Taking Mars' pulse at ETH Zurich?

NASA’s InSight mission has ended after four years of collecting unique science on Mars. Mission controllers at the agency’s Jet Propulsion Laboratory in Southern California were unable to contact the lander after two consecutive attempts, leading them to conclude that the spacecraft’s solar-powered batteries have run out of energy – a state engineers refer to as “dead bus.”

NASA had previously decided to declare the mission over if the lander misses two communication attempts. The agency will continue to listen for a signal from the lander, just in case, but hearing from it at this point is considered unlikely. The last time InSight communicated with Earth was Dec. 15, 2022.

“I watched the launch and landing of this mission, and while saying goodbye to a spacecraft is always sad, the fascinating science InSight conducted is cause for celebration,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate in Washington. “The seismic data alone from this Discovery Program mission offers tremendous insights not just into Mars but other rocky bodies, including Earth.”

Short for Interior Exploration using Seismic Investigations, Geodesy and Heat Transport, InSight set out to study the deep interior of Mars. The lander data has yielded details about Mars’ interior layers, the surprisingly variable remnants of its mostly extinct magnetic field beneath the surface, weather on this part of Mars, and lots of quake activity.

“It has been a lifetime adventure for our Swiss team, initiated already in 1997,” says Prof. Domenico Giardini, head of the Mars team at ETH Zurich. “We contributed to InSight the seismometer electronics, project management, the design and daily operation of the Marsquake Service and we finally had the chance for amazing research, we have been very lucky in our life as scientific researchers to be able to participate in a successful planetary exploration mission.”

The highly sensitive seismometer detected 1,318 marsquakes, including quakes caused by meteoroid impacts; the largest unearthed boulder-size chunks of ice late last year.

Such impacts help scientists determine the age of the planet’s surface, and data from the seismometer provides scientists a way to study the planet’s crust, mantle, and core.

“This was the first mission to explore the deep interior of another planet. We now know for example that the core is too big for our classical models, which changes the way we think about the terrestrial planets in our solar system and elsewhere in the universe”, says Simon Staehler, a Senior Scientist working in the Seismology and Geodynamics group led by Domenico Giardini at the Institute of Geophysics, ETH Zurich.

The seismometer was the last science instrument that remained on as dust accumulating on the lander’s solar panels gradually reduced its energy, a process that had begun before NASA extended the mission earlier this year.

“Insight has been such a huge part of our lives for the past four years” said John Clinton, Director of Seismic Networks at the Swiss Seismological Service, ETH Zurich, and head of Insight’s Marsquake Service. “Although the daily review of new data from Mars has now sadly come to an end, analysis of this amazing dataset will continue for years - we still have many outstanding puzzles to solve, and doubtless there are new discoveries to make.”

NASA InSight mission

InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) is an unmanned external Nasa Mars mission. In November 2018, the stationary lander, which is equipped with a seismometer and a heat probe, safely landed on the Martian surface. The geophysical instruments on the red planet permit exploration of its interior. A number of European partners, including France's Centre National d'Études Spatiales (CNES) and the German Aerospace Center (DLR), are supporting the InSight mission. CNES provided the Seismic Experiment for Interior Structure (SEIS) instrument to NASA, with the principal investigator at Institut de Physique du Globe de Paris IPGP). Significant contributions for SEIS came from IPGP; the Max Planck Institute for Solar System Research (MPS) in Germany; Imperial College London and Oxford University in the United Kingdom; and Jet Propulsion Laboratory (USA).

Author: From the editors / NASA

Researchers working in the Marsquake Service, at ETH Zurich have been analysing the measurements made by the NASA InSight mission’s seismometer on one of our neighbouring planets. For almost three years, the only seismic waves it detected on Mars were ones that propagated from the respective quake’s focus, or hypocentre, through the depths of the planet. However, the researchers were hoping all along for an event that would also generate waves travelling along the planet’s surface. Their wait was finally rewarded on 24 December 2021, when a meteorite impact on Mars yielded the type of surface waves they had been long anticipating.

Atypical characteristics in the quake readings led the researchers to suspect its source was near the surface, so they contacted colleagues who were working with a probe orbiting Mars. And indeed, images taken by the Mars Reconnaissance Orbiter in late December 2021 showed a large impact crater about 3,500 kilometres from InSight.

“The location was a good match with our estimates for the source of the quake,” says Doyeon Kim, a geophysicist and senior research scientist at ETH Zurich’s Institute of Geophysics. Kim is the lead author of a study that has just been published in the journal Science [https://doi.org/10.1126/science.abq7157]. The researchers were also able to pinpoint a meteorite impact at just under 7,500 kilometres (about 5,000 miles) from InSight as the source of a second atypical quake.

Because the hypocentre of each earthquake was at the surface, they generated not only seismic body waves similar to previously recorded marsquakes in which the hypocentres were at greater depth,  but also waves that propagated along the planet’s surface. “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,” Kim says.

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.

