The key is informed choice, and an ingenious device described in the Journal of Geophysical Research may be the very thing to help planners make up their minds. In the report, D. Vaniman of the Los Alamos National Laboratory, New Mexico and colleagues describe working prototypes of a device that could identify the precise mineral content of rock and soil samples collected by a robot sampler.

On Earth, the presence of certain minerals is a sign of life: conversely, life is more likely to live in environments where certain minerals are more common than others. This device should help the next generation of space missions narrow down places on Mars where life might lurk.

The device is named CHEMIN, to reflect its dual role as a combined chemical and mineral analyzer. It subjects a powdered sample of rocks or soil to a small, low-power source of X-rays. The way in which minerals fluoresce in X-rays (X-ray fluorescence, or XRF) gives clues to their elemental content. In addition, the device records the diffraction of X-rays (X-ray diffraction, or XRD) through the sample, producing a pattern of reflections that is as characteristic of individual minerals as a fingerprint is of the human to whom they belong. XRF and XRD are recorded simultaneously with a small digital camera, called a charge-coupled device (CCD).

More than 20 years ago, the Viking landers were capable of subjecting samples to XRF analysis. The machinery was crude by todays standards, but a great deal was learned. Nevertheless, XRD provides spot-on identification of minerals that can only be inferred from XRF, because XRF reveals elemental composition whereas XRD probes the mineral's unique crystal structure. CHEMIN would be capable of identifying all the minerals in a complex rock type, such as basalt.

Like all instruments destined for use in space, CHEMIN is small and light. It weighs less than a kilogram, has a volume of half a litre, and requires just two watts of power. CHEMIN could be used to investigate likely places spotted from orbit, to see if they are worth further investigation.

So, where might microscopic martians hang out? Mars has had a history of impressive volcanism, and there is a chance that it might still be active. The intrusion of magma beneath permafrost might have produced watery environments favorable for life. Bright spots in the floors of certain crater floors might represent salt pans, the remnants of once open water. Other craters preserve sediments that might have formed in river datas or by flooding: these could be sources of martian fossils. To help identify such sediments, the researchers tested CHEMIN's capabilities for identifying minerals which on Earth are associated with such environments and the microscopic life harboured within, such as carbonates, sulphates and certain forms of silica.