But it did: it suddenly moved.
Two years have passed since the Insight lander arrived on the red planet. Since then, the hammer drill installed on the surface by the marslander, an accident. Meanwhile, the seismometer that Insight is equipped with already has multiple marchquakes detected. But that’s not all this instrument notices. To the surprise of researchers, it appears that the seismometer also detects solar eclipses.
Solar eclipse on Mars
If you were on Mars, you would see the Mars moon Phobos moving from west to east across the sky every five hours. About once a year, Phobos is exactly between the sun and Mars. This creates one to seven solar eclipses within three days. As you can see from this, solar eclipses occur much more often on Mars than on Earth, when our moon crosses in front of the sun. “However, the eclipses on Mars are shorter – they last only 30 seconds – and are never total,” explains seismologist Simon Stähler.
Mars moon Phobos orbits Mars, causing solar eclipses. Image: jihemD / Wikimedia Commons / CC BY-SA 3.0
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In April 2019, the first series of solar eclipses were visible from Insight’s landing site, which has been in the Elysium Planitia region since November 2018. As expected, these solar eclipses were recorded by the Martian solar cells. “When Phobos faces the sun, less sunlight reaches the solar cells,” says Stähler. “In turn, they produce less electricity. We can measure the decrease in exposure to light caused by the shadow of Phobos. ” And indeed, the power yield decreased by no less than 30 percent. However, Insight’s weather instruments did not indicate atmospheric changes. But surprisingly, other Mars lander instruments did respond to the solar eclipses.
Seismometer
Both the seismometer and the magnetometer that Mars lander Insight is equipped with, were found to detect the solar eclipses. The fact that the magnetometer started is most likely due to the decrease in electricity from the solar cells. “We didn’t expect the seismometer to respond to the eclipse, however,” says Stähler. “It’s an unusual signal.”
Tilt
During the eclipse, the seismometer suddenly appeared to move and tilt in a certain direction. “This tilt is unbelievably small,” notes Stähler. But however small this effect was, it was still unmistakable. The question, of course, is why the seismometer suddenly moved. “The most obvious explanation would be Phobos’s gravity, similar to how Earth’s moon causes the tides,” says Stähler. But soon this idea was dismissed. If that were the explanation, the seismometer would have to move much more often; every five hours when Phobos passes, and not just during solar eclipses.
Cool ground
The researchers are leaning in their studies to another cause. “During an eclipse, the ground cools down,” says researcher Martin van Driel. “This deforms the ground, causing the instrument to tip.” And that indeed seems to be the best explanation. The infrared sensor measured a temperature drop of about two degrees Celsius during the short-term solar eclipse. This could cause the seismometer to tilt slightly.
Silver mine
This theory is reinforced by an earlier event on Earth. In the Black Forest Observatory, located in an abandoned German silver mine, researcher Rudolf Widmer-Schnidrig discovered a similar phenomenon. An employee forgot to turn off the light during a seismometer test. The heat emitted by the 60-watt bulb turned out to be enough to heat the top layer of granite in the ground. This caused the seismometer to tilt slightly to one side.
The slight tilt of Insight’s seismometer actually comes in handy now. The researchers believe that this tilt will allow them to map the orbit of Mars Moon Phobos more accurately. Insight’s position is the most carefully measured location on Mars. So by precisely determining the start and end times of the solar eclipse, the orbit of Phobos can be calculated very accurately. And that is important for future space missions. For example, the Japanese space agency JAXA plans to release in 2024 send a probe to the moons of Mars and bring samples from Phobos back to Earth. “To do that, they need to know exactly where they are flying to,” said Stähler.
More precise data about Phobos’ orbit could also shed more light on the inner workings of Mars. While our own moon moves a little further from Earth, Phobos is gradually falling back to Mars. It means that the moon will crash on the red planet in about thirty to fifty million years. “We can use this movement of Phobos to estimate how elastic, and thus how warm, the interior of Mars is,” explains researcher Amir Khan. Ultimately, the researchers hope to find out whether Mars was formed from the same material as Earth. They also hope to explain why Earth supports plate tectonics, a dense atmosphere and life, while Mars lacks all of these features.
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