At midnight on Mars, the red planet’s magnetic field sometimes starts to pulsate in ways that have never before been observed. The cause is currently unknown.
That’s just one of the stunning preliminary findings from NASA’s very first robotic geophysicist there, the InSight lander. Since touching down in November 2018, this spacecraft has been gathering intel to help scientists better understand our neighbouring planet’s innards and evolution, such as taking the temperature of its upper crust, recording the sounds of alien quakes, and measuring the strength and direction of the planet’s magnetic field.
As revealed during a handful of presentations this week at a joint meeting of the European Planetary Science Congress and the American Astronomical Society, the early data suggest the magnetic machinations of Mars are marvelously mad.
In addition to the odd magnetic pulsations, the lander’s data show that the Martian crust is far more powerfully magnetic than scientists expected. What’s more, the lander has picked up on a very peculiar electrically conductive layer, about 2.5 miles thick, deep beneath the planet’s surface. It’s far too early to say with any certainty, but there is a chance that this layer could represent a global reservoir of liquid water.
On Earth, groundwater is a hidden sea locked up in sand, soil, and rocks. If something similar is found on Mars, then “we shouldn’t be surprised,” says Jani Radebaugh, a planetary scientist at Brigham Young University who was not involved with the work. But if these results bear out, a liquid region at this scale on modern Mars has enormous implications for the potential for life, past or present.
So far, none of these datasets have been through peer review, and details about the initial findings and interpretations will undoubtedly be tweaked over time. Still, the revelations provide a stunning showcase for InSight, a robot that has the potential to revolutionise our comprehension of Mars and other rocky worlds across the galaxy.
“We’re getting an insight into Mars’s magnetic history in a way we’ve never had before,” says Paul Byrne, a planetary geologist at North Carolina State University who was not involved with the work.
A tale of two worlds
Earth has a major global magnetic field thanks to its rotation and churning, iron-rich, liquid outer core. We know that this field has been around for a while and that it has shifted about fairly dramatically across geological epochs, based on natural records of its strength and direction trapped in specific minerals within the crust. The history of Mars’s magnetic field is similarly archived in its crust, as scientists learned in 1997 thanks to data from the Mars Global Surveyor orbiter.
“The same zoo of magnetic minerals that exist on Earth exist on Mars,” says Robert Lillis, a planetary space physicist at the University of California, Berkeley, who wasn’t involved with the new research.
The orbiter detected the red planet’s magnetism from 60 to 250 miles above the surface, and it found that the crustal magnetic field is ten times stronger than Earth’s is when measured from the same height above the surface. This suggests that, once upon a time, Mars also had a major global magnetic field.
Unlike Earth, though, Mars got unlucky. Around four billion years ago, its convulsing outer core appears to have seized up, causing a collapse in its global magnetic field. Left with a weak magnetic shield to defend itself, an outpouring of radiation from the sun—known as the solar wind—gradually stripped away much of its ancient atmosphere, turning a potentially life-supporting, water-rich world into a cold desert.
Grasping why these two planets had such different fates requires the best possible measurements of Mars’ magnetic ghosts, but from orbit, the strength of this remnant magnetic field has poor resolution. It’s like looking at a crowd of people from far away: If many are wearing red shirts and a few are wearing blue, a camera at a distance will largely register the preponderance of red. But get close with the same camera, and those all-important blue hues will be more clearly seen.
“The same is true for magnetic measurements,” says Dave Brain, a researcher of atmospheric and space physics from the University of Colorado not involved with the work. “The closer you get, the more structure you are able to pick out.”
Mysteries at midnight
InSight’s magnetometer, the first placed on the Martian surface, gave scientists their best look yet at the crustal magnetic field, and it gave them a bit of a shock: The magnetic field near the robot was around 20 times stronger than what had been predicted based on past orbital measurements.
Brain, who is familiar with the InSight data, says that this strong, stable magnetic signal is coming from rocks near InSight, but whether they are deep underground or nearer to the surface is currently unclear. That identification matters, Byrne says, because if it’s coming from younger rocks near the surface, it would imply that a strong magnetic field persisted around Mars for longer than we currently think.
Perhaps even more puzzling, InSight also found that the crustal magnetic field near its location jiggled about every now and then. This wobbling is known as a magnetic pulsation, explains Matthew Fillingim, a space physicist at the University of California, Berkeley, and a member of the InSight science team.
These pulses are fluctuations in the strength or direction of the magnetic field, and they are not entirely unusual. Plenty of them happen on Earth and Mars triggered by upper atmospheric chaos, the action of the solar wind, and kinks in the planets’ magnetic bubbles, among other things.
What’s strange is that these Martian wobbles happen at local midnight, as if responding to the demands of an unseen, nocturnal timer.
InSight is near Mars’ equator, and in the same geographic position on Earth, at that time of night, you don’t see these types of magnetic pulsations. Night-time pulsations on Earth tend to happen at higher latitudes and are linked to the northern and southern lights. Right now, the ones on Mars have no clear source, but scientists have at least one suspect in mind.
Although it no longer has a potent global magnetic field, Mars is surrounded by a weak magnetic bubble created as the solar wind interacts with its thin atmosphere. This bubble is in turn compressed by the solar wind’s magnetic field, causing part of the bubble to take on a tail-like shape. At midnight, InSight’s spot on Mars is aligned with this tail, and as it passes through, the tail may be plucking the surface magnetic field like a guitar string.
If a high-altitude spacecraft, like NASA’s Mars Atmosphere and Volatile Evolution, or MAVEN, orbiter, can swing above InSight at just the right time, it might show this to be the case. For now, though, it’s a puzzle without an answer.
Making a splash
During one of the presentations about Mars’ magnetism, scientists also mentioned that features in the magnetic signals appear to be registering an electrically conductive layer somewhere beneath the Martian surface. While the team can’t yet pinpoint an exact depth, they think it wouldn’t be any deeper than 62 miles.
Tests in deserts on Earth have shown how magnetometers are able to tell you whether there is water at depth, Brain explains. The same applies to InSight’s magnetometer, and it is possible that the layer it spotted is an aquifer of water with dissolved solids, or an ice and water layer, that could stretch around the entire planet.
It’s unclear how long bodies of surface water persisted in lakes, rivers, and even oceans in Mars’ past, but there is some evidence that the subsurface contains briny reservoirs today. Mars’s crust also gets warmer as you go deeper, Radebaugh says. And given the strong evidence for widespread ground ice on Mars, it is reasonable to think that subterranean aquifers of liquid water exist, too.
But the devil is in the details, and all other causes of such a signal still need to be ruled out, Brain says. The InSight lander has a drill, but it can only dig down about 16 feet below the surface, so scientists may have to find other ways—perhaps via future Mars missions—to test the watery layer theory.
Whether this Martian aquifer’s existence is ultimately verified or rebuffed, Brain adds, the invaluable nature of InSight’s measurements, including its magnetic ones, is already clear. Even rooted to a single spot in Elysium Planitia, this robotic emissary is beginning to dig up all kinds of buried Martian marvels.