For more than two decades, scientists have wondered whether extraterrestrial life can thrive deep under the ice, which satellites in our outer solar system can boast. Space missions like Galileo on Jupiter and Cassini on Saturn came across evidence that some of the moons are hiding global oceans, fueled by the gravity of giant planets, in whose orbits they rotate. And ocean researchers, located much closer to home, discovered dynamic communities living in the dark around geological objects on the ocean floor.
Combine these two facts and easily imagine alien seas teeming with microbes. However, a new study looks deeper, into the breed itself, and suggests that these worlds may be dead inside – not only biologically, but also geologically.
“We wondered what it would be like if you were on a submarine and could fly over the ocean floor on Europe (Jupiter’s satellite),” says lead author Paul Byrne, a planetary geologist at North Carolina State University.
It was on the seabed that astrobiologists hoped to find heated, full of minerals, seawater, erupting into the ocean, like hydrothermal vents and black smoking places on Earth. In our oceans, these elements support the seething communities of microbes, which can feed on chemicals that form on the spot, where hot stones and seawater constantly mix. If such structures are found in the extraterrestrial ocean worlds, the prospect of finding life on planets remote from the Sun will become closer to reality.
“I was hoping that we would be able to characterize what a chain of volcanoes would look like, what the fault zones would look like – and suddenly they came to the conclusion: well, it looks like they will not be there,” says Byrne.
Before arriving at this conclusion, the scientists focused on the rock itself and determined what strength it would take to destroy the stone in two ways that we know on Earth: the usual faults that occur when the rock breaks and the faults that occur when the rocks are compressed, requiring more strength. The more power is required for rock destruction, the less geological activity occurs, and therefore less interactions between fresh rocks and alien waters, which in theory could support life.
Byrne and his colleagues focused on the four oceans of the world: the satellites of Jupiter Europe and Ganymede and Saturn – Enceladus and Titan. For each of these worlds, rock strength was calculated. Although there are many questions about these satellites, which we are not yet able to answer, it turns out that rock strength calculations, which are usually made on Earth for mining, are fine.
These calculations are based on the thickness of the cold, hard layer of rock that lies on top of a warmer and softer layer that cannot break. An analogy will help. “Imagine a Mars or Milky Way bar where chocolate touches caramel,” says Byrne. “Glaze can be considered as a brittle, hard layer.” The thicker it is, the harder it is to break it.
Then scientists added other quantities, such as the force of gravity of the body at a given depth, the weight of water and ice on the rocky surface of the moon. Even when they included a range of probable values with unknown input data, the final calculations for each moon fit into one range.
Byrne said that these initial results, which he presented at the conference, suggest that the breed is so strong that all these satellites do not have enough strength to regularly crush it. The point is the huge weight of water and ice lying on the rock. The breed is strong because even in the absence of strong gravity there is a lot of water on it.
Each moon, which the team studied, showed a different calculated rock strength, but the results were not particularly promising for possible aliens or geological presentations. “In Europe, it seems, it’s generally difficult to make some kind of crack or rift, and on Titan and Ganymede nothing happens at all,” says Byrne.
The strength of the Enceladus rock is not so high, because this moon is much smaller than the other three, and hence the entire water and ice above its rocky surface will be less. And the rocky core is more porous. If these pores line up, they can bring water to the depths. Perhaps Enceladus is the most promising companion of all.
In addition, on Enceladus, stone and water really interact — we saw plumes erupting into space in which Cassini found organic compounds.