The presentation will be live-streamed on this website from YouTube on Friday, starting at 7:30p. Participants will be able to ask questions via YouTube’s chat feature. Stay tuned.
Boise State Physics – First Friday Astronomy Event – Friday, Apr 3rd
“Rivers Across the Solar System”
Prof. Devon Burr, Astronomy & Planetary Sciences, Northern Arizona University
Online lecture begins 7:30pm – http://www.astrojack.com/ffa-rivers
Donate at http://give.boisestate.edu/astronomy
There were lots of great things about the movie “The Martian” (and a few inaccurate things), but one of the best things, for aeolian scientists like myself anyway, was the depiction of ubiquitous, enormous dust devils.
Mars loves to make dust devils. It’s relatively easy for sunlight to heat the atmosphere and get it churning, and thick dust deposits blanket enormous regions on Mars.
Dust devils on Mars help keep the atmosphere dusty, which warms the climate and helps drive weather. However, as surprising as it might be, we don’t totally understand how dust devils actually lift dust.
Sure it’s true that dust devils are windy, but when you actually plug the windspeeds measured in dust devils into the dust-lifting equations, the amount of dust they *should* lift can be much less than what they *do* lift. So some other mechanism besides just wind must help lift dust in devils.
One possibility is that dust devils act like vacuum cleaners and actually suck dust up off the martian surface. See, a lot of the dust sitting on the surface of Mars has been sitting there for a long time, not moving. As a result, the dusty surface can become vacuum-packed, trapping some gas in between the dust grains.
At the center of a dust devil is a small dip in the atmospheric pressure (created by the convecting air inside the dust devil). So when a dust devil skitters over the hermetically sealed dust surface, the trapped gas pressure can launch the dust into the air, where the devil can pick it up.
But this vacuum cleaner effect is still just a hypothesis, so to test this idea, the Experimental Astrophysics group at University of Duisburg-Essen, experts in astrophysical dust experiments, set up a test chamber to mimic the martian surface under a low-pressure (1% of Earth’s) martian atmosphere.
They created a thin layer of small dust grains on a membrane, with a pressure differential across the membrane, to see if a small pressure differential could really lift the dust grains up. The answer is yes!
Now whether this experiment accurately replicates conditions on Mars is not totally clear, but some of the measurements made by the soon-to-be-launched and recently named Mars 2020 rover Perseverance may help to test the idea.
In addition to collecting geological samples for later return to Earth, Perseverance will collect high-resolution imagery of dust and mineral grains on the surface of Mars. It will also continuously measure meteorological conditions, which we know from past missions can reveal the presence of dust devils. So in addition to telling us about the possibility of past life on Mars, Perseverance may also help us test whether there are dust devil vacuum cleaners on Mars.
As summer winds down and we prepare for the fall semester, I finally found the time to read the recent announcement about finding sub-surface water on Mars using the MARSIS radar onboard Mars Express.
Although evidence for liquid water on Mars has been reported for a long time, these reports are almost always about ancient flows or very modest, salty trickles (and the presence of water often turns out to be illusory). By contrast, if this most recent report holds up to scrutiny, there could be 10 billion liters of liquid water under the martian south pole.
That’s not much on the scale of the Great Lakes (the smallest one, Lake Erie, contains 10,000 times more water), but it’s more than a thousand times the volume of tanks at the Georgia Aquarium, which hosts more than 100,000 aquatic animals. So the martian lake could easily host a microbial zoo (although no direct evidence for that as of now).
As is common in polar regions on Earth, the martian water lies under kilometers of polar ice and is probably so cold it requires some kind of geological anti-freeze to keep from freezing solid (the kinds of mineral salts that can do the job are actually pretty common on Mars). The overburden pressure from all the ice also helps keep the water liquid.
But the fact that the lake sits underneath so much ice raises an obvious question: how did the scientists spot it in the first place? The answer is related to why the recent wildfires in the west, in addition to fouling the air, have given us very lovely sunsets.
When it first leaves the surface of the Sun, sunlight is colored white. But as it passes through the atmosphere, the light (which is a wave of electric and magnetic fields, after all) interacts electromagnetically with the atmospheric gas molecules, which themselves contain electric charges.
The closer the wavelength of the light ray is to the sizes of the molecules, the stronger this electromagnetic interaction and the more the ray can be diverted from its straight path.
Since blue light has a wavelength (500 nanometers) closer to the size of the atmospheric molecules than red light (700 nanometers, it is more readily diverted or scattered. At dusk, as the sun sets, its light has to pass through more and more of the Earth’s atmosphere. So more and more blue light is scattered away, leaving behind more red light and making the Sun look red. If you sprinkle in lots of smoke from a wildfire, you can enhance the coloration.
