What if you visited an icy moon?

Jupiter and Saturn have many moons. Some are covered in ice.

What would it be like to visit one? And what hides under the ice? Let's find out.

Far out in the solar system, where sunlight is weak, some moons are wrapped in shells of ice. NASA scientists think at least two of them, Jupiter's moon Europa and Saturn's moon Enceladus, hide liquid water oceans underneath.

So what if you could visit an icy moon? What would you walk on, what would you see in the sky, and could there be places where life could survive? This is a thought experiment: nobody has landed on one. Choose a path to explore the ice shell, the ocean below and the missions searching for answers.

Ocean worlds are bodies with liquid water beneath an icy crust. Europa and Enceladus are the best studied: NASA describes strong evidence for a salty ocean inside Europa and calls Enceladus's subsurface ocean a possible habitat, sampled directly by Cassini.

This thought experiment imagines a visit. The evidence comes from Voyager, Galileo and Cassini, telescopes on Earth, and the design of NASA's Europa Clipper. What a person standing on the ice would see or feel is an idea built on that evidence; no spacecraft has landed on either moon, and no life has been found on them.

Europa is a moon of Jupiter. It is almost as big as our Moon.

Enceladus is a small moon of Saturn. It sprays water into space!

Europa is one of Jupiter's four largest moons, about 3,100 kilometers across, or about 90 percent the size of our Moon. Its surface is water ice crossed by reddish-brown cracks, and it reflects 5.5 times as much sunlight as our Moon does.

Enceladus, a moon of Saturn, is much smaller, only about 500 kilometers across. NASA says it is the most reflective object in the solar system, bright white all over. In 2005, NASA's Cassini spacecraft discovered jets spraying water vapor and ice out of its south pole.

Europa, about 3,100 km across, orbits Jupiter every 3.5 days and keeps one hemisphere facing the planet. Its icy surface, with few craters, appears to be no more than 40 to 90 million years old, young in geologic terms; reddish-brown material along its fractures likely includes salts and sulfur compounds altered by radiation.

Enceladus, about 500 km across, orbits Saturn every 32.9 hours inside the E ring. Cassini found in 2005 that icy particles and gas erupt from warm fractures nicknamed tiger stripes at about 400 m/s; most falls back as snow, which helps make Enceladus the most reflective body in the solar system.

Imagine standing on Europa. The ground is smooth, cold ice with long red cracks.

Giant Jupiter might hang in the sky above you!

Imagine landing on Europa. Underfoot, you might find smooth ice with long cracks and ridges, some hundreds of meters tall. Because Europa keeps the same side facing Jupiter, from that side the giant planet might hang in the same part of the sky all the time.

You would need a lot of protection. NASA says Europa's surface is blasted by radiation from Jupiter, too strong for life on the surface. Scientists think the ocean is far below, under 15 to 25 kilometers of ice. These are ideas about a visit, built on what spacecraft have seen.

A visitor to Europa might stand on a surface of linear fractures 1 to 2 km wide that run for thousands of kilometers, ridges hundreds of meters tall, and chaos terrain, all seen by Galileo. Because Europa is tidally locked, Jupiter might appear fixed in the sky of the Jupiter-facing hemisphere; that follows from geometry, not from an observation.

The environment would be hostile: NASA notes intense radiation from Jupiter that surface life could not survive, though it may create fuel for life in the ocean below. Reaching that ocean would mean getting through an estimated 15 to 25 km of ice. None of this is modeled; the visit is a thought experiment.

Big planets pull on their moons. The pull stretches and squeezes the moon, again and again.

All that squeezing warms the moon inside, like when you squeeze a stress ball.

Europa's path around Jupiter is slightly stretched, not a perfect circle. So its distance from Jupiter keeps changing, and Jupiter's gravity tugs harder on the near side than the far side. NASA explains that this makes tides that stretch and relax the whole moon.

That flexing makes heat, called tidal heating, and likely cracks the surface. Enceladus gets the same treatment: another moon, Dione, keeps pulling Enceladus's orbit into a stretched shape, so Saturn's pull flexes it too. NASA says the heat from this flexing is likely enough to keep Europa's ocean liquid.

Europa's slightly elliptical orbit varies its distance from Jupiter, and the near side feels a stronger pull than the far side; the changing difference raises tides that flex the moon, likely creating the surface fractures and supplying tidal heat. If the ocean exists, NASA notes, that heat could drive volcanic or hydrothermal activity on the seafloor.

Enceladus is in a 2:1 orbital resonance with Dione, whose gravity keeps its orbit elliptical, so it too is tidally heated. The same process, at greater strength, makes Io, closest to Jupiter, the most volcanically active body in the solar system; NASA notes that distance from Jupiter determines how much tidal heating each of its large moons receives, and that the flexing likely makes enough heat to keep Europa's ocean liquid.

Spacecraft flew past these moons and took careful measurements.

Now a new spacecraft called Europa Clipper is on its way to Europa!

Scientists can't see under the ice, so they use clues. NASA's Galileo spacecraft found that Jupiter's magnetism behaved strangely around Europa, as if something salty and liquid were inside. That is still the best evidence for Europa's ocean.

At Enceladus, Cassini flew right through the jets and tasted them. Scientists found most of the chemical ingredients needed for life, and tiny grains that hint at hot water vents on the ocean floor. NASA's Europa Clipper launched in October 2024 and will reach Jupiter in April 2030 to study whether Europa could support life.

Galileo, orbiting Jupiter from 1995 to 2003, measured a disturbance in Jupiter's magnetic field near Europa implying an induced field from a conductive layer; NASA calls this the best evidence for a global salty ocean, perhaps 60 to 150 km deep and holding twice the water of Earth's ocean.

Cassini's gravity and wobble measurements showed Enceladus's jets draw on a global ocean. Silica nanograins in the E ring require water and rock reacting above about 90 °C, pointing to hydrothermal vents. Europa Clipper, launched October 14, 2024, will make 49 close flybys with nine instruments to determine whether Europa has places below its surface that could support life.