Using an Instrumented Drone to Sample Dust Devils
Dust devils are low-pressure, small (many to tens of meters) convective vortices powered by surface heating and rendered visible by lofted dust. The dust-lifting capacity of a devil likely depends sensitively on its structure, particularly the wind and pressure profiles, but the exact dependencies are poorly constrained. In this pilot study, we flew an instrumented quadcopter through several dust devils to probe their structures.
I’m sitting in my hotel room on the morning after the 2017 LPSC Meeting, trying process all the science that washed over me in the past week. And it was a lot, as you can see by checking out the twitter hashtag #lpsc2017.
From the migration of Martian dune ripples to the global seas on Enceladus, there’s no way for me to do it all justice in one blog post. So instead, I’ll talk about one of the talks that stood out most for me.
On the last day of the conference, Dr. Colin Dundas of USGS gave a mic-droppingly good talk in which he argued very convincingly that Mars’ recurring slope lineae are NOT the result of flowing water. This is a big deal for folks who study Mars but might sound a little arcane for non-Martians.
The animated image above consists of several pictures taken by the HiRISE camera onboard the Mars Reconnaissance Orbiter over several months. The very pronounced dark streaks extending out from the cliff face – those are the recurring slope lineae or RSLs. In many locations on Mars, they recur from one year to the next; they always show up on sloped surfaces; and they are long lines. Hence recurring slope lineae.
Under the current conditions at Mars’ surface, liquid water is not stable over long times, but these dark streaks sure look like small amounts of water running out over the surface, which got people very excited when they were first discovered.
In fact, just a few years ago, some scientists analyzed the spectra of some RSLs and argued they were highly salty brine flows. The salt is important because, even though pure water can’t exist on the surface of Mars, large amounts of salt can stabilize the water, at least for a little while.
This was a huge result, not just because it meant RSLs were small amounts of running water on Mars but because it raised the possibility of much larger sub-surface reservoirs of water. And where there’s lots of water, there could be life.
Well, Dundas’s talk throws all that into doubt. By analyzing the angles of the slopes on which the RSLs occur, Dundas showed that they almost every one of them ended on a slope of about 30 degrees.
Now, if RSLs are flowing water, they should keep flowing, even on small slopes. But if they are some sort of dry granular flow instead, you would expect them to stop once they reached the angle of repose, which is about 30 degrees.
Dark streaks on the sides of Martian dunes seen by HiRISE. These are NOT due to liquid water but resemble in many ways the RSLs.
To bolster his argument, Dundas showed several examples of dry granular flows on Mars that exhibited many of the same properties as RSLs – recurring dark streaks, running along sloped surfaces.
Continued work will either corroborate Dundas’s striking result or circumvent it, and given the number of folks lined up to talk with him after his talk, I’m sure the fans of liquid-flow RSLs will work hard to counter his arguments. And that’s how science progresses – one falsified hypothesis at a time.
Not quite as splashy, I also gave a presentation on our work trying to de-bias dust devil surveys. I’ve posted the presentation below.
All in all, my first LPSC was a great mix of science, warm weather, and warm friends.
Borrowed from @GijsMulders — https://twitter.com/GijsMulders/status/789626580058251264.
The last day of the meeting is always to hardest to write about because I’m usually so busy wrapping things up, I don’t have time to write (hence my writing this post from Boise on the Sunday AFTER the conference).
In any case, lots of talks and goodbyes on the last day, but one talk that stands out for me came from Andrew Hesselbeck Hesselbrock, one of David Minton‘s grad students at Purdue’s EAPS. The talk tackled one of the longest-standing mysteries in solar system science: Why hasn’t Phobos crashed into Mars yet?
Phobos (left) and Deimos (right).
Mars has two tiny moons, Phobos and Deimos, which visibly resemble asteroids but are probably not for a long list of reasons.
Phobos is close enough to Mars that Mars’ gravity is dragging the moon inward, similar to but in the opposite direction as the effect of the Earth’s gravity on the Moon. Phobos is so close, in fact, that astronomers expect it will spiral into Mars in just a few million years.
