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The Planetary Society Blog
By Emily Lakdawalla
DPS: Day 2: at the end, 7 p.m. local time.
Sep. 6, 2005 | 10:59 PDT | 17:59 UTC
OK, where are we? It's the end of another long day at DPS. Today was Mars, Mars, Mars, outer planets, extrasolar planets, and planetary system formation. There were lots of interesting Mars talks going on, but I decided that the Mars stuff would probably be extensively reported elsewhere, and that the rest of the solar system is often underrepresented, so I ended up going to more Neptune, Uranus, ESP, and other sessions than Mars stuff.
So, in the hour or so I have before I get kicked out of the building tonight, I'll sip my wine (I love European meetings) and try to get a few neurons marching in step long enough to make some sense of what I saw today.
Mark Lemmon talked about the dust devils he's been tracking (he has a great website devoted to dust devils). He talked about how for more than one Earth year after landing they never saw a dust devil from Spirit. During that time, they devised some special techniques to catch them. The first dust devils started appearing around sol 420. ("If they had been going on before sol 420, we would have noticed it.") Now, they are so large that they no longer need to apply their special image processing techniques to find them. "We can't take a picture in the middle of the day without seeing a dust devil," he said. When they started out at sol 420, they were small, about 20 meters in diameter, and they moved perpendicular to the dust devil track trends seen from orbit. "Nowadays we are seeing a different character. We're seeing stronger, more optically thick dust devils, moving parallel to wind streaks, many significantly larger than the first ones we saw," up to 100 or even 120 meters in diameter. But Mark thinks that it's going to get even better. "We are looking forward to the strongest dust devils now. The reason I say that is that they are finally aligned with the wind streaks now." They are seeing the dust devils start to influence the amount of dust in the air at the Spirit landing site, and expect that to strengthen over time.
Then I went to the outer planets talks. I must confess that I know a lot less about what's going on in outer planet science than I do about moons and Mars, so a lot of this was flying over my head, but I tried to note the kinds of things that the scientists thought were significant. I saw a talk by Glenn Orton on first results of Spitzer observations of Uranus and Neptune. His spectra are "difficult to match" with the usual library of gases you expect at these planets. The problem is that a spectrum is a single wiggly line. To figure out atmospheric compositions you try to match that wiggly line with the wiggly lines determined in the lab for pure gases. The wiggly line at Uranus and Neptune is dominated by hydrogen and methane. But once you account for hydrogen and methane, the Spitzer folks are left with several wiggles that they are having a hard time matching. The problem is, there are a lot of different ways you can match those wiggles--it's a poorly constrained problem. The best fit, Orton reported, contains a "zoo of things including acetylaldehyde, acetic acid, even formic acid!" The last item is so odd a suggestion for an outer planet atmosphere that he joked "I should pitch this to astrobiologists: perhaps Neptune has atmospheric ants?" (If you don't get the joke, formic acid is used by the ants to kill their prey and to ward off attackers.) When pushed by a questioner, though, he was careful to say that this is only a model, and a weird one at that: "I am NOT going to be quoted as saying I discovered formic acid at Neptune." He said that it's just an "intriguing puzzle."
Then there was a really interesting talk by Mark Hofstadter about "imaging the troposphere of Uranus at millimeter and centimeter wavelengths." This is cool because every time you can bring a different wavelength of the electromagnetic spectrum to bear on a target, you can learn something new. In the giant planets, the way it generally works is that the longer the wavelength you use, the deeper you can probe into the planet. When we look at the planets in the wavelengths we can actually see, we are only seeing the uppermost layer of the gases and clouds; these planets are fluid all the way down and must have a terribly complex structure when seen in three dimensions. There are LOTS of people writing down mathematical models for what goes on inside giant planets, but there is precious little data. Hofstadter's talk was the first time I'd seen this wavelength employed at Uranus, so it's a new window on the planet.
Anyway. He showed some gorgeous images that I hope he will release publicly some day, including the first map of Uranus produced at a 7-millimeter wavelength (this is all radio astronomy). But the kicker was a map of Uranus as seen at a wavelength of 1.3 centimeters. He saw two equatorial bands across Uranus, which have never been seen before. Remember the Voyager "bull's eye" view of Uranus? All the banding was toward the summer pole, nothing at the equator. But Uranus is beginning to head toward its equinox and is beginning to get interesting in the equatorial regions. More interesting, though, the equatorial stripes that Hofstadter saw at 1.3 centimeters were not nearly as pronounced at shorter (7-millimeter) or longer (2-centimeter) wavelengths. From this he concluded that the bands must be "vertically constrained," and that they were "probably ammonia clouds." But another Uranus astronomer, Imke de Pater, asked whether they could be hydrogen sulfide clouds, and he said that that should definitely be considered.
Heidi Hammel gave a talk about mid-intrared methane emission on Neptune and Uranus. She said that telescopic observations reported in 1977 by Macy and Stinton reported the detection of methane and ethane on Neptune but not Uranus. "But with Spitzer data," she said, "we have finally detected methane at Uranus." More interestingly, she said, "there is a suggestion of time variability in ethane measurements at Neptune. There's been a great deal of evolution of cloud structure on Neptune, changes in cloud patterns as a function of time." She said that Neptune's ethane emission brightened in many measurements performed between 1985 and 2003. But in 2004, she said, it had dropped down to the 1991 level. She showed more squiggly lines. In particular, she showed that the feature in the spectrum that was at the wavelength that ethane emits infrared radiation had a peculiar shape, a "notch in the spectrum." The notch was real, she said, and she suggested a couple of interesting possibilities for what could create it, including the presence of ethane ice in the Neptunian clouds. She wasn't saying she'd proven this, but she did invite the other scientists at the meeting to come up with an explanation that did a better job of explaining the "notch" in the Spizter spectra.
