Date: April 20, 2012
Title: Mysterious Lunar Swirls
Podcaster: Timothy Glotch
Organization: NASA Lunar Science Institute
Music: “Amelia” by Ben Bedford from his “Land of the Shadows” CD. http://benbedford.com/
Description: What are lunar swirls? On the surface of the Moon are several bizarre bright swirl patterns on lunar regolith, and some people have even compared these mysterious patterns to crop circles on Earth. To tell us about the real science research of lunar swirls is geoscientist Timothy Glotch from Stony Brook University in New York.
Bio: The NLSI brings together leading lunar scientists from around the world to further NASA lunar science and exploration.
Timothy Glotch is an associate professor in the Department of Geosciences at Stony Brook University in New York. http://www.stonybrook.edu/commcms/geosciences/people/faculty/glotch/index.html
Sponsor: Sponsorship for this episode of “365 days of Astronomy” is donated anonymously and dedicated to the men and women of NASA who strive to turn science fiction into science reality.
Nancy: What are lunar swirls? Hi this is Nancy Atkinson for the NASA lunar science institute. On the surface of the Moon are several bizarre bright swirl patterns on lunar regolith, and some people have even compared these mysterious patterns to crop circles on Earth. But to tell us more about the real science research of lunar swirls is Timothy Glotch, an associate professor in the Department of Geosciences at Stony Brook University in New York. Hi Tim and thanks for joining us.
Tim: Hi Nancy, my pleasure
Nancy: First of all, from a planetary geologist’s point of view, what is a lunar swirl?
Tim: Well swirls are identified on the Moon based on their albedo, which is basically just a fancy word for how bright they are. And so swirls are bright features on the Moon that, and they are called swirls for a reason – they have these kinds of swirly patterns — so from a planetary geologist’s standpoint they are bright swirly features on the Moon , and then we have to use some remote sensing instruments to try and figure out more about them and how they actually formed and got there.
Nancy: Could you tell us the history of lunar swirls – and where and when they were first observed?
Tim: Sure. They have been known since the Apollo era. So, as far back as the 60’s we’ve had good images from the Apollo astronauts and the lunar orbiter missions that identified the features both in the mare, which are the dark regions of the Moon and the highlands, the light regions of the Moon. Moving forward in time to the Lunar Prospector mission and the Clementine mission in the 90’s, we got all kinds of great spectral data and magnetic field data to try to understand these swirls a little bit more. And then up into the present-day with the Indian Chandrayaan 1 mission and NASA’s Lunar Reconnaissance Orbiter mission, we’ve learned a lot more about swirls. But we’ve known about them for a long time.
Nancy: What are some of the theories for how these swirls are formed?
Tim: You can kind of break the theories down into three separate groups. Lon Hood who is a planetary geologist at the University of Arizona first proposed a theory in the early 1980s that the swirls formed as a result of the deflection of solar wind by magnetic fields around the swirls. To understand this idea, you have to understand the concept of space weathering. Space weathering is that the solar wind impacts the surface of any airless body and the solar wind has lots of protons or hydrogen nuclei. The solar wind interacts with the surface of airless body and could create nano-phase iron in the very, very top layer, just a few nanometers of the regolith, the soil surface, and that darkens the surface, the regolith. So if you have a magnetic field which deflects the solar wind, everything around that magnetic field is getting darker due to space weathering, but the part of the surface that is protected by the magnetic field is not getting darker so it appears brighter overtime to your eyes. So, that was sort of the first idea that magnetic field was deflecting the solar wind, preventing the darkening.
A little bit later, a of group researchers that thought that maybe the swirls are result of the impacts of swarms of micrometeoroids. The idea here is that you have the swarms that breakup before they impact the lunar surface. They then disturb the regolith on the lunar surface and then reveal material underneath the surface while the rest of the surface has been space weathered. So this idea is also related to space weathering, but it is a little bit different. You have impacts disturbing the regolith and revealing this unweathered surface underneath.
And then finally there is a third idea that just came about really recently, a guy named Ian Garrick-Bethell, who at the time was a graduate student at Brown University and is now a professor at UC Santa Cruz. He suggested that the swirls are actually piles of dust made up of a mineral called plagioclase feldspar, which is a bright-colored mineral. That would explain why the swirls appear bright to us. The idea that he came up with is that as the terminator moves over the lunar surface, you get electrostatic lifting of this dust and placement of this dust along the swirl patterns, creating these swirly features.
So those are kind of the three basic ideas for how these swirls form.
