Seismic data collected in Elysium Planitia, the second largest volcanic region
on Mars, suggest the presence of a shallow sedimentary layer sandwiched
between lava flows beneath the planet's surface. These findings were gained in
the framework of NASA's InSight mission (Interior Exploration using Seismic
Investigations, Geodesy and Heat Transport), in which several international
research partners, including the University of Cologne, collaborate. The paper
'The shallow structure of Mars at the InSight landing site from inversion of
ambient vibrations' will appeared in Nature Communications on 23 November.
Geophysicist Dr Cédric Schmelzbach from ETH Zurich and colleagues, including
the earthquake specialists Dr Brigitte Knapmeyer-Endrun and doctoral
researcher Sebastian Carrasco (MSc) from the University of Cologne's Seismic
Observatory in Bensberg, used seismic data to analyse the composition of the
Elysium Planitia region. The authors examined the shallow subsurface to
around 200 metres in depth. Right beneath the surface, they discovered a
regolith layer of dominantly sandy material approximately three metres thick
above a 15 metre layer of coarse blocky ejecta -- rocky blocks that were
ejected after a meteorite impact and fell back to the surface.
Below these top layers, they identified around 150 metres of basaltic rocks,
i.e., cooled and solidified lava flows, which was largely consistent with
the expected subsurface structure. However, between these lava flows,
starting at a depth of about 30 metres, the authors identified an additional
layer 30 to 40 metres thick with low seismic velocity, suggesting it
contains weak sedimentary materials relative to the stronger basalt layers.
To date the shallower lava flows, the authors used crater counts from
existing literature. Established knowledge about the impact rate of
meteorites allows geologists to date rocks: surfaces with many impact
craters from meteorites are older than ones with fewer craters. Also,
craters with larger diameters extend into the lower layer, allowing the
scientists to date the deep rock, while smaller ones allow them to date the
shallower rock layers.
They found that the shallower lava flows are approximately 1.7 billion years
old, forming during the Amazonian period -- a geological era on Mars
characterized by low rates of meteorite and asteroid impacts and by cold,
hyper-arid conditions, which began approximately 3 billion years ago. In
contrast, the deeper basalt layer below the sediments formed much earlier,
approximately 3.6 billion years ago during the Hesperian period, which was
characterized by widespread volcanic activity.
The authors propose that the intermediate layer with low volcanic velocities
could be composed of sedimentary deposits sandwiched between the Hesperian
and Amazonian basalts, or within the Amazonian basalts themselves. These
results provide the first opportunity to compare seismic ground-truth
measurements of the shallow subsurface to prior predictions based on orbital
geological mapping. Prior to the landing, Dr Knapmeyer-Endrun had already
developed models of the velocity structure of the shallow subsurface at the
InSight landing site based on terrestrial analogues. The actual measurements
now indicate additional layering as well as more porous rocks in general.
'While the results help to better understand the geological processes in
Elysium Planitia, comparison with pre-landing models is also valuable for
future landed missions, since it can help to refine predictions,'
Knapmeyer-Endrun remarked. Knowledge of the properties of the shallow
subsurface is required to assess, for example, its load-bearing capacity and
trafficability for rovers. Besides, details on the layering in the shallow
subsurface help to understand where it might still contain ground water or
ice. Within the framework of his doctoral research at the University of
Cologne, Sebastian Carrasco will continue to analyse the effect of the
shallow structure of Elysium Planitia on marsquake recordings.
The InSight lander arrived on Mars on 26 November 2018, touching down in the
Elysium Planitia region. Mars has been the target of numerous planetary
science missions, but the InSight mission is the first to specifically
measure the subsurface using seismic methods.
Reference:
M. Hobiger, M. Hallo, C. Schmelzbach, S. C. Stähler, D. Fäh, D. Giardini, M.
Golombek, J. Clinton, N. Dahmen, G. Zenhäusern, B. Knapmeyer-Endrun, S.
Carrasco, C. Charalambous, K. Hurst, S. Kedar, W. B. Banerdt. The shallow
structure of Mars at the InSight landing site from inversion of ambient
vibrations. Nature Communications, 2021; 12 (1)
DOI: 10.1038/s41467-021-26957-7
No comments:
Post a Comment