Climate change on other planets   The Sun's rays pierce through the cloudy Venusian atmosphere and warm the planet's surface. As the heat rises from the surface it becomes trapped below the cloud layer - the greenhouse effect.     Artist inpression of Mars Express orbiter making a pass over the Martian surface at a lower point in its orbit.   This artist's concept of the Cassini-Huygens orbiter shows the Huygens probe separating to enter Titan's atmosphere. After separation, the probe drifts for about three weeks until reaching its destination, Titan. Equipped with a variety of scientific sensors, the Huygens probe will spend 2-2.5 hours descending through Titan's dense, murky atmosphere of nitrogen and carbon-based molecules, beaming its findings to the distant Cassini orbiter overhead. The probe could continue to relay information for up to 30 minutes after it lands on Titan's frigid surface, after which the orbiter passes beneath the horizon as seen from the probe. Release date: 22 March 2004