WASHINGTON, Jan 6 (Reuters) - Jupiter's moon Europa is on the short list of places in our solar system seen as promising in the search for life beyond Earth, with a large subsurface ocean thought to be hidden under an outer shell of ice. But new research is raising questions about ​whether Europa in fact has what it takes for habitability.

The study assessed the potential on Europa's ocean bottom for tectonic and volcanic activity, which on Earth facilitate ‌interactions between rock and seawater that generate essential nutrients and chemical energy for life. After modeling Europa's conditions, the researchers concluded that its rocky seafloor is likely mechanically too strong to allow such activity.

The researchers considered factors ‌including Europa's size, the makeup of its rocky core and the gravitational forces exerted by Jupiter, the solar system's largest planet. Their evaluation that there probably is little to no active faulting at Europa's seafloor suggests this moon is barren of life.

"On Earth, tectonic activity such as fracturing and faulting exposes fresh rock to the environment where chemical reactions, principally involving water, generate chemicals such as methane that microbial life can use," said planetary scientist Paul Byrne of Washington University in St. Louis, lead author of the study published on Tuesday in the journal Nature ⁠Communications.

"Without such activity, those reactions are harder to establish and sustain, ‌making Europa's seafloor a challenging environment for life," Byrne added.

Life may have emerged on Earth billions of years ago in the dynamic environment around seafloor hydrothermal vents. But Europa may lack these features.

"Based on our findings, the seafloor would probably not contain major tectonic landforms, such as ‍long ridges or deep troughs. There would likely not be any underwater volcanoes or seamounts, and we would not have any hydrothermal activity such as black smokers. Having said that, I hope I will stand corrected one day," said University of Georgia geologist and study co-author Christian Klimczak.

Europa, with a diameter of roughly 1,940 miles (3,100 km), is slightly smaller than Earth's moon. Its icy shell is believed ​to be 10-15 miles (15-25 km) thick, sitting atop an ocean perhaps 40-100 miles (60-150 km) deep.

The fourth-largest of Jupiter's 95 formally recognized moons, Europa is about a quarter Earth's diameter. ‌But its ocean of salty liquid water may contain double the water present in Earth's oceans.

Europa possesses traits suggesting potential habitability.

"There are three major factors thought to be critical for supporting life: liquid water, organic chemistry and energy," Byrne said.

"Europa's subsurface ocean satisfies the first requirement. We've identified organic chemicals on this moon's exterior icy shell, and there may very well be such chemicals inside the ocean. So that's the second requirement. And Europa's particular orbit means that Jupiter drives tidal heating within Europa - requirement three," Byrne said.

NASA in 2024 launched the robotic Europa Clipper spacecraft on a mission to examine whether Europa has conditions suitable to support life. The U.S. space agency is planning for Europa Clipper to conduct dozens of close ⁠flybys, starting in 2031.

"While geology operates similarly across the solar system, every planetary body that we have ​explored has been found to have some unique process to them. Given what we know about Europa, it ​is still the best place to look for extraterrestrial life," Klimczak said.

Jupiter's gravitational pull affects its numerous moons in different ways. Io, Jupiter's innermost large moon, is the solar system's most volcanically active body. Jupiter's gravity, coupled with gravitational forces from other moons, creates strong tidal forces on Io, ‍generating internal friction and heat. But Europa orbits ⁠much further from Jupiter than Io.

"The effect of this tidal heating drops off quickly with distance, so although there's enough tidal heating to keep Europa's ocean from freezing solid, according to our calculations at least there's not nearly enough to tectonically deform the ocean floor. So, in short, there probably isn't the kind of thing ⁠happening on Io taking place deep within Europa," Byrne said.

The study evaluated only Europa's current conditions.

"There are reasons to think that Europa might once have been much more geologically active than it is today, albeit a ‌few billion years ago. So perhaps for a time that world really was just not habitable but actually inhabited, before those conditions changed and the ‌chemical energy for life ran out," Byrne said.

(Reporting by Will Dunham, Editing by Rosalba O'Brien)