Since the end of May, Mars has been enshrouded by a dust storm covering nearly the entire planet. Although Mars has a dusty season each of its years, these “planet-encircling” dust storms only occur once every 3-4 Mars years, which is about 6-8 Earth years. Scientists still aren’t sure how these storms go global—but using an array of instruments on NASA’s orbiters and rovers, they’re aiming to find out. Currently, scientists think that the dust storm is starting to wane, but the atmosphere may not clear out for another month or two. Listen in as NASA Chief Scientist Jim Green discusses the Mars dust storm with a dust storm expert, Melinda Kahre.
Jim Green: Our solar system is a wondrous place with a single star, our sun, and everything that orbits around it--planets, moons, asteroids, and comets. What do we know about this beautiful solar system we call home? It’s part of an even larger cosmos with billions of other solar systems.
Hi, I’m Jim Green, NASA’s Chief Scientist, and this is Gravity Assist.
With me today is Melinda Kahre and she is from the NASA Ames Research Center. Now Melinda manages the Mars Climate Modeling Center, which produces global climate maps of Mars. I mean we are now predicting the weather on Mars much like NASA and NOAA did in the late 60's early 70's. We're making enormous progress understanding the climate of Mars. She's also an expert on Mars dust storms, so this is a fantastic opportunity to really understand what the heck in going on Mars today. Melinda welcome.
Melinda Kahre: Thanks Jim.
Jim Green: Melinda can you tell me what’s going on in the Mars Climate Modeling Center? What is it and how does it work?
Melinda Kahre: The Mars Climate Modeling Center is a group here at NASA Ames that develops climate codes, global Mars climate codes, to simulate the weather and climate on Mars.
Jim Green: Now that runs on super computers here, I mean Ames has some of the most fantastic, fast super computers from all of NASA.
Melinda Kahre: Yeah you know we need large computing power to be able to run our simulations which are very high resolution and have a lot of different physical processes that we are trying to accurately simulate for Mars so we need the super computer here at NASA Ames.
Jim Green: Now to accurately see what’s going on on Mars using your codes, you got to have input data and so we've got several orbiters and of course we’ve got weather stations on our rovers and landers on Mars in the past and two rovers Opportunity and Curiosity are still working. So what’s the most important data sets that you really have to ingest into the models?
Melinda Kahre: We use data in a variety of ways for numerical modeling. Some data sets we use directly in the model, like the observed topography of Mars is a good example of that and some data sets we use to compare model results to, to understand how realistic our data sets are. One really good example of that is the maps of atmospheric dust that are coming back from the orbiters.
Jim Green: Wow, and so you have certain grid sizes and you do a variety of modeling so that its temperature, pressure, wind velocity. What are some of the other parameters which are so important to making these models work?
Melinda Kahre: Yeah it’s everything that makes the climate of Mars run—so there are dynamical processes and physical processes. So on the dynamic side we have winds and temperatures, pressures, densities, and on the physics side we represent processes like dust lifting, cloud formation, radiative transfer, how radiation affects the temperatures in the atmosphere and things like that.
Jim Green: Wow, sounds complicated, but how do you guys make that work is really you bust it up into little modules and then connect them through the physics of how these things work together.
Melinda Kahre: Yeah, we really do it piecemeal. So we look at one little bit at a time and develop each little section individually.
Jim Green: You know let’s talk about what happening on Mars today; there’s a huge dust storm going on its a global dust storm you know and these dust storms going global, how often does that happen?
Melinda Kahre: Not that often. The last global dust storm that we've had was in 2007, so 11 years ago on Earth but six Mars years ago. Generally we see global storms on mars one out of every three years, but it’s been a while since we've seen one this big.
Jim Green: So we've been on a due-fer for a while
Melinda Kahre: We have.
Jim Green: So 2007 curiosity hadn't landed, but Spirit and Ppportunity were going strong so that global dust storm in 2007 they managed to survive.
Melinda Kahre: Yep they did, both of them, and they observed the amount of dust in the atmosphere throughout that storm.
Jim Green: Right and so that’s fascinating topic we can actually get observations from the ground, looking up, seeing how the sunlight dims over time. What's the composition of this dust?
Melinda Kahre: These dust grains are mostly silicates, so plagiaclase, feldspars, zeolite, and they also have some iron oxides which is why they look a little bit red.
Jim Green: So how big are they? Is it like grains of sand on a beach?
