QUESTION:
What contributes more to the cold temperatures on Mars -- the distance from
the Sun, or the thin atmosphere (~6 millibars vs. ~1000 millibars on Earth)?

ANSWER from Bob Haberle on August 12, 1997:
By far, it is the thin Martian atmosphere more than the planet's distance
from the sun that allows night time temperatures to dip into the minus
triple digits. Daytime ground temperatures probably reach values well above
freezing at the Sagan memorial station in spite of the fact that the planet
is about half again as far from the sun as Earth. But the thin atmosphere,
even though it is mostly CO2 a greenhouse gas, is simply not capable of
preventing the surface from cooling off at night. If Mars had a thick
atmosphere like the Earth's, the daily temperatures swings would be greatly
reduced.