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

Find out more : 

1. Explo­ring Mars’ struc­ture
   https://www.coursera.org/lecture/seismology-to-earthquakes/1–10-exploring-mars-structure-VfKNY
2. What are Mars­quakes ?
   https://www.coursera.org/lecture/seismology-to-earthquakes/2–8‑what-about-marsquakes-ac0pu

Refe­rences :

• P. Lognon­né, W.B. Banerdt, J. Clin­ton, R.F. Gar­cia, D. Giar­di­ni, B. Knap­meyer-Endrun, M. Pan­ning, W.T. Pike, Mars Seis­mo­lo­gy,Annual Review of Earth and Pla­ne­ta­ry Sciences 2023 51:1, 643–670, https://doi.org/10.1146/annurev-earth-031621–073318
• Lognon­né, P., Schim­mel, M., Stutz­mann, E., Davis, P., Drilleau, M., Sain­ton, G., et al. (2023). Detec­tion of Mars nor­mal modes from S1222a event and seis­mic hum. Geo­phy­si­cal Research Let­ters, 50, e2023GL103205. https://​doi​.org/​1​0​.​1​0​2​9​/​2​0​2​3​G​L​1​03205
• Lognon­né, P., W.B. Banerdt,  W.T.Pike, D.Giardini, U.Christensen, R.F.Garcia,et al., Constraints on the shal­low elas­tic and ane­las­tic struc­ture of Mars from InSight seis­mic data, Nature Geos­cience, 13, 213–220, doi : 10.1038/s41561-020‑0536‑y
• Lognon­né, P., W.B.Banerdt, D.Giardini, W.T.Pike, U.Christensen, P.Laudet, et al, SEIS : Insight’s Seis­mic Expe­riment for Inter­nal Struc­ture of Mars, Space Sci Rev, 215, 12, doi : 10.1007/s11214-018‑0574‑6
• Samuel, H., M. Drilleau, A. Rivol­di­ni, Z. Xu, Q. Huang, R. F. Gar­cia, V. Lekic, J.C.E Irving, J. Badro, P. H. Lognon­né, J. A. D. Connol­ly, T. Kawa­mu­ra, T. Gud­ko­va and W. B. Banerdt (2023). Geo­phy­si­cal evi­dence for an enri­ched mol­ten sili­cate layer above Mars’s core, Nature, 622, 712–717, doi : 10.1038/s41586-023–06601‑8
• Ver­dier, N., V. Ansan, P. Delage, K. S. Ali, E. Beu­cler, C. Cha­ra­lam­bous, E. Constant, A. Spi­ga, M. Golom­bek, E. Mar­teau, R. Lapeyre, E. Gau­din, C. Yana, K. Hurst, P. Lognon­né, and B. W. Banerdt (2023). Using wind dis­per­sion effects during the InSight tether burial acti­vi­ties to bet­ter constrain the rego­lith grain size dis­tri­bu­tion. Jour­nal of Geo­phy­si­cal Research : Pla­nets, 128, e2022JE007707. https://​doi​.org/​1​0​.​1​0​2​9​/​2​0​2​2​J​E​0​07707
• Xu, Z., Bro­quet, A., Fuji, N., Kawa­mu­ra, T., Lognon­né, P., Mon­ta­gner, J.-P., et al. (2023). Inves­ti­ga­tion of Mar­tian regio­nal crus­tal struc­ture near the dicho­to­my using S1222a sur­face-wave group velo­ci­ties. Geo­phy­si­cal Research Let­ters, 50, e2023GL103136. https://​doi​.org/​1​0​.​1​0​2​9​/​2​0​2​3​G​L​1​03136