22. July 2021
Marsquake measurements reveal new information

The Red Plan­et has a larg­er core and a thin­ner crust

Marsquake waves reveal the planet’s inner structure
Marsquake waves re­veal the plan­et’s in­ner struc­ture
Image 1/7, Credit: Chris Bickkle/SCIENCE

Marsquake waves reveal the planet’s inner structure

A plan­et’s in­ter­nal prop­er­ties and struc­ture can be stud­ied by mea­sur­ing seis­mic waves. These waves are caused by pro­cess­es such as quakes or me­te­orite im­pacts. On Mars, the S-waves that em­anate from a marsquake (red dot) and are re­flect­ed from the core are record­ed by the SEIS seis­mome­ter on NASA's In­Sight geo­phys­i­cal sta­tion (white dot). By analysing marsquakes, it be­came pos­si­ble to bet­ter es­ti­mate the size of the core (3700 kilo­me­tres in di­am­e­ter), the thick­ness of the crust (ei­ther 20 or just un­der 40 kilo­me­tres) and the struc­ture of the man­tle (sim­i­lar to but sim­pler than Earth’s up­per man­tle). In ad­di­tion, the strength of the re­flect­ed waves shows that at least the out­er part of the core must be molten as it is im­per­me­able to S-waves.
Comparing Earth and Mars
Com­par­ing Earth and Mars
Image 2/7, Credit: NASA/JPL-Caltech

Comparing Earth and Mars

Earth is twice the di­am­e­ter and 10 times the mass of Mars and is pre­dom­i­nant­ly cov­ered by oceans. Some­what less ob­vi­ous­ly, Earth's rigid out­er shell, the litho­sphere, con­sists of sev­en large plates and many small­er ones that in­ter­act with one an­oth­er (the yel­low lines on the Earth globe in­di­cate their bound­aries). Mars, on the oth­er hand, is a sin­gle-plate plan­et – its litho­sphere con­sists of on­ly one piece. Why the two plan­ets dif­fer so much in this re­spect is still un­clear. Un­der­stand­ing when and why Mars ex­pe­ri­enced a dif­fer­ent evo­lu­tion­ary path than Earth al­so helps us bet­ter un­der­stand why Earth is the way it is.
NASA's InSight lander on Mars
NASA's In­Sight lan­der on Mars
Image 3/7, Credit: NASA/JPL-Caltech

NASA's InSight lander on Mars

In­Sight is a geo­phys­i­cal sta­tion ap­prox­i­mate­ly six me­tres wide and one me­tre high and with a mass of around 700 kilo­grams. It has been in the Ely­si­um Plani­tia re­gion, a lit­tle north of the Mar­tian equa­tor, since 26 Novem­ber 2018. Two of its pri­ma­ry in­stru­ments are the French seis­mome­ter, SEIS (front left in the artis­tic rep­re­sen­ta­tion – in re­al­i­ty, the in­stru­ment is lo­cat­ed un­der the dome-shaped wind and ther­mal shield), for mea­sur­ing marsquakes, built with the co­op­er­a­tion of the Max Planck In­sti­tute for So­lar Sys­tem Re­search, and the Heat Flow and Phys­i­cal Prop­er­ties Pack­age (HP3, front right), de­vel­oped and built at DLR. The small­er im­age shows a 'self­ie' of the lan­der tak­en with the cam­era at the end of its robot­ic arm. Dust has set­tled on the so­lar pan­els, which has re­duced pow­er gen­er­a­tion and cur­rent­ly on­ly al­lows for lim­it­ed op­er­a­tion of the lan­der.
The SEIS marsquake station
The SEIS marsquake sta­tion
Image 4/7, Credit: NASA/JPL-Caltech

