Planetary Radio: Space Exploration, Astronomy and Science - How's the Weather on Mars?

Episode Date: June 7, 2004

How's the Weather on Mars?Learn more about your ad choices. Visit megaphone.fm/adchoicesSee omnystudio.com/listener for privacy information.See omnystudio.com/listener for privacy information.

Windy and cool today on Mars and on Planetary Radio. Hi everyone. We're glad you've joined us for another edition of our show. I'm Matt Kaplan. Cornell scientist Don Banfield may be the closest thing we have to a Martian weatherman. He's our guest this week. Bruce Betts tells us about a party in the sky in What's Up, along with our new space trivia contest. And as usual, we'll get started

with Emily, who says when it comes to planets, size does matter. I'll be right back. Hi, I'm Emily Lakdawalla with questions and answers. A listener asked, since the larger planets in the solar system are made of gas and the smaller ones are solid, I wonder if there is a limit to how large a planet can be before it can no longer remain solid. It is possible in principle for there to be a solid planet as massive as Jupiter, but in reality this is unlikely. The answer to this question has more to do with the composition than the mass of the planets. The reason that the most massive

planets are also gas giants is because they are made primarily of the most abundant stuff in the universe, hydrogen. Less massive planets are made of less common elements, such as those needed to make ice or rock. The gas giants are fluid, not solid, because hydrogen does not liquefy or solidify under the pressures and temperatures found in the planet's interiors. Of course, even Earth is not entirely solid. Deep below the solid crust and mantle is a fluid core. Are you confused yet? To understand more about what makes planets solid, liquid, or gas,

stay tuned to Planetary Radio. And speaking of big planets, we're going to have to have Don Banfield on the show at least one more time so we can talk about his studies of Uranus and Neptune. For today, we'll stick with this Cornell researcher's fascinating work on Mars, or rather just above Mars. Don has learned a lot about weather on the red planet. He hopes to learn much more in coming years through new instruments that will give us a much better understanding of how our solar system neighbor's weather differs from our home planet. Welcome to Planetary Radio, Don Banfield.

Hello. It's good to have you here. Let me start by asking you about Mars, the red planet. You've spent a good deal of time out in our area, our neck of the woods, Southern California, working with the Mars Exploration Rovers, and specifically the MINITES instrument, which we have talked about previously on this show. But when we talked about MINITES, it's always been about when the MINITES was pointed at the ground or a hill off in the distance. It turns out that MINITES has also been extremely useful in researching air movement,

what's happening in the atmosphere there on Mars. Right. Yeah, actually, we're very excited to have mini-tests there on the surface. It's very analogous to the Big Brother test that's in orbit right now on Mars Global Surveyor. When you use a spectrometer-like test from orbit and look down at the atmosphere, you can say quite a lot about the atmosphere above about 5 kilometers and up. But if you put one of those spectrometers down on the ground, like many tests sitting on the Mir rovers, and look up, you have very good information about the bottom maybe 3 kilometers of the atmosphere.

So it's very complementary to the Big Brother test in orbit going around. And since both test instruments, we're talking about its thermal emission spectrometer, so you're really looking at temperature changes in the air? Yeah, well, I guess the way they work, actually, is by looking at the infrared radiation that is emitted by the atmosphere itself, and it breaks it down into the spectrum. And if you look at different wavelengths, you can see the emission from different parts of the atmosphere. In much the same way that the mineralogy guys

want to look at the emission from different wavelengths of the rocks that will tell them something about the mineralogical characteristics of the rocks, we also want to look at different wavelengths when we're staring at the atmosphere because in certain wavelengths, the atmosphere is very opaque. And when you're looking from the bottom up towards the sky, as I'm doing right now, at the very opaque wavelengths, you're seeing the thermal emission from a distance not very far away, maybe 20 meters above the rover. As you step to other wavelengths nearby that where it's not quite as opaque, you see further and further up into the sky.

