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:  f.taylor@physics.ox.ac.uk 

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