[tig] NASA's colors for Mars

Peter Swinson peter_swinson
Sun Mar 7 14:55:09 GMT 2004

[editor?s note: From Peter Swinson, as mailed to your
administrator Rob, with the comment that this ties in very
neatly with the discussion I started in January
that I didn?t think the colors looked right. ]

 New Scientist 31 January 2004 
NASA has safely landed its rovers. But can it handle the conspiracy
theorists? David L Chandler investigates.

THE rolling landscape of red soil is strewn with dark rocks. NASA's brace
of rovers have certainly confirmed the picture we have come to expect of
Mars. But take a closer look at the images they have been sending back. Is
the Red Planet really, well, quite this red?
Welcome to the latest space conspiracy theory. The soft version of the
story claims that in a bid to make an ordinary looking, brownish Mars live
up to its billing, NASA has been naughtily tweaking the colours in Spirit's
digital images. The hard version has the evil NASA doctoring the colours so
the rest of us won't notice evidence of life, such as patches of green.
Leaving aside the question of why NASA would want to hide such a momentous
find, has it been taking liberties with the colours? Talk to NASA's image
experts and you discover that getting the colours right is a surprisingly
difficult  and, despite the technical wizardry involved, subjective   job.
In fact, truly accurate results, the specialists agree, are not going to
happen until people have been to Mars and seen its colours first hand.
That said, there are problems related to these Mars rover images, some of
them preventable. And in failing to make it clear just what we are seeing,
NASA has naively allowed conspiracy theorists a field day.
Although there are standard red, green and blue (RGB) filters on board that
can produce a fair approximation of "true" colour, these have hardly been
used. Instead, most of the colour images displayed so far have been taken
through green, blue and infrared filters (IR GB). When the infrared gets
rendered as red, the results are pretty close to true, but with some really
glaring exceptions. Blue and green, in particular, just don't come out
right. As far as we know, those colours don't exist anywhere on the surface
of Mars. If they did, we would have noticed them in the few images that
have been produced using a normal red filter.
But they are to be found on the spacecraft itself   hence the conspiracy
theories. Standard blue and green paints, it turns out, are extremely
reflective in the infrared, even though they hardly reflect any red light
at all. So the red yellow green blue colour targets installed on each
rover, as well as the bright blue NASA logo, look very strange indeed. So
does the blue insulation around much of the wiring. The blue paint reflects
more than three times as strongly in infrared as it does in blue. So when
the pictures taken with IR GB filters are printed as RGB, the result is
that the red pigment overwhelms the blue and you see a deep burgundy or
even, with the insulation, a hot pink. Similarly, the green reflects more
than twice as much infrared as it does green, so the green colour patch
ends up a sort of mustard colour.
Why have they been doing this? Jim Bell of NASA tells me that it's because
the important thing is to get the information the geologists need to
distinguish rock types, and to tell dusty rocks from clean ones. And for
that, infrared is much more useful than red; hence its use for the main
panoramic images we have been seeing.
That's just part of it, though. Because of the reddish dust that is always
in the air, the light falling on the surface of Mars is red to begin with;
the effect is likely to be rather like terrestrial lighting close to
sunset, when hills take on a pink or magenta hue. And the quality of Mar's
red light will depend greatly on the level of dust as well as the time of
day. That's a problem for NASA because the panoramas it has been showing us
are mosaics assembled from dozens of separate frames. It may take months of
fine tuning to get the colours consistent between frames.
Still, Bell says, compared to the initial images from Viking, which were
way too red, even the initial images from this mission have been closer to
what things would really look like there Better still for Bell, nobody is
in a position to argue with him. Until of course someone goes there to

Best regards

Peter Swinson.

I have no connection with NASA, as to the best of my knowledge they have no
space vehicle with adequate power to get me off the ground ! 


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