The SEIS marsquake station

A cam­era on the deck of the In­Sight plat­form filmed a ‘flip-book’ time lapse of the weath­er over a Mar­tian day. In the cen­tre of the im­age, the seis­mome­ter can be seen un­der its wind­shield. The in­stru­ment is so sen­si­tive that it can mea­sure the tiny vi­bra­tions of the Mar­tian soil gen­er­at­ed by the wind, dust dev­ils and the slight move­ments of the teth­er used to trans­fer da­ta. To re­duce this noise in the da­ta, the scoop of the robot arm will now be used to cov­er the teth­er with sand to bet­ter fix it in place.
Cross-section of the SEIS seismometer
Cross-sec­tion of the SEIS seis­mome­ter
Image 5/7, Credit: IPGP/D. Ducros

Cross-section of the SEIS seismometer

SEIS is a seis­mome­ter for mea­sur­ing move­ments in the Mar­tian soil at dif­fer­ent fre­quen­cies. It con­tains six seis­mic and oth­er aux­il­iary sen­sors. The in­stru­ment was de­vel­oped and built by the French space agen­cy (CNES) with the par­tic­i­pa­tion of the Max Planck In­sti­tute for So­lar Sys­tem Re­search in Göt­tin­gen. At the heart of the SEIS ex­per­i­ment are three pen­du­lums that re­act to even the small­est move­ments in the Mar­tian sur­face. The mo­tion of the pen­du­lums is record­ed elec­tron­i­cal­ly. Be­cause ma­te­ri­als ex­pand as they warm and con­tract when they cool, SEIS is equipped with a so­phis­ti­cat­ed ther­mal sys­tem in the form of sev­er­al in­su­la­tion shells, com­pa­ra­ble to the 'onion-skin prin­ci­ple' in mod­ern out­door cloth­ing. The shells re­duce the tem­per­a­ture fluc­tu­a­tions at the sen­sors so that mea­sur­ing con­di­tions al­ways re­main sta­ble in­side the in­stru­ment. The out­er­most white dome serves specif­i­cal­ly to pro­tect against the wind; a flex­i­ble sys­tem re­sem­bling a win­dow blind en­sures a tight seal at the bot­tom.
Traces of marsquakes from the past
Traces of marsquakes from the past
Image 6/7, Credit: ESA/DLR/FU Berlin

Traces of marsquakes from the past

Traces of tec­ton­ic stress­es and marsquakes are om­nipresent on our plan­e­tary neigh­bour. Pat­terns of al­most lin­ear and par­al­lel frac­ture struc­tures caused by the stretch­ing of the brit­tle crust and the re­sult­ing frac­tures are very com­mon, par­tic­u­lar­ly in the south­ern high­lands, which are more than three bil­lion years old. The stretch­ing cre­ates ad­di­tion­al space per­pen­dic­u­lar to the di­rec­tion of the act­ing tec­ton­ic force, in­to which en­tire blocks of crust sag and crustal seg­ments re­main be­tween these sagged blocks, cre­at­ing a char­ac­ter­is­tic land­scape pat­tern known in ge­o­log­i­cal ter­mi­nol­o­gy world­wide by the Ger­man terms ‘Horst and Graben’. Such ma­jor struc­tural changes are al­ways ac­com­pa­nied by strong quakes. Ex­am­ples on Earth are the East African Rift Val­ley or the Up­per Rhine Graben.
The mineral olivine
The min­er­al olivine
Image 7/7, Credit: Oliver Grobe, AWI (CC BY-SA 2.5)

The mineral olivine

Olivine is a green iron-mag­ne­sium sil­i­cate that oc­curs very fre­quent­ly in Earth’s up­per man­tle and in vol­canic melts with a low pro­por­tion of sil­i­con. Earth’s en­tire ocean floor con­sists of so­lid­i­fied lavas with a high pro­por­tion of olivine min­er­als, as do vol­canic re­gions such as the Eifel, Hawaii or Mount Et­na. The ex­am­ple of the typ­i­cal green olivine min­er­als on a basalt stone shown here comes from the vol­cano Mount Ere­bus in Antarc­ti­ca. On­ly at a depth of about 700 kilo­me­tres does the pres­sure in Earth's man­tle be­come so high that olivine changes in­to per­ovskite-group min­er­als, in which the same atoms are ‘pressed’ in­to a denser crys­tal lat­tice. The lat­est re­search re­sults de­rived from quake waves now show that the man­tle of Mars, which is just over 1500 kilo­me­tres thick, is more like the up­per, olivine-rich man­tle of Earth than the low­er man­tle.
  • The core of Mars is molten and larger than previously thought.
  • The Martian crust is thinner than previously estimated.
  • Marsquake measurements performed as part of NASA’s InSight mission have enabled new discoveries that have been published in the journal Science.
  • Focus: Space, exploration, Mars

