|ESA Science & Technology||29-Jun-2005 14:42:02|
Similar to tourists everywhere, the Mars Express lander, Beagle 2, will let those back home know what the weather is like when it lands on Mars around Christmas 2003. The season will be late winter in Mars' northern hemisphere. Even at Beagle's landing site not far north of the equator, temperatures will seem very chilly.
"Ground temperatures will range from a relatively balmy 0oC to -15oC during the day and fall to -80oC at night," says Francois Forget, from the Laboratoire de MÃ©tÃ©orologie Dynamique (LMD), Paris, France and a Mars Express interdisciplinary scientist. "During the day, air temperature just a metre above the ground could be 15oC below the ground temperature," he adds.
Astronauts visiting Mars, however, would barely notice such a large temperature difference between head and feet because the atmosphere is so thin. Atmospheric pressure at ground level is on average 0.7% that on Earth, equivalent to a pressure found about 35 kilometres above the Earth where only record-setting helium balloons have ventured. This thin atmosphere consists almost entirely of carbon dioxide with only very small fractions of nitrogen, argon, oxygen, and a few other rare gases.
Mars' atmosphere is also highly oxidising. Something in the air turns iron in the ground rusty, giving Mars its distinctive red colour, but no one knows precisely what. Ultraviolet light, which hits the ground because Mars has no ozone layer to stop it, is almost certainly one cause, but there could be others. Whatever the cause, scientists would have to design astronauts' spacesuits with this hazard in mind.
Water vapour, so important in driving weather and climate on Earth, is present in very small amounts on Mars. Liquid water cannot exist for long on the Martian surface because of the low atmospheric pressure. Water ice, however, is present at the north pole. Hidden by a covering of carbon dioxide ice for most of the year, it is exposed only during northern summer when the carbon dioxide heats up and reenters the atmosphere. "The Martian water cycle is not at all like Earth's," says Forget. "The north polar cap in summer is the main source of water vapour on the Red Planet. Some of the water ice sublimates (turns directly into vapour) and is carried to other latitudes. In northern summer, clouds can form, especially around the top of volcanoes." Heat rising from the tropics can also lead to the formation of cloud bands in equatorial regions, in much the same way as happens on Earth. Beagle 2 may witness some of these clouds overhead. However, no cloud on Mars ever produces rain, although it is possible that frost layers could temporarily form on the planet's surface.
The model works because the forces driving Mars' climate are similar to those driving that on Earth. Energy from the Sun pours into the atmosphere, which rotates with the planet. As on Earth, one of the dominant circulation patterns is caused by hot air rising in the tropics to be replaced by cooler air moving in from mid-latitudes. On both planets, this circulation, called a Hadley cell, gives rise to trade winds.
The shape of the planet's surface also influences wind direction. On Mars, the wind tends to move up slopes during the day and down them at night. "At the Beagle 2 landing site the predominant wind should be a south-easterly trade wind. But there will be daily variations due to the slope of the surface. It will feel a bit like a sea breeze on Earth," says Forget.
One major difference between the climates on Earth and Mars comes from the lower temperatures and high concentration of carbon dioxide in the atmosphere. Every winter, a substantial proportion of this carbon dioxide condenses as ice around the poles, so much so that atmospheric pressure decreases globally by about 30%. "There are two pressure maxima and two minima every year as the poles freeze and thaw," says Forget. This movement of carbon dioxide between each polar cap creates a global circulation with no equivalent on Earth. At the edge of the carbon dioxide ice cover, temperature contrasts set up strong winds.
A warmer, denser past
Mars' atmosphere and climate, however, have not always been like this. In the recent past (a few hundred thousand to a few million years ago), Mars' axis of rotation was tilted even further towards the Sun than today, resulting in an even greater difference between northern and southern seasons. Consequently, the water cycle was a hundred times more intense and the atmosphere two to three times thicker and possibly dustier. Small gullies were formed by liquid water able to flow locally for very short durations and there may have been glaciers in the tropics.
Several thousand million years ago, the atmosphere was much denser than now and there were probably rivers and possibly an ocean on Mars. Evidence to suggest that much of this early atmosphere has been lost to space has come from measurements made by spacecraft and Earth-based telescopes. Such measurements have shown the proportion of different types of the same gas in the atmosphere. For any atmospheric gas, for example nitrogen, more of the lighter type (or isotope) will float off into space than the heavier type. The sign that this has happened is an enrichment of the heavier type in the gas left behind. Measurements of isotope ratios in gases in Mars' atmosphere have confirmed this.
Mars' atmosphere may be cold, dry, thin, and inhospitable today - but that may not always have been the case.
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