Last Updated: Thursday, 16 March 2006, 21:36 GMT
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Doubt cast on Venus catastrophe
By Paul Rincon
BBC News science reporter, in Houston, Texas
Accepted views of how the
planet Venus evolved are
challenged by new age
dates for its surface.
Massive volcanism 500 million
years ago was thought to have
covered over much of the
planet's ancient features.
But work carried out at
Imperial College London, UK, Scientists age Solar System surfaces by
suggests a "volcanic counting craters
catastrophe" is not needed to explain the look of Venus's
surface.
The British team presented details of its research to a major
science conference in Texas, US.
Scientists will have an early opportunity to examine the new
ideas - Europe's Venus Express spacecraft is due to arrive at
the planet next month for a two-year investigation of Earth's
near-neighbour.
Crater counters
Researchers date planetary surfaces by looking at the
distribution of their impact craters.
On most planets and moons, impact craters tend to be
clustered on very old parts of the surface, due to the heavy
bombardment that is believed to have taken place in the
early Solar System.
But craters on Venus are distributed randomly over the whole
planet. This has led some scientists to the conclusion that
most of the surface is of similar age.
One way to arrive at this result is by rapid resurfacing - the
model long accepted by planetary scientists.
Timothy Bond and Mike Warner of Imperial College London
have now thrown that theory into doubt.
Using computer modelling, they came up with a suite of
possible scenarios that were compatible with the planet's
cratering record and surface features.
They concluded that there was no need to invoke massive
outpourings of lava over a short period. Instead, the planet's
present-day surface could be compatible with a slow decline
of volcanic activity, they argue.
Heat calculations
"The transition from a high rate of resurfacing to a low rate
could have lasted as long as two billion years," Timothy Bond
told the BBC News website.
Professor Warner added: "We
haven't shown that a very
short event isn't possible,
we've just shown that there
are a much wider range of
possibilities.
"A very short event is, a
priori, quite unlikely given
that there is a much wider
range of likely realities." Mission guide: Venus Express
Previous work suggests the volcanic upheaval 500 million
years ago covered up "almost all" of the ancient surface.
The models developed at Imperial College suggest about
26% of the planet's surface could be older than 700 million
years.
The findings agree with new models of heat loss from the
interior of Venus produced by Dr Richard Ghail, also of
Imperial College.
'Steady state'
Earth's surface is divided into many plates that move relative
to one another on convection currents in the mantle below.
At a type of boundary called a subduction zone, one plate is
dragged down below an adjacent plate and destroyed in the
mantle. At another, called a spreading ridge, two plates
move apart and grow as volcanism adds new material at
their edges.
These processes, called plate tectonics, continually cool the
Earth and keep it in balance - what scientists call a "steady-
state".
There is little evidence of plate tectonics on Venus.
Therefore, some scientists think heat might build up below
the Venusian crust, leading to occasional catastrophic
releases of magma along with rapid resurfacing of the planet.
However, Dr Ghail believes the surface features of Venus do
not necessarily reflect the rate of plate tectonics on the
planet.
Instead, he thinks high temperatures in the interior create a
weak zone between the crust and the mantle which
essentially decouples, or separates, them from each other.
This would allow more continual plate tectonic activity that
would leave little evidence on the surface.
"I think we're moving closer towards a steady-state model
for Venus," Dr Ghail told the BBC News website.
The researchers presented their results here at the Lunar and
Planetary Science Conference in Houston, Texas.
PLANETS COMPARED
VENUS EARTH
Distance from 108,200,000km 149,600,000km
Sun
Diameter 12,103.6km 12,756.3km
Mass 0.82 Earth masses 1.0 Earth masses
Rotation period 243 Earth days 23 hours 56 minutes
(day) (retrograde)
Year length 224.7 Earth days 365 days
Atmosphere 96% carbon dioxide 77% nitrogen
3% nitrogen 21% oxygen
Mean 464C 15C
temperature
Surface pressure 92 bars 1 bar
Moons 0 1
Global magnetic No Yes
field
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