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What Color is the Sky on Mars?

The first color panorama returned by the Spirit Mars Exploration Rover

The debate over the true color of the Martian sky has raged since the Viking landers touched down in the 1970s. Some images seem to show an Earth-like sky while others depict a monochrome wasteland overlooked by a dome of violent pink.

Various researchers have speculated that NASA's prevailing depiction of Mars -- excessively red and rather hellish-looking -- might be the deliberate result of over-contrasting surface imagery to "remove" suggestions of blue (and even hints of green that might indicate chlorophyll). After all, Mars is the "Red Planet," and images showing a red sky conform nicely with preconceived notions (though not all; in predictive illustrations of the Viking landers, space artists tended to show a blue sky with a foreground of reddish soil).

Depending on wind conditions, suspended dust in the Martian atmosphere can result in a pinkish hue. But persistent complaints, both from within NASA and from independent commentators, hold that JPL's consistently pink depiction of Mars' sky is in error. As Tom Paulson writes in the Seattle Post-Intelligencer, "Remote digital images taken during past Mars missions have had no reference, or calibration scale, to help scientists accurately determine the correct 'tint' of the images."

But why would JPL manipulate the contrast of Mars surface photos to produce an "ugly" Mars?

I've maintained that JPL's Martian "search for life" is largely a charade, in the sense that JPL Mars project scientists have vested intellectual and financial interests in the study of Martian geology. If Mars is in fact more Earth-like than it appears in the various images from the Viking, Pathfinder and Spirit missions, then it's possible that primitive life is surprisingly abundant -- and perhaps even detectable by cameras on the planet's surface.

The discovery of Martian biology could likely inflict severe damage on JPL's "monopoly" on Mars surface research. Notably, neither of NASA/JPL's Mars Exploration Rovers are remotely equipped to detect life. Many hoped that Britain's erstwhile Beagle 2 lander, specially equipped to test Martian soil for microbes, would challenge NASA's contention that Mars, while possibly once hospitable, is now lifeless.

A speculative "Earth-tone" view of the Spirit landing site. Could this depiction be more accurate than NASA's? Image courtesy Charles Creager.

There is intriguing evidence that Spirit's images, while stunning and informative, might not be wholly accurate renditions of the Martian surface. For example, the Spirit rover is topped with bright yellow cables for on-board electronics. These were photographed on Earth prior to the probe's departure under known lighting conditions. The same cables, photographed by Spirit at its landing site, look decidedly orange. Unless they have already been covered with a layer of red dust, which seems unlikely, one must logically question whether their current appearance is due to inaccurate color calibration by the JPL Mars Exploration Rover Team -- and, if so, whether the flawed calibration is an honest scientific mistake, a well-meaning but disingenuous attempt to make Mars look how it's "supposed" to look like, or deliberate subterfuge.

Fortunately, Spirit and Opportunity carry color calibration devices that, if used properly, could end this particular controversy.

A Curious Discrepancy on JPL's Website (Updated)

JPL has posted two radically different versions of the same Spirit image on its website. One is a forbidding blood-red; the other is a much more Earth-like sandy-pink. What gives? Is JPL using Spirit's color calibration target or not? See for yourself:


The image above looks as if it's been liberally dunked in red ink. JPL describes the image as follows:

"This latest color 'postcard from Mars,' taken on Sol 5 by the panoramic camera on the Mars Exploration Rover Spirit, looks to the north. The apparent slope of the horizon is due to the several-degree tilt of the lander deck. On the left, the circular topographic feature dubbed Sleepy Hollow can be seen along with dark markings that may be surface disturbances caused by the airbag-encased lander as it bounced and rolled to rest. A dust-coated airbag is prominent in the foreground, and a dune-like object that has piqued the interest of the science team with its dark, possibly armored top coating, can be seen on the right."

No indication is given to suggest that this is an underprocessed image, leaving viewers to assume it is an authentic depiction of our planetary neighbor.

