New observations of these active ocean worlds in our solar system have been captured by two NASA missions and were presented in two separate studies in an announcement at NASA HQ in Washington today.
Using a mass spectrometer, the Cassini spacecraft detected an abundance of hydrogen molecules in water plumes rising from the "tiger stripe" fractures in Enceladus' icy surface. Saturn's sixth-largest moon is an ice-encased world with an ocean beneath. The researchers believe that the hydrogen originated from a hydrothermal reaction between the moon's ocean and its rocky core. If that is the case, the crucial chemical methane could be forming in the ocean as well.
"Now, Enceladus is high on the list in the solar system for showing habitable conditions," said Hunter Waite, leader of the Cassini Ion and Neutral Mass Spectrometer team at the Southwest Research Institute in San Antonio and lead author of the Enceladus study.
"The presence of hydrogen established another reference point saying there is hydrothermal activity inside this body, and that's interesting because we know in our own oceans, those are very important places that are teeming with life, and they are probably one of the earliest places where life happened on Earth."
Additionally, the Hubble Space Telescope showed a water plume erupting on the warmest part of the surface of Europa, one of Jupiter's moons with an icy crust over a salty liquid water ocean containing twice as much water as Earth's seas. This is the second time a plume has been observed in this exact spot, which has researchers excited that it could prove to be a feature on the surface.
"This is significant, because the rest of the planet isn't easy to predict or understand, and it's happening for the second time in the warmest spot," said Britney Schmidt, second author on the Europa study.
Why is this exciting?
"This is the closest we've come, so far, to identifying a place with some of the ingredients needed for a habitable environment," said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate. "These results demonstrate the interconnected nature of NASA's science missions that are getting us closer to answering whether we are indeed alone or not."
The necessary ingredients for life as we know it include liquid water, energy sources and chemicals such as carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus.
But we've also learned that life finds a way in the harshest of Earth's environments, like vents in the deepest parts of the ocean floor. There, microbes don't receive energy from sunlight, but use methanogenesis, a process that reduces carbon dioxide with hydrogen, to form methane.
Europa and Enceladus are showcasing some of these key ingredients for life in their oceans, which is why researchers believe they are the best chance for finding life beyond Earth in our own solar system.
Previous results from the Cassini mission's flybys of Enceladus already had researchers intrigued. First, they could see plume material linked to interior water. They determined that the moon had a global ocean, and then a cosmic dust analyzer revealed silicon dioxide grains, indicating warm hydrothermal activity.
"This (molecular hydrogen) is just like the icing on the cake," Waite said. "Now, you see the chemical energy source that microbes could use. The only thing we haven't seen is phosphorus and sulfur, and that's probably because they were in small enough quantities that we didn't see them. We have to go back and look and search for signs of life as well."
Earth is considered an ocean world because those bodies of water cover the majority of the planet's surface.
Other ocean worlds in our solar system, besides Europa and Enceladus, potentially include Jupiter's moons Ganymede and Callisto; Saturn's moons Mimas and Titan; Neptune's moon Triton; and the dwarf planet Pluto.
It is believed that Venus and Mars were once ocean worlds, but the greenhouse gas effect and a vulnerable atmosphere, respectively, caused those planets to lose their oceans.
Although the Cassini mission, which began in 2004, comes to an end this year, Waite is eager for NASA to return to Enceladus and search for life, because he believes it is the best candidate for habitability.
Researchers want to confirm a very solid case for habitability by finding sulfur and phosphorus on Enceladus, as well as narrowing down the pH (potential of hydrogen) and reinforcing previous measurements. The second step would be looking for signs of life by flying a spectrometer through the plume, searching for rations of amino and fatty acids, certain isotopic ratios indicative of life and other relationships in molecules that indicate energy for microbial life, Waite said.
But first, we're going to investigate Europa's habitability.
NASA plans to further explore ocean worlds in our solar system, including through the recently named Europa Clipper mission, the first to explore an alien ocean. Waite believes that Europa is currently at a disadvantage because a mass spectrometer hasn't flown through its plume to collect data, and it's Europa's turn to have that experience.
"If there are plumes on Europa, as we now strongly suspect, with the Europa Clipper, we will be ready for them," said Jim Green, NASA's director of planetary science.
Although Waite favors Enceladus of the two ocean worlds, Schmidt believes that Europa could be the best case for life in our solar system beyond Earth.
Schmidt, an assistant professor at the Georgia Institute of Technology's School of Earth and Atmospheric Sciences, is also one of the architects of the project that became the Europa Clipper mission. She and two other researchers came up with the name while sitting in a hotel room during a conference.
The Europa Clipper, named for the innovative, streamlined ships of the 1800s, will launch in the 2020s and arrive at Europa after a few years.
