Using information obtained from around a dozen earthquakes detected on Mars
by the Very Broad Band SEIS seismometer, developed in France, the
international team of NASA's InSight mission has unveiled the internal
structure of Mars. The three papers published on July 23, 2021 in the
journal Science, involving numerous co-authors from French institutions and
laboratories, including the CNRS, the Institut de Physique du Globe de
Paris, and Université de Paris, and supported in particular by the French
space agency CNES and the French National Research Agency ANR, provide, for
the first time, an estimate of the size of the planet's core, the thickness
of its crust and the structure of its mantle, based on the analysis of
seismic waves reflected and modified by interfaces in its interior. It makes
this the first ever seismic exploration of the internal structure of a
terrestrial planet other than Earth, and an important step towards
understanding the formation and thermal evolution of Mars.
Before NASA's InSight mission, the internal structure of Mars was still
poorly understood. Models were based only on data collected by orbiting
satellites and on the analysis of Martian meteorites that fell to Earth. On
the basis of gravity and topographical data alone, the thickness of the
crust was estimated to be between 30 and 100 km. Values of the planet's
moment of inertia and density suggested a core with a radius of 1,400 to
2,000 km. The detailed internal structure of Mars and the depth of the
boundaries between the crust, mantle and core were, however, completely
unknown.
With the successful deployment of the SEIS experiment on the surface of Mars
in early 2019, the mission scientists, including the 18 French co-authors
involved and affiliated to a wide range of French institutions and
laboratories , together with their colleagues from ETH in Zurich, the
University of Cologne and the Jet Propulsion Laboratory in Pasadena,
collected and analyzed seismic data over one Martian year (almost two Earth
years).
It should be pointed out that to simultaneously determine a structural
model, the (arrival) time of an earthquake, and its distance, more than one
station is usually required. However, on Mars the scientists only have one
station, InSight. It was therefore necessary to search the seismic records
for the characteristic features of waves that had interacted in various ways
with the internal structures of Mars, and identify and validate them. These
new measurements, coupled with mineralogical and thermal modeling of the
planet's internal structure, have made it possible to overcome the
limitation of having a single station. This method ushers in a new era for
planetary seismology.
A single station, multiple findings
Another difficulty on Mars is its low seismicity and the seismic noise
generated by its atmosphere. On Earth, earthquakes are much stronger, while
seismometers are more effectively located in vaults or underground, making
it possible to obtain an accurate image of the planet's interior. As a
result, special attention had to be paid to the data. "But although Martian
earthquakes have a relatively low magnitude, less than 3.5, the very high
sensitivity of the VBB sensor combined with the very low noise at nightfall
enabled us to make discoveries that, two years ago, we thought were only
possible with earthquakes with a magnitude greater than 4," explains
Philippe Lognonné, a Professor at the University of Paris and the Principal
Investigator for the SEIS instrument at IPGP.
Every day, the data, processed by CNES, IPGP and CNRS, and transferred to
the scientists, was carefully cleaned of ambient noise (wind and deformation
related to rapid temperature changes). The international Mars Quake Service
team (MQS) recorded the seismic events on a daily basis: More than 600 have
now been catalogued, of which over 60 were caused by relatively distant
earthquakes.
Around ten of the latter contain information about the planet's deep
structure: "The direct seismic waves from an earthquake are a bit like the
sound of our voices in the mountains: They produce echoes. And it was these
echoes, reflected off the core, or at the crust-mantle interface or even the
surface of Mars, that we looked for in the signals, thanks to their
similarity to the direct waves," Lognonné explains.
An altered crust, a mantle revealed, and a large liquid core
By comparing the behavior of seismic waves as they traveled through the
crust before reaching the InSight station, several discontinuities in the
crust were identified: The first, observed at a depth of about 10 km, marks
the boundary between a highly altered structure, resulting from circulation
of fluid a very long time ago, and crust that is only slightly altered. A
second discontinuity around 20 km down, and a third, less pronounced one at
around 35 km, shed light on the stratification of the crust beneath InSight:
"To identify these discontinuities, we used all the most recent analytical
methods, both with earthquakes of tectonic origin and with vibrations caused
by the environment (seismic noise)," says Benoit Tauzin, Senior Lecturer at
the University of Lyon and a researcher at LGL-TPE.
In the mantle, the scientists analyzed the differences between the travel
time of the waves produced directly during the earthquake, and that of the
waves generated when these direct waves were reflected off the surface.
These differences made it possible, using only a single station, to
determine the structure of the upper mantle, and in particular the variation
in seismic velocities with depth. However, such variations in velocity are
related to temperature. "That means we can estimate the heat flow of Mars,
which is probably three to five times lower than the Earth's, and place
constraints on the composition of the Martian crust, which is thought to
contain over half the heat-producing radioactive elements present in the
planet," adds Henri Samuel, a CNRS researcher at IPGP.
