Jürgen Oberst

Flood Volcanism in the Northern High Latitudes of Mercury Revealed by MESSENGER

MESSENGER observations of Mercury’s high northern latitudes reveal a contiguous area of volcanic smooth plains covering more than ~6% of the surface that were emplaced in a flood lava mode, consistent with average crustal compositions broadly similar to terrestrial komatiites. MESSENGER observations from Mercury orbit reveal that a large contiguous expanse of smooth plains covers much of Mercury’s high northern latitudes and occupies more than 6% of the planet’s surface area. These plains are smooth, embay other landforms, are distinct in color, show several flow features, and partially or completely bury impact craters, the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement. These characteristics, as well as associated features, interpreted to have formed by thermal erosion, indicate emplacement in a flood-basalt style, consistent with x-ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites. The plains formed after the Caloris impact basin, confirming that volcanism was a globally extensive process in Mercury’s post–heavy bombardment era.

Topography of the Northern Hemisphere of Mercury from MESSENGER Laser Altimetry

Mercury Inside and Out The MESSENGER spacecraft orbiting Mercury has been in a ∼12-hour eccentric, near-polar orbit since 18 March 2011 (see the Perspective by McKinnon). Smith et al. (p. 214, published online 21 March) present the most recent determination of Mercurys gravity field, based on radio tracking of the MESSENGER spacecraft between 18 March and 23 August 2011. The results point to an interior structure that differs from those of the other terrestrial planets: the density of the planets solid outer shell suggests the existence of a deep reservoir of high-density material, possibly an Fe-S layer. Zuber et al. (p. 217, published online 21 March) used data obtained by the MESSENGER laser altimeter through to 24 October 2011 to build a topographic map of Mercurys northern hemisphere. The map shows less variation in elevation, compared with Mars or the Moon, and its features add to the body of evidence that Mercury has sustained geophysical activity for much of its history. Mercury’s topography indicates sustained geophysical activity for most of the planet’s geological history. Laser altimetry by the MESSENGER spacecraft has yielded a topographic model of the northern hemisphere of Mercury. The dynamic range of elevations is considerably smaller than those of Mars or the Moon. The most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains. Within this lowland is a broad topographic rise that experienced uplift after plains emplacement. The interior of the 1500-km-diameter Caloris impact basin has been modified so that part of the basin floor now stands higher than the rim. The elevated portion of the floor of Caloris appears to be part of a quasi-linear rise that extends for approximately half the planetary circumference at mid-latitudes. Collectively, these features imply that long-wavelength changes to Mercury’s topography occurred after the earliest phases of the planet’s geological history.

Mars Express HRSC Data Processing - Methods and Operational Aspects

Automated procedures for ground processing of Mars Express HRSC data have been developed and are applied systematically immediately after download in order to provide calibrated data sets as well as photogrammetric image and 3D data products within a short time frame. Multi-spectral orthoimages in scales of 10 - 50 m/pixel and digital terrain models of 200 m raster width are generated within days even for large orbits covering areas of several hundred thousand square kilometers. An even higher image resolution of up to 2.3 m/pixel provided by HRSCs Super Resolution Channel (SRC) extends the potential of the HRSC camera experiment.

Derivation and validation of high-resolution digital terrain models from mars express HRSC data.

The High Resolution Stereo Camera (HRSC) onboard the Mars Express mission is the first photogrammetric stereo sensor system employed for planetary remote sensing. The derivation of high-quality digital terrain models is subject to a variety of parameters, some of which show a significant variability between and also within individual datasets. Therefore, adaptive processing techniques and the use of efficient quality parameters for controlling automated processing are considered to be key requirements for DTM generation. We present the general procedure for the derivation of HRSC high-resolution DTM, representing the core element of the systematic derivation of high-level data products by the Mars Express HRSC experiment team. We also analyze test series applying specific processing variations, including a new method for signal adaptive image preprocessing. The results are assessed based on internal quality measures and compared to external terrain data. Sub-pixel scale 3D point accuracy of better than 10 m and a DTM spatial resolution of up to 50 m can be achieved for large parts of the surface of Mars within a reasonable effort. This confirms the potentials of the applied along-track multiple stereo imaging principle and allows for a considerable improvement in our knowledge of the topography of Mars.

Laser altimeter observations from MESSENGER's first Mercury flyby.

A 3200-kilometers-long profile of Mercury by the Mercury Laser Altimeter on the MESSENGER spacecraft spans ∼20% of the near-equatorial region of the planet. Topography along the profile is characterized by a 5.2-kilometer dynamic range and 930-meter root-mean-square roughness. At long wavelengths, topography slopes eastward by 0.02°, implying a variation of equatorial shape that is at least partially compensated. Sampled craters on Mercury are shallower than their counterparts on the Moon, at least in part the result of Mercurys higher gravity. Crater floors vary in roughness and slope, implying complex modification over a range of length scales.