"The reason we chose it is because the clipper ships were fast, American boats at the time that they were first used, when most shipping was achieved with large, slow vessels," Schmidt said. "We liked clipper for that reason, an ingenious way to solve the Europa mission problem: How do you get a long-lived mission at Europa with global coverage but not be in the radiation environment?"
Because that region of the solar system traps atomic particles from the sun, the radiation of the area around Jupiter is dangerous to spacecraft.
Schmidt will be an investigator for the ice-penetrating radar instrument that will be housed on the Europa Clipper. It will act like an X-ray, peering through the unknown thickness of Europa's icy crust the same way scientists use earthquakes to assess the interior of the Earth.
Other instruments will produce high-resolution images of the surface and measure the moon's magnetic field, temperature, atmospheric particles and even the depth and salinity of the ocean.
Waite is using his knowledge from the Cassini mission to work on an improved mass spectrometer for the Europa Clipper mission.
"Cassini and Enceladus really allowed us to see the kind of things we could do with mass spectrometers and, more importantly, with material that's coming up straight out of the ocean," Waite said. "It's a way of viewing the ocean without drilling into it. We didn't necessarily have to land; we could sit there and and sample to study quite a bit about these ocean worlds just from flying through the material that comes out of the interior. That's what the plumes are about on Europa as well. It's that connection to the interior ocean."
Europa or Enceladus?
Research suggesting the possibility of an ocean on Europa was published as early as 1977, after the Voyager mission saw long lines and dark spots, as opposed to a cratered surface similar to other moons. Then the Galileo mission reached Europa in 1996 and revealed for the first time that there was an ocean on another planet.
But because neighboring Mars has fueled imaginations and the possibility of exploring another planet for years, the general public hasn't been as captivated by Jupiter's moon. There are arguments that life could exist on Mars, but it was most likely habitable in the past, when it supported bodies of water and had a more hospitable climate and an atmosphere.
"The question is, do you want to study something that might have been habitable at one time, or do you want to study something that could be habitable right now?" Schmidt asked. "Europa has been pretty much Europa for 4.5 billion years, as long as the Earth has. So as far as what could have started and evolved there, that's a compelling question.
"If you think about early Earth and early Europa, they were probably very similar, at least at the ocean interface. They are almost the same place at that point in time. That's why I get excited about Europa. It could have been a place for life over the history of the solar system."
Regardless of which of the two ocean worlds is the better candidate for hosting life, both researchers believe that exploring ocean worlds is one of the best things we can do.
"Understanding the diversity of our solar system is pretty important and the possibility of applying what we know here to exoplanets, it just opens up the range of possibilities for life beyond our previous expectations," Waite said.
"These ocean worlds all over the outer solar system that are a little bit alien," Schmidt said, "they're the most compelling things that we have in the solar system."
NASA Missions Provide New Insights into 'Ocean Worlds' in Our Solar System
Two veteran NASA missions are providing new details about icy, ocean-bearing moons of Jupiter and Saturn, further heightening the scientific interest of these and other "ocean worlds" in our solar system and beyond. The findings are presented in papers published Thursday by researchers with NASA’s Cassini mission to Saturn and Hubble Space Telescope.
In the papers, Cassini scientists announce that a form of chemical energy that life can feed on appears to exist on Saturn's moon Enceladus, and Hubble researchers report additional evidence of plumes erupting from Jupiter's moon Europa.
“This is the closest we've come, so far, to identifying a place with some of the ingredients needed for a habitable environment,” said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate at Headquarters in Washington. ”These results demonstrate the interconnected nature of NASA's science missions that are getting us closer to answering whether we are indeed alone or not.”
The paper from researchers with the Cassini mission, published in the journal Science, indicates hydrogen gas, which could potentially provide a chemical energy source for life, is pouring into the subsurface ocean of Enceladus from hydrothermal activity on the seafloor.
The presence of ample hydrogen in the moon's ocean means that microbes – if any exist there – could use it to obtain energy by combining the hydrogen with carbon dioxide dissolved in the water. This chemical reaction, known as "methanogenesis" because it produces methane as a byproduct, is at the root of the tree of life on Earth, and could even have been critical to the origin of life on our planet.
Life as we know it requires three primary ingredients: liquid water; a source of energy for metabolism; and the right chemical ingredients, primarily carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. With this finding, Cassini has shown that Enceladus – a small, icy moon a billion miles farther from the sun than Earth – has nearly all of these ingredients for habitability. Cassini has not yet shown phosphorus and sulfur are present in the ocean, but scientists suspect them to be, since the rocky core of Enceladus is thought to be chemically similar to meteorites that contain the two elements.