A surprising result

“Until now, our knowledge of the Martian crust has been based on only a single point measurement under the InSight lander,” Kim explains. 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 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 can tell us about conditions billions of years ago and the timeline of impacts, which were particularly common in Mars’ early days.

Kim explains how the new measurement was made, “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 the different frequencies are 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 5 and 30 kilometres below the surface of Mars.

Greater seismic velocity explained

Why then was the average speed of the surface waves recently observed considerably higher than would be expected based on the earlier point measurement under the Mars InSight lander? Is this mainly due to the surface rock, or are other mechanisms in play? 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’s 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,” Kim explains.

Solving the mystery of the Mars dichotomy

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,” says 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.

A long wait for the wave

The ETH Zurich researchers 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”, says Kim.

Reference

Kim D et al.: Surface Waves and Crustal Structure on Mars. Science 27 October 2022, doi: 10.1126/science.abq7157 [https://doi.org/10.1126/science.abq7157]

Since 2018, when the NASA InSight Mission deployed the SEIS seismometer on the surface of Mars, seismologists and geophysicists at ETH Zurich have been listening to the seismic pings of more than 1,300 marsquakes. Again and again, the researchers registered smaller and larger Mars quakes. A detailed analysis of the quakes’ location and spectral character brought a surprise. With epicentres originating in the vicinity of the Cerberus Fossae - a region consisting of a series of rifts or graben - these quakes tell a new story. A story that suggests vulcanism still plays an active role in shaping the Martian surface.

Mars shows signs of geological life

An international team of researchers, led by ETH Zurich, analysed a cluster of more than 20 recent marsquakes that originated in the Cerberus Fossae graben system. From the seismic data, scientists concluded that the low-frequency quakes indicate a potentially warm source that could be explained by present day molten lava, i.e., magma at that depth, and volcanic activity on Mars. Specifically, they found that the quakes are located mostly in the innermost part of Cerberus Fossae.

When they compared seismic data with observational images of the same area, they also discovered darker deposits of dust not only in the dominant direction of the wind, but in multiple directions surrounding the Cerebus Fossae Mantling Unit. “The darker shade of the dust signifies geological evidence of more recent volcanic activity – perhaps within the past 50,000 years - relatively young, in geological terms,” explains Simon Stähler, the lead author of the paper, which has now been published in the journal Nature. Stähler is a Senior Scientist working in the Seismology and Geodynamics group led by Professor Domenico Giardini at the Institute of Geophysics, ETH Zurich.

Why study the terrestrial neighbour?

Exploring Earth’s planetary neighbours is no easy task. Mars is the only planet, other than Earth, in which scientists have ground-based rovers, landers, and now even drones that transmit data. All other planetary exploration, so far, has relied on orbital imagery. “InSight’s SEIS is the most sensitive seismometer ever installed on another planet,” says Domenico Giardini. “It affords geophysicists and seismologists an opportunity to work with current data showing what is happening on Mars today - both at the surface and in its interior.” The seismic data, along with orbital images, ensures a greater degree of confidence for scientific inferences.

One of our nearest terrestrial neighbours, Mars is important for understanding similar geological processes on Earth. The red planet is the only one we know of, so far, that has a core composition of iron, nickel, and sulphur that might have once supported a magnetic field. Topographical evidence also indicates that Mars once held vast expanses of water and possibly a denser atmosphere. Even today, scientists have learned that frozen water, although possibly mostly dry ice, still exists on its polar caps. “While there is much more to learn, the evidence of potential magma on Mars is intriguing,” Anna Mittelholz, Postdoctoral Fellow at ETH Zurich and Harvard University.

Last remnants of geophysical life

Looking at images of the vast dry, dusty Martian landscape it is difficult to imagine that about 3.6 billion years ago Mars was very much alive, at least in a geophysical sense. It spewed volcanic debris for a long enough time to give rise to Tharsis Montes region, the largest volcanic system in our solar system and the Olympus Mons – a volcano nearly three times the elevation of Mount Everest.

The quakes coming from the nearby Cerberus Fossae - named for a creature from Greek mythology known as the “hell-hound of Hades” that guards the underworld – suggest that Mars is not quite dead yet. Here the weight of the volcanic region is sinking and forming parallel graben (or rifts) that pull the crust of Mars apart, much like the cracks that appear on the top of a cake while its baking. According to, Stähler it is possible that what we are seeing are the last remnants of this once active volcanic region or that the magma is right now moving eastward to the next location of eruption.

This study involved scientists from ETH Zurich, Harvard University, Nantes Université, CNRS Paris, the German Aerospace Center (DLR) in Berlin, and Caltech. 

Reference

Stähler SC, Mittelholz A, Perrin C, Kawamura T, Kim D, Knapmeyer M, Zenhäusern G, Clinton J, Giardini D, Longnonné, P, Banerdt WB: Tectonics of Cerberus Fossae unveiled by marsquakes. Nature 2022, doi: DOI:10.1038/s41550-022-01803-y