Io, the innermost of Jupiter's large satellites and the most volcanically active body in the solar system, with plumes of matter rising up to 186 miles (300 km) above the surface is considered a prime candidate as a hotspot for extreme extraterrestrial life.
"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 said.