"Confirmation that the chemical energy for life exists within the ocean of a small moon of Saturn is an important milestone in our search for habitable worlds beyond Earth," said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
The Cassini spacecraft detected the hydrogen in the plume of gas and icy material spraying from Enceladus during its last, and deepest, dive through the plume on Oct. 28, 2015. Cassini also sampled the plume's composition during flybys earlier in the mission. From these observations scientists have determined that nearly 98 percent of the gas in the plume is water, about 1 percent is hydrogen and the rest is a mixture of other molecules including carbon dioxide, methane and ammonia.
The measurement was made using Cassini's Ion and Neutral Mass Spectrometer (INMS) instrument, which sniffs gases to determine their composition. INMS was designed to sample the upper atmosphere of Saturn's moon Titan. After Cassini's surprising discovery of a towering plume of icy spray in 2005, emanating from hot cracks near the south pole, scientists turned its detectors toward the small moon.
Cassini wasn't designed to detect signs of life in the Enceladus plume – indeed, scientists didn't know the plume existed until after the spacecraft arrived at Saturn.
"Although we can't detect life, we've found that there's a food source there for it. It would be like a candy store for microbes," said Hunter Waite, lead author of the Cassini study.
The new findings are an independent line of evidence that hydrothermal activity is taking place in the Enceladus ocean. Previous results, published in March 2015, suggested hot water is interacting with rock beneath the sea; the new findings support that conclusion and add that the rock appears to be reacting chemically to produce the hydrogen.
The paper detailing new Hubble Space Telescope findings, published in The Astrophysical Journal Letters, reports on observations of Europa from 2016 in which a probable plume of material was seen erupting from the moon’s surface at the same location where Hubble saw evidence of a plume in 2014. These images bolster evidence that the Europa plumes could be a real phenomenon, flaring up intermittently in the same region on the moon's surface.
The newly imaged plume rises about 62 miles (100 kilometers) above Europa’s surface, while the one observed in 2014 was estimated to be about 30 miles (50 kilometers) high. Both correspond to the location of an unusually warm region that contains features that appear to be cracks in the moon’s icy crust, seen in the late 1990s by NASA's Galileo spacecraft. Researchers speculate that, like Enceladus, this could be evidence of water erupting from the moon’s interior.
“The plumes on Enceladus are associated with hotter regions, so after Hubble imaged this new plume-like feature on Europa, we looked at that location on the Galileo thermal map. We discovered that Europa’s plume candidate is sitting right on the thermal anomaly," said William Sparks of the Space Telescope Science Institute in Baltimore, Maryland. Sparks led the Hubble plume studies in both 2014 and 2016.
The researchers say if the plumes and the warm spot are linked, it could mean water being vented from beneath the moon's icy crust is warming the surrounding surface. Another idea is that water ejected by the plume falls onto the surface as a fine mist, changing the structure of the surface grains and allowing them to retain heat longer than the surrounding landscape.
For both the 2014 and 2016 observations, the team used Hubble's Space Telescope Imaging Spectrograph (STIS) to spot the plumes in ultraviolet light. As Europa passes in front of Jupiter, any atmospheric features around the edge of the moon block some of Jupiter’s light, allowing STIS to see the features in silhouette. Sparks and his team are continuing to use Hubble to monitor Europa for additional examples of plume candidates and hope to determine the frequency with which they appear.
NASA's future exploration of ocean worlds is enabled by Hubble's monitoring of Europa's putative plume activity and Cassini's long-term investigation of the Enceladus plume. In particular, both investigations are laying the groundwork for NASA's Europa Clipper mission, which is planned for launch in the 2020s.
“If there are plumes on Europa, as we now strongly suspect, with the Europa Clipper we will be ready for them,” said Jim Green, Director of Planetary Science, at NASA Headquarters.
Hubble's identification of a site which appears to have persistent, intermittent plume activity provides a tempting target for the Europa mission to investigate with its powerful suite of science instruments. In addition, some of Sparks' co-authors on the Hubble Europa studies are preparing a powerful ultraviolet camera to fly on Europa Clipper that will make similar measurements to Hubble's, but from thousands of times closer. And several members of the Cassini INMS team are developing an exquisitely sensitive, next-generation version of their instrument for flight on Europa Clipper.
NASA confirms two moons in the Solar System are venting oceans into space
The prospects for life existing in our Solar System beyond Earth and finding it within a decade or two improved with two scientific findings announced Thursday by NASA. The space agency confirmed the presence of hydrogen in plumes emanating from Saturn's small moon Enceladus, and it also reported that plumes are very likely to exist on Jupiter's moon Europa.
Both of these findings are significant. It means not only that most of the ingredients required for life must exist in the oceans of Enceladus but also that a pair of probes being planned to explore Europa will have a much better chance of finding any life there. In something of an understatement, NASA's Jim Green, who oversees the agency's planetary exploration plans, said, "This is a very exciting time to be exploring the Solar System."
