You may think volcanoes, earthquakes, and tsunamis are unique to earth. You’d be wrong. Apollo astronauts set up seismometers on the moon way back in 1969. That data coined the phrase ‘moonquake’ and set forth a new frontier in geophysics. Since then we’ve used satellites and probes to study the compositions of all our celestial neighbors. The results are stunning.
Quakes were discovered on the Sun in the 90s, originating in the photosphere – the Sun’s outer shell. Some of those reached a magnitude of 11.3. To put that in perspective, that’s as powerful as two-million nuclear bombs. Hour-long tremors on the moon can be triggered by tides here on Earth. And they could be devastating for any human settlement there. Volcanoes of ice, water, and gas, known ascryovolcanoes likely dot the moons of Saturn and Jupiter.
Things get crazy on Mars. Olympus Mons, in the planet’s western hemisphere, is a shield volcano like those in Hawaii and the tallest mountain in the solar system. It’s comparable in size to Washington and Oregon combined, with a height more than twice that of Everest. But when it comes to the forces at work below the Martian surface, it gets controversial.
Martian geophysics entered a new era in 2012 when UCLA professor An Yin discovered what could be a planet-wide crustal divide. Yin believes Mars could have two distinct tectonic plates. Compared to the fifteen major plates on Earth this seems mundane. But it may have not always been this way.
Since the late 90s many scientists believed Mars was seismically active early in its history, with tectonic plates and quakes much like Earth. The surface of the planet is clearly divided between a low, flat northern hemisphere and a high and rocky southern hemisphere. It was proposed these northernlowlands are the remnants of oceanic crust. As the older southern highlands subducted underneath,new oceanic crust would form via ridges like those beneath the Atlantic.
This theory would explain both the north’s lower elevation and dearth of impact craters. Oceanic crust on Earth is significantly thinner and denser than continental crust, composed magnesium- and iron-rich mafic rocks. It is because of this greater density that it ‘floats’ lower on the earth’s mantle. Scientists expect these same principles could apply to Mars. The lack of impact craters also supports this. If the northern plate is the product of a constructive boundary – basically a crack where magma can reach the surface and form new land – it would be younger than the southern highlands. Hence, it had less time to accumulate craters.
These processes were assumed long dead. The core of Mars cooled long ago, spurred by a smaller diameter than Earth and less radioactive isotopes in its rocks to keep things warm. As the core cooled, so did the Martian mantle. Material could no longer melt to form new crust. Tectonics ground to a halt. So we thought.
This brings us back to 2012. Studying the Valles Marineris – a crack in Mars’ surface nearly nine- times the size of the Grand Canyon, the largest in the solar system, actually – Yin noticed something interesting. It seemed the ground on either side was moving opposite to the other. On Earth, this type of motion almost always signals a transform fault. Two plates slide past each other, creating earthquakes along the way. This process is particularly infamous in Southern California. There the Pacific Plate is sliding northwards parallel to the North American Plate, forming the San Andreas Fault.
With Mars’ Valles Marineris, Yin found craters offset on either side of the canyon. He foundsteep cliffs nearly identical to those formed by faults here on Earth. And he found chains of volcanos, normally a product of a plate sliding over a volcanic hotspot. His research was soon published in geophysics journal Lithosphere where it gained international attention.
The question became: Is Mars was still active? NASA launched the InSight lander in 2018 to answer just that. Packed with seismometers, the lander set out to record marsquakes and map the planet’s deep interior.
On April 6, 2019 it found what it was looking for. The InSight lander recorded a two-second rumble for NASA analysis. Its estimated magnitude? A weak 2.7. That’s so small you’d probably never feel it. But scientists anticipate more. “We expect Mars to have much bigger quakes eventually. Just like on the Earth, there are lots of small events and fewer big ones,” says Dr. Ingrid Daubar, a planetary geoscientist working on the InSight program. “We're waiting and listening for ‘the big one,’ which could happen any time.”
What remains to be known is where the marsquake came from. Yin’s proposed transform fault is a candidate, but so is planetary cooling – the process of Mars shrinking as its primordial heat radiates away. “The signals could be from either of those sources, or a variety of other possibilities,” says Daubar. “We're still studying the data to try to figure out exactly what could have caused them.”
“Getting the first seismic signals from InSight is important because it shows us that not only is everything working with the spacecraft and the instruments, but also that Mars is actually seismically active,” she explains. “Once we fully analyze these events, they'll be able to tell us a lot about the interior structure of the planet, how it's like Earth or different, and what that means about the planet's formation and evolution.”
The InSight mission won’t wrap up until autumn of 2020. Perhaps by then we’ll have a new host of information, painting an image of Mars unlike any we’ve seen before.
