Solar eclipse measured on Mars, affects interior

Surprise on Mars
If the moon Phobos obscures the sun, the seismometer tilts to the side, hardly measurable, and thus registers the transit of the moon in front of the sun. Credit: NASA/JPL

NASA’s InSight mission provides data from the surface of Mars. Its seismometer, equipped with electronics built at ETH Zurich, not only records marsquakes, but unexpectedly reacts to solar eclipses as well. When the Martian moon, Phobos moves directly in front of the sun, the instrument tips slightly to one side. This miniscule effect could aid researchers in determining the planet’s interior.

An observer standing on Mars would see the planet’s moon Phobos cross the sky from west to east every five hours. Its orbit passes between the sun and any given point on Mars about once each Earth year. Each time it does so, it causes from one to seven solar eclipses within the space of three days. One place where this happens is the site of NASA’s InSight lander, stationed in the Elysium Planitia region since November 2018. In other words, the phenomenon occurs much more frequently than on Earth, when our moon crosses in front of the sun. “However, the eclipses on Mars are shorter—they last just 30 seconds and are never total eclipses,” explains Simon Stähler, a seismologist at ETH Zurich’s Institute of Geophysics. Photos taken by NASA’s two Mars rovers, Opportunity and Curiosity, also show a sharp-edged lump against the backdrop of the sun.

Photographs are not the only way to observe these transits. “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,” Stähler explains. An analysis of the data from InSight should indicate whether similar effects are also detectable on Mars.

Waiting for 24 April 2020

In April 2019, the first series of solar eclipses were visible from InSight’s landing site, but only some of the data it recorded was saved. Initial indications from that data prompted Stähler and an international research team to prepare excitedly for the next series of eclipses, due on 24 April 2020. They published the findings from their observations in August in the journal Geophysical Research Letters.

As expected, InSight’s solar cells registered the transits. “When Phobos is in front of the sun, less sunlight reaches the solar cells, and these in turn produce less electricity,” Stähler explains. “The decline in light exposure caused by Phobos’s shadow can be measured.” Indeed, the amount of sunlight dipped during an eclipse by 30 percent. However, InSight’s weather instruments indicated no atmospheric changes, and the winds did not change as expected. Other instruments; however, delivered a surprise: both the seismometer and the magnetometer registered an effect.

Surprise on Mars
The moon Phobos orbits Mars. Credit: jihemD/Wikimedia Commons/ CC BY-SA 3.0

Unusual signal from the seismometer

The signal from the magnetometer is most likely due to the decline in the solar cells’ electricity, as Anna Mittelholz, a recent addition to ETH Zurich’s

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