THE vivid reds and pinks of Mars provide vital clues about the planet鈥檚 mineral composition. Unfortunately these colours are becoming increasing difficult to pick out in the pictures being sent back by the rovers Spirit and Opportunity. But a clever redesign of a crucial component should mean that colours in the pictures sent back by the next Mars lander should stay true for far longer.
The troublesome piece of equipment is a saucer-sized disc covered in patches of brightly coloured polymer that sits near the panoramic camera on the two rovers. By regularly taking photographs of this 鈥渃alibration target鈥� and comparing the colours with those in photographs taken on Earth, researchers can tell how the dust-filled atmosphere is affecting colours and adjust the images of the landscape accordingly.
鈥淭he targets take the varying lighting conditions out of the equation,鈥� says Mark Lemmon of Texas A&M University, who leads the camera team on NASA鈥檚 next Mars lander, called Phoenix. Accurate colour calibration allows minerals to be identified from the pictures. But after 14 months on Mars, the calibration targets have become masked by dust. Researchers must use a computer model that estimates and subtracts the effect of dust on the target when processing images and this introducs inaccuracies.
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Designing a coloured target that does not get clogged by dust is a tricky task. The material it is made from has to be resilient enough to cope with the vacuum of space while on its way to Mars, and then with the harsh conditions on the planet鈥檚 surface. The target also has to reflect light evenly in all directions without causing glare, and to do this it must be capable of being cratered with 100-micrometre pockmarks.
The only material known to meet all these criteria is a slightly sticky silicon-rich polymer called RTV655. The pockmarks trap dust and this, combined with the material鈥檚 stickiness, has meant the targets have become caked.
鈥淎fter 14 months on the surface of Mars, the rovers鈥� colour calibration targets have become masked with dust鈥�
Using a different material for the targets is not an option. 鈥淢aterials that have space-qualified properties are very difficult to find,鈥� says Firouzeh Sabri, a materials scientist at the University of Florida, Gainesville. To get round the problem, she and colleagues have added a layer of transparent RTV655. This fills the dimples without altering their optical properties. To further improve the target鈥檚 dust resistance, they have coated this layer with a transparent nanolayer of gold.
NASA鈥檚 Phoenix lander, complete with the improved calibration target, is due for launch in August 2007.
