Hello, nwr. Many years ago I used to be a graduate student in the planetary sciences. Hopefully my information isn't too far out of date.
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We often hear that Venus is hot due to a runaway greenhouse effect, and that Mars is cold because of the lack of greenhouse gasses. No doubt I am quite naive about this, and ignorant of much of atmospheric science, but it has always seemed to me that Venus is hot because it is closer to the sun, and Mars is cold because it is farther from the sun.
Venus has the hottest surface temperature of all the planets, despite the fact that Mercury is closest to the sun.
Venus is indeed hot because it is closer to the sun than the earth, yet the surface temperature of Venus is far, far hotter than it should be given its distance.
If there were enough CO2 in the atmosphere of Mars (I've seen that it needs enough CO2 to create an atmospheric pressure of 100 times the earth's atmospheric pressure), Mars would be a warm, livable planet, although with an atmosphere that would be too crushing and toxic for most earth life.
It is known that (from well-established models on stellar evolution) that the sun was much, much cooler in the past, yet there has always been liquid water here on the earth's surface. This is because there has always been enough CO2 in the atmosphere to keep the surface temperature in the liquid water range (with the possible exception of the so-called "snow ball earth" that existed shortly before the Cambrian). In fact, I seem to recall doing the calculation that currently at the distance the earth is from the sun, the earth temperature should be below freezing -- the minute amounts of CO2 in the atmosphere is what keeps us toasty.
It is known that in about 4 billion years the sun will enter its red giant phase, expand, and vaporize the inner planets including the earth. However, the earth will already be dead by then: the sun is gradually heating up; as it heats up, the amount of CO2 in the atmosphere will decrease (by a theorized geochemical negative feedback cycle); but in a little over a billion years there will be no more CO2 to be removed. Then temperatures can only increase, initiating the run-away greenhouse effect we see on Venus.
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