The findings buttress a recent focus by NASA on bulking up a program to explore these ocean worlds in the outer Solar System, including Enceladus, Europa, and Saturn's methane-covered moon Titan. This has been a principal aim in particular for Texas Republican John Culberson, who serves as chairman of the House subcommittee over NASA's budget.
"This is truly exhilarating news," he told Ars Thursday afternoon. "The findings on Enceladus and Europa reaffirm that the best place to look for extant life in the Solar System is on these ocean worlds. Almost certainly life could have evolved there, like we see in hydrothermal vents here on Earth." Culberson added that he would continue to push not only for an orbiter and lander to be sent to Europa but for a dedicated mission to Enceladus to follow thereafter.
Planetary scientists weren't particularly excited about the small, icy moon of Enceladus before the Cassini spacecraft arrived in the Saturn system about 13 years ago. However, as Cassini began making observations throughout the Saturn system, scientists soon found evidence of plumes emanating from cracks in the moon's surface. Subsequent observations confirmed the existence of those plumes, plus a large ocean below the surface of Enceladus.
So as the Cassini mission neared its end, scientists planned a number of closer observations of Enceladus, including flybys through the plumes themselves. In 2015, the spacecraft made its closest approach to within 49km of the moon's surface. During that flyby, the spacecraft detected a significant amount of molecular hydrogen in the plume.
In their subsequent analysis, scientists ruled out a number of explanations for the hydrogen and concluded that it most likely formed from the interaction between warm water near a rocky core of the moon, akin to the hydrothermal vents in Earth's oceans. On Earth, large communities of microbes thrive near these vents, subsisting through a process known as methanogenesis. These organisms use carbon dioxide and hydrogen to create methane, a chemical reaction that imparts a jolt of energy for the microbe. This is how they can survive without any Solar energy.
Could such a process be unfolding within the oceans of Enceladus? Definitely, scientists said Thursday during a briefing held by NASA. Astrobiologists believe life as we know it requires water, chemical elements to make the building blocks of cells, and chemical energy. Saturn's small moon, which is only about 500km in diameter, has all three. "Now all we need to know is if Enceladus has had enough time to evolve life and make an imprint," said Mary Voytek, an astrobiology senior scientist at NASA Headquarters.
One of the four Jovian moons observed by Galileo, Europa is quite a bit larger, about the size of Earth's Moon. Scientists are confident that the ice-covered Europa also harbors a large ocean, likely much larger than that of Enceladus. And while planetary scientists are eager to explore the ocean of Europa and its potential for life, they're not sure how they're going to get through the moon's crust of ice, which is probably at least a few kilometers thick.
Using the Hubble Space Telescope in recent years, astronomers have found some evidence that, like on Enceladus, plumes may also be exiting from Europa's internal ocean into space. But they weren't sure—spying water vapor at such a great distance was tenuous at best even for the large space telescope. But on Thursday, William Sparks, an astronomer with the Space Telescope Science Institute in Baltimore, said new evidence from Hubble provides very strong evidence for plumes.
Over the course of 12 different observations, Hubble found evidence of water vapor emanating from Europa's surface two times from the same location on the moon, near its equator, in 2014 and 2016. It was a four-sigma result, that is, there is a 99.99 percent chance the observations were not due to random chance. Moreover, the plume location lines up squarely with where the Galileo probe, which flew through the Jovian system in the 1990s, observed thermal "hot" spots on the surface of Europa. "It’s really intriguing," Sparks said. "It’s quite astonishing in fact."
The confluence of plumes at a thermal hot spot could be due to liquid water, below the ice but not too far from the surface, Sparks said. Alternatively, the plumes themselves, venting as much as 100km into space, could be raining a fine, mist vapor back onto the surface of Europa that is changing the moon's thermal character over a local area. Regardless, as NASA is planning to send both an orbiter and a lander to Europa during the 2020s, the existence of plumes would make the task of sampling the vast ocean below much simpler for scientists.
All of this talk about oceans is exciting, but what kind of life might exist there? Certainly, no one expects to find whales, sharks, or other large sea creatures in the oceans of Europa or Enceladus. Chemosynthetic microbes seem far more likely. But really, we have no idea. If any life does exist, it would be exotic.
As to which ocean offers the best prospects for life, NASA astrobiologist Voytek said she would still bet on Europa. That Cassini found such high concentrations of hydrogen in the plumes of Enceladus may well mean that there is nothing in the moon's oceans consuming it. There is also evidence that Enceladus could be considerably younger than the moons of Jupiter, giving life less time to form there. "My money for the moment is still on Europa, but it could be on any of these moons," she said. "It would be great if life were on all of them.