Mars is by far the most clement of neighboring worlds: much colder than the Earth, but at its warmest reaching temperatures a human could withstand (that is, if the atmosphere weren't intolerably thin). Aside from some of the distant moons of Jupiter and Saturn (Europa and Titan, most notably), which present us with new destinations in the search for life, Mars has always been the natural place to look. It's rocky, and not insufferably hot like Mercury or Venus. And between the water ice caps and ancient river valleys, astronomers have long thought that Mars may have been much warmer in the past... a place that could have been a perfect hatchery for life.
But was it really warm? As Physorg explains, we still don't know. There are a few obstacles that stand in the way of a warm-Mars hypothesis. For one thing, Mars is much farther away from the Sun than Earth (about 50% farther away). We also know that the Sun has grown much brighter over time, which means Mars would have received much less heat in the distant past. And since it has only about 11% the mass of Earth, the greenhouse effect that keeps Earth and Venus warm doesn't work nearly as well for Mars.
But not to fear, SETI enthusiasts: as we pointed out a few days ago, the search for life on Mars continues, regardless of what we might discover about Mars' ancient climate.
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