The news earlier this year from Saturn’s moon Enceladus was thrilling. The possibility of life elsewhere in the solar system suddenly seems highly plausible.
Less than two years ago, scientists of the Cassini-Huygens project announced that the Cassini probe orbiting Saturn had found evidence that Enceladus has an ice-covered ocean enveloping the little moon. It was discovered through analyzing the moon’s wobble, which was different from what it would have been if Enceladus were solid. The motion "can only be accounted for if its outer ice shell is not frozen solid to its interior, meaning a global ocean must be present," says the Sept. 15, 2015, announcement on jpl.nasa.gov.
"The finding implies the fine spray of water vapor, icy particles and simple organic molecules Cassini has observed coming from fractures near the moon's south pole is being fed by this vast liquid water reservoir."
Since then, the spacecraft has continued its exploration of Saturn and its moons. It dipped repeatedly toward Enceladus, sampling the plumes of water vapor and ice. Its Oct. 28, 2015, "deep dive" was to bring the spacecraft within 30 miles of the surface.
NASA announced earlier this spring a monumental finding gained through that dive. Cassini project scientists published a paper in the journal Science describing the probe's detection of molecular hydrogen gas "pouring into the subsurface ocean of Enceladus from hydrothermal activity on the seafloor" (see nai.nasa.gov). The spectrometer aboard determined that about 98 percent of the material in the spray is water, while molecular hydrogen makes up 1 percent "and the rest is a mixture of other molecules including carbon dioxide, methane and ammonia."
The significance of the hydrogen is that it must come from hydrothermal vents on its ocean’s floor. This shows that the vents provide enough energy to sustain life. Bacteria, the bottom of the food chain, can consume molecular hydrogen and release methane. Molecular hydrogen is like "a candy story for microbes," wrote Hunter Waite, the study's lead author.
On our planet, deep-sea hydrothermal vents were discovered in 1977. Scientists in the submersible Alvin discovered vents on the floor of the Pacific Ocean; like cracks in a volcano, the vents spew hot, "mineral-rich fluids from beneath the sea floor," according to the Woods Hole Oceanographic Institution, Woods Hole, Massachusetts. The vents are heated by magma that underlies volcanic regions. Their fluids provide a base for colonies of underwater life.
The institute adds, "As they pour out of a vent, the fluids encounter cold, oxygenated seawater, causing another, more rapid series of chemical reactions to occur. Sulfur and other materials precipitate, or come out of solution, to form metal-rich towers and deposits of minerals on the seafloor. The fluids also contain chemicals that feed microbes at the base of a unique food web that survives apart from the sun. Instead of relying of photosynthesis to convert carbon dioxide into organic carbon, the bacteria use chemicals such as hydrogen sulfide to provide the energy source that drives their metabolic processes and ultimately support (s) a wide range of other organisms such as tubeworms, shrimp, and mussels.
"Vents also support complex ecosystems of exotic organisms that have developed unique biochemical adaptations to high temperature and environmental conditions we would consider toxic."
At the bottom of the Pacific Ocean where sunlight cannot penetrate, an entire ecosystem thrives on the energy and chemicals boiling out from hot hydrocarbon vents. An educated guess is that the vents aren’t far different from those of Enceladus.
More than 886 million miles from the sun, in the terribly cold depth of space, under the ice crust of the moon Enceladus, geothermal activity keeps water from freezing. The vents enrich that ocean with energy and chemicals.
Jet Propulsion Laboratory scientists in Pasadena, California, pointed out that the life forms we know depend on liquid water, a source of energy for metabolism and certain chemical ingredients. These are mainly carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. So far, the last two chemicals haven’t been detected at Enceladus, but scientists "suspect them to be (present), since the rocky core of Enceladus is thought to be chemically similar to meteorites that contain the two elements" (see jpl.nasa.gov).
Also announced April 13 in the same news release was that last year the Hubble Space Telescope photographed a plume jetting 62 miles above the surface of Europa, a moon of Jupiter where another ocean is believed to exist under the ice. It was the second time a plume was observed there; the first, spotted near the same location in 2014, sent spray 50 miles high.
If I were running NASA, I would propose a major change in priorities. I would insist on establishing a moon base because of its obvious scientific value and its relative ease of access.
But instead of expending so much effort and money to visit Mars — where an ocean may have existed billions of years ago — I would focus on exploring Enceladus and Europa, which have oceans today. The moons aren’t as romantic as Mars, which is famous in science-fiction lore for its imaginary civilizations. They are far smaller too — the red planet is 4,220 miles across while Enceladus' diameter is only 310 and Europa's is just under 2,000 miles. Also, they are much more remote.
Humans probably will never land on those moons, given their distance and the dangerous radiation from their huge nearby planets. Exploration would be by robotic spacecraft and landers, which necessarily lack the glamour of a brief landing and flag-raising on Mars.
On the plus side, the cost of robotic explorers is minute, compared that of crewed planetary voyages, and their malfunctions don’t kill astronauts. The technology is available now to sample the moons by landers and orbiters.
One of the greatest questions that could be answered by science — maybe the greatest question — is whether life exists outside Earth. At no place in our solar system are we more likely to find an affirmative answer than at Enceladus and Europa.