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By Emily Lakdawalla

Mars rover finds puddles on Mars?

Jun. 11, 2007 | 07:22 EDT | 11:22 UTC

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UPDATE JUNE 12: New Scientist and Ron Levin have now retracted the original story.

Last week, New Scientist Space posted a provocative story titled "Mars rover finds 'puddles' on the planet's surface." The story concerned a presentation made at the 2007 Institute of Electrical and Electronics Engineers (IEEE) Aerospace Conference by Lockheed Martin physicist Ron Levin. The article reads in part:

A new analysis of pictures taken by the exploration rover Opportunity reveals what appear to be small ponds of liquid water on the surface of Mars....

This would be an amazing find, if true! How could the mission's scientists have missed this? And how could liquid water possibly persist in the sub-zero temperature and near-vacuum pressure at Opportunity's landing site? The article goes on to explain the basis for Levin's claim:

Along with fellow Lockheed engineer Daniel Lyddy, Levin used images from the Jet Propulsion Laboratory's website. The resulting stereoscopic reconstructions, made from paired images from the Opportunity rover's twin cameras, show bluish features that look perfectly flat. The surfaces are so smooth that the computer could not find any surface details within those areas to match up between the two images.

Here is the image that accompanied the article, showing smooth bluish areas lying in what appear to be local lows between bumpy rock surfaces:

Click to enlarge >

"Puddles" on Mars?
A computer algorithm applied to stereo Pancam images of an area of Burns Cliff, within Endurance Crater, identified some very smooth areas (blue). Credit: NASA / JPL / Cornell / Ron Levin

The blue color is, of course, false color and probably arose from the color table that Levin and Lyddy used to encode the results of their image analysis algorithm -- it's not the blue color that argues for water, it's the smoothness and the way that the smooth areas fill topographic lows. At least I'm pretty sure that's what they're claiming; unfortunately, I couldn't find a copy of the conference abstract online. I found a list of the papers here, but that's it; apparently the conference proceedings have not yet been published. So all I have to go on to investigate this extraordinary claim -- that Opportunity saw puddles of water on the Martian surface -- is what was written in the New Scientist article. That, and the images from the rover, which are out there for everyone to investigate.

It's kind of hard to know what's going on in that image without some context. For the rovers, you have to know when an image was taken in order to learn something about where the image was taken. Fortunately, a sharp-eyed observer tracked down the temporal context, saving me some work. It was on sol 290; you can see the image in approximate true color on Daniel Crotty's wonderful website of Mars Exploration Rover Multispectral Color Imagery. (The color is only approximate, because this image was taken using filters 2, 5, and 7 on the rover's left eye, which see infrared, green, and blue-violet, respectively; the color space reaches to both shorter and longer wavelengths than human eyes can actually detect.) Here's Daniel's color version of the image:

Click to enlarge >

"Puddles" image in approximate true color
This image was taken by Opportunity on sol 290 as it explored Burns Cliff inside Endurance Crater. Credit: NASA / JPL / Cornell / Daniel Crotty

OK, so now we know when the image was taken, so we can begin to figure out its spatial context. The excellent route map by Phil Stooke shows where Opportunity was on sol 290. Sol 290 was during the period when Opportunity had been carefully, carefully driven out along the steeply sloping wall of Endurance Crater to investigate Burns Cliff. Here's that same image, in its spatial context (helpfully provided to me by Michael Howard):

Click to enlarge >

Opportunity's view from sols 287 to 294, on Burns Cliff
This view was created in Michael Howard's Midnight Mars Browser from approximate true color images generated by Daniel Crotty. At the time, Opportunity was poised at the toe of Burns Cliff inside Endurance Crater. Credit: NASA / JPL / Cornell / Daniel Crotty / Michael Howard

The white square shows you where the image comes from. It's in the middle of Opportunity's Burns Cliff panorama, on some of the steepest slopes that Opportunity saw before arriving at Victoria crater! Those can't be puddles -- unless the amazing "liquid" that puddles here on Mars in a freezing near-vacuum also has antigravity properties. A less sensational hypothesis is that the smooth "fluid" that fills local lows between rocks on this sloping surface is fine dust.

It's kind of astonishing to me that anyone could present a paper of this nature without having checked the spatial context. They must have assumed that the surface that the rover was observing was horizontal, like so many surfaces in Meridiani Planum. It may even have been "horizontal" with respect to the rover's deck -- but the rover was sitting on the same slope that it was photographing at the time the images were taken. It shows you how dangerous it can be to attempt to do science from partial data. The images are only one kind of data returned from the rover; along with the images come data from many other instruments, including, for example, tilt sensors that would tell you exactly how much of a slope the rover was sitting on when it took that image.

I think this story also serves as a cautionary tale to taking too seriously papers that are presented at conferences. These "papers" are, for the most part, not reviewed by experts in the field before they are presented or published in the conference proceedings. Conferences are a great place to get updates from researchers on the current status of ongoing research, but the papers that scientists present there are just progress reports on work that hasn't yet been submitted to critical and skeptical scrutiny by other experts in the field. This is one claim that will never make it past peer review.

I wouldn't have even written about this, except that an uncle of mine -- not a technogeek -- asked me this weekend about these puddles of water that were just discovered on Mars. So this story is clearly getting some play in some kind of mainstream media. I don't think it does any huge amount of harm -- in fact, it had the benefit of causing my uncle to strike up a conversation with me about Mars and what's going on there right now. Still, it's apparently being discussed quite seriously in a lot of places, and I wanted to go on record to point out that this particular "water on Mars" claim is not supported by evidence.

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