The Solar System Has a Lot of Water

There have been a couple of recent discoveries of water in new places in the solar system. Last week a team using data from India’s Chandrayaan-1 probe announced the discovery of water ice on the surface of the Moon, and last month researchers using radar data from the European Space Agency’s Mars Express mission announced the discovery of liquid water under the south polar ice cap on Mars. These aren’t the first discoveries of water on other worlds – water is actually pretty common.

The solar system has a lot of water. It probably hasn’t escaped your attention that we have water here on Earth. The surface of our planet, after all, is mostly covered by deep oceans. If you looked at photos of the other planets in the solar system you’d be forgiven for thinking there wasn’t much water out there, but everything in the solar system formed from the same material, so if there’s water here it makes sense that water is out there too. (Water is made from hydrogen and oxygen, which are the most common and third most common elements in the Universe, respectively.) The evidence backs that up. We’ve found liquid water and water ice on many of the moons of the solar system, and we’ve seen signs of water on Mars before (though the recent announcement is the first direct evidence of water which is permanently liquid there).

The Pacific Ocean, photographed by the Galileo spacecraft. [Credit: NASA/JPL]

We don’t know exactly where all the water on Earth came from. Our water either formed with the planet and was here all along, or it fell to Earth later (on comets, for example). We don’t know how much water was here at the beginning and how much arrived later, but we can narrow it down by measuring the ratio of different hydrogen isotopes (regular hydrogen vs deuterium) in water on Earth and comparing it to the water found on comets. If the isotope ratios on comets were roughly the same as on Earth, then that would be evidence that most of our water came from comets. If not, it would mean that most of the water was probably here from the beginning. The water that didn’t come from comets would have come from geological sources – certain kinds of minerals (known as hydrates) contain water chemically bound inside their molecules. Heating processes, such as volcanic activity and the natural decay of radioactive substances, can release that water over time as vapour. In reality water on Earth is a mix from both sources. The question is how much water came from each one. So far, the measurements we’ve taken on comets don’t match the water on Earth very closely, but we’ve only taken a handful of measurements so this might not represent all comets very accurately.

Water is very important for the search for life. Besides being interesting for its own sake, the presence (or not) of water is interesting because finding water makes it more likely that we will find life. All life on Earth requires liquid water and on Earth we find life literally everywhere we find water. Even the most inhospitable places are at least home to bacteria, from the extreme heat of the Grand Prismatic Spring, to Lake Vostok deep under the Antarctic ice, to the highest altitude deserts. That doesn’t guarantee that there’s life in the water on other worlds (far from it), but it seems if we’re going to find life anywhere, it will be somewhere with water. Because of this, NASA’s strategy for finding life is to “follow the water”. 

The colours of Yellowstone’s Grand Prismatic Spring come from bacteria [Credit: Adam McMaster]

Liquid water might be the most interesting for finding life, but most of the water in the solar system isn’t liquid (even if you include Earth’s oceans). The majority of water in the solar system is in supercritical form in Uranus and Neptune . That means the water is at such a high temperature and pressure (in this case due to gravity) that it doesn’t exist in a distinct liquid or gas form – it’s kind of both. Two thirds of the mass of Uranus and Neptune is supercritical water mixed with small amounts of things like methane and ammonia. There’s also a lot of water ice in the solar system, much of it on comets and asteroids, as well as on various moons and dwarf planets. Comets are famous for being balls of ice and rock. When they come close to the sun they heat up and release plumes of water vapour, which is how we know the water is there. Plumes of vapour have also been observed from the dwarf planet (formerly an asteroid, and before that a planet – reclassifications happen) Ceres in 2014. That’s interesting, because where you see erupting plumes of water vapour there must be liquid water, but it’s not necessarily a permanent ocean – it could just melt and erupt occasionally before refreezing.

So where is all the liquid water? Besides Earth, liquid water is found on some of the moons around the giant planets, and now we know there is also at least a small amount on Mars. There are a few places which might have liquid water, such as Jupiter’s moons Callisto (based on magnetometer observations from the Galileo spacecraft) and Ganymede (based on clever observations by Hubble of the moon’s aurorae, which show how the water is affecting the moon’s magnetic field), and Saturn’s moon Titan (based on gravitational measurements from Cassini). There are three places where we can be more certain that there is permanent liquid water: Mars, Europa, and Enceladus. 

Possible flows of water in Newton Crater on Mars [Credit: NASA/JPL-Caltech/Univ. of Arizona]

As mentioned above, we now know (based on radar observations) that there is a small lake of liquid water under Mars’ south polar cap. The lake shows up as a bright line which is different from the surrounding rock in the radar image. The water is liquid all the time, which means it is likely to be very salty (otherwise it would freeze). There is some question about whether it could be too salty to support life, but extremophiles on Earth usually surprise us in that kind of situation so it’s not impossible that something lives there. Even before this latest discovery we had evidence that there could be liquid water on Mars at least some of the time. The Mars Reconnaissance Orbiter has observed what could be melting water flowing downhill on the surface. Just based on the valleys and channels we see on the surface, we know that Mars probably once had large amounts of liquid water on its surface in the past.

Chaos terrain on Europa [Credit: NASA/JPL]

Jupiter’s moon Europa is covered in ice, and there is good evidence that under the surface the water is liquid. You might think that it would be too cold in the outer solar system for liquid water to exist, but thanks to the moon’s proximity to Jupiter it is heated by tidal forces stretching and compressing it as it orbits. The ice on the surface repeatedly melts and freezes, cracking as it moves around. This type of surface is referred to as “chaos” terrain. There is also evidence of geysers, with water erupting from the ice and spraying into space. It could be possible to sample water from such geysers to find evidence of life, without even having to land on the surface.

Lastly, Enceladus. Like Europa, the surface is covered in ice, with features which are nicknamed “tiger stripes” near the south pole. These are ridges created by cracks in the ice. Also like Europa, Enceladus has geysers of water driven by tidal heating, with plumes sending subsurface water into space. Cassini measured this water, determining that it comes from a salty subsurface ocean.

Geysers on Enceladus [Credit: NASA/JPL-Caltech/Space Science Institute]

As we’ve seen there’s a lot of water out there, which is both not all that surprising and rather exciting. It’s not surprising because the ingredients for creating water were all over the solar system when it formed, and it’s exciting because of how important water is for life on Earth. The more places we find liquid water, the more places we might find life. We find liquid water even in places that are far from the Sun, where you might reasonably expect to find worlds that are frigid and sterile – thanks to tidal heating and cryovolcanism, liquid water can exist basically anywhere. And so could life.

Further reading 

• Extraterrestrial liquid water (Wikipedia)
• Origin of water on Earth (Wikipedia)
• The Solar System and Beyond is Awash in Water (NASA)
• Suddenly, It Seems, Water Is Everywhere in Solar System (The New York Times)