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Doubt cast on Venus catastrophe George 03-16-2006
Posted by George on March 16, 2006, 8:05 pm
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 http://news.bbc.co.uk/2/hi/science/nature/4815230.stm

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, suggests a "volcanic
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."

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.



Posted by Stuart on March 17, 2006, 12:53 pm
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George wrote:
> http://news.bbc.co.uk/2/hi/science/nature/4815230.stm
>
> By Paul Rincon
> BBC News science reporter, in Houston, Texas
>

<snip>

> 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 doubt that is true. Delamination will bring hot mantle closer to the
surface, weakening the crust above it resulting in volcanism.

Delamination, even where it has been proposed to occur on Earth, is
accompanied by volcanism as well.


STuart


Posted by George on March 17, 2006, 5:07 pm
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>
> George wrote:
>> http://news.bbc.co.uk/2/hi/science/nature/4815230.stm
>>
>> By Paul Rincon
>> BBC News science reporter, in Houston, Texas
>>
>
> <snip>
>
>> 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 doubt that is true. Delamination will bring hot mantle closer to the
> surface, weakening the crust above it resulting in volcanism.
>
> Delamination, even where it has been proposed to occur on Earth, is
> accompanied by volcanism as well.
>
>
> STuart

That's what I thought as well. We see it in the Andes, and I think, at
Vesuvius, for instance.

George



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