NASA’s InSight Lander is a multipurpose mission to study Mars's interior like never before and has already reported the discovery of some exciting firsts, such as the first reported marsquakes. On April 24 this year, the industrious lander experienced a solar eclipse, and its instruments reported the effects of that, too. A new study in Geophysical Research Letters reports the curious effect solar eclipses appear to have on the lander's seismometer.

During the eclipse, Mars’s moon Phobos passed in front of the Sun, blocking some of the sunlight. This was recorded by InSight’s cameras and solar panel. Phobos blocks around 40 percent of the light, so that detection was not surprising. But it was certainly unexpected that the eclipse would register with both the magnetometer and seismographer. In fact, the seismometer even ended up slightly tilted.

“When Phobos is in front of the sun, less sunlight reaches the solar cells, and these in turn produce less electricity,” lead author Dr Simon Stähler, from ETH Zurich, said in a statement. “The decline in light exposure caused by Phobos’s shadow can be measured. But we didn’t expect this seismometer reading; it’s an unusual signal.”

Eclipses on Mars are a lot more common than on Earth. Phobos crosses the Mars sky every five hours, its orbit passing between the Sun and any region on Mars at some point once every year. When it does, it causes between one and seven eclipses within a three-day period. InSight experienced three in a day, which allowed it to get these interesting and shocking measurements multiple times.

The changes detected by the magnetometer were easy enough; they are due to solar cells electricity. Currents induce magnetic fields after all. The seismometer signal is a lot more peculiar. The whole instrument tilted by a tiny fraction. The researchers compared it to a coin resting on a table, being lifted on why side by adding just two atoms under one edge.

“The most obvious explanation would be Phobos’s gravity, similar to how Earth’s Moon causes the tides,” Stähler said, “but we quickly ruled this out.”

If the instrument was sensitive enough to detect Phobos it would do so every five hours when it passes over the Elysium Planitia where InSight is located. The fact that it happened only during the eclipses meant it had to have a different explanation. Mars eclipses last only around 30 seconds, which is enough for the top few millimeters of soil to cool down, and then takes about 1.5 minutes to warm back up to normal. The researchers think this sudden change impacted the seismometer, which ended up being slightly tugged in one direction.

“During an eclipse, the ground cools," explained Martin van Driel from the Seismology and Wave Physics research group. "It deforms unevenly, which tilts the instrument.” 

Studying Phobos and its eclipses is more than just cool curiosity. It helps scientists understand how the interior of Mars works. The moon is slowly getting closer to the Red Planet, and will eventually come crashing down in 30 to 50 million years. It might even have happened before.

“We can use this slight slowdown to estimate how elastic and thus how hot the Martian interior is; cold material is always more elastic than hot,” explained Amir Khan, also at ETH Zurich’s Institute of Geophysics

Understanding the interior of Mars will give us insights on how rocky planets form and why Earth and its neighbors are so different.