Martian Solar Eclipse - Strange Activity Recorded

Phobos Transit as viewed from Mars causing Solar Eclipse.
(Credit: Curiosity rover/NASA)

Solar eclipse on Mars is quite different to that on Earth since Phobos, one of the moons of the Red Planet, has an orbit that makes it closer to Mars as compared to our Moon. 

It is merely 6000 km from the Martian surface. As a matter of fact, Phobos revolves around Mars much frequently - thrice every sol (solar day on Mars) and its speed is greater than the speed of rotation of Mars!

Phobos. (Credit: NASA)

So, if you happen to spend your vacation on Mars, you will notice a chunk of rock travelling from western horizon, all the way to the eastern part, three times a day.

Transit of Phobos. (Credit: NASA/JPL-Caltech)

What happens during Martian Solar Eclipse?

By virtue of Phobos' speed, the eclipse lasts for only 30 seconds at a maximum.

It is within this small fraction of time that NASA's InSight lander detected a strange activity - among the other scientific instruments, the seismometer tilts during the solar eclipse by a small portion towards one side. 

The seismometer is responsible for detecting quakes, that is, any movement of the surface due to forces residing inside the body, however feeble.

 

Mars does not have tectonic plates like our planet does, it was shaped by gradual cooling of molten rock, thereby forming a 'static' crust around a 'rocky' mantle, so 'marsquakes' cannot possibly be a result of tectonic forces. 

In order to get an idea of what might be causing the tilt of the seismometer, let us first understand what happens on Earth during a solar eclipse and then compare it with that happening in the 'rusty world'.

A Tale of Two Worlds and their Eclipses

Solar eclipse on Earth has come a long way from being treated as an omen in the early ages to being celebrated as a celestial event, lately. 

With proper filters and viewing procedures, anyone from any part of the world, can enjoy the view for a couple of minutes, but for the people in the region of totality, it's a mesmerizing view. 

As the Moon gradually covers up the Sun and the bright halo dazzles the sky, Earth, at the same time, goes through a phase. Seismologist of ETH Zurich, Simon Stähler, describes the phase:

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.

The change in weather during eclipse was experimentally tested by Giles Harrison and Suzanne Gray, atmospheric scientists the University of Reading, UK, who analyzed the weather data before and after the 1999 eclipse and observed a significant drop in wind speed! 

The same was also reported by the residents in the area under observation. Harrison concluded: 

When the eclipse influences temperature, it seems to also cause the wind to drop and change direction. This is perhaps what people have remarked about.

Summing up, solar eclipse causes change in wind speed on Earth, but nothing as such happens on Mars. 

None of InSight spacecraft's sensors detected any deviation in the atmosphere during Phobos transit. 

However, two of the onboard instruments showed inconsistent readings - seismometer and the magnetometer. 

The scientists figured out that the decrease in current obtained from the solar array was likely the cause, but it isn't enough to describe the tilt of seismometer.

Is it just a Glitch?

Curiosity rover relishing the view. (Credit: NASA/JPL)

By now, the thought that the tilt is just a technical error must have crossed your mind and Why not! It's just a machine after all, operating in a planet where dust storms are frequent and the weather is not quite friendly. 

The moons' tidal force, that is, their gravitational pull, can also affect the parameters.

The signals suggesting the tilt was noted not once, but thrice during Phobos' transit. These signals were faint, but real without a doubt.

Animation of Phobos and Deimos' orbit around Mars,
compressed to a smaller timescale.

Moreover, such signals do not correspond to any of the seismic activity recorded earlier on the planet. Two possible explanations to this unique issue can be -

• Contraction of the tether (cable) connecting the seismometer with the lander.
• Change in atmospheric temperature, hence change in density affecting the instrument.

The first case will result to a tilt definitely, but that will be in a direction opposite to what has been observed. For the second case, we can rule out that possibility as discussed earlier, no deviation in atmospheric conditions were observed during Phobos' transit.

BUT the infrared radiometer detected a minuscule drop in surface temperature during the longest transit, after which the surface was re-heated to the pre-transit temperature. 

Seismologist Martin van Driel of ETH Zurich shed some light on this matter:

During an eclipse, the ground cools. It deforms unevenly, which tilts the instrument.

 

Bottom-line

Phobos passing Deimos in real time. (Credit: NASA/JPL)

Few experiments performed to understand factors affecting seismometer's reading suggest that the instrument is quite sensitive to heat changes in seismic pillar. 

Another series of experiments were performed to replicate the seismometer's tilt that happens during the transit of Phobos on Mars. The data obtained is being used to decipher the mysteries of Phobos and its parent, Mars as well. 

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