLA profiles. The Love numbers k2 and h2 are key constraints on planetary interior models. We further develop and a...

Digital Terrain Models (DTMs) of Europa in Geotiff format. The stereo DTMs cover 7 regions of Europa based on Galileo images. Two regions (Conamara Chaos and Yelland) are covered with two different resolutions. For a description of the methodology see Giese et al. 1999. The geological regions are described in Steinbruegge et al. 2020. Please also s...

We propose the co-registration of local laser profile segments to high resolution Digital Terrain Models (DTMs) as an approach for obtaining seasonal CO 2 ice cover height variations on Mars. The co-registration is parameterized in instantaneous MOLA pointing angles involving a rigorous laser altimeter geolocation model. Thereby, the height bias of...

The BepiColombo Laser Altimeter (BELA) is on its way to Mercury on board the Mercury Planetary Orbiter (MPO), one of the two spacecraft of the BepiColombo mission. It will arrive at Mercury in Dec 2025 and start measurements of Mercury's surface and environment. The goal of this study is to analyze the performance of BELA by using a comprehensive i...

We assess the surface roughness of Europa by reevaluating Galileo stereo images. Roughness provides important information about surface textures, which are related to geologic processes operating at a variety of scales. Roughness also sets important performance requirements for various remote sensing instruments on board the upcoming icy satellite...

The BepiColombo mission, which is a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), was launched successfully on October 20, 2018 from Kourou, French Guyana. The spacecraft is currently on its 7 yr cruise to Mercury. The main science campaign at Mercury will begin, however, no earlier than spring...

Context. The Love number h2 describes the radial tidal displacements of Mercury’s surface and allows constraints to be set on the inner core size when combined with the potential Love number k2. Knowledge of Mercury’s inner core size is fundamental to gaining insights into the planet’s thermal evolution and dynamo working principle. The BepiColombo...

Having information on the global shape of a planet as well as having precise topography information is a prerequisite for many advanced scientific studies as well as for future robotic exploration. To date there are numerous methods to obtain topographic information including laser altimetry and stereo imaging. A currently less explored method is a...

Context. The Love number h2 describes the radial tidal displacements of Mercury’s surface and allows constraints to be set on the inner core size when combined with the potential Love number k2 . Knowledge of Mercury’s inner core size is fundamental to gaining insights into the planet’s thermal evolution and dynamo working principle. The BepiColomb...

We point out some errors in the most recent report from the International Astronomical Union (IAU) Working Group on Cartographic Coordinates and Rotational Elements (Archinal et al. 2018). We correct a sign error in Figs. 1 and 2. We also correct the equation for the prime meridian position (W) of Mars’ satellite Phobos in Table 2.

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...

The accurate measurements of seasonal and long-term Martian polar CO 2 frost elevation and volume changes can serve as important constraints in Mars climate models and can help tap into the density evolution of the CO 2 snow (Matsuo and Heki, 2009). The traditional approach to this problem is the cross-over analysis, but this method may suffer from...

We propose the co-registration of laser profiles to high resolution Digital Terrain Models (DTMs) as an approach for retrieving surface elevation changes at the poles of Mars. The edge of this method is validated using reprocessed and simulated Mars Orbiter Laser Altimeter (MOLA) profiles. The ultimate goal is to apply this method to MOLA and HRSC...

Many of the moons of the giant planets are trapped in a 1:1 spin-orbit resonance. Up to now this special rotation state is confirmed by observations for 8 satellites of Jupiter, 15 satellites of Saturn, 15 satellites of Uranus, and 7 satellites of Neptune[1]. These rotation states are a result from the strong tidal torque exerted on the satellites...

We analysed the influence of Mercurys gravity field on the orbit of the Mercury Planetary Orbiter (MPO) which is the part of the European-Japanese mission BepiColombo. The gravity field of Mercury was determined from radio tracking data of the NASAs spacecraft MESSENGER. Due to the highly eccentric orbit of MESSENGER, the calculated gravitational h...

We provide an overview of quality characteristics of the regional data products of the HRSC Mars Chart (HMC-30) series, and present new data on coordinate accuracy relative to the MOLA global reference, which were not available from previous HRSC DTMs. Finally, details of the HMC-30 global map layout and tiling scheme are reported.

Jupiter’s moon Ganymede might be in possession of a subsurface ocean located between two ice layers. However, from Galileo data it is not possible to unambiguously infer the thickness and densities of the individual layers. The upcoming icy satellite mission JUICE (JUpiter ICy moons Explorer) will have the possibility to perform more detailed inves...

Laser altimeters (LAs) have been extensively used in a wide range of planetary research [1,2,3,4,5]. Besides the peculiarities of the measure�ment process of an altimeter, the data reduction can be also rather involving, in particular, when relativistic effects are taken into account. This paper discusses the geoprocessing step, which converts the...

Laser altimeters (LAs) have been extensively used in a wide range of planetary research. Besides the peculiarities of the measurement process of an altimeter, the data reduction can be also rather involving, in particular, when relativistic effects are taken into account. This paper discusses the geoprocessing step, which converts the time of fligh...

