As NASA missions venture deeper into our solar system and search for new worlds, they are uncovering water in some unexpected places. While water is just one piece of the puzzle in our quest to find habitable planets and extraterrestrial life, it connects many seemingly unrelated worlds in surprising ways.
“NASA’s scientific discoveries in recent years, especially those related to water, continue to inspire us to explore our origins and the potential for life beyond Earth,” said Ellen Stofan, the agency’s chief scientist. “In our lifetime, we may finally answer the age-old question: Are we alone in the universe?”
Water’s chemical elements—hydrogen and oxygen—are among the most abundant in the cosmos. Astronomers detect signs of water in vast molecular clouds between stars, in the disks surrounding newborn stars that may form planetary systems, and in the atmospheres of exoplanets orbiting distant suns.
There are numerous celestial bodies believed to harbor liquid water beneath their surfaces, along with many more where water exists as ice or vapor. Water can be found in comets, asteroids, and dwarf planets like Ceres. The atmospheres and interiors of the gas giants—Jupiter, Saturn, Uranus, and Neptune—are thought to contain vast amounts of water, while their moons and rings are rich in water ice.
Perhaps the most astonishing water worlds are the five icy moons of Jupiter and Saturn, which show compelling evidence of subsurface oceans: Ganymede, Europa, and Callisto orbiting Jupiter, and Enceladus and Titan orbiting Saturn.
Using NASA’s Hubble Space Telescope, scientists have recently gathered compelling evidence that Ganymede harbors a saltwater subsurface ocean, likely nestled between two layers of ice.
Europa and Enceladus are believed to have liquid water oceans beneath their surfaces, in contact with mineral-rich rock. These environments may contain the three essential ingredients for life as we know it: liquid water, key chemical elements for biological processes, and energy sources that could support living organisms. NASA’s Cassini mission has revealed Enceladus as an active icy world, featuring geysers, and recent studies suggest hydrothermal activity on its ocean floor—an environment that could potentially support life.
NASA spacecraft have also detected signs of water in the permanently shadowed craters on Mercury and the Moon, which act as time capsules, preserving icy remnants of ancient impacts.
While some areas in our solar system appear to be abundant with water, others show evidence of significant water loss.
On Mars, NASA’s missions have found clear signs that the Red Planet once had water flowing on its surface for extended periods in the distant past. Curiosity, the Mars Rover, uncovered an ancient streambed that formed under conditions once conducive to life.
Using ground-based telescopes, NASA scientists have recently estimated the amount of water Mars has lost over time. Their findings suggest that Mars once had enough liquid water to fill an ocean covering nearly half of the planet’s northern hemisphere, with some regions reaching depths of over a mile (1.6 kilometers). But where did all that water go?
While some of it remains in Mars’ polar ice caps and underground, much of the planet’s early atmosphere was likely stripped away by the solar wind—a stream of charged particles from the Sun. This process caused Mars to dry up over time. NASA’s MAVEN mission is currently investigating this theory as it orbits Mars.
The loss of water on Mars is closely tied to the interaction between the planet’s atmosphere and the solar wind. Data from NASA’s solar missions, including STEREO, the Solar Dynamics Observatory, and the upcoming Solar Probe Plus, are crucial in helping scientists better understand the planet’s climatic history.
Studying the distribution of water throughout our solar system offers valuable insights into how planets, moons, comets, and other bodies formed 4.5 billion years ago from the gas and dust surrounding the early Sun. The region closer to the Sun was hotter and drier, while farther out, conditions were cold enough for water to freeze. The boundary between these two areas, known as the “frost line,” lies near Jupiter’s current orbit. Even today, this line marks the point where the ice on most comets begins to melt, triggering their brilliant displays of water vapor, ice, dust, and other chemicals—materials thought to form the building blocks of the outer solar system’s icy worlds.
Scientists believe that in the early days of our solar system, it was too hot for water to condense into liquid or ice on the inner planets. Instead, water likely had to be delivered by comets and water-rich asteroids. NASA’s Dawn mission is currently studying Ceres, the largest object in the asteroid belt between Mars and Jupiter. Researchers suspect that Ceres has a water-rich composition, similar to the bodies that may have brought water to the inner rocky planets, including Earth.
Jupiter plays a crucial role in solving the puzzle of our solar system’s formation. As the first planet to form, Jupiter contains much of the material that didn’t become part of the Sun. The leading theories about its formation depend heavily on the amount of water the planet contains. To help solve this mystery, NASA’s Juno mission, which began in mid-2016, is measuring this critical component.
Looking beyond our solar system, studying other planetary systems as they form gives us a glimpse of our own solar system’s “baby pictures,” with water playing a key role. For instance, NASA’s Spitzer Space Telescope has detected evidence of water-rich comets bombarding a young solar system, much like the early bombardment that Earth and its neighbors experienced.
As we continue studying exoplanets—planets that orbit other stars—we are getting closer to discovering other water-rich worlds. Our understanding of habitability is closely tied to water: every star has a “habitable zone,” a range of distances where conditions are just right for liquid water to exist. NASA’s Kepler mission, designed with this in mind, searches for planets in the habitable zones around a variety of stars.
Kepler’s data, which recently confirmed its thousandth exoplanet, have revealed that the most common planet sizes are those just slightly larger than Earth. Astronomers believe many of these planets could be entirely covered by deep oceans. Kepler’s successor, K2, continues to monitor starlight for signs of new worlds.
Looking ahead, NASA’s upcoming TESS mission will focus on nearby, bright stars in the solar neighborhood to search for Earth- and super-Earth-sized exoplanets. Some of the planets TESS discovers may have water, and NASA’s next flagship observatory, the James Webb Space Telescope, will analyze the atmospheres of these intriguing worlds in remarkable detail.
While it’s easy to forget, the story of Earth’s water—from gentle rains to raging rivers—is closely tied to the broader narrative of our solar system and beyond. After all, Earth’s water came from somewhere. Every planet in our solar system received its water from the same shared source. So, the next glass of water you drink might have once been part of a comet, a moon with a subsurface ocean, or even a long-lost sea on Mars. And in the night sky, countless exoplanets, formed through similar processes to our own world, may host alien seas, where waves lap against distant shores.
Preston Dyches
Jet Propulsion Laboratory, Pasadena, CA
818-354-7013
[email protected]
Felicia Chou
NASA Headquarters, Washington, D.C.
202-358-0257
[email protected]
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