What does all this have to do with martian lakes? The MARSIS instrument used to find the subsurface lake uses very red radar light, with wavelengths tens to hundreds of meters long. Similar to red sunlight, such long wavelengths can easily pass through even solid rock since they’re much larger than the rocky molecules that make up the martian surface.
This explanation simplifies things a lot, but the upshot is that MARSIS could see the lake as a very unusual bright patch underneath all that polar ice.
What’s next? It’s possible that continued data collection and analysis will turn up other subsurface lakes on Mars. If Mars’ south pole is brimming over with these icy lakes, it could be an especially good habitat for martian microbes. So maybe the effort to find martian life should explore using the same ice drilling technology being considered for exploring the oceans of Jupiter’s moon Europa.
I’m sitting in the hotel lobby at the Woodlands Marriott, waiting for my supershuttle to IAH and recouperating from my second Lunar and Planetary Sciences Conference, LPSC. Before I’m whisked away back to Boise, I thought to write about a few of the fascinating and mind-blowing things I saw this week.
First of all, LPSC is an annual conference that focuses on the geology, geochemistry, and geophysics of planetary science. There’s a lot of focus on solar system bodies with solid surfaces, as opposed to the annual DPS meeting, which has a bit broader focus.
I arrived on Sunday evening and dove immediately in, helping with the First-Timers’ presentation review, an opportunity for new-comers to the meeting to have more senior folks provide feedback on their posters or oral presentations.
Monday dawned cloudy, and I sat through several talks about our sister planet Mars. One that stuck out for me was one about field experiments to explain the perennially mysterious gully formations found on martian slopes with sleds of dry ice.
Tuesday saw me in a session on Saturn’s moon Titan, a cornucopia of geology and atmospheric physics. One particularly impressive talk discussed work to understand how methane deluges on Titan modified the surface.
— Brian Jackson (@decaelus) March 20, 2018
Tuesday evening, I presented our group’s work flying drones through active dust devils.
Wednesday was packed with talks on sediment transport experiments and analyses, attempting to decipher the martian aeolian cycle, including a neat study of time-lapse imagery of martian dunes.
— Brian Jackson (@decaelus) March 21, 2018
Thursday was packed with Pluto and results from New Horizons. One talk that stuck in my mind was an analysis of landslides on Pluto’s moon Charon, which, frankly, was a little bit of a tear-jerker. Hard to believe that not one hundred years ago, we didn’t even know Pluto existed. Now we’re trying to understand the system’s geology.
— Brian Jackson (@decaelus) March 22, 2018
Friday morning wrapped up the meeting with a series of talks about glacial geology on Mars, including a fabulous presentation on mysterious geomorphic features on Mars. Even though these features look for all the world like glacial flow, they appear on totally flat ground, where flow shouldn’t be possible.
— Brian Jackson (@decaelus) March 23, 2018
And now to catch the shuttle.
Dr. Colin Dundas of the USGS’s Astrogeology Group based in Flagstaff spotted these buried ice cliffs during his daily scan of the regularly collected images taken by the HiRISE camera onboard Mars Reconnaissance Orbiter (MRO) currently circling Mars. (The camera itself is pretty stunning – it produces orbital images of Mars at high enough resolution that you could almost read the headline on a martian newspaper, assuming they had newspapers.)
In scanning through the daily haul of images, Dundas spotted striking blue strata in the walls of steep cliffs just a few meters below the dusty martian surface that sure look like water ice. Follow-up spectral observations by CRISM instrument on MRO confirmed the cliffs were indeed almost completely pure water ice, with no more than with less than 1% dust.
Ice on Mars isn’t particularly surprising – astronomers have known (or at least suspected) there is water ice at the poles of Mars for more than 100 years, and a mountain of data has indicated vast stores of ice in Mars’ subsurface, especially near the poles. But key questions about this ice have persisted: Was the ice recently deposited, and how much dust is mixed in?
Since these newly discovered cliffs are so pure, though, Dundas and colleagues suggest that they were probably deposited as snow before being buried. Mars’ current climate isn’t really conducive to water snow, and so the ice was probably deposited millions of years ago, when Mars’ axis had a very different tilt resulting a very different climate from now. The fact that the ice cliffs occur much nearer to the equator than might be expected also points to formation during a previous climatic epoch.
The implications of these cliffs for Mars’ climate history aren’t entirely clear, but their importance for exploration of Mars is hard to overstate. As Dundas et al. say in their paper, the cliffs would very likely serve as a resource for future human visitors. The water could be combined with gases in the martian atmosphere to make rocket propellent and even oxygen.
So there are large deposits of ice in the subsurface of Mars? Maybe “Total Recall” wasn’t so much science fiction as science prophesy.
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.
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.
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 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.