And this was all the morning! Over lunch, as happened yesterday, I shifted to press conferences, gobbling down half a sandwich in the five minutes between the meetings and the start of the press conferences.
Sushil Atreya gave a long presentation about the current status of the understanding of methane in Mars' atmosphere. There is methane at Mars but there's controversy over how much. That story hasn't changed a whole lot since last year (see A. J. S. Rayl's report from last year's DPS meeting.) Two data sets (Mars Express PFS and one from Krasnopolsky) indicate a global average of 10 parts per billion, and a third (from Mike Mumma) indicates 250 parts per billion, an absolutely enormous difference. "If you take 250 ppb and pass it through PFS," Sushil said, "Mars would light up like a light bulb. But none of this has been seen."
"Methane is quite important," Sushil said, "because methane on Earth is almost entirely from biology, only 0.2% from volcanoes. In earth’s volcanoes, sulfur emissions are about 100 to 1000 times more than methane. On Mars, sulfur dioxide has not even been detected, so a volcanic source is out." Cometary sources are also out, he said, because models indicate that only one comet hits Mars in 62 million years. Even if a comet did hit Mars recently, then the methane in the atmosphere should be globally distributed evenly. But it has regional variations. So comets are out. There are two non-biological ways to make methane on Mars, Sushil said, both of which involve serpentinization of basalt. What's that, you ask? Basalt is the most common igneous rock on Mars and it's made of olivine and pyroxene, two minerals that tend to form at relatively high temperatures and pressures. If you expose them to water, they can react and form a new mineral called serpentine. That process can liberate methane and hydrogen.
But what really got Sushil going was the fact that we really don't understand the loss mechanisms of methane from the atmosphere. And one loss mechanism that hasn't been talked about much is the loss due to the presence of hydrogen peroxide. He presented a cool theory that described sand grains jumping (or "saltating") in the martian wind. Dust devils can build up huge triboelectric charges this way, but he said you don't even need dust devils to make a significant charge -- you can do it with your standard, everyday, ubiquitous martian wind. The charges can be up to 25 kilowatts per meter, and can generate hydrogen peroxide at a rate a thousand times what is produced photochemically, from the Sun interacting with Mars' upper atmosphere. The peroxide is so reactive that it doesn't last long in the atmosphere. But the mechanism generates so much peroxide that it would be everywhere, ready to eat up any chemical that's ready to be oxidized. And methane is certainly ready to be oxidized. What does that all mean? I haven't the foggiest idea.
In the afternoon I wandered into some talks having to do with outer planetary satellite systems. One young guy, a graduate student named Zhang, was talking about the Neptunian satellite system. His claim was that you could determine the masses of the inner Neptunian satellites by studying their orbital dynamics. Outer planet satellite systems are dynamic places; the orbits are not stable over long time periods, especially if you've got eccentric satellites among them. But we only see their conditions at a snapshot in time. Zhang's claim was that the inclinations of the orbits of the inner satellites Larissa, Galatea, Despina, and Thalassa are a "fossil record" of their passage through orbital resonances with the largest inner satellite, Proteus. His mathematical models produced inner satellite densities that were less than that of water, which would be significant, if it could be confirmed by another observational method. But it's just a model, and models are a dime a dozen in planetary science.
An aside here: I was talking with Imke de Pater in the foyer between talks -- she's a planetary astronomer who studies Uranus, Neptune, and Titan. She has data on these places, and has used her data to constrain some models of what's going on inside them. She told me that she's been approached by some extrasolar planetary scientists who are interested in her trying to apply her models to figure out the kinds of things that could possibly be happening in extrasolar planetary systems. She laughed and told me, "I say to them, 'I'd rather look at data than models!'"
I think that is all that I will be able to report today. I've got a few more notes on some extrasolar planetary stuff but that is way beyond my area of expertise and I think I'll make a hash of it if I try to write any of it down, especially in my currently fatigued state. Tomorrow, we have Deep Impact and icy satellites talks to look forward to. There has been a lot of consternation surrounding Deep Impact today, because the Deep Impact press conference from the DPS meeting is scheduled for tomorrow at lunchtime, but Science magazine decided to hold a press conference tonight and lift the embargo on the Deep Impact papers that are apparently being published there this week. I am going to hold any comments on the Deep Impact results until tomorrow. And no, I have not heard a crater size yet!
Funny story there -- on Sunday afternoon, when I was mixing and mingling with people during meeting registration, I ran into Deep Impact principal investigator Mike A'Hearn. The first words out of his mouth when he spotted me were, "no, I am not going to tell you a crater diameter now!" Oh well. As soon as I hear anything about that, I'll tell you.
One final note -- JPL has released a science plan for the Titan-7 encounter, which includes a RADAR pass by Titan. I've seen the plan, but I am just not going to be able to post my usual preview story about it this week. I'll have to catch up with all of that next week.
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