That is the thermal emission that's coming from atmospheric layers higher and higher up. And for many tests, if you step away from this absorption feature that is very strong at 15 microns, you can see something like between 20 meters at the closest up to maybe 2 to 3 kilometers and higher at the farthest. How much have we learned about the atmosphere of Mars, and in particular how it moves around, what we call here on Earth wind? Right. Well, let's see. We've learned quite a lot. There were instruments orbiting Mars in the past. There was the Mariner 9 Iris, which was actually a pretty similar spectrometer to TESS,

and that allowed us to get atmospheric temperature observations. But the difference between MGS and TESS and Mariner 9 and Iris back in the early 70s is that MGS is on basically a polar orbit. It's very much like the Earth's weather satellites. Whereas Mariner 9, way back in the 70s, was on an eccentric orbit and

it wasn't on as short of a period orbit. So it observed much more erratically the atmospheric temperature. With MGS that's still orbiting, it has been for five or six or more years. Yeah, very successfully. Yeah. We have basically a single example of an Earth weather satellite. And we've used that very well to sort of track the atmospheric temperatures and how they change day to day. This is because in a polar orbit,

a satellite gets to look at every place on a planet over some period of time? Yep, that's right. In fact, MGS takes something like 12 orbits per day, and they're evenly spaced in longitude. And so basically you get a full picture of the atmosphere actually twice a day because you pass each latitude going north and then again going south. You really get two full pictures of the planet a day in terms of the atmospheric temperature. So why is wind an important thing for us to learn about on Mars?

Well, there's a bunch of reasons for that to be interesting. The most obvious one is when you look at orbital images of Mars, you see things like dunes and drifts and dust streaks and dust storms. Basically, those are just clues telling you that the atmosphere of Mars is really the big agent of dynamic change on Mars. The current agent of geologic change on Mars is the wind. Since there's no running water anymore. That's right.

So for that reason, it's certainly interesting to understand the prime force that's shaping the geological terrain that we see on Mars now. Another reason that might be good to know the winds is if you want to safely put a lander down on the surface. This was a big issue for the Mir rovers in that their delivery system with the parachutes and the retro rockets and the bags, it wasn't as robust against wind shear as it might have been. It was actually sort of delicate in that sense.

If the rovers had come down and were moving horizontally too fast, then the bag could have been ripped when they hit a sharp rock or something like that. So the lack of our knowledge of the winds in the lower atmosphere on Mars turned out to be one of the big risks of putting the MER rovers down on the ground. We tried to model that to understand what the winds might be as they came down into Gusev and Meridiani, but there just isn't all that much ground truth data that we could have used to verify that the models that we used were accurate. I guess I mentioned that we have test data from Mars Global Surveyor for the last six years or so.

But remember that that just tells us about the top 5 to 60 kilometers of the atmosphere. It doesn't tell us about the bottom 5 kilometers. So there's a paucity of data right down there near the surface where really all the action is happening. And that's where you would like to learn a lot more about wind patterns, how air flows around the red planet. And that's something that I definitely want to talk with you about because it has to do with your strong interest in putting a different kind of anemometer, basically a wind speed gauge, on the surface of Mars. Yeah, it's good to keep in mind that actually the Mars rovers, the MER rovers, the Mars Exploration Rovers, don't have a wind gauge on them.

In fact, they have a camera which lets us see clouds, dust devils, that sort of thing, although we haven't seen any dust devils yet. And they have the mini-tests, which lets us see the temperature of the atmosphere. But that's a very indirect measure of what the winds might be. In fact, we don't really have any good way to say what the winds are at the Mer Rover site. In the past, there were wind sensors, both on Pathfinder and the Viking landers, but those were somewhat limited in their capabilities. They were basically hot wire anemometers where they heated up a wire and watched it cool down with the wind.

