IT HAS only been a few days, but Cassini鈥檚 pictures of Saturn鈥檚 rings have already revealed mysterious structures, and its images of Titan have reversed our ideas about the moon鈥檚 surface.
Cassini entered Saturn鈥檚 orbit on 1 July, and came as close to the planet鈥檚 rings as it ever will. The spacecraft鈥檚 cameras saw the expected waves and ripples in Saturn鈥檚 rings caused by the gravity of its moons, and also a host of ringlets with unexpectedly sharp edges 鈥� a mystery, as collisions between particles should smooth out the edges. They also found some ring particles that seem to be clumping together. 鈥淚 don鈥檛 know what this is. I literally don鈥檛 have a clue,鈥� says Carolyn Porco, head of the imaging team at the Space Science Institute in Boulder, Colorado. Saturn鈥檚 rings are regarded as a model for the disc of gas and dust from which planets formed, so these clumps could reveal something about how planets came about.
Measurements of the spectra of light from the rings show that most of them are 99 per cent pure water ice. But there are also some dirty bits. Saturn鈥檚 F ring, for example, has spectra resembling those of the dirt on Phoebe, Saturn鈥檚 outermost major moon. It may be that these parts of the ring system have a different origin from the rest. Phoebe is thought to have been trapped by Saturn鈥檚 gravity after it wandered in from the outer solar system; similar bodies may have got closer still and been torn apart. Maybe the F ring is the remains of an unlucky sister of Phoebe.
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Cassini has also passed within 340,000 kilometres of Saturn鈥檚 giant moon Titan. By looking at wavelengths of light that can cut through Titan鈥檚 orange haze, the Cassini team saw surface features as small as 10 kilometres across, including a giant H, possibly shaped by tectonic processes, and what could be an impact crater more than 1000 kilometres wide. The images also show a cluster of methane clouds floating near the moon鈥檚 south pole.
Until Sunday, planetary scientists thought that the bright areas on Titan were relatively pure water ice, and the darker areas were hydrocarbons. But spectral analysis has shown that it is the other way around. The large bright region known as Xanadu might be a sea of methane instead of mountains of ice. 鈥淭he story has changed completely in the last 10 hours,鈥� says team member Elizabeth Turtle of the University of Arizona at Tucson.