Melinda Kahre: Smaller than sand. We're talking one and a half microns, two microns, so think really fine ash in size.
Jim Green: More like house dust, right?
Melinda Kahre: That's right.
Jim Green: You get in the house and you scrape that dust off the shelves and that could be what we see like on Mars.
Melinda Kahre: Yep definitely dust particles, pretty small, small little guys.
Jim Green: You know one dust phenomena that we were really surprised to see when Spirit and Opportunity landed because we thought well, dust will settle out, it will coat the solar panels and the rovers those two small rovers will die after about 90 days but really neat phenomena happened in the atmosphere that changed all of that. What was that?
Melinda Kahre: There are small dust storms—dust devils and also little wind gusts that came through and swept the dust from the top of the rovers and off those solar panels.
Jim Green: Yeah its actually fascinating when you see the power coming from the solar panels on Spirit and Opportunity go down, down, down, down, down, as it accumulates dust, but then one of these little dust devils goes by and it just peaks right up again and then they start moving.
Melinda Kahre: Mars helps out.
Jim Green: Yeah, it’s really fascinating. You know that when we think about these dust storms the dust is really small and then can be lofted but there’s also dust that’s slightly larger and does get lofted a little bit but then comes down, there’s a certain way that dust moves over the planet.
Can you describe a little bit how that happens?
Melinda Kahre: Yeah there are a few different ways that particles move depending on their size. So if a particle is really fine like a dust grain then it can be lofted directly into the atmosphere it can enter into suspension which means that it can blow around without coming back down to the ground too quickly.
If you get a bit bigger than that you start talking about sand sized particles, 100 microns or so, and those particles will leave the ground but they won't get very far before in the air before they fall back down due to gravity. When those particles come back they can then hit the ground and splash other, smaller particles into the air and into suspension.
The final process is called "Creep," and those are large particles so those particles don't really leave the ground but they can roll and bump along the surface a little bit. We think that saltation actually is the most likely mechanism for injecting dust into the atmosphere.
Jim Green: That’s when they leave the ground and come back and come back and throw more particles up, it’s sort of like a cascade.
Melinda Kahre: Yeah it focusses on sand sized particles because those are the easiest particles to move as it turns out. Particles are too big they don't leave the ground because of gravity, but if they're too small then they stick together on the ground and they’re hard to get off the ground. So sand moves most easily and then it can come back down and splash small particles up.
Jim Green: So how high can these dust storms get?
Melinda Kahre: In altitude?
Jim Green: Yeah in altitude.
Melinda Kahre: We see direct evidence of dust getting up to 60 kilometers or more.
Jim Green: I mean that's phenomenal when you think about it. That’s so very high. You know I’ve also heard about sometimes these get charged and they will have a little spark, a little tiny lightning like event that occasionally happens. There seems to be some indication from our orbiters that that’s happening.
Melinda Kahre: Yeah and that that changed particle idea might also affect how dust is lifted off the ground, some charge effects may change how easy or hard it is to lift particles.
Jim Green: Yeah that’s really fantastic. Now once these get lofted, the dynamics in the atmosphere change, too. What's the biggest thing that happens to the atmosphere during a dust storm?
Melinda Kahre: Well when you put these small dust particles into the atmosphere they absorb solar radiation. So they absorb radiation and then they warm the atmosphere and then they radiate in the infrared so the atmosphere actually warms up. The middle of the atmosphere warms, and that warming causes changes to the dynamics, to the wind fields, and the circulation is generally ramped up when you put a lot of dust into the atmosphere.
Jim Green: You know so during diurnal cycle the temperature on mars can change like 150-160 degrees F, and then with the dust lofted the change isn’t so much, maybe a third of that.
Melinda Kahre: Yeah the diurnal cycle is suppressed when you look at surface temperatures, and that’s because when you put dust into the atmosphere it absorbs solar radiation during the daytime, so not as much sunlight comes down to the ground for warming. So the daytime temperatures are cooler with dust in the atmosphere, but then at night those dust particles are radiating in the infrared down to the surface and the surface is actually warmed the night. So the surface is warmed at night and cooled during the day, so the diurnal cycle is quite a bit smaller.
Jim Green: You know that whole effect is what Spirit and Opportunity enabled them to survive these long period dust storms. They go into hibernation, these rovers, and then fortunately the temperature doesn't get so low it destroys the circuitry, so having that dust also warming the atmosphere at night is critical.