It's basically like licking your finger and feeling which side is cooled down by the wind. It's not a very accurate way to figure out what the wind speeds are. It's very easily perturbed by the sun shining, for example, on the detector itself. We have been working on building or designing, I guess, a better anemometer for Mars. The idea that we're hoping to use is called sonic anemometry. These things are commercially available on Earth, actually. It's the flagship way to measure winds on Earth with very high accuracy and very high speed. But it's tricky to use them on Mars because they are based on sound.

As you might imagine, if you start reducing the atmospheric pressure, basically putting a very thin atmosphere, it's difficult to make sound. So that's the challenge of building this kind of anemometer. Don, I'm going to stop you there because I want to hear more about this very interesting use of Earth technology and how you're adapting it to Mars. Don Banfield is a planetary scientist and senior research associate at Cornell University. We're going to talk with him a little bit more right after this. This is Buzz Aldrin.

When I walked on the moon, I knew it was just the beginning of humankind's great adventure in the solar system. That's why I'm a member of the Planetary Society, the world's largest space interest group. The Planetary Society is helping to explore Mars. We're tracking near-Earth asteroids and comets. We sponsor the search for life on other worlds, and we're building the first-ever solar sail. You can learn about these adventures and exciting new discoveries from space exploration in the Planetary Report. The Planetary Report is the Society's full-color magazine.

It's just one of many member benefits. You can learn more by calling 1-877-PLANETS. That's toll-free, 1-877-752-6387. And you can catch up on space exploration news and developments at our exciting and informative website, PlanetarySociety.org. The Planetary Society, exploring new worlds. Planetary Radio returns with planetary scientist Don Banfield. We're talking to him in his office at Cornell University.

Don, we were starting to talk about this acoustic anemometer that you have proposed for Mars based on principles that I guess have worked for some time here on Earth. But you said it's going to be a challenge adapting this instrument to the Martian environment. Yeah, it definitely is. On Earth, they use what's called piezocrystals, which are these little special crystals that when you put voltage across them, they vibrate. And it turns out that if you put enough power into them, they vibrate pretty well and will couple to Earth's atmosphere and make noise. But if you were to put them on Mars, because the

atmosphere is so thin, you can make these crystals vibrate all you want, but they basically don't couple to the Martian air. So if you were to take an Earth sonic anemometer, drag it to Mars, and turn it on, well, the little crystals would vibrate, but they wouldn't really shake the air enough to make acoustic waves. And that's the fundamental technique that we use to measure the wind with the sonic anemometry.

So we had to come up with a new set of transducers. And we actually found them. There's a commercial set out there, and they use these for non-destructive testing of plywood in the lumber industry, where they ping the plywood without touching it through the air with these sonic waves, and then look at the reflection that comes back to make sure the plywood is good. Turns out we can use those, take them to Mars, and ping them towards each other, and by doing that, we can measure the wind speed.

So these are, they put out a pretty strong ping, I take it. They do, and actually what's different about them is that they very efficiently couple to the air. These piezotransducers don't efficiently couple to the air, but these special ones that we've found for use on Mars, they're just much more efficient in terms of coupling to a low-density atmosphere. One of the most interesting little lines I saw in the copy of the proposal that you sent me talked about how acoustic horns on these transducers may, I guess, improve this coupling even further and that they really may end up looking a little bit like those outdoor PA speakers we're all used to seeing? Right. We've thought about that.

We actually haven't gone down that route very far because it turns out we really don't need to. The drivers themselves seem to work pretty well without these acoustic horns. Yeah, the horns are basically the same concept that you saw in the old-fashioned Victrolas, where they were basically trying to couple between the needle on the record and the air itself, which had very different acoustic competences. And so they had this horn that would match between the needle and the air. We were going to try to do the same thing at Mars, but it turns out we don't have to. The devices work pretty well without that. So it's always good to keep your instrument as simple as possible.

I assume that you're going to want to test these, but you have to do it on Earth. How do you simulate the Martian environment? Right. Well, we have a tank, actually at Ball Aerospace, who I'm working on this with. They can suck the pressure down in that tank and fill it with CO2 to 6 millibars. So it basically, as far as the transducers are concerned, looks exactly like Mars. So you even fill it with carbon dioxide to better simulate Mars. That's important because it turns out that nitrogen, Earth air, doesn't absorb sound nearly as efficiently as CO2 does.

