Mars‘ moons are rather different than our planet’s Moon. Phobos, the one that has a bigger size among the two, is much closer to its planet, and compared to the Moon’s 27-day orbit, Phobos rotates around Mars perpendicular to the planet’s equator trice every sol or Martian day.
This means that solar eclipses are much more common there than those on Earth. Phobos passes by the Sun for an annular or partial eclipse on Mars’ most sols, but it never transits for more than 30 seconds because it moves at incredible speed.
However, during this short time, the Mars InSight lander was able to record something odd happening.
To the surprise of Mars scientists, during Phobos eclipses, the vehicle’s seismometer spins, just an infinitesimal little bit towards one side. Experts at ETH Zurich’s Institute of Geophysics were, in fact, analyzing data from Mars InSight to find out if some of the impacts of eclipses here on Earth also happen on Mars.
“When Earth experiences a solar eclipse, instruments can detect a decline in temperature and rapid gusts of wind, as the atmosphere cools in one particular place and air rushes away from that spot,” explained seismologist Simon Stähler of ETH Zurich.
Mars InSight is geared with temperature and wind sensors, but these registered no atmospheric changes throughout Phobos transits. However, the solar cells did.
“When Phobos is in front of the Sun, less sunlight reaches the solar cells, and these, in turn, produce less electricity,” Stähler said. “The decline in light exposure caused by Phobos’s shadow can be measured.”
Still, that was the extent of the ‘expected;’ the unexpected was the fact that both the magnetometer and the seismometer registered peculiar readings, but in particular the latter.
“We didn’t expect this seismometer reading; it’s an unusual signal,” Stähler said. “Imagine a 5-franc coin; now, push two silver atoms under one edge. That’s the incline we’re talking about: 10^-8.”
The signal doesn’t seem to be false positive, as it is recorded for three transits.
What Phobos’ Future Could Hold
The team believed that it might be seismic aftermath to the moon’s tidal pull as it passed. However, when they analyzed it in comparison to other readings of seismic activity on Mars, the signal had no similarities with other such activities.
A possibility is that the tether linking the seismometer to the vehicle contracted. Still, this would have created a tilt in the opposite direction to what it was witnessed. A change in atmospheric temperature could have also generated a density change that triggered the seismometer, but no such change was observed.
There was one more signal. An infrared radiometer registered a slight drop in surface temperature during the longest transit, and then a period of about a minute and a half in which the ground warmed back up to its previous temperature.
This is the most probable cause of the odd reading, the team says.
“During an eclipse, the ground cools,” said seismologist Martin van Driel of ETH Zurich. “It deforms unevenly, which tilts the instrument.”
This new information could be used to learn more about Phobos and Mars, the researchers said. Mars InSight’s location is very precisely mapped, and knowing when a Phobos eclipse starts and ends at that place could help experts more accurately curb its orbit. That, then, could help them understand what Phobos’ future could hold.
The research has been published in the scientific journal Geophysical Research Letters.