Trail Dust - C Instruments

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These measurements will help understand Mars' interior structure, and the evolution of terrestrial planets more generally. At the noise model review at JPL in November , I welcomed the InSight team's plans to test a seismometer in 'field conditions' outdoors. Unlike the rather simple instrument on the Viking lander, the ultra-sensitive InSight seismometer will be placed directly on the ground on Mars, and will be protected by a wind and thermal shield.

Still, it will not be deeply buried in a concrete vault, as conventional Earth-based seismic stations are.

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This makes the InSight seismometer susceptible to surface tilt effects due to wind stresses and horizontal variations in atmospheric pressure. In fact, when I made a study of seismic noise backgrounds in connection with a Venus lander study a few years ago , I found the most useful data on these effects were not in traditional geophysics papers but in the literature on military applications of seismology — in perimeter security and treaty verification, where understanding such noise is particularly important. For example, seismometers were air-dropped on the Ho Chi Min trail to detect enemy supply convoys in the Vietnam war, and a world-wide network of seismic and infrasound stations is operated by the Comprehensive Test Ban Treaty Organization CTBTO to detect clandestine nuclear explosion tests.

I had fun showing the disturbances from distant earthquakes, from the astronauts ascending the ladder of the Apollo lunar module, and a nearby Leopard 1 main battle tank all on the same plot. Anyway, while the Noise Model Review did — as such reviews often do — a fine job of addressing the "known unknowns", such a field test offered the possibility to flush out really unexpected effects, which I thought to be very important. As I found when putting together a book on Space Systems Failures , you don't know what you don't know, and that's what usually catches you.

The tests also offered the prospect of getting seismic data in a setting where dust devils would be present, and I offered to help by augmenting their field installation with an array of pressure loggers that I've used in previous dust devil surveys e. Indeed, the InSight mission includes wind, pressure and magnetic field sensors to document these environmental variables. These sensors will provide an interesting and powerful record of the meteorological and magnetic environment in their own right.

They are also critical for subtracting out these noise effects from the seismometer signal. In particular, wind causes the solar arrays to flap slightly, and wind pushes sideways on the lander, causing different loads on its three feet. These varying loads cause the ground to deform slightly, which appears as a tilt signal on the seismometer.

Similarly, variations in the atmospheric pressure cause small deformations on the ground, since pressure is simply the weight of the air column.

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Embraced by the seismometer team as a Collaborator albeit an unfunded one , I had the opportunity to look through the seismic data together with Sharon Kedar of the Jet Propulsion Laboratory, who led the testing campaign. He looked in the seismic record, and I examined the data from my pressure loggers. While there were many seismic events with a variety of causes, not always known — Goldstone is in the Ft. Irwin military training area , and many dust devil vortices in the pressure data, matching them up was complicated by the fact that the data were recorded on different systems which were not perfectly synchronized.

But a pair of vortices ten minutes apart, recorded at My pressure loggers, distributed in a cross formation about 60 meters in cardinal directions from the seismic station you can actually just see them as dark dots near the seismic instrument corral in Google Earth if you go to The fact that all loggers showed it means it was a rather large vortex, perhaps meters in diameter or so, and close analysis suggested it passed from the west to the east, somewhat south of the seismometer. The seismometer indicated a horizontal acceleration of about 2 microns per second squared or 0.

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Just from the units you can tell what a sensitive measurement this is! The North-South axis showed a somewhat symmetric dip, while the East-West had more of a 'heartbeat'. A little thought explains this effect. The dust devil vortex is a low-pressure system.

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A 1 mbar pressure drop in SI units is 1 hectoPascal, or Pascals. Thus on every square meter of ground inside the dust devil, there is N less weight of air pushing on the ground than elsewhere. Add that up over a dust devil say 10m in diameter, and you have about 10, N, or the weight of a small car. So a large dust devil can cause a very significant change in the loading of the ground, and it is no surprise the ground deforms by a tiny amount.

In essence, the dust devil sucks on the ground, pulling it upwards like a tablecloth pinched between thumb and forefinger. So the ground tilts away from the dust devil. Naomi Murdoch, a postdoc working in France on the InSight seismometer team, put together a model of this tilt, using classical theory on elastic deformations. From a seismic survey done at the playa, we had a good idea of the rigidity of the playa sediments. They make for a rather soft material, geologically speaking, and perhaps not too unlike the regolith we expect to land on at Elysium on Mars.

We pretended the dust devil acted as a point load, equal to the pressure drop integrated over an area the pressure drop actually declines smoothly with distance from center , which allowed an analytic solution to the deformation problem. And it seemed that the numbers added up: As for the shape of the tilt signatures, assuming the dust devil moved in a straight line actually many follow slightly cycloidal paths , then in the cross-track direction, the seismometer tilts more and more towards the close-approach point, and then settles back as the devil gets further away. In the along-track direction, the seismometer tilts more and more towards the oncoming devil, then at closest approach must snap over into the opposite downwind direction before relaxing back to vertical as the devil recedes.

The signals from the two seismometer axes which may be in an arbitrary direction relative to the dust devil path are a combination of these two. In addition to the tilt signal, there are also short-period fluctuations in the seismic data and in pressure data.

Detecting Dust Devils with Insight

These fluctuations are presumably caused by azimuthal variations in dust devil structure: Indeed, early dust devil observers noted that multiple cores can exist, dancing around in a circle like Dervishes. Kindle Cloud Reader Read instantly in your browser. Product details File Size: April 25, Sold by: Share your thoughts with other customers. Write a customer review. Amazon Giveaway allows you to run promotional giveaways in order to create buzz, reward your audience, and attract new followers and customers.

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