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The Mars Pathfinder mission has scored at least one scientific hit to add to its set of technological achievements: the Sojourner rover has analysed the surface of a rock called Barnacle Bill, and found it to be unexpectedly high in silicates.

Sojourner travelled about 3 metres to the rock on 6 July, extending its alpha proton X-ray spectrometer (APXS), which uses a radioisotope source to bombard the rock surface with alpha particles, and then takes an energy spectrum of the X-rays, protons and alpha particles that return.

The relative proportions of most of the important elements can be determined in this way. In the case of Barnacle Bill, the measurements imply a silicate content of roughly 58 per cent. This is too high for the most basic category of volcanic rock, basalt, which was expected to be prevalent on Mars's surface.

Barnacle Bill's composition appears to be similar to that of andesite, which on Earth is found above subducted oceanic crust. Either the crust melts, or the mantle melts after the crust has injected water into it; and the melt then erupts from volcanoes to become andesite.

So did Mars once have plate tectonics, similar to those on Earth? If that could be proved, it would be a considerable discovery because, unless one counts the ice plates of Jupiter's moon Europa, there is no planetary body — apart from the Earth — with evidence for plate tectonics.

Unfortunately, there are other ways to make a rock with this composition. Slow cooling in a large magma chamber, for example, could achieve this, especially in the presence of water. (And a great deal of water is believed to be locked in the rocks of Mars, either in the top 10 km of porous crust, or perhaps in the mantle.) Such a chamber could even have been created by the impact of a large comet or asteroid.

More samples would clearly be welcome. On Wednesday 9 July, the rover drove to another rock, ‘Yogi’. But a driver's miscalculation meant that it travelled too far, and was left leaning up against the rock, with the APXS at an unusable angle.

By last Saturday, the controllers at NASA's Jet Propulsion Laboratory in Pasadena had succeeded in ordering it to reverse, but only after a few days of confused communication. The first command was thought to have gone astray last Thursday because of a timing error, so it was resent on Friday. Then on Friday evening the lander's computer reset itself and forgot what it was supposed to tell Sojourner. In fact the original command had gone through.

Sojourner should now have had another go at analysing Yogi before heading for a third target, possibly a rock called Casper, whose paleness may mean an even higher silicon content — or just a fine dust coat.

Meanwhile the lander is collecting weather information and preparing a colour panorama of the area. Up-to-date information can be found at http://mpfwww.jpl.nasa.gov and at many other Web sites around the world.

Figure 1: Bill's barnacles: pale soil mottles the face of this otherwise dark volcanic rock, highlighting its rough texture.
figure 1

NASA

Singled out for optical spectroscopy, the soil looks like oxidized volcanic rock on Earth.

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