Mars' mystery cloud explained

Wayne Jaeschke

This photo from amateur astronomer Wayne Jaeschke shows cloud cover on the right side of the Martian disk, with the tops of the planet's huge shield volcanoes sticking through the clouds. For more from Jaeschke, check out his Exosky website.

By Alan Boyle

A week ago, amateur astronomers were marveling over a curious cloud that they spotted on the Mars ? and now the professionals are focusing in on an explanation.

The cloud was intriguing because it was most noticeable along the very edge of the Martian disk, and seemed to project high into the atmosphere. Some suspected that it might be a cloud of dust thrown up by an impact on the Red Planet. So, over the past week, professionals and amateurs have been working together to collect imagery and analyze the hazy spot.

"It's most likely a condensate cloud/haze, H2O in composition," Bruce Cantor, senior staff scientist at Malin Space Science Systems, said in an email that was circulated to other experts. "Similar type of phenomena have been seen in early-morning orbital observations in the past."

Cantor pointed to an earlier example of morning-limb clouds, observed by NASA's Mars Global Surveyor in the planet's northern hemisphere in 2003.

Amateur astronomer Wayne Jaeschke, who first observed this month's Martian cloud, appreciated getting the word.

"That's very interesting, as my first report on the subject suggested that it was a high-altitude water-ice cloud," Jaeschke told me in an email. "I wouldn't be surprised if that's what the consensus turns out to be."

Checking scenarios
Jaeschke said that he's been in contact with other astronomers who are looking at data from the Mars Color Imager, or MARCI, which is one of the instruments on NASA's Mars Reconnaissance Orbiter. "To date, the data shows that there was no abnormal dust activity at Mars' southern latitudes, further reducing the possibility that this was some sort of high-altitude dust storm, impact strike, or other similar phenomena," he said.

The fact that MARCI saw no abnormal cloud activity during its passes at 3 a.m. and 3 p.m. local Mars standard time suggests that the mystery cloud was a transient feature ? for example, morning clouds that dissipated by the afternoon on Mars. "Still, researchers are suspect of normal cloud activity, due to the large size of the phenomenon and apparent altitude," Jaeschke said.

One of the more exotic scenarios suggests that the morning clouds were lit up by localized auroral activity, sparked by a recent string of solar storms. "Mars doesn't have a magnetic field similar to that on Earth, but Mars Global Surveyor mapped 'umbrella-like' localized fields back in 2004," Jaeschke said.

THEMIS on the case
The High Resolution Imaging Science Experiment, or HiRISE, the powerful camera on Mars Reconnaissance Orbiter, is designed to take up-close looks at the Martian surface, but not the atmosphere. So HiRISE is unlikely to shed any additional light on the cloud question. But the team for the Mars Odyssey orbiter's Thermal Emission Imaging System, or THEMIS, has been trying to get pictures of the cloud, as well as the clouds hanging around Mars' big shield volcanoes.

"Of the nine images we targeted over the region with that large cloud, only four have been downlinked so far," Jonathon Hill, a member of the THEMIS mission operations team, told me in an email on Wednesday. "And unfortunately, it looks like the cloud either moved or is so think that we can't really see it when we're zoomed in that close."

Today, Hill provided another email update:

"We've downlinked a couple more of the images we targeted over the region with the large high-altitude clouds, but unfortunately they're all very clear without any sign of cloud activity.

"I'm starting to suspect that the clouds people have been photographing are just so wispy and thin that when we look at them zoomed in at about 100 meters per pixel, there's just not enough cloud structure for us to make out. But it is a cool example of how, even though we have a camera in orbit, we have a very limited perspective, which is why we need to combine data from multiple instruments, including ground-based observations, to study the planet as a whole.

"Next week we have some passes over the large Tharsis volcanoes, so we're planning images of their summits, where there's usually a lot of cloud activity this time of year. The good thing about those clouds is that they are anchored by the summits, so we know exactly where they'll be. Hopefully we'll be able to see some structure in them.

"I'll definitely keep you updated. Our atmospheric scientists can't wait to get some good visible/infrared images of these late spring clouds!"

Bottom line? The likeliest explanation for the mystery cloud seems to be the one Cantor came up with: It's a seldom-seen but far from unprecedented manifestation of Martian morning weather. For more of the expert amateur opinion, check out the Unmanned Spaceflight website, the Cloudy Nights online forum and the Mars Observers group on Yahoo.

Where in the cosmos?
Jaeschke's picture of Mars, featuring the cloud cover surrounding the Red Planet's monster volcanoes, served as this week's "Where in the Cosmos" picture puzzle on the Cosmic Log Facebook page. It didn't take long for my Facebook friends to figure out what the picture showed, and even name the four big volcanoes (Olympus Mons, Ascraeus Mons, Pavonis Mons and Arsia Mons). For solving this week's mystery so quickly, Rick Casey and Shelton Howard will be getting some 3-D glasses in the mail, plus a 3-D picture of yours truly. Keep your eyes peeled for next week's "Where in the Cosmos" puzzle on Facebook.

The Martian mystery cloud was one of the subjects discussed during this week's Space Hangout, hosted by Pamela Gay with Emily Lakdawalla, Ian O'Neill and yours truly as commentators.

More about amateur astronomy:

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Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.

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