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The Shallow Sky

Worlds of Controversy

Akkana Peck


“Life on Mars! Whoopee! So everyone wants to see methane on Mars, and it makes for great headlines.”

“But any Earth-based measurement of Martian methane has to cope with the fact that Earth’s atmosphere has far more methane than Mars.”



Controversy is alive in the shallow sky, and we got a good taste of it in local talks in January.

First, Dr. Jeff Moore, of NASA Ames, was the featured speaker at the monthly SJAA meeting. His topic: ice volcanoes on Saturn’s moon Titan. Or rather, a lack of ice volcanoes. Volcanoes of methane, ammonia and water ice erupting in slow motion may sound cool – but it turns out the evidence for them isn’t as convincing as the news stories might have you believe.

The early Cassini photos did look promising, with what looked like flow patterns on the flanks of a low mountain. But measurements with Cassini’s altimeter showed that they weren’t at all what they seemed: what looked like the summit of a volcano in the visual image wasn’t the highest point at all, and most of the mountainous region was a relatively flat plain. It was all a trick of the light.

JPL clings to the story – Jeff played us a pro ice volcano infomercial they made after he first questioned the data. But he demonstrated how every ice volcano candidate so far can be explained as a product of erosion by Titan’s wind and methane rain. Jeff compared Titan’s surface to the surfaces of several Galilean moons, and to various types of erosion on Earth, concluding that Titan may be, in effect, “Callisto with weather.”

Jeff’s colleague at NASA Ames, Dr. Kevin Zahnle, has been in the news as well – on the cover of The Economist for a paper he wrote questioning common assumptions about methane in Mars’ atmosphere.

I’m sure you’ve seen articles on Martian methane. Methane doesn’t last long in the atmosphere – only a few hundred years – so if it’s there, it’s being replenished somehow. On Earth, one of the most common ways to produce methane is through biological processes. Life on Mars! Whoopee! So everyone wants to see methane on Mars, and it makes for great headlines.

The problem, according to Kevin, is that the Mars measurements show changes on a scale much shorter than hundreds of years: they fluctuate on a seasonal basis. That’s tough to explain. Known atmospheric oxidation processes wouldn’t get rid of methane fast enough, so you’d need to invent some even more exotic process – perhaps methane-eating bacteria in the Martian soil? – to account for the drops.

Worse, the measurements showing methane aren’t very reliable. The evidence is spectroscopic: methane absorbs light at several fixed wavelengths, so you can measure methane by looking for its absorption lines.

But any Earth-based measurement of Martian methane has to cope with the fact that Earth’s atmosphere has far more methane than Mars. How do you separate possible Mars methane absorption lines from Terran ones? There’s one clever way: you can measure Mars at quadrature, when it’s coming toward us or going away from us, and any methane spectral lines would be red- or blue-shifted compared to the Terran ones. But then the lines overlap with other absorption lines from Earth’s atmosphere. It’s very difficult to get a reliable measurement. Of course, a measurement from space would avoid those problems, so the spectrograph on the ESA Mars orbiter has been pressed into service. But there are questions about its accuracy.

The published evidence so far for Martian methane just isn’t convincing, especially with those unlikely seasonal fluctuations. That doesn’t mean there’s no methane there; it means we need better data. The next Mars Rover, dubbed “Curiosity”, will include a laser spectrometer which can give us much more accurate methane measurements. Curiosity is set to launch this fall and arrive at Mars in August of next year.

Good stuff! But there was one more planet controversy in January. Just a few days after the SJAA meeting, San Joseans were treated to two talks on Pluto. On Jan 18, Dr. Alan Stern, principal investigator of the New Horizons mission to Pluto, spoke at the SETI institute about Pluto and the mission. Then the next day, Mike Brown spoke at Foothill College about his new book, “How I Killed Pluto and Why It Had it Coming!” It was an entertaining pair of talks. Dr. Stern couldn’t resist adding a few jibes at Brown while explaining why Pluto’s importance in the solar system shouldn’t be underestimated. Brown’s talk was less technical and told the story of the discovery of Eris and other Kuiper-belt objects. Both talks were recorded, though neither video seems to be online yet as I write this.

Sick of controversy and just want to look at some planets?

Saturn rises in the early evening in March and is at its best a few hours after midnight. The ring tilt is a bit less than ten degrees. Take a look at Saturn and at Titan – bet you don’t see any ice volcanoes!

Mercury becomes visible in the evening sky in the second half of March, joining Jupiter in the evening twilight. Mercury reaches greatest elongation from the sun on the 23rd, and then closes rapidly, shrinking to a crescent by month’s end as it grows in size.

Mars, Uranus and Neptune aren’t well positioned for observing this month. Pluto is a morning object, rising a few hours after midnight, so it doesn’t get very high before dawn overtakes it.

Venus is a morning object, as is Neptune. On the mornings of the 26 and 27, look for Venus and Neptune less than half a degree apart. They’ll pass much closer than that, only a tenth of a degree apart – but it happens during daylight here, when Neptune is invisible.

Daylight savings time begins on March 13.

Finally, look for the Zodiacal Light as a faint band rising up along the ecliptic as the sky darkens after sunset in the last two weeks of March.


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