A recent Space.com article revisits the notion of terraforming the red planet. The notion of modifying our second closest planetary neighbor is certainly seductive. Atmospheric heat trap techniques offer a relatively cost effective means of accomplishing the task.
The concept raises some thorny ethical questions that must be addressed before we start thinking of Mars as a second Earth. Unlike Venus, which harbors no possibility of life, the Martian environment still holds out the possibility that native organisms continue to exist either on or beneath the surface. Should terraforming be prevented by the possibility of life? If single celled organisms are discovered, does that pose an insurmountable ethical barrier to terraforming?