Phobos and Deimos have probably been orbiting Mars for about the age of the solar system, 4.6 billion year. So if this orbital decay were the whole story, it would be mean we just caught Phobos right at the end of its life, about as likely as catching someone driving from Boise to New York City right as they pass through the Holland Tunnel*. Hesselbeck Hesselbrock suggested in this talk that we’re actually seeing a recurring phase in a much more dramatic story for Phobos.
Instead of steadily spiraling in toward Mars for 4.6 billion years, Phobos (or at least a proto-Phobos) already spiraled in toward Mars before, millions of years ago. But when the satellite got close enough to Mars, Mars’ gravity ripped it apart and formed a disk of rubble around the planet. Soon after forming, this disk spread out, some moving toward Mars (and ultimately impacting the surface) and some moving away. Eventually, the bits that moved outward moved far enough away from Mars that they re-coalesced. In fact, Hesselbeck Hesselbrock speculated that Phobos has actually been reincarnated many times in this way, every time a little smaller than before, until we were left with the bitty moon we see today.
As crazy as this hypothesis sounds, it could answer several puzzles of the Martian system, including accounting for cyclic sediment deposits on Mars’ surface — the deposits form every time Phobos falls aparts and bits rain down on Mars’ surface.
Again, the annual DPS meeting astounds and amazes. Looking forward to Provo next year.
The distance from Boise to New York City is about 2,475 miles, and the Holland Tunnel is about 9,000 feet long. Assuming a uniform driving speed, the probability of catching our driver in the tunnel is roughly equal to 9,000 feet/2,475 miles ~ 0.1%. The probability of catching Phobos during a 10 million year window over the age of the solar system is about 0.2%. Of course, you’re a little more likely to catch our driver in the Holland Tunnel, given NYC’s traffic.
Fourth day of the DPS meeting, and I found myself sitting through some great plenary talks.
First up was Kleomenis Tsiganis‘s Farinella Prize lecture “Flavors of Chaos”, a rapid-fire tour of the intricate and complex web of gravitational interactions among planets and asteroids in our solar system.
Tsiganis’s described how, using a combination of computational and pencil-and-paper techniques, we can pick at the threads in this cosmic network to tease out the early history and evolution of our solar system.
For instance, the orbits of asteroids in the asteroid belt provide subtle clues that, billions of years ago, Jupiter moved inward almost to the orbit of Mars before backing out near to its current orbit, a celestial maneuver referred to as “The Grand Tack“.
This presentation was followed by Leigh Fletcher‘s Urey Prize talk about the menagerie of seasonal changes we observe in the atmospheres for all the outer planets, from Jupiter to Neptune.
The talk was full of beautiful images of the roiling and boiling of planetary atmospheres and concluded with Fletcher’s plea to send another mission to the Uranus or Neptune before he’s too old to participate (some plans from NASA have a mission launching to Uranus or Neptune sometime in the late 2020s/mid-2030s).
Finally, we had a tag-team talk from Ashwin Vasaveda and Sanjeev Gupta about new results from Mars Curiosity rover. In addition to the stupefying images, the thing that impressed me most about the talk was just the level of detail to which we can infer the geological history of Gale Crater, where Curiosity landed.
Gupta described how the tilt of beds of sedimentary rock could be used to infer the presence of a river delta spilling out into the crater, which suggests the existence of a long-lived (millions of years) lake in the crater, probably billions of years ago when Mars was warmer and wetter.
Mars will soon make its closest approach to Earth in over a decade, and Boise State’s Physics Dept will host an astronomical viewing party to celebrate on Tuesday, May 31 from 8:30p till 11p.
The event will kick off in the Multi-Purpose Classroom Building, room 101 on Boise State’s campus with a public talk on the latest science of the red planet from local planetary scientist Dr. Josh Bandfield of the Space Science Institute.
Then at 9:30p the event will move to the Boise State quad, where telescopes will be set up to view Mars, Jupiter, and Saturn.