The Mars Pathfinder landing site and the Viking control point network

We recomputed the Viking Mars control point network and derived Mars-fixed three-dimensional coordinates of 3739 selected, globally distributed landmarks. Mars Pathfinder and Viking Lander 1 coordinates as well as data from reanalyzed spacecraft occultation measurements are used as fixed control information in the analysis. The network benefits greatly from recently restored Viking orbiter trajectory data and new planet rotational parameters derived from Pathfinder lander tracking. In our attempt to predict the coordinates of the Viking Lander 1 site from given Pathfinder coordinates, we find an offset of ≈4–5 km of this lander location with respect to the previously reported data. We suggest that this probably is explained by erroneous identification of this landing site in the images. Ninety percent of our landmark coordinates have positional errors of less than 1800 m. This network thus removes previously noted severe offsets between Mars-fixed coordinates and cartographic products.

High stress shallow moonquakes: evidence for an initially totally molten moon

Abstract Thermoelastic stress calculations show that if the moon was initially molten only in the outer few hundred kilometers as in the magma ocean model of the moon, the highlands crust should be aseismic. In contrast, if the moon was initially totally molten, high stress (1 to ≳ 3 kbar), shallow (0 to ∼ 6 km deep), compressional moonquakes should be occurring in the highlands crust. Calculations of the minimum stress drops made for the 28 observed shallow moonquakes suggest that 3 of them probably have stress drops in the kbar range. Thus, these very limited seismic data are consistent with the model that the moon was initially totally molten.

Contribution of Lunar Laser Ranging to Realise Geodetic Reference Systems

Lunar Laser Ranging (LLR) has provided observations for more than 36 years. There is enormous science potential of LLR to further our understanding of the dynamics of the Earth-Moon system (e.g.Earth orientation parameters (EOP) or the secular increase of the Earth-Moon distance: 3.8 cm/year) and to determine relativistic quantities such as the equivalence principle or any time variation of the gravitational constant. Here, we discuss the potential of LLR to contribute to the realisation of various reference systems, i.e. the terrestrial and selenocentric frame, but also thedynamic realisation of the celestial reference system, where most benefit is obtained from the long-term stability of the lunar orbit. Because of the tight link budget, only a handful terrestrial laser ranging stations are capable to routinely carry out the distance measurements (at cm level of precision). Therefore, we propose a next-generation lunar ranging experiment. Lunar landers shall deploy laser ’beacons’pointing at Earth.We estimate that the received pulse strength froma50 mJ Laser is 3 orders of magnitude larger than at classical LLR. Such laser shots could be received by most existing Satellite Laser Ranging (SLR) stations and measurement accuracies at mm level can then be accomplished. The contribution to the realisations of the aforementioned geodetic reference systems could be further improved.If in addition radio transponders were deployed at the same locations, a strong tie to the kinematic VLBI system couldbe established.

The Mapping Performance of the HRSC/SRC in Mars Orbit

The images obtained by the HRSC (High Resolution Stereo Camera) on Mars Express show excellent potential for topographic mapping of the planet. The derived stereo models agree with topographic data obtained earlier by MOLA (Mars Orbiter Laser Altimeter) on the Mars Global Surveyor: For the image scenes from the Mars Express commissioning phase that were studies, we find absolute difference in heights as small as 50 m and laterial positional differences along MOLA tracks of about 100 m. We show that HRSC effectively fills the gap between the MOLA tracks. SRC (Super Resolution Channel) images as well placed at their nominal geometric positions and reveal further detail within the HRSC context images. However, many of the images, fall short of the expected image quality for reasons to be examined.

The shape of Mars before Global Surveyor: Results from reanalysis of the Viking control point network

Three-dimensional coordinates of 3739 globally distributed control points derived from photogrammetric analysis of Viking Orbiter image data are studied with respect to the shape and large-scale morphology of Mars. Spheres, spheroids, spherical functions, and a Digital Terrain Model (DTM) of 50 km grid spacing are fitted to the data. Prominent topographic features in the terrain model include the Tharsis volcanoes, Olympus Mons, Alba Patera and prominent mare and highland regions as well as the major impact basins. While the global dichotomy is clearly visible in our data, our model does not reveal any distinct signature associated with the presumed dichotomy boundary at this global scale. Rather, it suggests a smooth topographic transition from southern to northern hemisphere. This model is unique among existing topographic data sets for Mars, as it combines global coverage, a spatial resolution sufficient to resolve regional topography, and absolute elevation data more accurate than in previous control point network analyses. Formal errors and a comparison with the first released topographic profiles obtained by Mars Global Surveyors Laser Altimeter (MOLA) suggest that 70% and 90% of the DTM grid elements represent the topography of the planet to better than 1000 m and 2000 m, respectively.