Ok so what's different about our other rover, Curiosity, it’s that it’s got a radio isotope power system and it could just rain dust, it doesn't care.
Melinda Kahre: It doesn’t need the sunlight.
Jim Green: It doesn't need the sunlight, so that’s one big advantage of having the RTG we call the radio thermal generator.
Ok so where is the dust coming from on Mars? Are there specific places where they start?
Melinda Kahre: Dust storms are observed to start on a lot of different locations on Mars. Not every global storm start in the same place, although a lot of them start in the southern hemisphere.
This current dust storm started in a storm in the north that traveled across the equator and started igniting lifting centers in the southern hemisphere. So we don’t actually have a great handle on where the surface dust actually is on mars. We have good indications that there are dusty locations—the low thermal inertia regions near the equator have thick dust deposits. We also know that there’s a fair amount of dust in the polar regions.
Jim Green: You know at the end of these dust storms you’ve redistributed the dust across Mars once again, so it’s a matter of when the conditions are right to loft them in the new locations that they reside.
Melinda Kahre: Yeah it’s a complex system for sure.
Jim Green: Yeah we have to keep track of where the dust goes.
You know Another thing that fascinates me, is that Mars has a fairly elliptical orbit, and my understanding is that he dust storms start when the sun and Mars are the closest.
Melinda Kahre: That’s right, Mars is closest to the sun near the southern summer solstice, and that tends to be when the circulation is the strongest to begin with.
Jim Green: So that’s kind of the season we’re entering right now, is the early spring into the summer in the southern hemisphere.
Melinda Kahre: That’s right.
Jim Green: So how long do these global dust storms last? What can we expect to happen next?
Melinda Kahre: They last months, actually it takes a while for them to get going and once they reach their peak and dust is literally enshrouding the whole planet, we seem to see the lifting centers stop operating and then dust falls out slowly over time, and it can take months for that dust to come back down.
Jim Green: So recently I heard that this particular dust storm had reached its peak and maybe waning, so we are still going to have several more weeks of this dust.
Melinda Kahre: Yep, it’s going to be up there for a while. The large particles will fall first, and then the small particles will fall out more slowly.
Jim Green: Now some people have noticed that Mars looks a little more yellow at night when they look up from Earth and see it; it’s not as red at it used to be and it that because of the dust storm?
Melinda Kahre: Yeah you can't really see the surface of Mars right now when you look at it with your naked eye or through a telescope when you look from Earth. The whole atmosphere is full of dust which does change the color a little bit.
Jim Green: Does anything like this happen on Earth? I don't know that we expect global dust storms, but some of the physics must be the same.
Melinda Kahre: Yeah you know we don't experience global dust storms here on Earth, but we do have dust storms on Earth and dust devils on Earth, and the physics is the same. We use what we learn from Earth and apply it to other places and that’s exactly how we attempt to understand how these things operate on Mars.
Jim Green: You know one of the fascinating things that we most recently found out about these earth dust storms is that as dust is listed and moves intercontinental, it actually takes microbes with them.
Melinda Kahre: Yeah.
Jim Green: This is an exciting time for an Earth science when we make observations like that, and we don't know if that’s happening on Mars, it probably isn't, we don’t believe there’s much life, if any, on the surface of Mars. It’s fascinating to think about how dust can also be used to loft life and move it around the planet.
Well you know one of the things that I always do, because I’m always fascinated about how each and every on of us get into the field of science that we’re doing, and there’s always some event that happened in our life that really propelled us forward; straightened us out to move us in a direction that allows us to become the scientist we have become today. And I call that a "Gravity Assist."
So Melinda what is your gravity assist?
Melinda Kahre: Well you know it’s actually hard for me to identify just one because I fell I’ve had lots over the years. I got hooked on science in school when I was pretty young, probably in sixth or seventh grade, I took Earth science and physics and I loved science, I liked the process of science. Then I started taking an astronomy class in college and got hooked on astronomy and then I moved on into planetary when I went to grad school. Actually the 2001 global storm happened the summer I started grad school, and among other reasons, was a why I got interested in studying Mars.
Jim Green: Wow alright well it comes full circle then. Well thank you very much I’ve really enjoyed taking with you about what’s going on at Mars. Really wonderful.
Melinda Kahre: Thank you very much it’s been fun.
Jim Green: I’m Jim Green, and this is your Gravity Assist.