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Pink planet: the martian sky is coloured pink by ice particles formed on dust in early morning. Credit: NASA

After spending more than a month exploring the surface of Mars, the lander Pathfinder is being allowed by the US National Aeronautics and Space Administration to rest at night, in order to cut down on the charging and discharging that depletes its batteries.

The hope is that the lander can keep working for several more months, using solar power. Sojourner is still taking chemical measurements of rocks and soil patches. But over the past couple of weeks, more attention has been focused on the lander's measurements of martian weather. Weather conditions have been found to be steady, with large regular daily swings in pressure and temperature.

But two meteorological oddities have been observed. One is rapid fluctuations in both temperature and pressure. Ground temperatures can change by 20°C within a few minutes, and pressures can also alter over the same timescale. These fluctuations are probably very local, and may be caused by ‘dust devils'.

The second oddity is a striking temperature drop within a few feet of the surface. Unlike the Viking landers, Pathfinder can measure temperature and wind speed as a function of height, using thermocouples and wind socks at different heights on its mast. It has found that the temperature drop from ground level to five feet above the surface can be more than 40°C.

Such information is vital in characterizing the ‘boundary-layer’ interactions between the martian atmosphere and surface, including factors such as turbulence and rate of heat flow from the surface. Little information has previously been available about such issues, and the findings have significant implications for understanding the behaviour of the atmosphere as a whole, as well as for the way in which wind is involved in erosion and deposition of material on the surface.

Understanding how the winds change, and how they are affected by local terrain and by tides, could be of more than just academic interest. “These local winds will be important if we ever go ballooning on Mars in order to cover substantial distances,” says Brian Toon of the University of Colorado.

The dust carried by the winds into the atmosphere also plays an important part in controlling the weather by absorbing heat from the Sun. Two weeks ago, the flight team released pictures of sunrises and sunsets, showing the atmospheric dust.

Some of these pictures also show clouds of water vapour in the early morning — confirmation of the idea that pre-dawn condensation into clouds helps to move water around the planet. In photographs taken in the early morning, the sky is coloured pink by ice that has condensed on dust particles some 16 km above the surface (see above).

The main reason for the stability in the weather conditions experienced by Pathfinder so far is that it has landed during a season of low pressure, when there is minimal atmospheric activity. But the martian autumn will bring dust storms, promising to make the weather more interesting to observers on Earth.