QUESTION:  
The last LHST program said that the average  temperature on Mars
has dropped 20 degrees in the past ten years. What could have caused this
big drop?

ANSWER from Ed Colbert on May 16, 1996:
One way to begin thinking about this is to ask yourself: What causes 
temperature changes on the Earth?  If you watch the weatherman on TV
one evening he will probably show a large map of the cloud cover
(i.e. the atmosphere).  So, this is a clue.  Has there been any 
dramatic changes in the atmosphere of Mars over the past ten years?

Mars has a dynamic atmosphere, but it's rather thin compared with the earth.
There are many dust storms there (big ones).  Dust absorbs sunlight, but
heat (infrared light) passes through it. That's another clue.

Most questions like this don't have an absolute answer, rather they are
questions that scientists get paid (sometimes) to figure out over a long
time by making observations of Mars, theorizing on what might be happening
and seeing if it all makes sense (the scientific method).  It's fun!