Conclusive evidence of life on Mars, however minute or unintelligent, will bring fulfillment of one of humanity's greatest fantasies. Both worlds coalesced from a cloud of dust and gas swirling around the sun 4.6 billion years ago to form molten cores in a solid crust. Essential for life, it once flowed freely on the surface of the now dry red planet. Some wonder if subterranean reservoirs persist. Peaks higher than any found on Earth once spewed lava on Mars, and their heat could have helped sustain life.
Some earthly bacteria thrive in hellish environments, such as hot vents on the sea floor.
While Earth is wrapped in a thick, warm cloak of nitrogen and oxygen, Mars has only a threadbare covering of carbon dioxide. But billions of years ago, it may have sported a denser atmosphere. Some suspect that Martian weather was once more clement. But a climate colder than the Antarctic is now the norm. Life may have hung on by descending below the permafrost.