Insights into the subsurface structure of the Caloris basin, Mercury, from assessments of mechanical layering and changes in long-wavelength topography

The volcanic plains that fill the Caloris basin, the largest recognized impact basin on Mercury, are deformed by many graben and wrinkle ridges, among which the multitude of radial graben of Pantheon Fossae allow us to resolve variations in the depth extent of associated faulting. Displacement profiles and displacement-to-length scaling both indicate that faults near the basin center are confined to a ~ 4-km-thick mechanical layer, whereas faults far from the center penetrate more deeply. The fault scaling also indicates that the graben formed in mechanically strong material, which we identify with dry basalt-like plains. These plains were also affected by changes in long-wavelength topography, including undulations with wavelengths of up to 1300 km and amplitudes of 2.5 to 3 km. Geographic correlation of the depth extent of faulting with topographic variations allows a first-order interpretation of the subsurface structure and mechanical stratigraphy in the basin. Further, crosscutting and superposition relationships among plains, faults, craters, and topography indicate that development of long-wavelength topographic variations followed plains emplacement, faulting, and much of the cratering within the Caloris basin. As several examples of these topographic undulations are also found outside the basin, our results on the scale, structural style, and relative timing of the topographic changes have regional applicability and may be the surface expression of global-scale interior processes on Mercury

Insights into the subsurface structure of the Caloris basin, Mercury, from assessments of mechanical layering and changes in long-wavelength topography

The volcanic plains that fill the Caloris basin, the largest recognized impact basin on Mercury, are deformed by many graben and wrinkle ridges, among which the multitude of radial graben of Pantheon Fossae allow us to resolve variations in the depth extent of associated faulting. Displacement profiles and displacement-to-length scaling both indicate that faults near the basin center are confined to a ~ 4-km-thick mechanical layer, whereas faults far from the center penetrate more deeply. The fault scaling also indicates that the graben formed in mechanically strong material, which we identify with dry basalt-like plains. These plains were also affected by changes in long-wavelength topography, including undulations with wavelengths of up to 1300 km and amplitudes of 2.5 to 3 km. Geographic correlation of the depth extent of faulting with topographic variations allows a first-order interpretation of the subsurface structure and mechanical stratigraphy in the basin. Further, crosscutting and superposition relationships among plains, faults, craters, and topography indicate that development of long-wavelength topographic variations followed plains emplacement, faulting, and much of the cratering within the Caloris basin. As several examples of these topographic undulations are also found outside the basin, our results on the scale, structural style, and relative timing of the topographic changes have regional applicability and may be the surface expression of global-scale interior processes on Mercury

On the origin of mascon basins on the Moon (and beyond)

Mascon basins on the Moon are large craters that display significant positive free-air and Bouguer gravity anomalies. An important question is why is not every large crater a mascon, as less than half have been previously determined to be. We detrend the free-air, topographic, and Bouguer gravity anomalies and find that most large basins (28 of 41) display mascon characteristics (e. g., strong positive Bouguer anomalies narrower than the surface rims). Negative free-air gravity annuli surrounding the central highs generally are absent in the Bouguer gravity, implicating surface topography. We propose that beneath a forming large basin, the relatively narrow transient crater drives mantle uplift, while upward and inward collapse forms the surface topography. Furthermore, the nonmascon basins are all ancient and heavily degraded, indicating a postimpact evolutionary process. Our results suggest that mascon formation is the standard for large impacts on the Moon and by extension on other terrestrial planets

Crater Floor Slope as a Measure of Long-wavelength Changes in Topography on Mercury

During the course of three flybys and an orbital mission phase that began on 18 March 2011, the MESSENGER spacecraft has been performing a detailed survey of Mercury in order to characterize the planet’s origin and evolution. Precise topographic information about the surface of Mercury is being collected by the Mercury Laser Altimeter (MLA), largely over the northern hemisphere where the spacecraft slant range from the surface is less than 1500 km. Complementary knowledge of surface relief is gained through stereographic imaging by the Mercury Dual Imaging System (MDIS). Analysis of stereographic images returned during MESSENGER’s first flyby of Mercury revealed, and orbital MLA profiles have confirmed, the presence of unexpected long-wavelength topography within and adjacent to the Caloris impact basin. In particular, basin topography is far from radially symmetric, and portions of the northern basin floor lie at higher elevation than the nearby basin rim. The anomalously high areas of basin floor appear to be part of a larger-scale topographic variation that extends outside the basin. To assess the nature and development time of this long-wavelength topography we examine surface features that may have been tilted during its formation. In particular, we investigate the idea that the slopes of the floors of nominally flat-floored impact craters within and near the Caloris basin may, depending on their age, reflect changes in long-wavelength slopes associated with the large-scale topography. Whereas floor slopes for individual craters may be the result of any of several volcanic, tectonic, or impact processes, a large-scale organization of slope direction and magnitude can be an indicator of a common origin. Results from the measurement of crater floor slopes from MLA profiles across the northern Caloris region of Mercury reveal that a majority of flat crater floors profiled by MLA have along-track slopes between ~0.25 and 1.5°. Moreover, the magnitudes and along-track slope directions of these crater floors are generally spatially correlated with the long-wavelength slope of the Caloris floor topography. Ongoing collection of topographic profiles by MLA will serve to extend the statistical sample of craters that may have been influenced by the development of this large-scale feature as well as permit estimation of cross-track slopes for some craters, both crucial for understanding its development. Results to date also suggest that measurement of post-impact tilting of crater floors may provide a means more generally to assess the existence and development of comparable late-stage long-wavelength surface deformation across the planet

Topography of the Northern Hemisphere of Mercury from MESSENGER Laser Altimetry

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

Topography of the Northern Hemisphere of Mercury from MESSENGER Laser Altimetry

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