LAST MONTH, the spacecraft Magellan first pierced the clouds of Venus
with its radar. Now it is returning images that are sharper than any before,
and that are changing geologists’ ideas of the planet. Previously, the planet’s
atmosphere, composed of carbon at almost 100 times the pressure of Earth’s,
has permitted only a fuzzy view of the surface.
Magellan is circling Venus in a near-polar orbit at an altitude that
varies between 290 and 8000 kilometres. Since 15 September, it has been
sending back images taken by its synthetic aperture and altimetry radars.
Each time the spacecraft completes an orbit – almost eight times every 24
hours – it images a longitudinal sliver of the surface about 20 kilometres
wide and 7000 kilometres long as the planet rotates beneath it. When pieced
together, adjacent slivers form a mosaic of Venus.
‘This planet is an amazing place,’ says Joe Boyce, chief scientist on
the Magellan project. With less than two per cent of the planet mapped,
Venus is offering surprises. The surface is cross-hatched with faults and
lava flows, pockmarked with large impact craters, and etched with long,
belt-like ranges of mountains. ‘Magellan is like a microscope on Venus,’
says James Head of Brown University. ‘We are getting 10 to 100 times the
resolution of previous views, and there is a lot of action there.’
For instance, the craters left by meteorite impacts are large, and there
are very few small ones. According to Head, this is because the planet’s
atmosphere is so thick that all but the largest meteorites burn up.
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An example is a crater in the Navka region in an area dubbed the ‘crater
farm’. The crater, which is 12 kilometres long and 9 kilometres wide, is
flat and hummocky, with terraced walls. Its appearance is unique in the
Solar System, Head says. ‘We think the morphology may be due to the break
up of the projectile in the atmosphere,’ he says. ‘When it gets to the surface
it may be more like a pancake than an equidimensional meteorite.’
The lack of small craters poses a problem for planetary geologists.
Usually a count of craters per unit of surface provides a ‘clock’ to date
a planet’s origin. Scientists assume that small meteorites strike the surface
at a relatively steady rate throughout its history. But on Venus the clock
simply is not running.
On the other hand, the ejecta – the dust and debris around the rim of
craters – may help solve some mysteries on Mars as well as Venus. Magellan
has found bright patterns of ejecta surrounding craters like the petals
of a flower. Craters on Mars also show this pattern, and some geologists
have suggested that they were created by water. Craters on a waterless world,
such as the Moon, have uniform, or ‘ballistic’, patterns of ejecta.
‘Any time water moves,’ says Bruce Marsh of Johns Hopkins University,
‘the edges of it are tongue-like, like waves lapping on a beach.’ Venus
has no water, so if the scientists continue to find the petal pattern across
Venus, the Martian theory could be in serious trouble.
At the moment, geologists are speculating that the ejecta patterns might
be caused by Venusian wind – which has a speed of only a metre or two per
second – or perhaps by volatile gases released from beneath the surface
when the meteorite struck.
Because there is no water on Venus, there can be no water erosion. So
geologists can trace the planet’s history by its multilayered geological
features. They remain as sharp as the day they were created.
Already, for example, tectonics is becoming visible on Venus. On Earth,
tectonic plates leave telltale marks – faults and subduction zones in areas
where plates are drawing apart, and linear mountain ranges such as the Himalayas
in areas where there is compression.
According to Boyce, Venus’s surface shows plenty of fracturing, but,
he says, these are tensional fractures and not necessarily associated with
plates.
But in an image of the Danu Montes mountain belt, which is two to three
kilometres high, there are ridges that probably are a sign of compressional
deformation. Nearby troughs suggest that the mountain range may be collapsing
under its own weight. There are also sinuous features nearby and some ponding
of dark material, along with small crater chains. These indicate that volcanoes
were active when the mountains were formed.
One image shows a flat-topped dome ‘like an upside down cereal bowl’,
says Head. The top has been fractured but there is no sign of lava flow
– perhaps an intrusion in which molten material has solidified.
Maria Zuber, a geophysicist at Goddard Space Flight Center in Maryland,
says she has mostly seen signs of extension, and few signs of compression.
Something must give if the crust expands, says Zuber. For this reason, she
suggests that rising plumes of hot rock from the mantle might be causing
the surface to ‘bow up’ in places. And tension fractures could be caused
by cooling after volcanic activity.
Another observation made possible by the unprecedented resolution of
Magellan’s images – about 100 square metres per pixel – involves the size
and spacing of surface features. The distribution of features is correlated
to the thickness of the crust. ‘It’s like pushing the edge of a carpet,’
explains Zuber. ‘The width and spacing of the wrinkles depends on the carpet’s
thickness.’ Magellan’s radar has spotted smaller, more closely spaced features
than any previously seen. Zuber suggests that the crust of Venus may not
be uniform, but, like an onion, made of many thin layers.