Mars' surface is known in great detail through exploration using orbiting spacecraft. But until now its interior structure could only be derived indirectly or simulated using computational models. With the participation of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), NASA's InSight mission has provided surprising new discoveries. The core of our planetary neighbour is larger than previously thought, and the overlying mantle has a structure similar to Earth's upper mantle. Finally, the crust of the Red Planet's surface is thinner than previously estimated. The Seismic Experiment for Interior Structure (SEIS), a seismometer developed under French leadership, acquired the data to address this geological puzzle over a period of more than two years. The results were published on 22 July 2021 in the journal Science.

Martin Knapmeyer, a geophysicist at the DLR Institute of Planetary Research in Berlin who was involved in the evaluation of the data, is excited by the breakthrough: "Models of the structure of Mars have existed for almost 100 years. However, the last decades have seen little progress with regard to the dimensions of the crust and core. Only seismology can measure these directly. In the past, they had to be inferred from other observations."

"According to our measurements, the Martian core has a diameter of approximately 3700 kilometres," explains Ana-Catalina Plesa, also from the DLR Institute of Planetary Research. "That is about half the diameter of Earth's core and is rather at the upper end of the size range indicated by all previous estimates," explains the scientist, who was involved in two of the three studies that have been published. Mars' diameter is 6770 kilometres, about half that of Earth's.

"A larger core also means a lower density than previously assumed," Plesa adds. "The lower density shows that the iron-nickel melt has a larger proportion of lighter elements such as sulphur, carbon, oxygen, or perhaps even hydrogen mixed into it." The scientists have determined the density of the core to be six grams per cubic centimetre. At nine to 13 grams per cubic centimetre, the density of Earth’s core is much higher. In addition, analysis of the seismograms shows that the Martian core must be molten – at least in its outer zone. The investigation of the core, led by Simon Stähler of the Swiss Federal Institute of Technology (ETH) in Zurich, thus confirms earlier measurements from satellite geodesy, which, however, were unable to determine the core size precisely.

Mars, like Earth, has a shell structure

We know that Earth is made up of concentric shells. A thin crust of light, brittle rock is followed by the thick mantle of heavy, plastically deformable rock, whose circulatory movements shift the continental plates around the globe. Below this is Earth's core, which consists largely of iron and nickel. A similar structure was assumed for the other Earth-like bodies of the inner Solar System, such as the Moon, Mercury, Venus and Mars. One of the main scientific goals of NASA's InSight mission is to study the planet's shell structure. InSight is a geophysical station that has been located in the Elysium Planitia region near the Martian equator since November 2018. The seismic data form InSight now allows absolute quantifcation of the thicknesses of the inner shells of Mars, and puts constraints on their possible compositions.

Crust thinner than previously thought

"There are also interesting new findings about the Martian crust and the mantle underneath, located between the crust and the core," emphasises Brigitte Knapmeyer-Endrun from the Bensberg Earthquake Station at the University of Cologne, the first author of the Science study on the thickness of Mars' crust, in which Plesa and Knapmeyer are also involved. As recently as 2018, a crustal thickness somewhere between 19 and 90 kilometres had been predicted for the landing site of the InSight mission. Knapmeyer-Endrun can now narrow this down: "The data now only allow two possibilities: Either the crustal thickness at the landing site is approximately 20 kilometres, or it is just under 40 kilometres, which is indicated by an additional weak signal."

