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JUPITER’S closest moon, Io, has a new volcano. Astronomers in Hawaii
discovered it on Christmas Eve while testing a new infrared camera.

Planetary geologists have known that Io has volcanoes ever since NASA’s
Voyager spaceprobes sent back spectacular images in 1979. Vulcanism on Io
is thought to be produced by tidal forces: Jupiter’s gravity is stronger
on the nearside of the moon than on the farside, and this pulls the moon
apart.

After receiving Voyager’s pictures, astronomers continued to monitor
Io from Earth. But it is difficult for them to resolve any detail on the
moon’s surface, because it appears so small in the sky – only 1.2 arcseconds
across.

However, late last year, astronomers at NASA’s Infrared Telescope Facility
on Hawaii were installing a new infrared camera, known as ProtoCAM. To test
the camera to the limit, the scientists decided to look at Io. On Christmas
Eve, Mark Shure, the designer of the camera, and Mike Ressler, a graduate
student, tuned in to a wavelength of 3.8 micrometres and pointed the telescope
at Io.

Through luck, and a wise choice of wavelength, the astronomers saw details
as small as 0.4 arcseconds across. This was one-third the diameter of Io
in the sky.

Immediately, the astronomers saw Loki, the large volcano discovered
by the Voyager probes. It was a bright spot against the darker disc of Io.
As they watched, Io fell into shadow, eclipsed by Jupiter.

Later, at the Institute of Astronomy in Honolulu, John Spencer processed
the data by computer to brighten the images taken by ProtoCAM. Opposite
Loki on the face of Io, he saw another bright spot, only a twentieth as
bright as Loki.

Spencer analysed closely the images taken as Jupiter moved in front
of Io, and was able to confirm that it was not a trick of the camera’s electronics,
nor of the Earth’s atmosphere. It was a new volcano.

‘Now we will be able to monitor this new volcano regularly,’ says Spencer.
‘Watching it will tell us a lot about how much molten magma is available
in Io at any one time. We will also be able to tell whether the amount of
vulcanism detected by Voyager is typical of Io’s behaviour.’