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Top Comments: The Search for Life on Jupiter's Moons [1]

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Date: 2023-05-14

Photographic compilation of Jupiter and its four largest moons: from the upper left, Callisto, Ganymede, Europa and Io.

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In the next two years, two planetary probes will be launched to explore three of Jupiter’s four Galilean moons: Europa, Ganymede, and Callisto. While all three of these moons have very cold, icy surfaces, heating from the moons’ interiors have melted the ice below the surface, and the resulting subsurface oceans may contain all the factors necessary for life to arise and possibly thrive.

On April 13, 2023, the European Space Agency launched a rocket carrying a spacecraft destined for Jupiter. The Jupiter Icy Moons Explorer – or JUICE – will spend at least three years on Jupiter’s moons after it arrives in 2031. In October 2024, NASA is also planning to launch a robotic spacecraft named Europa Clipper to the Jovian moons, highlighting an increased interest in these distant, but fascinating, places in the solar system.

[The fourth Galilean moon, Io, is the odd moon out: it has little or no water on its surface. Because Io is the closest to the giant planet, Jupiter’s tidal forces heat up the planet to quite high temperatures and induce volcanism. Hardly a day passes without some kind of eruption on Io. This is not the sort of environment where we would expect to find life.]

It is now certain that the three Galilean moons with icy surfaces have oceans beneath their surface. Jupiter’s strong magnetic field induces a smaller magnetic field in each of these moons; the reason is that ionic compounds are dissolved in these oceans, and these moons moving through Jupiter’s magnetic field induces the charged particles to move, which in turn generates a magnetic field. Liquid water and the presence of certain ions are necessary for life as we know it to exist, so for those looking for extraterrestrial life, these are initial signs that life may be lurking below one or more of these icy surfaces.

However, life has requirements beyond water and certain ions. We know that life on Earth also requires the presence of nucleotides (from which DNA and RNA are made) and amino acids (from which proteins are made). Further, there must be an energy source. Unfortunately, detecting the presence of nucleotides and amino acids in the subsurface oceans is beyond the capabilities of current technology, but it is possible to detect the presence of an energy source. Now, much of life on Earth is powered directly by the Sun through photosynthesis in plants, but this option is out of the question on the moons of Jupiter, because little to no sunlight penetrates their icy surfaces. However, even with life on Earth, we know that the Sun is not the only possible power source. The oldest life on Earth is powered by very hot deep-sea thermal vents. If Jupiter’s moons are sufficiently geologically active, there may be similar sorts of heating occurring at the bottom of their oceans as well. The probes being launched will have sensors capable of determining whether these moons are geologically active.

Scientists will also observe small variations in the moons’ gravitational pulls by tracking subtle movements in both spacecrafts’ orbits, which could help determine if Europa’s seafloor has volcanoes that provide the needed energy and chemistry for the ocean to support life.

These probes will not start taking measurements for at least 8 years (2031), so we’ll have to wait a while, but it will be exciting to see what they will observe.

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