SUCCESSFUL DEPLOYMENT OF THE SEIS SEISMOMETER
Before NASA’s InSight mission the internal structure of Mars was not well understood. The models were merely based on measurements taken by orbiting satellites or the analysis of Martian meteorites that had fallen on Earth. The crust’s thickness, using just measurements of gravity and topography, was estimated at between 30 and 100 km. The values for the moment of inertia and the density of the planet suggested a core with a radius of between 1,400 and 2,000 km. Details of the planet’s internal structure and the depth of the borders between the crust, mantle and core were totally unknown.
With the successful deployment of the SEIS experiment on the surface of Mars in early 2019, the mission’s scientists (18 French co-authors involved and affiliated with a number of institutions and laboratories, 3 of them at ISAE-SUPAERO, and their colleagues at ETH Zurich, the University of Cologne and the Jet Propulsion Laboratory in Pasadena), collected and analyzed seismic data covering one Martian year (or nearly two terrestrial years). These new measurements, combined with mineralogical and thermic
“The SEIS seismometer sends us continuous recordings of Martian ground movements in which marsquakes appear as short periods of strong vibrations on the planet’s surface – strong compared to the minutely small vibrations recorded between marsquakes. These minutely small vibrations provide a great deal of information on the Martian subsurface. Called “seismic noise”, they have notably enabled us to probe the structure of the Martian crust under the SEIS seismometer down to its interface with the mantle. Once the crust’s structure under the seismometer is known, we can construct models of the crust on the scale of the entire planet using the gravitational field and Martian typography,” explained Nicolas Compaire, doctoral student in planetology at our Institute and in charge of the SEIS seismometer data analysis used to determine the internal structure of Mars.
AN ALTERED CRUST, A MANTLE REVEALED AND A BIG LIQUID CORE
By comparing behaviors of seismic waves when they cross the crust before reaching the Insight station, several discontinuities were identified in the crust: the first, observed at a depth of some 10 km, marks the separation between a highly altered structure resulting in an ancient circulation of fluids and a crust with little alteration. The second discontinuity, at approximately 20 km and then a third, less marked one at around 35 km, reveal the crust’s stratification under InSight.
In the mantle, the differences between the travel time of the waves generated directly during a marsquake and that of the waves generated during these direct waves’ reflection on the surface were analyzed. Using these differences, a single station can determine the structure of the upper mantle, and notably the variation in seismic velocities with depth. These speed variations are connected to the temperature.
In the third study, the scientists looked for waves reflected by the surface of the Martian core, measuring the radius of which is one of the main results of the InSight mission.
“Recordings of ground movements produced by the most powerful marsquakes point to the presence of waves reflected on the Martian core. The amplitude of these waves confirms that the core is liquid. Furthermore, travel time between the marsquake and the SEIS seismometer was used to estimate the size of the core, which has a radius of between 1,790 and 1,870 km. These values suggest a larger core with a lower density than the values predicted using other geophysical methods,” explained Raphaël Garcia, Professor of Planetary Geophysics at our Institute and head of detection and analysis of seismic waves reflected on the core of Mars.
Mélanie Drilleau, research engineer in geophysics also at the ISAE-SUPAERO campus, in charge of constraining internal structure of Mars, explained that, “To achieve this result, we tested several thousand models of mantles and cores in relation to the phases and signals observed.” Despite the weak amplitudes of the signals related to the reflected waves (called ScS), an excess of energy was observed for cores with a radius of between 1,790 km and 1,870 km. Such a size indicates the presence of light elements in the liquid core and has major consequences on the mantle’s mineralogy at the mantle/core interface.
OBJECTIVES ARE MET AND NEW QUESTIONS EMERGE
After more than two years of marsquake monitoring, the first model of the internal structure of Mars all the way to the core was obtained. Mars thus joins the Earth and Moon in the club of telluric planets and satellites whose deep structure has been explored through seismology. “Thanks to the InSight mission and the seismic waves produced by marsquakes, we have managed to constrain the crust’s thickness under the SEIS seismometer, estimate the profile of seismic waves in the mantle and determine the size of the core. This is extremely precious information that will help us to understand why Mars and the Earth evolved so differently, even though they are neighbors,” Mélanie Drilleau continued.
And as is often the case in planetary exploration, new questions have now come up: is the alteration of the crust over the first 10 kilometers a generalized phenomenon or is it limited to InSight’s landing area? What impact will these first models have on theories concerning Mars’s formation and thermal evolution, especially for the first 500 million years when Mars had liquid water on its surface and intense volcanic activity?
“These results give rise to new questions on the composition of the mantle and the Martian core, and therefore on the processes of planetary formation and evolution. Understanding these processes is the key to understanding why certain worlds are inhabitable whereas most of them are not. After years of observation of the surface of Mars, for the first time we can take a look under the hood and discover how the red planet’s heat machine works.” Raphaël Garcia concluded. With the InSight mission’s two-year extension, new data will consolidate and further improve these models.
For further information:
Conference by Mélanie Drilleau on the initial scientific results of the analysis of Mars data
[First year of the Martian InSight mission>https://www.isae-supaero.fr/fr/actualites/premiere-annee-de-la-mission-martienne-insight-des-resultats-scientifiques/]: surprising scientific results
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