Finally, in the third study, the scientists looked for waves reflected off
the surface of the Martian core, the measurement of whose radius was one of
the main achievements of the InSight mission. "To do this," explains Mélanie
Drilleau, a research engineer at ISAE-SUPAERO, "we tested several thousand
mantle and core models against the phases and signals observed." Despite the
low amplitudes of the signals associated with the reflected waves (known as
ScS waves), an excess of energy was observed for cores with a radius between
1,790 km and 1,870 km. Such a large size implies the presence of light
elements in the liquid core and has major consequences for the mineralogy of
the mantle at the mantle/core interface.
Goals achieved, new questions emerge
More than two years of seismic monitoring has resulted in the very first
model of the internal structure of Mars, right down to the core. Mars thus
joins the Earth and the Moon in the select club of terrestrial planets and
moons whose deep structures have been explored by seismologists. And, as
often happens in planetary exploration, fresh questions emerge: Is the
alteration of the top 10 km of crust general, or is it limited to the
InSight landing zone? What impact will these first models have on theories
of the formation and thermal evolution of Mars, in particular for the first
500 million years when Mars had liquid water on its surface and intense
volcanic activity?
With the two-year extension of the InSight mission and the additional
electrical power obtained following the successful cleaning of its solar
panels carried out by JPL, new data should consolidate and further improve
these models.
References:
“Upper mantle structure of Mars from InSight seismic data” by Amir Khan,
Savas Ceylan, Martin van Driel, Domenico Giardini, Philippe Lognonné, Henri
Samuel, Nicholas C. Schmerr, Simon C. Stähler, Andrea C. Duran, Quancheng
Huang, Doyeon Kim, Adrien Broquet, Constantinos Charalambous, John F.
Clinton, Paul M. Davis, Mélanie Drilleau, Foivos Karakostas, Vedran Lekic,
Scott M. McLennan, Ross R. Maguire, Chloé Michaut, Mark P. Panning, William
T. Pike, Baptiste Pinot, Matthieu Plasman, John-Robert Scholz, Rudolf
Widmer-Schnidrig, Tilman Spohn, Suzanne E. Smrekar and William B. Banerdt,
23 July 2021, Science.
“Seismic detection of the martian core” by Simon C. Stähler, Amir Khan, W.
Bruce Banerdt, Philippe Lognonné, Domenico Giardini, Savas Ceylan, Mélanie
Drilleau, A. Cecilia Duran, Raphaël F. Garcia, Quancheng Huang, Doyeon Kim,
Vedran Lekic, Henri Samuel, Martin Schimmel, Nicholas Schmerr, David
Sollberger, Éléonore Stutzmann, Zongbo Xu, Daniele Antonangeli, Constantinos
Charalambous, Paul M. Davis, Jessica C. E. Irving, Taichi Kawamura, Martin
Knapmeyer, Ross Maguire, Angela G. Marusiak, Mark P. Panning, Clément
Perrin, Ana-Catalina Plesa, Attilio Rivoldini, Cédric Schmelzbach, Géraldine
Zenhäusern, Éric Beucler, John Clinton, Nikolaj Dahmen, Martin van Driel,
Tamara Gudkova, Anna Horleston, W. Thomas Pike, Matthieu Plasman and Suzanne
E. Smrekar, 23 July 2021, Science.
“Thickness and structure of the martian crust from InSight seismic data” by
Brigitte Knapmeyer-Endrun, Mark P. Panning, Felix Bissig, Rakshit Joshi,
Amir Khan, Doyeon Kim, Vedran Lekic, Benoit Tauzin, Saikiran Tharimena,
Matthieu Plasman, Nicolas Compaire, Raphael F. Garcia, Ludovic Margerin,
Martin Schimmel, Éléonore Stutzmann, Nicholas Schmerr, Ebru Bozdag,
Ana-Catalina Plesa, Mark A. Wieczorek, Adrien Broquet, Daniele Antonangeli,
Scott M. McLennan, Henri Samuel, Chloé Michaut, Lu Pan, Suzanne E. Smrekar,
Catherine L. Johnson, Nienke Brinkman, Anna Mittelholz, Attilio Rivoldini,
Paul M. Davis, Philippe Lognonné, Baptiste Pinot, John-Robert Scholz, Simon
Stähler, Martin Knapmeyer, Martin van Driel, Domenico Giardini and W. Bruce
Banerdt, 23 July 2021, Science.
Tags:
Space & Astrophysics