So we've got another strike against us. Not only is the atmosphere very low density, but CO2 is very hard to make noise in as well. Mars doesn't make things easy. No, it doesn't. But, you know, it's kind of fun in that sense. You talked about how fast this new type of anemometer will be. What's the significance of that? And how much faster are we talking about compared to the ones that have been on Mars previously?

You mentioned Viking and Pathfinder. Right. So for Viking and Pathfinder, those both relied on thermal changes in the detector. And so they had a time constant that was of order a second or maybe a couple seconds. So they couldn't record changes that were any faster than that. But our technique relies on counting the time it takes for sound to go in one direction versus another. And the sound speed is very fast. It's 250 meters per second, roughly, on Mars.

So if you're talking about an instrument that's only maybe 15 centimeters across, then you can take observations that may be even as high as 500 hertz. But we'll probably average that down to something like 20 hertz or 20 wind speed samples per second. Now, that's interesting, because what we really want to do with our instrument is look at the turbulent eddies that are going past the station that we would have

or the rover that we're going to bolt this onto. And that's interesting because if you look at the turbulent eddies, you can measure a number of quantities, one being the fluxes of heat and momentum from the atmosphere to the surface. Basically that's how much heat the surface is losing to the atmosphere or how much force the atmosphere is putting on the surface, basically to move sand grains, that kind of thing, or lift dust devils. The other thing that would be very interesting to look at

would be to look at the eddy transport of water vapor away from the surface. And you can do that by, if you have a very fast wind gauge, and you look at when the wind is blowing upwards, if it's slightly more humid than it is when the wind is blowing downwards, then that means that there's a flux of water out of the surface. So you need a little hygrometer as well. Yep, exactly. And we're planning to tie our fancy wind gauge in with a very fancy hygrometer

that can respond also at these high speeds. So here we are, if we're lucky, if all of this comes about, with what's going to amount to a very nice little weather station on Mars sometime in the next few years, depending on which mission you manage to get these instruments on. Right. We'll keep trying. But talk about the weather on Mars. Are we talking about a system that is as complex as the very complex patterns we see on Earth?

I guess I would describe it as not quite as complex. It certainly has high and low pressure systems that roll past a single point. We saw that back with Viking. And now we can see it with MGS and TESS from orbit. But what's different about Mars weather, the amplitude of the high and low pressures and the temperature perturbations are very similar to what they are on Earth, but it's very regular. It turns out that a lot of times, if you knew what the weather was today, although it would be warmer and colder in the days coming,

you'd be able to predict whether it was going to be warmer or colder as long as 100 days in the future. So there are variations. There are slower variations on Mars, and they're very coherent. That is, they persist for very long times. On Earth, the weather gets chaotic with a timescale of something like 10 days. But on Mars, it seems to run very regular for up to maybe 100 days. Wow.

John, we are almost out of time. Thanks very much for joining us, and good luck on getting that acoustic anemometer on one of these future missions, and also with your research on Uranus and Neptune. Yeah, thanks. Don Banfield has been our guest. He is a planetary scientist

and a senior research associate at Cornell University in Ithaca, New York. We're going to continue with Planetary Radio, specifically Bruce Betts and What's Up, right after this return visit from Emily. I'm Emily Lakdawalla, back with Q&A. There are lots of factors besides size that affect whether a planet is primarily solid, liquid, or gas, and these factors interact in complicated ways. Composition is a second factor. Planets made of light elements are more likely to be gas, while planets made of heavy elements are more likely to be liquid or solid.