Nancy: You’ve been working with other scientists with the Diviner instrument on the Lunar Reconnaissance Orbiter and data from the recent Chandrayaan mission to study the swirls – what are some of the new findings in your research?
Tim: So, we’ve been using Diviner, which is a mid-IR radiometer, so it measures infrared radiation between about 8 and 400 microns. These are wavelengths that are much longer than our eyes can see. We specifically are using the shorter wavelength channels, around 8 microns, to look at the effects of these swirls and see if we can see any differences between the swirls and the surrounding terrain. It turns out that Diviner is sensitive to this space weathering process. What we notice when we use Diviner to look at the lunar swirls is that the swirls do appear to be less space weathered than the surrounding terrain, which is really fascinating and it kind of supports either the magnetic field model or it supports the impact swarm model.
Now the other thing we can do is use Diviner to measure temperatures at nighttime. This is a really critical capability of Diviner. What we do is look at Diviner data acquired when the swirls are exposed to space at night and we can look for any temperature differences between the swirls and the surrounding terrain. Now, based on the properties of soils and rocks, some materials cool down faster than others at night, and we can say something about the physical properties of the lunar regolith. And it turns out that the temperature differences between the swirls and the surrounding terrain are not really all that great. There is a very small temperature difference, maybe about 1 degree Celsius or so. This suggests to me and some of my colleagues that there is no event in the recent past, or in the distant past that really strongly disturbed the regolith. If we had a meteorite swarm or microcomet swarm impacting the lunar surface, you’d expect the disturbed regolith to have a thermal anomaly or some sort of physical difference that would manifest itself in temperature differences in the Diviner nighttime data. But we don’t see that. So, from the standpoint of Diviner, we are moving, converging towards this original model that Lon Hood proposed, which is that the magnetic fields are deflecting the solar wind, preventing space weathering from the solar wind impacting the lunar surface.
Now, other researchers have also been using other instruments on LRO and the Chandrayaan spacecraft to study lunar swirls. If you look at the Chandrayaan 1 Moon Mineralogy Mapper data, and this is something that a researcher named Georgiana Kramer has done, you can look at the hydroxyl or OH on the surface. The theory is that the OH forms on the surface also due to the interaction of the solar wind with the surface. But if you look at the swirls, they tend to be OH deficient. So this is another piece of evidence that magnetic fields are preventing the interaction of the solar wind with the lunar surface.
Another researcher named Catherine Neish has been using radar data from the Mini-RF instrument on LRO to try and study the swirls as well. What she has found is that the swirls are really surficial features. There is no thick layer of material that characterizes the swirls compared to the surrounding terrain. So these swirls really just appear to be very surficial features. This is also consistent with the Diviner data. We can do some thermal modeling with the Diviner data and our thermal modeling suggests that any layers associated with the swirls, there really is a distinctive layer – it has to be less than a centimeter thick, or less than that. We’re talking about something that is just on the very, very surface.
So the Moon Mineralogy Mapper data, the Diviner data and the Mini-RF data are all consistent and they are all converging on the idea that the solar wind is being prevented from interacting with the lunar surface. So that is really exciting when you have different types of scientific instruments which measure different physical and chemical properties of something, whatever you are look at, in this case the lunar surface and all those different measurements are pointing towards the same answer, that is pretty exciting….Because that doesn’t always happen in science.
Nancy: Even though it sounds like you are getting close to an answer, it still is somewhat of a mystery. What do you see as the future of this research – would there be a particular type of instrument on a future mission that would help answer these questions?
Tim: There is a lot of really interesting work going on now. One of the other researchers I mentioned earlier, Ian Garrick-Bethell, has proposed an interesting space mission, a microsatellite that would fly thought the magnetic field of one of the main swirls called Reiner Gamma, and it would impact the surface but it would take measurements of the magnetic field all the way down to the surface. Something like that would tell us a lot about how the magnetic field is interacting with the solar wind. So something like that would be relatively low cost and a really interesting mission to tell us a lot more about the swirls. But other than that, what you really want to do is you want to return a sample. You want to go to a swirl, pick up some of that soil and maybe pick up some soil outside of the swirl and do one to one comparison to see if the swirl material is actually less space weathered than everything around it. Something like that is probably al little bit farther off but that would give us a definitive answer.
Nancy: Well, Timothy Glotch, that you so much for joining us today to talk about the current research on lunar swirls. We’ll have more information listed in the show notes on our website.
Tim: My pleasure, thank you.
End of podcast:
365 Days of Astronomy
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