In 2005 planetary scientists worldwide were electrified when the Cassini spacecraft sent back images of its close flyby of Enceladus. In the southern polar region of the icy moon, which orbits Saturn in the second outermost ring, enormous plumes were visible, shooting hundreds of kilometers into space. The mission plan was immediately changed, and the probe was steered directly through these plumes. Among other things, the onboard sensors detected water vapor and grains of ice. How could that be?

Enceladus, with a diameter of just over 500 kilometers, the sixth largest of Saturn’s known moons, is covered by an ice crust many kilometers thick. Temperatures on the surface are more than just frigid, reaching as low as minus 240 degrees Celsius.

According to Frank Postberg, a professor of planetary sciences at the Department of Earth Sciences at Freie Universität, certain salts in the ice plumes indicated that there must be liquid saltwater below them. Shortly thereafter scientists were able to confirm the existence of hydrothermal activity on the moon, which made it clear that under the ice shell there must be a subsurface ocean with hot springs at the bottom. This was indicated by small silicate particles detected by Cassini, in addition to the ice. According to Postberg, that is only possible if hot water exists there because silicon dioxide dissolves readily in the hot water and precipitates into tiny particles upon cooling. The particles are then emitted into space by the plumes.

A moon at the edge of the solar system with liquid water! At least five other ocean moons are known in our solar system. Enceladus, however, is special because its cryovolcanoes provide a kind of access point to its water. Gases and particles from deep within are being blown directly into space through cracks in its icy shell. And what’s more, this has likely been happening continuously for thousands of years.

Hydrothermal vents, chimney-like structures through which hot, mineral-rich water emerges, also exist in the deep sea on Earth. These white smokers are considered places where life may have originated: first small organic molecules, and later the first single-celled organisms. Something similar could have happened on Enceladus. The European Space Agency (ESA) is planning to send a probe there to actively search for traces of life. It will do so in a virtually “clean” manner to preserve the pristine state of the moon and avoid introducing any microbes from Earth. The journey will take approximately ten years.

Postberg says that the Cassini mission led to a paradigm shift and explains, “Just thirty years ago, scientists considered our solar system to be completely hostile to life. Liquid water, without which we cannot imagine life, does not exist on either Mars or Venus.” Venus is far too hot, and Mars extremely dry and cold. However, 3.5 billion years ago, in the early history of Mars, there must have been large quantities of liquid water there.

The Mars rovers did not land on the bottom of a former lake, several hundred meters deep, without reason. Life could have developed there as well. The rovers are searching for fossils. Lena Noack, a professor of planetary geodynamics at Freie Universität, points out that they certainly do not expect to find skeletons of dinosaurs or fish, but rather microfossils of single-celled organisms. The conditions did not likely allow for enough time for complex life to develop, but microfossils could be found on the oceanic moons.

Three more moons – Ganymede, Callisto, and Europa – orbit Jupiter, and the ESA mission JUICE (Jupiter Icy Moon Explorer) is already en route to them. Expected arrival: six years from now. The NASA probe Europa Clipper is expected to arrive a year earlier. Europa is considered the most promising of the three ocean moons in the Jovian system in terms of habitability. “Habitable conditions” does not mean that a planet would be habitable for humans, but rather that the conditions are present for life to develop there. And that is precisely what scientists are searching for far beyond our solar system.

Noack’s research focuses on exoplanets. So far, there are few data on the atmospheres of these very distant celestial bodies. Noack explains, “We are trying to understand on which planets we can expect to have habitable conditions, but also what else besides water and carbon is necessary for life, for example, sulfur, nitrogen, and oxygen.”

The latter are a major component of Earth’s present-day atmosphere and, for Earth, a fairly clear indication of life from an anthropocentric perspective because we, and all other living beings on Earth, consist of water and carbon-based molecules. Noack points out, however, that this does not mean that the presence of these elements would be a clear indication of life on other planets.

When we think of “life,” we usually think of plants, trees, and animals. Postberg notes that such complex life has only developed since the beginning of the Cambrian Explosion 541 million years ago. As he further explains, “The first single-celled life forms emerged as early as four billion years ago. At the same time, there were also large amounts of water and moderate temperatures on Mars.”

All that came to an end 3.5 billion years ago, as geological markers prove. Presumably, that was not enough time for anything substantial to develop on Mars, but perhaps it was sufficient on planets outside our solar system? Views on this have also changed dramatically in the last thirty years, according to Noack. Previously, scientists assumed that only a few stars were orbited by a planet, but we now know that at least half of all stars have at least one planet.

Noack says, “In our galaxy alone, the Milky Way, more than 5,000 planets have already been detected. With 200 billion stars in the Milky Way, that is just the tip of the iceberg, and that is nothing compared with everything else that exists out there.”

The Department of Earth Sciences is involved in the ESA’s PLATO mission, which is headed by Heike Rauer, professor of planetary sciences at the German Aerospace Center (DLR). In 2026, the mission will send a probe equipped with twenty-six cameras deep into space with the goal of discovering thousands more planets.

The probe will specifically search for Earth-like rocky planets located in the habitable zone around other stars that might possess surface water. Water is essentially the common thread running through all of Freie Universität’s planetary science projects, whether focusing on icy moons, Mars, exoplanets, or the Earth’s moon. Liquid water is a prerequisite for the emergence and existence of life, but also a prerequisite for the survival of extraterrestrial life, for example, for people from Earth, should they someday decide to make a stopover on their way to Mars and establish a base on the moon.

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This article originally appeared in German in the Tagesspiegel newspaper supplement published by Freie Universität Berlin on October 11, 2025.