Saturday, May 11, 2019

Courtesy of NASA/JPL

Martian Dust in the Wind

It’s no secret that dust storm on Mars can have devastating affects on technology. We all remember that it was a global dust storm that caused the end of Opportunity. Still, it is also the Martian wind that cleans the solar panels of the rovers and landers. The same wind that kicks up all the dust on the Red Planet’s surface is what kept both Spirit and Opportunity running well past their initial expiration dates.

Known as dust clearings, these are also expected for the newest inhabitant of Mars, the InSight Lander…but with a twist. The craft’s weather sensors will provide crucial science data every time this type of cleaning happens. In fact, the mission has already had a glimpse of this.

On Sol 65 (February 1st), InSight detected a passing wind vortex (dust devil). While InSight’s cameras weren’t able to snap an image of this vortex, the lander’s two large solar panels did experience very small “bumps” in power. This equated to about 0.7% on one panel and 2.7% on the other. This suggests that a tiny amount of dust was, in face, lifted from their surfaces.

In the cases of Spirit and Opportunity, dust-clearing wind gusts occasionally boosted power by as much as 10%, leaving the solar panels visibly cleaner. And while this wasn’t the case for InSight, the recent event has given scientists their first measurements of wind and dust as it interacted “live” on the Martian surface. Since InSight is the first solar-powered lander to include meteorological sensors that are recording around the clock, these dust cleanings could provide valuable information for the design of future solar-powered missions, as well as how these winds sculpt the landscape.

Engineers regularly calculate how much dust is covering the panels (dust factor) when analyzing InSight’s solar power. While they aw no change in the dust factor around the time of this passing vortex, they did see a clear increase in electrical current, which suggests the wind did lift a small amount of dust.

The instruments aboard InSight that are key to measuring these cleanings are collectively known as the Auxiliary Payload Sensor Suite (APSS). During the first vortex event, APSS measured a steady increase in wind speed and a sharp drop in air pressure. These are signature signs of a passing dust devil. When the wind direction changed by about 180°, this indicated that the dust devil likely passed directly over the lander. APSS also measured a peak wind speed of 45mph (20 meters per second), as well as the biggest air pressure drop ever recorded by a Mars surface mission: 9 pascals, or 13% of ambient pressure. This pressure drop suggests that there may have even been stronger winds that were too turbulent for sensors to record.

“The absolute fastest wind we’ve directly measured so far from InSight was 63mph (28 meters per second), so the vortex that lifted dust off our solar panels was among the strongest winds we’ve seen,” said InSight participating scientist, Aymeric Spiga of the Dynamic Meteorology Laboratory at Sorbonne University in Paris. “Without a passing vortex, the winds are more typically between about 4-20mph (2-10 meters per second), depending on time of day.”

What’s really interesting is that the dust lifting happened at 1:33pm local Mars time. This is consistent with dust devils seen here on Earth. On both planets, the highest levels of dust devil activity are usually seen between noon and 3pm when the intensity of sunlight is strongest, and the ground is hot compared with the air above it.

InSight landed on November 26, 2018 in Elysium Planitia, a windy region on the Martian equator. Since then, each of InSight’s dinner-table sized solar panels has gathered a thin layer of dust which has caused a 30% fall in power output. This is due not only to the dust but to the fact that Mars is moving farther from the Sun in its orbit. Daily operations aboard the lander require roughly 1500 watt-hours per sol, and right now the panels are producing about 2700. So, while the mission’s power engineers are still waiting for the kind of dust cleaning that Spirit and Opportunity experience, they still have ample power incase they don’t see one for a while.

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