From: JPL apparently uses the Spirit's color calibration system only when producing "definitive" pictures of the Spirit landing site. According to a correspondent, technical difficulties prohibit the use of true color in most of Spirit's snapshots, forcing project scientists to make educated guesses. Reportedly, no pretense is made that such images are properly color-calibrated. But this rather important revelation came in the form of a NASA TV broadcast; JPL's website itself is annoyingly bereft of such disclaimers.

My purpose in bringing the above contradiction to light is not to accuse JPL of a deliberate coverup, although the possibility of "academic sanitizing," described in the previous installment, should not be dismissed. But it seems a simple enough effort to let the Internet-viewing public know which images are deemed chromatically accurate and which are not, if only to avoid unnecessary cynicism and unchecked "conspiracy" allegations.


Astronomer Tom Van Flandern on Mars Color Calibration

[The following is from an email from Dr. Tom Van Flandern, quoted with permission. Italics mine. --M.T.]

The PanCam has two independent cameras, each with a different set of filters so scenes can be imaged at more than a dozen diverse wavelengths from ultraviolet to infrared. Most single images are taken through a single filter, and therefore have filtered colors accordingly. For 3D views, both cameras must use the same filter, and they only have two filters in common.

For color-corrected mosaics, Spirit has a color wheel from Earth that allows the scientists to adjust the various combined images to true color. This is because the corrections change with lighting conditions. Images of the same scene taken at least at three different wavelengths are required to get true color, even with the color wheel. Otherwise, one must make an educated guess of the true color of something in the image, and adjust everything else accordingly.

A few true color images have been shown. The rest are taken through specific filters for specific purposes (such as identifying specific minerals), and make no pretense to being true color. Even the "color corrected" views involve some guesswork unless based on at least three wavelengths and compared to the color wheel. But the emphasis in these early stages is on getting lots of quick looks, not on getting the colors right. The panorama being assembled is being made "true color". But the daily images are mostly not.

Rectangular Holes Baffle Martian Rock-Hounds

Similar rectangular holes on rocks near Spirit.

Efrain Palermo and other independent researchers are puzzling over odd rectangular openings seen in the vicinity of Spirit. The holes appear surprisingly geometric and might be due to a prior freeze/thaw cycle, indicating the existence of water.

Alternatively, it's just possible that the openings are due to some unrecognized biological phenomenon. The holes are conspicuously deep. Shelters for hardy Martian vermin?


Blue Craters on Mars?

MGS image of the failed Beagle 2's landing site.

The Mars Global Surveyor orbiter has captured a provocative image of the intended landing site for the silent Beagle 2 probe, which was to have tested the Martian surface for signs of life. In the image (above), strange blue features can be seen inside impact craters. If these are in fact blue and not due to improper color calibration, perhaps an exotic explanation is in order. Could the anomalous blue patches represent large-scale organic activity? To see the image in high-resolution, click here.


A Very Weird Rock

Close-up of the "Rectangle Rock," featuring rectangular opening and unusual "varnish" on top.

Possibly the most intriguing of the various rocks photographed thus far by the Spirit rover is a roughly columnar chunk with a clearly defined rectangular opening. The close-up view above suggests that the rectangle could be surprisingly deep. Also of interest is the piece lying in the dirt directly behind the "Rectangle Rock"; it appears to have cleanly broken off of the main body, allowing researcher Bryan Butcher to reconstruct its likely original appearance. Notice the unique speckled "varnish" effect on the rock's top. Interestingly, the "non-varnished" surfaces of the Rectangle Rock are noticeably darker than other rocks in the area.

Plausible reconstruction of the "Rectangle Rock" by Bryan Butcher.

Some have inevitably claimed that the abundant interestingly shaped rocks at the Spirit site The displaced rear portion of the Rectangle Rock, as demonstrated by Butcher, may have been caused by expanding water ice -- perhaps in the relatively recent past -- in which case one would assume JPL would take an active interest in such fractures. Disappointingly to growing numbers of Martian rock-hounds, Spirit has been piloted away from the unusual rectangular opening, leaving us to wonder how deep it goes and what might have caused it to form.

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