For almost three years, the only seismic waves InSight detected on Mars were ones that propagated from the respective quake’s focus, or hypocentre, through the depths of the planet. However, on 24 December 2021, a meteorite impact on Mars yielded the type of surface waves scientists had been long anticipating.
Dr Anna Horleston, from the University of Bristol’s School of Earth Sciences is a Planetary Seismologist and co-leads the frontline team of the Marsquake Service for NASA’s InSight Mission to Mars. She said: “Although we’ve catalogued over 1300 marsquakes during the mission, the signals from the meteorite impacts were clearly different. We knew immediately that we were seeing something new.”
“This is the first time seismic surface waves have been observed on a planet other than Earth. Not even the Apollo missions to the Moon managed it,” Dr Doyeon Kim, lead author of the study explained.
What makes the seismic surface waves so important to researchers is that they provide information about the structure of the Martian crust. Seismic body waves, which travel through the planet’s interior during a quake, have so far provided insights into Mars’s core and mantle, but have revealed little about the crust away from the lander itself.
“Until now, our knowledge of the Martian crust has been based on only a single point measurement under the InSight lander,” Dr Kim said. The result of the surface wave analysis surprised him. On average, the Martian crust between the impact sites and InSight’s seismometer has a very uniform structure and high density. Directly below the lander, however, the researchers had previously detected three layers of crust that implied a lower density.
The new findings, published today in Science journal, are remarkable because a planet’s crust provides important clues about how that planet formed and evolved. Since the crust itself is the result of early dynamic processes in the mantle and subsequent magmatic processes, it reveals conditions billions of years ago and the timeline of impacts, which were particularly common in Mars’ early days.
Dr Kim said: “The speed at which surface waves propagate depends on their frequency, which in turn depends on their depth.” By measuring changes in velocity in the seismic data across different frequencies, it is possible to infer how the velocity changes at different depths, because each frequency is sensitive to different depths. This provides the basis for estimating the average density of the rock, because the seismic velocity also depends on the elastic properties of the material through which the waves travel. This data allowed the researchers to determine the structure of the crust at depths of between roughly five and 30 kilometres below the surface of Mars.
In general, volcanic rocks tend to exhibit higher seismic velocities than sedimentary rocks. Also, the paths between the two meteorite impacts and the measurement site pass through one of the largest volcanic regions in Mars’ northern hemisphere.
Lava flows and the closure of pore spaces from heat created by volcanic processes, can increase the velocity of seismic waves. “On the other hand, the crustal structure beneath InSight’s landing site may have been formed in a unique way, perhaps when material was ejected during a large meteoritic impact more than three billion years ago. That would mean the structure of the crust under the lander is probably not representative of the general structure of the Martian crust,” Dr Kim added.
The new research could also help solve a centuries-old mystery. Ever since the first telescopes were pointed at Mars, it has been known that a sharp contrast exists between the planet’s southern and northern hemispheres. While the dominant feature of the southern hemisphere is a plateau covered by meteorite craters, the northern hemisphere consists mostly of flat, volcanic lowlands that may have been covered by oceans in the planet’s early history. This division into southern highlands and northern lowlands is called the Mars dichotomy.
“As things stand, we don’t yet have a generally accepted explanation for the dichotomy because we’ve never been able to see the planet’s deep structure,” said Domenico Giardini, ETH Zurich Professor of Seismology and Geodynamics. “But now we’re beginning to uncover this.” The initial results appear to disprove one of the widespread theories for the Mars dichotomy: the crusts in the north and in the south are probably not composed of different materials, as has often been assumed, and their structure may be surprisingly similar at relevant depths.
The InsSight Science team are expecting further results soon. In May 2022, InSight observed the largest marsquake to date, with a magnitude of 5. It also recorded seismic surface waves generated by this shallow event. This happened just in time, since the InSight mission will soon be coming to an end now that the lander’s solar panels are covered in dust, and it is running out of power. An initial analysis of the data confirms findings that the researchers obtained from the other two meteorite impacts. “It’s crazy. We’d been waiting for so long for these waves, and now, just months after the meteorite impacts, we observed this big quake that produced extremely rich surface waves. These allow us to see even deeper into the crust, to a depth of about 90 kilometres”, added Dr Kim.
This press release has been adapted from ETH Zurich which can be found here.
Paper:
‘Surface Waves and Crustal Structure on Mars’ by Kim D, Banerdt WB, Ceylan S, Horleston A, et al in Science. DOI: 10.1126/science.abq7157a