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.
Proponents cite four clues as part of the evidence the Martian meteorite once hosted life.
They are similar in size and texture to carbonates formed with the help of bacteria on Earth. Their chemical makeup is hard to explain unless living organisms are involved somehow. A recent paper suggested the globules arose as very hot fluids interacted with volcanic rock. The globules could have formed through inorganic processes, as could the minerals in them. The crystals have the same shape and composition as magnetofossils made by Earth-dwelling bacteria, which may have used magnetite to orient themselves with the Earth's magnetic field.