Beneath its cold, dusty surface, Mars is buzzing. The quiet, steady hum beats periodically to the rhythm of earthquakes shaking across the planet, but the source of this alien music remains unknown.
This Martian buzz is just one of a series of new mysteries and discoveries detected by NASA's InSight lander. The work, described in a set of five studies published today in Nature Geoscience and Nature Communications, offers a glimpse into the surprising activity found above and below the surface of the Red Planet.
InSight touched down on Mars in November 2018, after a terrifying descent to a flat, featureless expanse near the planet's equator. Since then, the spacecraft has been using an extremely sensitive seismometer and a host of additional instruments to take readings that help scientists unravel the geological activity and internal structure of Mars.
"It's such a relief to finally be able to stand up and scream, to look at all these amazing things we're seeing," says Bruce Banerdt, the InSight mission's principal investigator.
In addition to the strange buzz, InSight's latest batch of data describes the first active fault zone discovered on Mars, patterns and pulses in modern magnetic fields, and hints at the planet's magnetic past. Taken together, the information obtained from Mars is vital to discovering how all rocky planets form and evolve over time.
"You can't model it from Earth alone, you need more data points," says Suzanne Smrekar, deputy principal investigator for the InSight mission. "It's super exciting that we're seeing some of these things and that we're trying to understand Mars."
Curious origins of the earthquake
One of InSight's main goals is to measure seismic activity on Mars. The first few months of hearing tremors were worryingly quiet. But finally, on April 6, 2019, the first detected "marsquake" (marsquake) rumbled across the red planet, thrilling ground-based seismologists.
Since that detection, marsquakes have kept coming, with more than 450 recorded so far, Banerdt says. While the level of activity was no surprise, the small events seem to be happening more and more frequently. The increase in small tremors could be a seasonal effect, but since we've just started collecting records, that's still one of many mysteries the team is working to solve.
Scientists aren't sure exactly where all these marsquakes come from, either. Here on Earth, the planet's surface frequently shakes due to the slow march of plate tectonics. Stress builds up in the crust and a sudden release generates an earthquake. But Mars doesn't have global plate tectonics, so geologists are considering other possible triggers.
Two particular tremors detected by InSight are helping scientists get closer to the answers. The pair rang loud and clear, clocking in between magnitude three and four, allowing researchers to trace them back to their area of ??origin, a series of deep cuts in the surface known as the Cerberus Fossae, which formed 10 million years ago or less.
Expansive lava flows and floodwaters once erupted from these surface fissures, and some fluids may still remain subsurface. As such, cooling and contracting magma pockets, or the movement of molten rock or even water through the subsurface, are possible sources for the Martian rumble pair, says Smrekar.
To determine the cause of the marsquakes and get a better picture of the subsurface, scientists will need to detect more tremors, perhaps even a few bumps.
"We'd love to see a [magnitude] five," says Smrekar. "And who knows? It's a waiting game.
The Martian song that never ends
InSight also detected a mysterious seismic signal that constantly buzzes in the background of tremors.
The earth has many constant background hums, the most frequent of which comes from the lapping of the oceans and the crashing of waves against the shore. But at 2.4 hertz, the Mars drone is higher in pitch than most natural hums on Earth, which tend to drop below 1 hertz, says Stephen Hicks, an earthquake seismologist at Imperial College London. who was not involved in the new studies.
The analysis suggests that Mars's hum is unrelated to the planet's roaring winds, and appears to strengthen with the blast of a distant earthquake. The effect is a bit like ringing a bell by screaming nearby, explains Joshua Carmichael, a quantitative geophysicist at Los Alamos National Laboratory who was not involved in the research. Your voice is a mix of frequencies, and if one matches the resonance of the bell, your screams can make it ring.
Perhaps the buzzing is related to the geology beneath InSight, which somehow amplifies that particular tone, says Banerdt. InSight is located in an ancient crater, as it is filled with dust and sand, which could sound by trapping earthquake waves. However, the structure is also not excited by turbulent winds, and the basin seems a bit small to generate this particular ring, says Banerdt.
The buzzing and shaking could be coming from two separate sources, says Hicks. The InSight lander itself could even be causing the mysterious resonance, Banerdt adds.
"It's extremely disconcerting," says Banerdt. "We don't have a consensus idea of ??what this is."
A sharp magnetic image
In addition to revealing oddities about Mars' current seismic activity, the latest InSight data has uncovered something unexpected about the planet's magnetic field: The field surrounding the lander is 10 times stronger than predicted by satellites.
On Earth, our magnetic field is powered by what's known as a geodynamo, created as the molten iron core inside the constantly churning planet some 1,800 miles below our feet. This rotation generates a global magnetic field that surrounds the planet and protects it from solar radiation.
Mars once had a geodynamo, too, but it stopped billions of years ago, perhaps allowing solar flares to strip away a once-thick atmosphere. Only holes of Mars magnetism still remain, in the form of magnetic minerals locked in the planet's rocks. As such, the planet has a mosaic of magnetic fields of varying strengths covering its surface.
Orbiters have provided a blurry image of that mosaic field as it rises above the ground. But now, InSight has provided the equivalent of a high-resolution image, and the magnetic field is much stronger than expected. While the lander's measurement is just a single data point, the information could help unravel the strength of Mars's past geodynamo, which in turn could answer questions about when and why Mars went from a warm, hot world. humid to a cold and dry orb.
"This measurement was our first little taste, a single point, of how strong the magnetization might be," says Robert Lillis, a planetary space physicist at the University of California, Berkeley, who was not part of the study team.
The new studies also provide a tantalizing clue to the age of the geodynamo. The researchers traced the signals to a unit of rock miles underground that is believed to be about 3.9 billion years old, about 200 million years younger than when the Mars geodynamo is thought to have shut down.
Did the geodynamo churn in the core of Mars for longer than previously believed? It's hard to say for sure because of large uncertainties in the age of the rock units, warns Catherine Johnson, first author of the new study on Mars' magnetic fields and a geophysicist at the University of British Columbia and the Planetary Science Institute. But perhaps future InSight discoveries will offer more clues.
the mysterious pulse
InSight also detected a bit of wobbly magnetism about Mars' constant magnetic field: shifts from day to night and signals that occasionally pulse in a weak pulse.
Daily variations of the magnetic field are common on Earth. On our planet's side, solar radiation hits the upper atmosphere, creating charged particles that interact with winds and Earth's magnetic field lines to generate electrical currents. As a result, the magnetic field on the day side of our planet gets a little extra boost, while the night side is slightly weaker. This diurnal pattern is also seen on Mars, something scientists did not expect to detect from the surface.
The Martian magnetic field appears to have shorter periods of oscillation that take place around midnight and sometimes at dawn or dusk. They don't appear every day, with no apparent rhyme or reason behind the pattern, Johnson notes. The invisible conductor of this magnetic orchestra is likely well above the surface of the Red Planet. There, streams of charged particles from the sun flow around the planet, partially deflected by Mars's weak atmosphere and irregular magnetic field.
Turbulence in these solar winds could generate ripples in Mars' magnetic field, and this process or similar effects could be the source of the pulsations InSight measures on the surface.
"We don't really understand where they come from," says Johnson. The paired analysis of InSight on the ground and an orbiter called MAVEN in the sky could help provide a break in the case.
Meanwhile, all InSight data from the new studies is now publicly available, with new batches released every three months. Banerdt hopes that other scientists will join his team in their continued efforts to pore over the numbers.
"The more minds you have thinking about these things," he says, "the more likely you are to find good answers." (Text and photos: National Geographic)