A third factor is temperature. Everyone knows that high temperature may cause solids to become liquids or even gases. But a fourth factor, pressure, can have the opposite effect to temperature. High pressure causes materials to solidify even when they are very hot, which is why Earth's inner core is solid while its outer core is liquid. But sometimes temperature and composition can beat size and pressure. Even though the pressure near the center of Jupiter is tens of millions of times as high as the surface of the Earth, hydrogen still doesn't solidify. It turns into a strange fluid metal instead. In principle, size, composition, temperature, and pressure could balance out to give you an all-liquid planet. A planet with an

Earth-like composition and a massive and opaque atmosphere could be liquid throughout because of the greenhouse effect. There's no all-liquid planet out there now, but a long time ago, Earth could briefly have been such a place. Got a question about the universe? Send it to us at planetaryradio at planetary.org. And now here's Matt with more Planetary Radio. Bruce Fetz, the Director of Projects for the Planetary Society, joins us once again, as he does every week, for What's Up?

What's up, Doc? I haven't said that in like a year and a half. I figured it was time. Where's that wascally wabbit? I'm really excited about doing What's Up this week. Thank you, Elmer. Go for it. Not my best impersonation. What's Up this week thank you elmer go for it not my best impersonation what's up this week we have so much exciting stuff i can hardly stand it we've got a venus transit the first venus transit of the sun since 1882 venus will be crossing across the face of

the sun as seen from the earth this will be visible from most places, but not where we are, so I'm just not going to talk about it. Just kidding. Okay, no, really, it will be visible for many of you. In fact, even more than I implied last week, I was a little misleading. The Eastern North America kind of people, and even Eastern South America people,

will have a chance to see it rising with the sun. In transit, across the the sun at sunrise, those of you in Europe and Asia will be seeing it in the middle of the day. And over in Eastern Asia, Australia, you'll pick it up around sunset. So go to our website, planetary.org slash radio. We'll give you links to where you can find out more about when to look for the Venus transit from where you live. Now, we also still have some other special things to see up there. We've got two comets still visible with binoculars, Comet NEAT-C2001Q4, otherwise known as a bucket full of marmots, although it's getting really dim now. It's really fading out, but it's visible after sunset in the

west, and you can go to our website again to find a link to find out exactly where to look, but you're going to want to use binoculars to find it, as is usually the case with comets. And comet Linear C2002 T7 Bob is visible, and is actually brighter, but about magnitude 3, for those of you who play the astronomy magnitudes game, and also can be seen after sunset in the west-ish, and you can also find finder charts for that, but use the binoculars. If you're a dark site, you might pick up the tail and might be able to see it naked eye. A suburban site, you're probably going to need the binoculars. You may just see a fuzzy blob. Disclaimer, bucket full of marmots and Bob,

not recognized as actual astronomical names by any official body. And you can also see other exciting things with their official names in the night sky, such as you can still see Saturn and Mars also in the West. It's just a party in the West after sunset. It really is. Party in the sky. It is. We've lost Venus.

Venus passed out and is falling across the sun. God, is she a lush. Saturn and Mars are close to each other in the West, Saturn being much, much brighter, Mars dimmer. You can see them below the twins, those wonderful twins, Castor and Pollux and Gemini in the West. And the brightest-looking thing up there in the night sky now is Jupiter, now that we've gotten rid of that pesky Venus lush.

So Jupiter up in the West, high in the west at night, brightest thing up there. Good stuff. Go see them. Go see the planets. Go see comets. And if you possibly can't see that Venus transit that no one alive has seen, but if you miss it, hey, there's another one in a few more years this time around. And I hope that's one we're going to get on the west coast too.

You'll check on that. We'll take a road trip. Planetary radio road trip. All right. How about we move on to this week in space history on June 8th of 1625. You remember it well, don't you, Matt? I certainly do. Giovanni Cassini was born, for whom the Cassini mission to Saturn was named.

And what excellent timing, considering what you're going to be telling us about in about two seconds here. Random Space Fact! Cassini spacecraft will finally, after a seven-year voyage, be flying past something in the Saturnian system. In this case, Saturn's moon, Phoebe. My random space fact is about Phoebe. It takes a really long time to go around Saturn.

