Io, Europa, Ganymede & Callisto (i.e: the four moons of jupiter in photo order)Surface features, such as furrows, grooves and impact craters are illustrated on this 360 degree view of the moon. The best imagery from NASA's Voyager 1 and 2 and Galileo spacecrafts were used to create it. Galileo Galilei discovered it in 1610.
Could Ganymede, Europa and/or Callisto support life?
The Jupiter Icy moons Explorer – JUICE is the first Large-class mission chosen as part of ESA’s Cosmic Vision 2015-2025 program. It will be launched in 2022 from Europe’s spaceport in Kourou, French Guiana, on an Ariane 5, arriving at Jupiter in 2030 to spend at least three years making detailed observations. Jupiter’s diverse Galilean moons – volcanic Io, icy Europa and rock-ice Ganymede and Callisto – make the Jovian system a miniature Solar System in its own right.
With Europa, Ganymede and Callisto all thought to host internal oceans, the mission will study the moons as potential habitats for life, addressing two key themes of Cosmic Vision: what are the conditions for planet formation and the emergence of life, and how does the Solar System work?
"Where there is an ocean, life could arise. In this respect, after Mars, the Europa satellite is probably the most intriguing place in the solar system," says Lev Zelyony, Head of the Russian Space Research Institute.
Fascination with Europa began centuries ago in 1610 when Galileo discovered four Jovian satellites: Io, Callisto, Ganymede, and Europa. About forty years ago, modern astronomer Gerard Kuiper and others showed that Europa's crust was composed of water and ice. In the 1970s, space exploration of Jupiter's satellite system began with the Pioneer and Voyager fly-by missions which verified Kuiper's vision.
Europa is about the same size as our Moon. At first glance, it looks like a dead, frozen wilderness. The entire surface is cloaked in a layer of ice. Yet, photographs of the moon taken by the Galileo probe revealed some exciting features in the ice. A complex web of lines criss-cross the globe. Only one other place in the solar system looks like that - the frozen Arctic seas on Earth. It is believed that the icy surface of Europa could be floating on a vast liquid ocean, just like the Arctic icebergs. All the ingredients for life are likely present.
scale: Ganymede next to Earth
If a liquid ocean does exists, there must be a heat source stopping it from freezing over. However, the ice that covers the surface is probably too thick to allow sunlight through. So what is keeping Europa warm?
Europa is just one of many moons that revolve around Jupiter. Jupiter's huge gravity holds it in orbit. But Europa is also pulled at from the other moons. Different forces, all pulling in different directions, yank Europa's surface out of shape as it travels through space, known as 'tidal forces'. These tidal forces heat up the moon's core. If the forces are strong enough, the core could be hot and melt the frozen water surrounding it.
It's even possible that the ocean floor may contain hotspots called 'hydrothermal vents', or cracks in the ground that spout warm, nutrient-rich water. Hydrothermal vents on Earth are rich in life. Astrobiologists are hoping that Europa could be similarly fertile.
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.
"Everyone right away tends to categorically exclude the possibility of life on Io," said astrobiologist Dirk 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 said.
JUICE will continuously observe Jupiter’s atmosphere and magnetosphere, and the interaction of the Galilean moons with the gas giant planet.It will visit Callisto, the most heavily cratered object in the Solar System, and will twice fly by Europa. JUICE will make the first measurements of the thickness of Europa’s icy crust and will identify candidate sites for future in situ exploration.
The spacecraft will finally enter orbit around Ganymede in 2032, where it will study the icy surface and internal structure of the moon, including its subsurface ocean.Ganymede is the only moon in the Solar System known to generate its own magnetic field, and JUICE will observe the unique magnetic and plasma interactions with Jupiter’s magnetosphere in detail.
“Jupiter is the archetype for the giant planets of the Solar System and for many giant planets being found around other stars,” says Prof. Alvaro Giménez Cañete, ESA’s Director of Science and Robotic Exploration.“JUICE will give us better insight into how gas giants and their orbiting worlds form, and their potential for hosting life.”
This announcement is the culmination of a process started in 2004 when ESA consulted the wider scientific community to set Europe’s goals for space exploration in the coming decade.The resulting Cosmic Vision 2015-2025 programme identified four scientific aims. What are the conditions for life and planetary formation? How does the Solar System work? What are the fundamental laws of the Universe? How did the Universe begin and what is it made of?