Mars: unusual signals cause surprise

ETH Zurich researchers wanted to observe a solar eclipse on Mars. By chance, they made an amazing discovery in the process.

The Mars moon Phobos: When it passed the sun, researchers from ETH Zurich were able to detect an unexpected signal. ESA/DLR/FU Berlin (G. Neukum)

The Mars moon Phobos: As it passed the sun, researchers from ETH Zurich were able to detect an unexpected signal. ESA/DLR/FU Berlin (G. Neukum)

Sometimes pure coincidence helps. The seismometer on Nasa’s Insight spacecraft on Mars is actually designed to measure only Martian quakes. But a few months ago, the instrument, whose electronics were built at ETH Zurich, caused a surprise.

It responded to a solar eclipse- in a curious way that the researchers would not have expected, as ETH now announces.

Mars: Solar eclipse lasts only 30 seconds

But from the beginning: Researchers wanted to observe a solar eclipse on Mars in April 2020. The moon Phobos orbits Mars from west to east every five hours. In its orbit, it passes directly in front of the Sun at each location on the planet’s surface about once per Earth year. One to seven solar eclipses then occur within three days. The phenomenon can be observed much more frequently than on Earth. „However, eclipses on Mars are shorter, lasting only 30 seconds, and the occultation is never complete", explains Simon Stahler, seismologist at the Institute of Geophysics at ETH Zurich.

NASA’s Insight spacecraft has been at the landing site in the so-called Elysium region, an extensive plain on Mars, since November 2018. This is where the researchers wanted to observe the solar eclipse. Already in April 2019, the probe had recorded a first series of solar eclipses, but only part of the data was stored. Stahler and an international group of researchers subsequently prepared for the next eclipse series on 24. April 2020 ahead.

Sudden gusts of wind: Direct effect on environment

The so-called transits, i.e. the passing of the moon by the sun, can be seen not only in pictures. They also have a direct, tangible effect on the environment. „On Earth, you measure a drop in temperature and rapid gusts of wind during an eclipse because the atmosphere gets colder in one place at a time and and the air moves away from there", explains Stahler. Using data from the Nasa probe, researchers wanted to find out whether similar effects could also be detected on Mars.

Nasa rover Curiosity has captured images of a transit series on Mars: Phobos passes by the Sun as an angular chunk. Photo: NASA/JPL-Caltech/MSSS

Nasa rover Curiosity has captured images of a transit series on Mars: Phobos passes by the sun as an angular chunk.

Job search for engineers

And indeed, the spacecraft’s solar cells registered the transit the scientists had expected. „If Phobos is in front of the sun, less sunlight reaches the solar cells and they produce less electricity", explains Stahler. „This is the way to measure the decrease of the light irradiation due to the occultation.“ But: The weather station of Insight showed no atmospheric changes, the winds did not turn as expected. On the other hand, the seismometer showed an effect and thus took on an unexpected significance in the analysis of the solar eclipse.

Signal on Mars is "a curiosity"

The magnetometer also showed a signal, but this is most likely due to the current drop in the solar cells, as geophysicist Anna Mittelholz was able to show. The seismometer signal cannot be explained in this way, but is "a curiosity", says Stahler.

What happened: The seismometer tilted minimally in a certain direction during the eclipse. „The tilt is really extremely small", says Stahler, who explains it quite figuratively: "Imagine a five-franc piece and push two silver atoms underneath it on one side. That would result in this inclination of 10 to the power of minus 8.“

A tiny effect, but still clearly measurable. „The most banal explanation would be that the gravitational pull of Phobos is responsible for this, just as the Earth’s moon causes the tides", says Stahler. However one could exclude this thesis fast. If tidal forces were responsible, the seismometer signal should be measurable whenever Phobos passes by- so every five hours.

ground cools down minimally

The most probable cause for the toppling: "During the eclipse the ground cools down. It deforms unevenly, triggering the tilt.", says Martin van Driel of the Seismology and Wave Physics Group. In fact, an infrared sensor on Mars was able to measure a cooling of the ground by two degrees. The cold wave penetrated only micro- to millimeters into the ground during the 30-second eclipse, but the effect was enough to cause the tipping.

Important finding for future Mars missions

This finding could become enormously important for future space missions. Because the scientists can use the tiny tilt signal now possibly, in order to determine the course of Phobos more exactly than so far possible: If one knows, When a Phobos transit begins and ends exactly at Insight’s landing site, the moon’s orbit can be precisely calculated. The Japanese space agency Jaxa, for example, plans to send a probe to the Martian moons in 2024 to collect samples from Phobos. „To do this, you have to know exactly where you want to go", says Stahler.

In addition, more precise data could be obtained from Phobos’ The orbit will give more information about the interior of the planet Mars. Its relation to the planet is completely different from that of our Moon to the Earth. Our moon is gaining angular momentum and continuously moving away from Earth. Phobos, on the other hand, is slowing down and approaching Mars. In 30 to 50 million years, it will crash onto its surface. „We can use this slight deceleration to estimate how elastic, and thus how hot, Phobos is Mars’ interior, because cold material is always more elastic than hot material," says, explains Amir Khan of ETH’s Institute of Geophysics.

Why is Mars hostile to life?

The researchers want to know whether Mars was formed from the same material as Earth, or whether different building blocks could provide an explanation for why the two planets are so different: Earth has plate tectonics, a dense atmosphere and conditions conducive to life- not on Mars.

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