It's way out there from Saturn. It takes about 18 months for Phoebe to make complete one orbit around Saturn. It orbits retrograde, the opposite of most of the moons, meaning opposite of the direction that Saturn itself is spinning. And it's about 220 kilometers across. Cassini spacecraft will be seeing it about 1,000 times better in terms of resolution than Voyager 2 did. It should be fascinating.

Plus, with whole new instruments that are going to get a chance at the Saturnian system, offering much more in the way of spectroscopy. We've got radar. It's good stuff, Maynard. Gosh, Bruce, if only there was a place on the web where I could learn more about the Cassini mission. Funny you would ask, Matt. It turns out

that on the Planetary Society website, we've debuted a whole new section, planetary.org slash Saturn, that will tell you all you want to know about the Cassini mission, about Saturn, about Saturn's moons, has some beautiful graphics, latest images from the mission, has wonderful size comparisons of the different moons that vary so radically in size of the planet itself. Neat. Go there. See it.

Learn about it. Gee, Bruce, I'll go there today. Really? Okay, good, man. Do you have any more Saturn stuff for us? I will. We'll be coming back to it in just a few moments. But first, let's take a trip to the inner solar system with last week's trivia contest. That's right.

We asked you, what does the acronym MESSENGER stand for? MESSENGER, a mission to Mercury. The old five passed Venus as well, launching within a couple months. Well, Bruce, we got all sorts of correct entries this week. Well, that's great. I'm so excited. Oh, my God. It's the top 40 gene.

It's recessive, but it comes to the surface periodically. Did we get any entries from Britney Spears this week, Matt? No, we did not. Well, you know. That's unusual. I think she finally got discouraged because she was wrong every other time. But Messenger, Bruce, Messenger, as many, many people told us, stands for, here it is, told us, stands for, here it is, Mercury, M-E, Surface, Space, Environment, E-N, Geochemistry,

G-E, and Ranging. And that's MESSENGER. What a cool acronym. And our winner this week is Jason Martin from one of my favorite towns in the world, San Francisco, California. Jason, you will be getting that Planetary Radio t-shirt. Thanks so much for entering. I also want to mention Ryan Caron, one of our regulars. Ryan, whose name wasn't picked this week, even though he did have the right answer,

he suspects that they added the ranging capability to this spacecraft just so they could use that extra cool acronym, MESSENGER, MERCURY, get it? Anyway, Ryan. How did he find out? I don't know. He just suspects. What do we got for next week? I don't know. He just suspects. What do we got for next week? We can't discuss anything more about that.

So we're going to move on to next week's trivia contest, which is, strangely enough, about Saturn, and strangely enough, about Phoebe. Phoebe, June 11th, that encounter. Don't miss it. How far away from Saturn is Phoebe on average? And you can just round this puppy off, because it's so darn far out,

you can round it off to the nearest million kilometers, or if you choose, nearest million miles. How do people enter our contest? Go to planetary.org slash radio, where not only can you find all the spiffy links to see these things in the night sky, well, not Phoebe, but all these other things, but you can find out how to enter our contest and win the glorious Planetary Radio t-shirt.

And do try to get that entry in by noon Thursday, June 10, so that you'll be sure to be considered in this week's or next week's, actually this week's contest. And our good friends at WMUH in Pennsylvania, folks, remember, you guys have got to go to the website to be able to enter during the week the contest is open. So go to planetary.org slash radio. I think we're done. I just want to remind people they can send their answers anytime. We just won't give you any prizes.

That's right. With that thought, everyone look up in the night sky and think about squiggles. Squiggles. Squiggles. Thank you and good night. What a great word. Bruce Betts is the director of Projects for the Planetary Society, joins us each week here on What's Up, part of Planetary Radio.

That's all the time we have. Remember, don't try to view the Venus transit of the sun without proper eye protection. We may just be a radio show, but we want our listeners to keep all their senses as long as they can. Have a great week.