杏吧原创

Going on holiday and worried about the weather? Then spare a thought for the
travellers of tomorrow, who might visit Mars rather than St Moritz, and Titan
instead of Tenerife. The Solar System offers plenty of sun, sand and perhaps the
odd sea, along with some glorious sights. But there are also nightmares in
store鈥攕torms that engulf entire planets, temperatures to make your blood
boil and acid rain that could eat you alive.

Take Mars for example. A mere 78 million kilometres from Earth, it is one of
the obvious destinations for the first interplanetary tourists. The journey time
is only six months for spacecraft like the American Mars Pathfinder, due for
launch later this year. Any tourists would need to wrap up warm鈥攖he
average surface temperature is a chilly 鈭�23 掳C, about the same as the
North American Arctic in winter. But during the long Martian summers,
temperatures near the equator can climb higher than 0 掳颁.

On a fine day, a small yellow Sun burns fiercely in a deep blue sky. Near the
horizon, a pink haze hovers, caused by fine red dust that becomes suspended in
the atmosphere during storms. Despite the cold, visitors would have to be
careful in the Sun鈥攋ust a few minutes without sunblock could result in
serious burns. 鈥淭he ultraviolet radiation is intense,鈥� says Fred Taylor, head of
atmospheric, oceanic and planetary physics at the University of Oxford. Mars
lacks the protective ozone layer that absorbs dangerous levels of radiation on
Earth.

Mars also hosts some of the biggest storms in the Solar System. These
tempests are not like those on Earth, where high winds generally affect no more
than a few hundred square kilometres and blow themselves out after a few hours
or days. Martian winds of up to 40 metres per second create ferocious dust
storms that reduce visibility to zero. The storms can engulf the entire planet
and sometimes last for months. In 1971, when the Soviet Mars 3 became the first
probe to make a soft landing, it was unlucky enough to touch down in the midst
of one these maelstroms. It fell silent after broadcasting for only 20
seconds.

The most exciting Martian weather occurs at the poles. About a third of the
planet鈥檚 atmosphere, which is over 95 per cent carbon dioxide, freezes every
winter. This extends the polar caps of frozen CO₂ more than halfway
towards the equator. 鈥淵ou鈥檇 have CO₂ snowstorms that are much more
violent than those on Earth,鈥� says Taylor. Unfortunately, much of this activity
would not be visible because the Martian poles, like those on Earth, spend the
winter months in almost total darkness.

Far better to visit the the polar caps in spring. As the planet heats up, the
huge mass of CO₂ accumulated during the winter begins to return to the
atmosphere. What makes this spectacular, says Taylor, is that carbon dioxide
sublimes rather then melts: the solid ice turns directly into a gas. Tourists
could watch huge mountains of ice vaporise as the ice continent raced away from
them at several kilometres per day. However, Taylor warns that the rapidly
changing temperatures and pressures would create winds fierce enough to put off
all but the bravest weather-watchers.

For an even more hostile climate, why not try Venus? About 50 kilometres
above the surface, the temperature and pressure may be similar to those at the
Earth鈥檚 surface, says Taylor, but lemon-yellow clouds of sulphuric acid droplets
fill the skies, sometimes breaking out into thunder and lightning. There may
even be acid rain, just as on Earth鈥攖hough the Venusian variety would be
80 per cent concentrated sulphuric acid.

Sulphurous clouds

The sulphurous clouds hang in a thick atmosphere of CO₂, which
creates a surface pressure nearly 100 times that on Earth. The temperature
reaches 450 掳C, hot enough to melt lead. CO₂ is a notorious
greenhouse gas that traps heat. Even so, says Taylor, these temperatures are
mysteriously high. 鈥淚n theory, Venus should only be a little bit warmer than
Earth,鈥� he says. 鈥淲hen you do the sums, it鈥檚 difficult to get it that hot.鈥�

Another Venusian weather curiosity is the high winds that whip around the
equator at more than 60 times the surface rotation rate. 鈥淲here has this
momentum come from?鈥� asks Taylor. The reason for the speed of the winds may lie
in the unchanging seasons on Venus. Because the planet鈥檚 axis is almost
perfectly perpendicular to the plane of its orbit, the amount of sunlight
received at each latitude stays constant through the Venusian year. 鈥淭his locks
up the weather patterns,鈥� says Taylor. The lack of change perhaps allows the
winds to build up momentum over a long period.

The Venusian winds accelerate as they move towards the north and south poles,
just as spinning ice skaters move more quickly when they draw in their arms.
This process creates giant vortices that sit permanently over each pole. Unlike
the hurricanes on Earth or anywhere else in the Solar System, the Venusian
vortices have two eyes. Researchers are mystified. 鈥淲e haven鈥檛 been able to
model hurricanes with two eyes and nobody knows how they form,鈥� says Taylor.

The planet鈥檚 atmosphere is so thick that it absorbs visible light, ensuring
that no surface features can be seen from Earth. But the NASA spacecraft
Magellan visited the planet in 1990 and spent four years mapping the whole
surface by radar. The maps show that much of the planet consists of gently
rolling featureless plains. 鈥淚t鈥檚 rather like the bottom of the ocean,鈥� says
Taylor. There are also mountains that would tower above Everest, and active
volcanoes.