Global maps of the gravity field and topography of Mars allow this point measurement at the InSight landing site to be extrapolated to the entire planet. This shows that the average thickness of the Martian crust is between 24 and 72 kilometres. "Mars research has been waiting decades for an 'anchor point' to calibrate the global maps," says Martin Knapmeyer, explaining the significance of the result. The wide range between minimum and maximum values for crustal thickness is related to the distribution of radioactive elements in the planet's interior, which generate heat through their decay and ultimately drive the geological processes. A 'thicker' crust is more consistent with the abundance of heat-generating radioactive elements in the crust as observed at the surface, while a 'thinner' crust can be explained by a greater concentration of such elements at depth. "Determining the thickness of the crust based on the InSight data not only helps us understand what Mars looks like today, but also provides us with important information about its thermal evolution," says Ana-Catalina Plesa.

Mantle of Mars 'a simpler version of Earth's mantle'

In Earth's mantle, the pressure required for the formation of the mineral bridgmanite is present from a depth of approximately 700 kilometres. Bridgmanite is a silicate magnesium oxide (MgSiO3) from the mineral family of perovskites. Such perovskites make up four fifths of Earth's mantle and are only formed under extremely high pressures. The new measurements now show that this pressure is only reached in the iron core of Mars, and thus Mars’ entire mantle should be dominated by the mineral olivine [(Mg,Fe)2SiO4], similar to Earth's upper mantle.

Marsquake waves show layer boundaries

The InSight scientists obtained the new results by analysing various seismic waves generated during quakes. During marsquakes, as in earthquakes, energy is released in the form of waves, referred to as 'P' and 'S' waves for historical reasons. P-waves are pressure waves, like sound waves in the air. S-waves, on the other hand, oscillate perpendicular to the direction of propagation, like a guitar string. Since P-waves propagate at greater speed, the distance to the quake's origin can be calculated from the time interval between the arrival of the different wave types. Seismic waves pass through planets and are reflected and refracted at different layer boundaries in the interior.

The InSight mission seismometer has detected more than 700 identified marsquakes since the beginning of 2019. The P- and S-waves travelling directly from the source to the landing site are easily identified. However, to study the inner structure of the planet, the scientists need other signals. Since the liquid core is impermeable to S-waves, thus they produce strong echoes. The thickness of the crust can be determined by means of an effect that only occurs in solid bodies. At the boundary between two types of rock, such as that between the crust and the mantle, there is a partial conversion of P-waves into S-waves, where the delay between the two is larger for a thicker crust.

DLR's participation in NASA's InSight mission

The InSight mission is being carried out by NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, on behalf of the agency's Science Mission Directorate. InSight is part of NASA's Discovery Program. DLR is contributing the Heat Flow and Physical Properties Package (HP3) experiment to the mission. It operated until the beginning of 20210114_the-mars-mole-has-reached-the-end-of-its-journey The German Space Agency at DLR, with funding from the German Federal Ministry for Economic Affairs and Energy, supported a contribution by the Max Planck Institute for Solar System Research to the French main instrument SEIS (Seismic Experiment for Interior Structure). DLR researchers are involved in the evaluation of the SEIS data.

Detailed information on the InSight mission can be found on DLR's dedicated mission page.

Contact
  • Falk Dambowsky
    Ed­i­tor
    Ger­man Aerospace Cen­ter (DLR)
    Me­dia Re­la­tions
    Com­mu­ni­ca­tions and Me­dia Re­la­tions
    Telephone: +49 2203 601-3959
    Fax: +49 2203 601-3249
    Linder Höhe
    51147 Cologne
    Contact
  • Ana-Catalina Plesa
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Plan­e­tary Physics
    Rutherfordstraße 2
    12489 Berlin
    Contact
  • Martin Knapmeyer
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Plan­e­tary Physics
    Rutherfordstraße 2
    12489 Berlin
    Contact
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