Overall, Mercury is a very hot planet; it gets ten times as much solar
radiation as the Earth. At midday, the temperature can reach 430OC, hot
enough to melt lead and zinc. The atmosphere is so thin that very
little heat is captured, and the temperature can plummet to -170OC at
Some scientists think that Mercury has ice at its north pole. Not
much sunlight reaches the north pole, and the radar photographs from
1975 show something that looks like ice covering an area of about 400
square kilometres. Again, the atmosphere is so thin that the little
heat can be trapped at the north pole to melt this ice.
The seasons in Mars's southern hemisphere include short, very hot
summers, and longer, cold winters. The Martian orbit is less circular
and more elliptical than Earth's, which means for part of the year the
planet is a lot closer to the sun. The southern hemisphere, tilted
towards the sun when Mars is closest, has a hotter summer than the
other hemisphere. The northern hemisphere is tilted towards the sun
when Mars is farther away, and so its summers are not as hot.
Planets travel fastest when closest to the sun, so the hemisphere with the hotter summer will also have a shorter summer.
(republished on 07/19/06)