Polar Vortex on Venus
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Transcript of Polar Vortex on Venus
enus shines brilliantly in the evening
sky, a regular reminder that she is
our nearest planetary neighbour and
readily accessible to our spacecraft. Yet, like
the abyssal regions of Earth’s oceans, and for
strikingly similar reasons, the lower
atmosphere and surface of Venus are little
explored. We know there are massive
mountains, steep valleys, wandering dry
river beds of Amazonian proportions,
sweeping decks of cloud and bizarre
weather systems on a planet that, in terms
of its solid body, is intriguingly like our
own – superficially at least. We also know
that the surface of Venus is subjected
to crushing atmospheric pressure and
correspondingly searing temperatures, a
consequence (probably) of the slow
planetary rotation rate and scores of
vigorously active volcanoes, fuelling
greenhouse warming that makes the
threatened hothouse Earth of a century or
so hence look like a beach in Bermuda.
No wonder there was no mention by
President Bush of Americans on Venus in 30
years. The Europeans, with their ambitious
‘Aurora’ programme, don’t want to go there
either. We’ll be skiing on Mars and Mercury
(yes, we can) before we walk on sultry
Venus. A planet with rivers of molten tin and
mountains capped, not with snow but with
deposits of heavy metal, is no place to hunt
for extremophiles, either, at least most of us
don’t think so. Small, frozen Mars just plain
seems like more fun than Earth-sized, hot,
over-excited Venus behind her drab veil of
lemon-tinted clouds.
Why should we be so keen on getting to
know Venus? On the face of it,
we aren’t, not any more;
after a flurry of Russian and American
probes in the 1960s, 70s and 80s – nearly
30 spacecraft altogether – we have seen
only the NASA Magellan mission, that
mapped the surface in great detail using
radar more than a decade ago, make the
short journey to Venus. Now we have Venus
Express, under construction for a November
2005 launch, and a plethora of studies for
new orbiters, floating balloons, showers of
entry probes, landers and even sample
return, one or two of which have a good
chance of following the European visitor to
the evening star in the next 10 years.
A perplexing planet A daunting list of scientific problems awaits
these missions on arrival. Why does a planet
so similar to the Earth in size and distance
from the Sun exhibit such a different surface
climate? Venus actually absorbs considerably
less energy from the Sun than our own
planet. It requires an extremely potent case
of the greenhouse effect to push the surface
temperature so high as to be a major
challenge to the modellers and theoreticians.
The sulphur-laden clouds help, but probably
have to be continually reinforced by the
emission from dozens of active volcanoes.
We don’t know why Venus should be so
much more volcanically active than the
Earth: the surface must be plastered with
lava on a huge scale. The river valleys
presumably were cut by lava, but it would
have to be remarkably runny and persistent
stuff to erode such long, deep features.
Some have gone so far as to suggest that
they were cut by water in a past era when
Venus was cooler, and had a more Earth-like
climate. It’s hard to see how Venus could
have been less volcanically active in the past.
Whether or not Venus (and Mars) once
had Earth-like, watery climates, it certainly
behoves us to understand how all four
10
Deep weather
Venus’ cloud tops look
flat and bland from the
outside, but probing with
infrared light reveals vast
weather systems deep in
the atmosphere. The
cloud patterns associated
with these have yet to be
studied in enough detail
to reveal how Venusian
meteorology relates to,
and differs from, the
Earth’s, and why
feature >> For more on the Venusian environment,
V
Venus unveiledFred Taylor
As the red planet takes the stage in planetary exploration,
the exotic evening star waits in the wings
As the red planet takes the stage in planetary exploration,
the exotic evening star waits in the wings
terrestrial planets evolved, and how they
work now. The dense Venusian atmosphere
exhibits strong zonal winds that we strain
to explain with any known theory of
atmospheric dynamics – just as radiative
energy-balance models, similar to those we
depend upon for global warming forecasts
on the Earth, are not, without more data, up
to the challenge posed by Venus. This is
something that ought seriously to concern
us – it is underlining the fact that terrestrial
climate models, on which much depends,
do not really work, except under the specific
conditions which they were ‘tuned’ to match.
Many of the problems with predicting
Earth’s climate have to do with the oceans,
and the way in which they transport massive
amounts – several terawatts – of energy
around the globe to balance the uneven
heat input from the Sun. The equivalent on
Venus is the deep, dense atmosphere below
the clouds. Free of continental barriers, this
appears at first sight to form a simple
‘Hadley cell’ – the elementary equator-to-
pole overturning that was thought in the
17th century to represent the logical
behaviour of the Earth’s atmosphere. The
American Pioneer missions to Venus in the
1970s showed that was too naive for Venus
as well as the Earth. Both atmospheres
have large components of zonal angular
momentum that destabilise the flow, forming
waves, turbulence and vast weather systems.
On Venus, these take a bizarre form. Two
huge vortices dance around the poles,
sucking cold air downwards towards the hot
surface. They are surrounded by a wide river
of even colder air that separates the polar
regime from lower latitudes, where the
dominant behaviour involves huge
convective systems that may resemble
terrestrial thunderstorms, but on a much
larger scale. There are many analogues to
our terrestrial climate system, but at the
same time they are bizarrely different, and
our understanding of our own planet is too
poor to stretch to fit even this, our nearest
neighbour and closest planetary analogue.
New missions neededNew data is the only answer. Venus Express
is coming soon and will be a brilliant start,
targeting many of the basic questions about
why the climate is so defiantly different from
Earth’s. Venus Express is bound to be
followed by larger orbiters, entry probes,
landers, surface-skimming aircraft, and
sample return. If, as some of us suspect, the
large greenhouse effect requires constant
renewal by active volcanoes to maintain the
high temperatures and pressures we see
today, these may fluctuate and eventually
decline to more Earth-like levels, making
manned bases a practical proposition. Who
knows what we will find there then?
Assuming we survive our own problems
with an unruly greenhouse first.
Professor F. W. Taylor is Halley Professor of
Physics at Oxford University.
Email: [email protected]
F
11
Snow-capped mountains
As on Earth, the mountain tops on Venus are cooler than
the plains below, leading to condensation caps on the peaks
reminiscent of those familiar on the Earth. The temperature
of condensation is much too high for water, however;
it matches that of the low-melting-point metal, tellurium.
More recent research suggests the more prosaic lead
sulphide is a more likely candidate on chemistry grounds
Venusian Rivers
Mars is not the only Earth-like planet
with ancient river valleys – this one is
in the Ishtar region on Venus. In
fact, Venus has, or had, the longest
rivers in the Solar System, hundreds
and even thousands of miles long,
and averaging over a mile wide, a
formidable piece of erosion by a
fluid of unknown
composition
go to: www-atm.physics.ox.ac.uk/user/fwt/WebPage/Venus%20Review%204.htm
The lower atmosphere andsurface of Venus are little explored Polar dipole: Pioneer Venus revealed
the strange ‘hot dipole, cold collar’
circulation in the atmosphere
over the poles of Venus