For decades, planetary scientists assumed Mars was dead.
Geologically, that is. Smaller than Earth, the planet would have cooled faster than ours after it formed. It was, for a time, quite volcanically active. However, as the thinking goes, when the interior temperature gradually dropped, so too did the planet’s ability to generate large-scale geologic activity—such as huge volcanoes and marsquakes.
Recent discoveries, however, belie that belief. It just so happens that Mars is only mostly dead; scientists have found that a large region on Mars has been prone to quakes and even mild volcanic activity in recent geologic times, indicating something is brewing beneath the surface. But what?
Looking over data from several robotic Mars missions, a team of planetary scientists has come to the astonishing conclusion that an immense tower of hot material moving upward in the planet’s mantle is pushing on the crust from beneath, creating pressure that is cracking the surface and causing tectonic activity. Called a mantle plume, it may be a relatively new feature in the interior of Mars, and one that has analogues on—or, rather, under—Earth. It may even have implications for extant life on Mars—or more accurately, underneath it. The work was published in Nature Astronomy.
Mars was once a heavily volcanic planet. The surface is still dotted with these ancient mounds, including one called Olympus Mons. This monster is over 600 kilometers in diameter—roughly equal to the length of the state of Colorado—and towers 21 kilometers above the average surface elevation of its planet, about two and half times as high as Mount Everest. Though other volcanoes on Mars are smaller, they are still huge, and terribly old.
Large-scale volcanism started before Mars was even a billion years old, and were active for roughly a billion years thereafter. Globally, volcano building stopped after that. There’s evidence of some lava flows on Olympus Mons that date back to only a few million years ago, but these are small-scale events and likely sporadic. By three or so billion years ago, the era of active volcano construction on Mars was over. For comparison, most of the active volcanoes on Earth are less than a million years old.
Until recently scientists considered that the end of the story of volcanism on the Red Planet. However, spacecraft orbiting Mars have recently captured high-resolution images that show the final chapter hasn’t yet been written. In a region called Cerberus Fossae there are large numbers of cracks in the surface (fossae are trenches or fissures), and one such feature has dark streaks of material running alongside it for dozens of kilometers. Measurements from orbit show the material is loaded with pyroxene, a mineral associated with explosive volcanic events called pyroclastic flows. Startlingly, these outflows may have occurred only some tens of thousands of years ago. That’s recent, and points toward ongoing activity beneath the surface.
Moreover, in 2018, NASA’s InSight lander touched down on Mars in a region called Elysium Planitia, about 1,600 kilometers from Cerberus Fossae. A mission to help measure what’s going on under the Martian surface, InSight has a seismometer that has detected hundreds of small marsquakes over the past few years, and several that were fair-to-middling in energy. The overwhelming majority of them appear to have come from the direction of Cerberus Fossae. Again, this indicates the Martian mantle may not yet be completely dead.
In the recent Nature Astronomy study, the scientists focused on this region of Mars. Much of the surface the planet shows compression features like wrinkle ridges, formed when the surface of a planet contracts as it cools. However, Elysium Planitia is a bulge on the surface: evidence for extension, a stretching of the crust as the local area expands. The cracks making up Cerberus Fossae are fissures where the crust has split apart from this extension. The scientists also note that the floors of impact craters that formed many millions of years ago are tilted away from the center of the bulge, which would be expected if they formed before the surface was pushed upward. Together, these findings indicate that whatever caused the uplift is relatively young.
All of this evidence is consistent with a mantle plume. The basic idea of a plume is familiar, if you’ve ever watched water boil or a hot-air balloon in flight: in a fluid, hot material rises and cold material sinks, a process called convection. The core of a planet is hot, and the mantle above it is somewhat cooler, so the material heated at the base rises.
The curveball here is that much of the mantle of Mars (and Earth) is actually solid; it’s a misconception that it’s a liquid. But convection can work even in a solid. The silicate material making up the bulk of a mantle is crystalline, and there can be flaws and breaks in the crystal pattern. Under the huge pressures deep underground, atoms from the material below can fill in these cracks in the structure in a process called dislocation creep. In this way, hotter material closer to the core can rise up slowly, essentially flowing. It’s an extremely slow process; the Earth’s mantle flows at a rate of something like two centimeters a year. That’s half the rate your fingernails grow.
It’s not clear exactly how mantle plumes form. At the base of the mantle above the core, a hotter-than-average spot can create a region of stronger convection, where the material flows in a more constrained column. This plume rises to the surface over tens or hundreds of millions of years, and when it gets near the crust the pressure is much lower and the solid material can liquefy. It spreads out, forming a mushroomlike cap that pushes upward on the crust, causing an extension feature like the one seen in Elysium Planitia.
This scenario would explain essentially all the anomalies in Cerberus Fossae: the uplift, the cracks, the volcanic eruptions, the earthquakes. Measurements of Mars’s gravity field even show that the field is slightly weaker under Cerberus Fossae, which would be consistent with lower-density mantle pushing up under the crust. This indicates that the uplift is supported very deep underground.
The scientists used computer models to simulate the geophysics of Mars, and found that a plume some 95–285 degrees Celsius hotter and slightly less dense than the surrounding mantle centered almost directly under the fossae would do the trick. It would form a cap spread out over about 2,500 kilometers, and push the crust up about a kilometer, again matching Cerberus Fossae. It would also be a young feature: The activity in and around Cerberus Fossae appears to have started around 350 million years ago, long after every other large-scale engine inside the planet had effectively shut down.
Although the plume model is an excellent match to the observed data, the scientists acknowledge that there could be other explanations. For example, a slightly lower density blob of mantle material could just be sitting there under the region, which would explain the gravity readings, though it wouldn’t explain the uplift or anything else. The idea that covers the most ground, literally, is a mantle plume.
If the hypothesis is correct, then this is important news. For one thing, many of the conclusions scientists have drawn about the Martian interior based on InSight’s seismic measurements assume that Elysium Planitia is boring—just another spot on Mars. If it’s sitting on the cap of a tremendous plume of hot, low-density material, that changes how we should interpret InSight’s data.
And though it’s a bit of a stretch for now, the plume could have implications for life, too. Scientists assume water under the surface on Mars takes the form of ice, but a warm mantle plume could heat pockets of water enough to be liquid. On Earth, life needs liquid water, so it may not be too silly to consider the possibility of biology deep under the surface of Mars.
In which case Mars may not be entirely dead either geologically or in the more common biological sense. We’ve only just begun to understand the true nature of the Red Planet, and the more we look, the more we find it still has a little kick left in it.
This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.