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Phobos' Shadow: Experience an Eclipse on Mars with NASA's InSight Lander

NASA's InSight Mars lander focuses its sensitive instruments on the Martian surface and below, but it took a moment to appreciate celestial events last week: two quick eclipses of the sun by Mars' moon Phobos.

InSight, which landed on the Red Planet in November, caught views of one eclipse with its Instrument Deployment Camera on March 5 and the other with its Instrument Context Camera on March 6. Its radiometer also showed a 1 degree Celsius drop in temperature for both short eclipses, as well as a third one. InSight's cameras are essential to setting up and operating its array of instruments that track Mars weather, seismic activity and below-surface heat.

While cameras from the Mars spacecraft have captured a Phobos eclipse before — including Curiosity tracking the moon's progress across the sun — this is the first time this type of observation was made with a radiometer.

Related: InSight Celebrates 100 Days on Mars by Watching the Sun Set

A view from the InSight Mars lander's Instrument Deployment Camera, on the lander's robotic arm, as Mars' moon Phobos passed in front of the sun March 5, 2019. The camera was able to capture a brief darkening of the ground around it over the 24.3 second eclipse. (The camera captures a frame of video every 50 seconds.) (Image credit: NASA/JPL-Caltech)

"The cooling is by about 1 degree C and is thus somewhat larger than expected and certainly better than with the most pessimistic estimates (that would have said we will not be able to see it at all)!" Tilman Spohn, an instrument lead for InSight, said in a blog post from the German space agency DLR. "So the team is happy and rejoicing about the first eclipse on Mars ever observed with a radiometer."

Just like meteor impacts with Mars and Marsquakes can help InSight's seismometer learn about the planet's composition, this natural eclipse helped the radiometer learn about the makeup of the material near the planet's surface based on how it conducted the change in heat.

"There is a quantity that scientists call the thermal inertia," Spohn said. "The quantity depends on the thermal conductivity of the near surface material, its density and its heat capacity." A small value would lead to a bigger observed effect from the brief eclipse, and would suggest low thermal conductivity or a more porous material.

"The team will now have to analyze the data and come up with a model of the uppermost millimeters or so of the surface material," he added. "So it is part of our efforts to measure the geophysical parameters of Mars."

The view from InSight's Instrument Context Camera under the lander's deck when Mars' moon Phobos passed in front of the sun on March 6, 2019, causing a 26.7 second eclipse. (This camera also captures a frame of video every 50 seconds.) (Image credit: NASA/JPL-Caltech)

InSight deployed its instruments one by one after its landing last year, and it placed its final instrument, a heat probe "mole," on Feb. 12. The mole has since hit a rocky obstacle as it drills itself into the Martian surface, and Spohn said in the blog post that the team has been meeting to figure out next steps for the instrument. The thermal conductivity measurement, as well as measurements from the seismometer as the mole tries to propel itself downward, should help the researchers figure out its exact predicament.

Email Sarah Lewin at slewin@space.com or follow her @SarahExplains. Follow us on Twitter @Spacedotcom and on Facebook

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