Contact Prof. Brian Jackson (bjackson@boisestate.edu — @decaelus) with questions.
Event poster available here.
Endolithic (“within-rock”) life
Today, in physics, we hosted Prof. Nancy Chanover of New Mexico State Astronomy for our departmental seminar. Chanover gave a fascinating talk on her work developing acousto-optical tunable filters (AOTFs) to look for life in exotic terrestrial environments and on Mars.
AOTFs involve the application of an oscillating radio signal to a birefringent crystal. By applying the right frequency to the crystal, the crystal can be made to filter out light of very specific colors. Measuring the filtered light that comes out allows one to measure the colors of a object — is there more red light that filters through than blue, for instance?
One big advantage of these AOTFs is that they can produce spectra of rocks, minerals, anything that is colored, without any moving parts, and no moving parts is a big plus when you send an instrument to another planet.
In her talk, Prof. Chanover discussed her group’s work to develop AOTFs and techniques to analyze the emergent spectra and identify minerals on planets or moons in our solar system. Different minerals can have distinctive colors, and so taking the spectrum of a Mars rock, say, could allow us to identify its composition, without having to vaporize the rock to chemically analyze it.
The same technique could be used to look for Martian life. In some cases, extremophiles on the Earth leave tell-tale coloration in rocks (see figure at left), and so Martian life (if it exists) might do the same. Prof. Chanover’s group is looking for the distinctive spectral signatures of terrestrial biota in hopes of sending an AOTF to Mars and looking for life there, particularly in caves, which might be especially hospitable for life.
As a precursor to exploration of Martian caves, Chanover discussed her work attaching an AOTF to a robot developed by the Jet Propulsion Lab that climbs walls using footpads inspired by geckos, a LEMUR. This project involved several unforeseen challenges — as she said, on one trip, she struggled to say “acousto-optical tunable filter” in Spanish to a dubious Mexican border guard on the way to a field site in Mexico. The life of a planetary scientist.
Pressure variations (in hectoPascal, hPa) vs. local time for one dust devil pressure dip. The blue curve shows our model fit.
Dust devils occur in arid climates on the Earth and ubiquitously on Mars. Martian dust devils have been studied with orbiting and landed spacecraft, while most studies of terrestrial dust devils have involved manned monitoring of field sites, which can be costly both in time and personnel. As an alternative approach, my colleague Ralph Lorenz and I performed a multi-year in-situ survey of terrestrial dust devils using pressure loggers deployed at El Dorado Playa in Nevada, USA, a site known for dust devil activity.
When a dust devil passed over our pressure sensors, it appeared as a pressure dip in the time series, as illustrated in the figure. By modeling these signals, we learned a lot of about dust devils. For instance, in spite of expectations, we found signals that looked a lot like dust devils that occurred at night and even in the winter. So do dust devils happen year-round, day and night? More work will help us figure it out.
Our paper on this study will appear soon in the Journal of Geophysical Research Planets.
From https://emps.exeter.ac.uk/physics-astronomy/research/astrophysics/phd-opportunities/modelling-shock-waves/.
On Friday, everyone in our research group gave a little update on what they’ve been up to.
Liz and Jennifer talked about Parmentier et al.’s (2013) paper on the meteorology of hot Jupiters and how condensates are transported throughout these dynamic atmospheres.
Emily talked about working through the first few chapters of Murray & Dermott’s classic Solar System Dynamics. She will eventually study the orbital dynamics of systems of exoplanets very close to their host stars.
Brenton discussed his reading of Balme & Greeley (2006) on dust devils in preparation for working with me on terrestrial and Martian dust devils. A very exciting possibility, Brenton and the rest of the group said dust devils are common just south of Boise. Good chance we can do some in-situ monitoring locally.
Nathan spoke briefly about looking for more very short-period planets using data from the Kepler and K2 missions.
In attendance were Liz Kandziolka, Jennifer Briggs, Emily Jensen, Brenton Peck, Nathan Grigsby, Trent Garrett, and Tiffany Watkins.
