In the image above Two sulfurous eruptions are visible on Io from the robotic Galileo spacecraftthat orbited Jupiter from 1995 to 2003. At the image top, over Io's limb, a bluish plume rises about 140 kilometers above the surface of a volcanic caldera known as Pillan Patera.
In the image middle, near the night/day shadow line, the ring shaped Prometheus plume is seen rising about 75 kilometers, or about 46 miles, above Io while casting a shadow below the volcanic vent. The plume is visible in every image ever made of the region dating back to the Voyager flybys of 1979, presenting the possibility that this plume has been continuously active for at least 18 years.
"Everyone right away tends to categorically exclude the possibility of life on Io," said astrobiologistDirk Schulze-Makuch at Washington State University. Conditions on Io might have made it a friendlier habitat in the distant past. If life did ever develop on Io, there is a chance it might have survived to the present day, Schulze-Makuch suggested.
"Life on the surface is all but impossible, but if you go down further into the rocks, it could be intriguing," he said. "We shouldn't categorize it as dead right away just because it's so extreme."
Computer models suggest Io formed in a region around Jupiter where water ice was plentiful. Io's heat, combined with the resulting possibility of liquid water, could have made life plausible.
“There must have been quite a lot of water on Io shortly after formation, judging from the amount of water ice on Europa and Ganymede,” said Schulze-Makuch.
Jupiter's radiation would have stripped this water from Io's surface, perhaps within 10 million years. At this point life could have retreated underground, where water might still be abundant, and geothermal activity and sulfur compounds could provide microbes with sufficient energy to survive.
Although no organic molecules have been detected on the moon’s surface, that does not mean they do not exist underground, Schulze-Makuch said. Any organic compounds that once existed on the surface or that may today still emanate from the subsurface -- which probably were naturally present in this region of space during Io's formation -- would get quickly destroyed by Jupiter's radiation.
The many lava tubes thought to exist on Io could serve as an especially favorable environment for life, Schulze-Makuch suggested, by protecting organisms from radiation. The lava tubes also could provide thermal insulation, trapping moisture and providing nutrients such as sulfurous compounds. Microbes are common in lava tubes on Earth, from ice and volcano zones in Iceland to hot sand-floored tubes in Saudi Arabia, and lava tubes are the most plausible cave environment for life on Mars, he added.
The primordial soup that any life on Io might have originated from was likely based on water, but the solvent of choice for organisms there might have drastically changed later on as the moon transformed. Hydrogen sulfide is one choice, as it is reasonably abundant in Io's shallow subsurface and remains liquid from negative 123 to negative 76 degrees F (-86 to -60 degrees C), falling within the environmental conditions that would prevail there. While it is not especially efficient as a solvent for ions, it does dissolve many substances, including many organic compounds. Other possibilities include sulfur dioxide and sulfuric acid.
"I'm exploring with colleagues whether sulfur compounds could work as solvents of life," Schulze-Makuch noted. Given the wild extremes Io can swing through as it orbits Jupiter, one possible survival strategy for life in this challenging environment would be to remain dormant most of the time, only reverting back when nutrients were rich. "It'd be much easier for life to take a beating if it goes dormant regularly," Schulze-Makuch
Source: The Daily Galaxy via Washington State University