One strange fact recorded by Magellan is that the Venusian mountains reflect
radar signals much more strongly than ordinary rock. Last year, Robert Brackett,
Bruce Fegley and Raymond Arvidson, researchers at Washington University in Saint
Louis, suggested that the high reflectance may be due to a thin coating of metal
over the mountain slopes.

This metallic frost could have come from volcanic eruptions. On Earth,
volcanoes release vapours containing metallic compounds of lead, copper and tin.
Brackett says the same process on Venus could coat the mountains over a period
of 10 million years or so. Who knows鈥攖he metallic frost might one day lead
to a new generation of holiday activities. If the metals are magnetic, climbers
could use magnets to prevent themselves falling. Futuristic skiers could create
frictionless runs by pumping a current through the surface and levitating on the
resulting magnetic field.

Farther out in the Solar System, the planets are very different. Jupiter,
Saturn, Uranus and Neptune are not solid lumps but giant gaseous balls
consisting mostly of hydrogen and helium. The largest of these is Jupiter, which
lies roughly 780 million kilometres from the Sun and boasts the most famous
weather pattern in the Solar System鈥攖he Great Red Spot.

The red spot is a huge oval storm more than twice the diameter of the Earth.
鈥淓verything on Jupiter is on a bigger scale than on Earth,鈥� says Vivien Moore, a
physicist specialising in the Solar System at Imperial College in London. The
storm was raging on Jupiter even before astronomers first began peering at the
planet through their telescopes around 300 years ago. Just how the storm has
become so large and why it has raged for so long is not clear.

Only the very outer layers of the Jovian atmosphere are visible from Earth,
but these are enough to demonstrate that Jupiter has the most spectacular cloud
systems in the Solar System. No one can explain their beautiful colours, or even
how they form. Cloud formation on Earth requires small particles in the
atmosphere around which water condenses. 鈥淭hese particles are spewed into the
air by things like volcanoes,鈥� says Moore. 鈥淚t鈥檚 not clear what could play a
similar role on Jupiter.鈥�

While the weather on the gas giants is completely alien, the scenery on
Titan, the largest of Saturn鈥檚 19 moons, may be a little more familiar to
Earthlings. Titan has unearthly orange skies, coloured by complex organic
molecules formed by photochemical reactions. But pictures from the Voyager 1
spacecraft in the early 1980s showed that the surface holds large reserves of
liquid methane, possibly in lakes and oceans that separate large continents.
鈥淚t鈥檚 possible that methane plays the same role on Titan as water does on
Earth,鈥� says Moore.

All will be revealed in 2004, when a small probe called Huygens will
parachute into Titan鈥檚 atmosphere as part of a joint mission by the European
Space Agency and NASA, due for launch next year. No one knows what Huygens will
find, so it has been designed for any eventuality. 鈥淚t鈥檚 possible that the probe
could touch down on land or in the middle of a sea,鈥� says John Zarnecki of the
University of Kent. Zarnecki leads a team building one of six experiments on
board the probe.

Waves on the methane seas are likely to be much bigger than on Earth, where a
wind speed of 6 metres per second creates waves of less than a metre high.
Zarnecki and his team have calculated that on Titan, where gravity is much
weaker, a similar wind would create waves up to 10 metres high. Boating holidays
may be out of the question, but Titan could be a surfer鈥檚 dream.

With a surface temperature of 鈭�180 掳C, Titan might even have
methane rain and snow. Ralph Lorenz at the University of Arizona in Tucson has
calculated that the weak gravity on Titan could mean that raindrops there might
be more than a couple of centimetres in diameter. And if there is rain, why not
rivers meandering across the continents and rainbows in the orange skies?
Hydrocarbon icebergs might be floating in the oily seas. Titan might one day
become a deep space Costa del Sol, with spacesuited holiday-makers crowded along
the shores of dirty methane seas.

Last resort

But for anyone who wants to get off the beaten interplanetary track, Pluto
might be the ideal resort. This rocky planet is on average almost 6000 million
kilometres from the Sun, keeping its average temperature down to 鈭�230
掳颁.

In its elongated 248-year orbit, Pluto sometimes comes closer to the Sun than
Neptune. In 1988, during one of these warmer periods, astronomers observed the
planet as it passed in front of a star. They were surprised to find that it has
a diffuse atmosphere containing nitrogen, carbon monoxide and methane, which
extends about 10 times farther from the surface than the Earth鈥檚 atmosphere.

But the onset of winter on Pluto could cause dramatic changes. Lorenz says
that as the planet recedes from the Sun, the atmosphere may completely freeze to
the surface. As the residual atmosphere thins, the convection processes that
carry heat from the equator to the poles would eventually fail. The dying
atmosphere would then heat much more at the equator than the poles鈥攁
process that could set up supersonic winds between them. Eventually, Pluto鈥檚
atmosphere would disappear, leaving only the icy surface of a barren
planet鈥攖he ultimate end to the ultimate holiday season.