We live in a universe of water. It is everywhere on Earth, throughout the solar system, and has been detected in the far reaches of space. While there are critical concerns for fresh water availability on Earth, water in all its forms (liquid, ice, and vapor) is ubiquitous in the cosmos. And where there’s water, there can be life.
There are nine “oceans” in the solar system; however, Earth is the only planet with liquid water on its surface. The other oceans are found under thick covers of surface ice. If both liquid and frozen water are counted, Earth ranks fifth in volume of its oceans. Most of the solar system’s oceans are on the satellites (moons) of planets, although some research suggests that Jupiter and Saturn may have the largest oceans.
Our world ocean contains 321 million cubic miles of salt water. A huge volume considering that all 7 billion people on Earth could fit into just one cubic mile. Remarkably, fresh water accounts for a tiny 3.5 percent of all our planet’s water. Of that amount, only 1 percent is available to humankind. The rest is locked in landbound ice, primarily in the Antarctic and Greenland.
The largest ocean in the solar system is on Ganymede, Jupiter’s largest and most massive moon. Confirmed in 2001, its estimated water volume is eight times greater than Earth’s world ocean. Most impressive are the calculated ice thickness of 62 miles and ocean depth of 93 miles.
The most studied moon is Europa, a good exemplar of an “other ocean.” The fourth largest of Jupiter’s 79 moons, it was first described by Galileo in 1610. In the mid-1990s, Europa was one of the earliest places NASA studied using spacecraft flybys looking for water and life in our solar system. While smaller than Earth’s moon, it ranks fourth in water volume of the nine solar system oceans. Europa’s icy surface is about 10 miles thick and covers an ocean 60 miles deep. It’s estimated to have twice the water volume found on Earth.
Direct measurements of these extraterrestrial oceans are not possible without the use of landers such as Curiosity on Mars. But remote data from spacecraft flybys, land-based telescopes, and several other analytical means have helped in gathering data. Using this multipath approach, it has been possible to infer a great deal about the presence of water on the bodies studied.
By 2022, NASA plans to launch the $2 billion dedicated spacecraft Europa Clipper, which will orbit that body. The voyage will take six years. Later, it hopes to send landers that will carry ice-penetrating probes and small submersibles for exploring the ocean below the ice. In recent fiscal years, funding for the ambitious orbiter program has been slowed or not allocated, making it difficult to establish launch dates. The lander will be much more expensive, so no tentative launch dates have been set.
NASA has been a serious investor in the development and deployment of through-ice probes for the lander. An example is in the Antarctic, the one place on earth where there is thick ice cover over an isolated “ocean.” At the Russian Vostok Station in the East Antarctic, the ice is up to two miles thick. Under the ice is the 3,300-foot-deep Lake Vostok, the 19th largest lake in the world. In 2014, a prototype probe penetrated the ice in the lake and found some evidence of life. The technical and operational challenges have been formidable, such as how to drill a deep hole with a heated probe without the hole freezing behind it and immobilizing the drill. Ongoing is the development of a submersible to explore the ocean beneath the ice, and how to send its data to the surface.
More important than finding and assessing quantities of water is the search for life forms in the solar system. For life to occur, the essentials are water and organic compounds together in an environment that helps to incubate the origins of life. The NASA Astrobiology Program, begun in the 1990s, is supporting extensive work to detect signs of life. Under this program in 2018 they launched the “Oceans Across Space and Time” project to find and assess water in our solar system.
The best estimate is that the volume of liquid water in our solar system is 25 to 50 times greater than that of water on Earth. If NASA’s plans to send multiple landers to planetary moons in the next decade are successful, a new type of scientist may evolve: Planetary Oceanographer.