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

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