Thousands of tiny, perfectly rounded spheres have been seen on the dark soil around the Mars rover Opportunity.
The bead-like particles were seen in high-resolution images taken by the craft鈥檚 panoramic camera, and two of them were seen close up by its microscopic imager.
鈥淭he spheres are unlike anything we鈥檝e ever seen on Mars,鈥� said lead scientist Steven Squyres, and are intriguing because 鈥渢here are only so many ways to make round things鈥�.
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They could be frozen drops of lava ejected by a volcanic eruption (called lapillae), frozen drops of rock melted and thrown out by a meteorite impact (called tektites), or, most excitingly, particles formed by slow chemical accretion in a body of water (called oolites).
Further close examination by the rover鈥檚 suite of instruments should help to pinpoint the mechanism of formation. For example, if the rover finds a cracked sphere and sees pearl-like concentric layering, the particles would be oolites.
However, some of the spheres appear to have holes in them, which suggests the escape of trapped gases from a molten blob and hence a volcanic or impact origin.
Field trip
In addition to the spheres, there are other pebbles on the surface that have a reddish colour. While much of Mars is covered in red dust, the underlying rocks are mostly dark, making these pebbles unusual. They may be weathering out of the outcrop of bedrock six metres from the lander.
In the early hours of Thursday morning, Opportunity moved about three metres forward, bringing it about halfway to the right-hand edge of the long outcrop. It is expected to complete the trip on Friday.
The rover will then spend several days travelling slowly along it, taking high-resolution, full-colour stereo images and infrared spectra of the finely layered rocks. These images will be then used to guide the science team in deciding which areas to investigate up close with the rover鈥檚 multiple instruments.
A detailed image already taken by the mini-TES infrared spectrometer shows some parts of the 22-metre crater in which Opportunity landed are very rich in hematite while others have virtually none. The presence of the iron-oxide mineral, which often forms in liquid water, was a major factor in choosing Opportunity鈥檚 landing site.
鈥淓very shape, texture and colour has a story to tell鈥� about the geological history of the area, said Squyres. 鈥淚n the coming days, we鈥檒l unravel the mysteries.鈥