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AS THE Hubble Space Telescope orbits the Earth, it is giving planetary scientists new views of Mars and Venus. The latest Hubble observations reveal that Mars is becoming drier and cloudier, and that the amount of sulphuric acid raining down on the surface of Venus has decreased tenfold since the late 1970s.

Although Hubble gets most headlines for its studies of distant galaxies, the orbiting telescope can also be used to study our planetary neighbours. When scientists were limited to ground-based telescopes they could only study Mars and Venus in detail when the planets’ orbits brought them close to the Earth. But thanks to the sharp-eyed Hubble telescope, they can now observe the planets throughout most of their orbits.

Planetary scientists already knew that the Martian climate is very variable: radio observations have shown that Mars’ atmosphere is on average 20 °C cooler than in 1976, when the planet was visited by NASA’s two Viking landers. Hubble’s wide field/planetary camera has now shown that this temperature shift has caused the formation of large quantities of high-altitude cirrus clouds. This makes sense, says Steven Lee of the University of Colorado, as these clouds are composed of ice crystals.

The formation of icy clouds should make the Martian atmosphere much drier, as it will remove water vapour. Another of the Hubble’s instruments has confirmed this picture – its faint object spectrograph has shown that there has been a threefold increase since 1992 in the quantity of ozone in the Martian atmosphere. This is consistent with a drying planet, says Lee, because water vapour helps to destroy ozone.

Planetary scientists believe that these changes are linked to a lull in the dust storms that ravage Mars. The storms suspend large quantities of fine dust in the planet’s upper atmosphere, and these particles absorb radiation from the Sun, warming the atmosphere. During the current lull, says Lee, much of the dust will have fallen out of the atmosphere, allowing it to cool.

The planet Venus was studied by another group of scientists, led by Larry Esposito of the University of Colorado, who used a Hubble instrument called the Goddard high-resolution spectrograph. This observes in the ultraviolet part of the spectrum, which can reveal the amount of sulphuric acid in a planet’s atmosphere. The latest observations, made in January, show that the Venusian atmosphere contains just a tenth of the amount of sulphuric acid it did in 1978, when the planet was orbited by the Pioneer Venus spacecraft.

Esposito and his colleagues believe that shortly before the pioneer Venus mission Venus was rocked by a giant volcanic eruption which spewed massive amounts of sulphuric acid into the atmosphere. Since then, the researchers suggest, this sulphuric acid will have gradually rained onto the Venusian surface.

The volcano may be a mountain called Maat Mons. NASA’s Magellan spacecraft, which last year completed a radar survey of the Venusian surface, found highly reflective rocks near Maat Mons which could be recent lava flows.