Enlarge / InSight captured seismic waves released when an impact formed this crater in 2021. But a similar event can't be tied to any craters.

Earth has earthquakes. Mars has marsquakes . There is just one difference: marsquakes are most frequently caused by meteoroid crashes since the Red Planet lacks the tectonic plates that shift pieces of crust on Earth. So what caused the most intense marsquake ever when there has been no evidence of a collision?

Vibrations from the 4.7 magnitude quake sent tremors through the Martian crust for six hours (if not more) and were captured by NASA’s InSight lander in May 2022. Otherwise known as S1222a, this marsquake was assumed to have been caused by a meteoroid impact , so an international team of researchers immediately began searching for evidence of a fresh crater. The problem was that none existed. That's when the team, led by planetary geophysicist Benjamin Fernando, began thinking that something was potentially going on beneath the surface.

“We undertook a comprehensive search of the region in which the marsquake occurred,” Fernando and his team said in a study recently published in Geophysical Research Letters . “We did not identify any fresh craters in the area, implying that the marsquake was likely caused by geological processes.”

Enlarge / Image showing a dust storm over the northern hemisphere of Mars. (credit: ISRO)

If the explorers from Journey to the Center of the Earth were to journey to the center of Mars instead, they definitely wouldn’t come across the subterranean oceans or live dinosaurs they encountered in the movie, but they would probably see something different from our planet’s core.

Earth has a mantle of rock that moves like a sluggish liquid. Beneath the mantle is a liquid iron outer core and solid iron inner core. Because Earth and Mars are both rocky planets, and might have even had similar surface conditions billions of years ago, does that mean we should expect the same interior on Mars? Not exactly.

When two teams of researchers used data from NASA’s InSight lander and other spacecraft to get as close to the core of Mars as they could in a lab, they found that the red planet is not much like Earth on the inside. Data from NASA’s InSight lander’s SEIS (Seismic Experiment for Interior Structure) project had previously suggested that Mars has a large core that is not very dense. But the new analysis, which included additional seismic signals, indicates that what was once thought to be the surface of the Martian core is actually a thick molten rock layer. The actual core of Mars is most likely much smaller.

Enlarge / A self portrait of InSight's hardware on the red planet. (credit: NASA/JPL-Caltech)

To say Mars is a bizarre planet might be something of an understatement. It has nearly no atmosphere, has an unstable liquid metal core that causes it to wobble on its axis constantly, and as a frozen desert, is an oxymoron in itself. As if Mars wasn’t strange enough, data from NASA’s InSight Lander (RIP) has now revealed that the red planet is spinning faster and faster every year.

The increasing spin went unknown until a research team found evidence of acceleration through InSight’s RISE (Rotation and Interior Structure Experiment) instrument. That same team, led by radio scientist Sebastien Le Maistre of the Royal Observatory of Belgium, who is also the principal investigator of RISE, had previously found that the core of Mars is most likely a glob of molten metal. Looking further into RISE data from InSight’s first 900 days on Mars, they saw that the planet’s spin was accelerating by a fraction of a millisecond per (Earth) year, or about 0.76 milliseconds. Martian days are gradually growing shorter. But why?

What lies beneath—or above

RISE’s main objective was to see how much Mars wobbled as its orbit was pushed and pulled by the gravity of the Sun. This would determine whether the core was more likely to be solid or liquid. However, RISE also had another task, which was measuring the length of a Martian day. Days on Mars, known as sols, are about a half-hour longer than Earth days at 24 hours and 37 minutes. RISE measured both the rotation rate and wobbling of Mars with reflected radio waves. When it received a radio signal from NASA’s Deep Space Network (DSN), it would reflect those waves right back at Earth. The difference between the frequency of the signal sent out by the DSN and the signal that bounced back to Earth told the InSight team how the lander was moving along with Mars.

Enlarge / Artist's view of what InSight looked like after landing. (credit: NASA/JPL-Caltech )

Mars appears to be a frozen expanse of red dust, gaping craters, and rocky terrain on the outside—but what lies beneath its wind-blasted surface? NASA’s InSight lander might have discovered this before it took its proverbial last breaths in a dust storm .

Whether the core of Mars is solid or liquid has been long debated. While there is no way to observe the Martian core directly, InSight tried. Its seismometer, SEIS, was the first instrument to find possible evidence of a liquid core. In the meantime, its RISE (Rotation and Interior Structure Experiment) instrument had been measuring minuscule changes in the planet’s rotation as it orbited, “wobbles” in its axis caused by the push and pull of the Sun’s gravity.

“Our analysis of InSight’s radio tracking data argues against the existence of a solid inner core and reveals the shape of the core, indicating that there are internal mass anomalies deep within the mantle,” write the researchers behind the instrument in a study recently published in Nature.