We propose the co-registration of laser profiles to high resolution Digital Terrain Models (DTMs) as an approach for retrieving surface elevation changes at the poles of Mars. The edge of this method is validated using reprocessed and simulated Mars Orbiter Laser Altimeter (MOLA) profiles. The ultimate goal is to apply this method to MOLA and HRSC...

GALA (GAnymede Laser Altimeter) is one of the payload instruments of the JUICE (JUpiter ICy moons Explorer) project to be launched in 2022 to the Jovian icy moons Ganymede, Europa, and Callisto. GALA is developed through an international collaboration between Germany, Japan, Switzerland, and Spain. With the performance model of GALA, we have sought...

GALA (GAnymede Laser Altimeter) is one of the payload instruments of the JUICE (JUpiter ICy moons Explorer) project to be launched in 2022 to the Jovian icy moons Ganymede, Europa, and Callisto. GALA is developed through an international collaboration between Germany, Japan, Switzerland, and Spain. With the performance model of GALA, we have sought...

Laser altimetry is a powerful tool for addressing the major objectives of planetary physics and geodesy. Through measurements of distances between a spacecraft and the surface of the planetary bodies, it can be used to determine the global shape and radius: global, regional, and local topography: tidal deformation: and rotational states including p...

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...

We re-evaluated the surface roughness of Europa at scales between 30 m and 5 km using stereo-pair images collected by the Solid State Imager (SSI) aboard the Galileo mission with a resolution between 9 m and 255 m per pixel. These images have been adjusted relative to each other to obtain consistent mosaics used for geomorphological mapping. Overal...

We investigate the potential for the Ganymede Laser Altimeter (GALA) on board the JUICE (JUpiter ICy moons Explorer) mission to measure the tidal deformations and the rotational state of Ganymede, in particular physical librations. A subsurface ocean would cause the upper ice shell to librate decoupled from its interior. Two distinct iterative leas...

Thanks to the NASA MESSENGER mission, our understanding of the planet Mercury has never been greater, and the dual-spacecraft ESA–JAXA BepiColombo mission promises further breakthroughs in Mercury science. Yet there is only so much that can be accomplished from orbit. Here, we detail outstanding questions related to several aspects of Mercury’s ch...

We use co-registration techniques [1] to study rotation parameters of Mars. The technique involves the alignment of two topographic data sets in the three-dimensional space. The method is conceived to operate with the pair of data constituted by laser altimetry profiles (LA) and gridded Digital Terrain Models (DTMs). The transformation is performed...

We are carrying out an instrument study for a next-generation laser altimeter on a spacecraft orbiting Mars. The science focus is on the geodetic control and morphology of polar ice caps of the planet. We discuss the scientific goals and the technical requirements of the instrument as well as possible spacecraft orbits and mission scenarios.

We computed interior structure models of Mercury and analyzed their viscoelastic tidal response. The models are consistent with MErcury Surface, Space Environment, GEochemistry, and Ranging mission inferences of mean density, mean moment of inertia, moment of inertia of mantle and crust, and tidal Love number k2. Based on these constraints we predi...

We use data from the Lunar Orbiter Laser Altimeter (LOLA) to retrieve the h 2 tidal Love number of the Moon and find 0.040 < h 2 < 0.044. The applied algorithm solves simultaneously for h 2 and the shape of the Moon, parametrized using cubic splines on a global rectangular grid. The result can give valuable constraints for the Moon's interior and v...

We analyse images taken by the MErcury Surface, Space ENviorment, GEochemistry, and Ranging (MESSENGER) spacecraft for the camera’s thermal response in the harsh thermal environment near Mercury. Specifically, we study thermally induced variations in focal length of the Mercury Dual Imaging System (MDIS). Within the several hundreds of images of st...

We produce high-resolution (222 m/grid element) Digital Terrain Models (DTMs) for Mercury using stereo images from the MESSENGER orbital mission. We have developed a scheme to process large numbers, typically more than 6000, images by photogrammetric techniques, which include, multiple image matching, pyramid strategy, and bundle block adjustments....

We explore the perspectives of the BepiColombo Laser Altimeter (BELA) onboard ESA / JAXA's Mercury mission BepiColombo and present an updated semi-analytical instrument performance model, in which we estimate signal-to-noise ratio, single shot probability of false detection, range errors and the accuracy of pulse width reconstruction. The model is...

We present a co-registration technique which performs the alignment of two topographic data sets in the three-dimensional space. The method is conceived to operate with the pair of data constituted by laser altimetry profiles (LA) and gridded Digital Terrain Models (DTMs); the transformation is performed from the coordinates of the laser profile to...

Laser altimetry is a powerful tool for addressing the major objectives of planetary physics and geodesy, and have been applied in planetary explorations of the Moon, Mars, Mercury, and the asteroids Eros, and Itokawa. The JUpiter Icy Moons Explorer (JUICE), led by European Space Agency (ESA), has started development to explore the emergence of habi...