Mars: how studying earthquakes changed our vision of the red planet

Ter­res­tri­al seis­mol­o­gy pro­vides impor­tant infor­ma­tion on the inte­ri­or of our plan­et, includ­ing how it formed and evolved. The same applies to extrater­res­tri­al plan­ets with the very first seis­mic sig­nals mea­sured on the plan­et Mars report­ed by sci­en­tists fol­low­ing NASA’s InSight (Inte­ri­or Explo­ration using Seis­mic Inves­ti­ga­tions, Geo­desy and Heat Trans­port) mis­sion, launched in 2018. After four years of oper­a­tion on the Red Plan­et, researchers have detect­ed around 1,300 seis­mic events, the analy­sis of which will rev­o­lu­tionise our understanding.

Mars is not the first extrater­res­tri­al body on which sci­en­tists have detect­ed seis­mic activ­i­ty. Five seis­mome­ters oper­at­ed on the Moon between 1969 and 1977, mea­sur­ing tens of thou­sands of “moon­quakes”. Like the Moon, Mars lacks tec­ton­ic plates and should there­fore be much less active than the Earth in this respect.

“Of the 1,300 seis­mic events detect­ed on Mars, a good ten were caused by mete­orite impacts,” explains Philippe Lognon­né, pro­fes­sor at Uni­ver­sité Paris-Cité and Insti­tut de Physique du Globe de Paris (IPGP) who heads the team that built SEIS (Seis­mic Exper­i­ment for Inte­ri­or Struc­ture of Mars), InSight’s main seis­mome­ter. Of all the quakes, at least 50 are rel­a­tive­ly large, mea­sur­ing more than 2.5 on the Richter scale, with the largest mea­sured hav­ing a mag­ni­tude of 4.7. There are also almost a thou­sand small­er tremors, with a low­er mag­ni­tude of between 1 and 2.

Before InSight, it was thought that this type of seis­mic activ­i­ty was main­ly linked to the cool­ing under­way in the inte­ri­or of the plan­et, which caus­es side-to-side con­trac­tions and leads to an accu­mu­la­tion of stress points. These are released by rup­tures in the crust, gen­er­at­ing earth­quakes. InSight showed that only a small pro­por­tion of the earth­quakes could be explained in this way, how­ev­er. Indeed, the InSight researchers dis­cov­ered that half of the large Mar­t­ian tremors detect­ed all orig­i­nat­ed in a region called Cer­berus Fos­sae. “We know that vol­can­ism was active between 500 000 and 1 mil­lion years ago in this region,” explains Philippe Lognon­né. “Thanks to our mea­sure­ments, we also know that these tremors are almost con­tin­u­ous: not a month goes by with­out a small earthquake.”

“That half of the tremors detect­ed occur in this region implies that the vol­ca­noes that erupt­ed there in the past are prob­a­bly not extinct, but sim­ply dor­mant. This is the first major dis­cov­ery we made and, to be hon­est, no one in our team expect­ed this result. Until now, we thought that the seis­mic activ­i­ty on Mars came sole­ly from the plan­et cool­ing and its crust crack­ing in dif­fer­ent direc­tions. It’s amaz­ing to think that Mars is a plan­et that still has vol­canic tec­ton­ics asso­ci­at­ed with vol­ca­noes that could very well become active again in the future.”

The sec­ond major dis­cov­ery made by InSight is that the Mar­t­ian man­tle does not have the same sub­di­vi­sions as Earth’s low­er man­tle. “In oth­er words, and even though the entire Mar­t­ian man­tle con­tains more or less the same types of rocks and min­er­als as the Earth’s upper man­tle, Mars does not have a low­er man­tle, but rather a kind of mag­ma ocean at its base.” Although this result is less sur­pris­ing, since it is plau­si­ble, it had nev­er been dis­cov­ered in anoth­er tel­luric plan­et. This unex­pect­ed dis­cov­ery shows that at the base of Mars’ man­tle, the liq­uid is not the same as that in the core.

“Again, few mem­bers of the team expect­ed this,” says Philippe Lognon­né. “We now realise that the actu­al struc­ture of Mars is very dif­fer­ent to that of Earth’s. On our plan­et, you first have a crust, then a sol­id man­tle divid­ed into two parts. At the base of the sol­id man­tle, there is a liq­uid core of iron and light ele­ments, and under­neath, a sol­id core of iron. On Mars, it’s not like that at all: we have a crust, a sol­id man­tle, but at the base of the sol­id man­tle, we have a molten man­tle that cov­ers a metal­lic core that is also molten and much rich­er in light ele­ments. Final­ly, we’ve had some rather nice sur­pris­es in terms of mete­orite impacts,” he explains. For exam­ple, a 150-m-wide crater not only gen­er­at­ed superb seis­mic waves but also revealed the pres­ence of pock­ets of ice beneath the sur­face. This dis­cov­ery could have impli­ca­tions for future manned mis­sions to Mars, as we now know that under­ground ice can be found in cer­tain places.”

InSight’s SEIS instru­ment col­lects infor­ma­tion about the Mar­t­ian crust by detect­ing seis­mic waves from sources such as tremors and mete­orite impacts that rever­ber­ate through­out the plan­et. As these waves trav­el through the inte­ri­or of Mars, they change speed and direc­tion at the bound­aries between the dif­fer­ent mate­ri­als in the crust. This means that, when mea­sured by SEIS, seis­mic waves from the same source can be detect­ed at dif­fer­ent times, depend­ing on the paths they have tak­en to reach the probe.

The speed at which seis­mic waves trav­el through rocks of dif­fer­ent den­si­ties varies accord­ing to their com­po­si­tion, the inter­sti­tial space and what fills that space – be it gas, water or ice. By analysing the dif­fer­ent times that seis­mic waves com­ing from the same sources reach the probe, researchers can deter­mine the com­po­si­tion of the plan­et’s interior.

“We use seis­mic waves as a kind of light to illu­mi­nate the inte­ri­or of the plan­et,” explains Philippe Lognon­né. “By analysing the data mea­sured by the seis­mome­ters, we can then ‘image’ the inte­ri­or, deter­mine the thick­ness of the main parts of the plan­et, the crust, the man­tle and the core, observe what is liq­uid and where and when water exists. These are the main areas of seis­mol­o­gy in gen­er­al and I think that with InSight we have made some first-rate dis­cov­er­ies in these fields. We can’t say that InSight has rev­o­lu­tionised seis­mol­o­gy in terms of data analy­sis tech­niques, but this sen­sor has cer­tain­ly been rev­o­lu­tion­ary in the sense that it has pro­vid­ed new data on a plan­et nev­er before vis­it­ed,” he concludes.

Interview by Isabelle Dumé

Ref­er­ences:

P. Lognon­né, W.B. Banerdt, J. Clin­ton, R.F. Gar­cia, D. Gia­r­di­ni, B. Knap­mey­er-Endrun, M. Pan­ning, W.T. Pike, Mars Seis­mol­o­gy,
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