OK, I'll admit it: I have a huge crush on Enceladus. But I happen to think my feelings for Saturn's icy moon are justified. It's a brilliant white snowball. It's got a global ocean beneath its surface. It's got freakkin' ice volcanoes. Best of all, based on samples collected during today's historic flyby, we might soon know if Enceladus is habitable.
Today, NASA's Cassini probe nose-dived through the plume of water ice spewing out of fissures at Enceladus' south pole. It was collecting samples and decoding their chemistry to assess the habitability of the moon's subsurface ocean. What Cassini finds as a result of this flyby could well determine whether NASA sends a dedicated life-seeking mission to Enceladus in the future.
"This is a very big step in a new era of exploring ocean worlds in our solar system," Cassini program Curt Niebur said during a NASA teleconference on Monday. "These are bodies with great potential to provide oases for life."
In anticipation of all the awesome science to come, let's review what we know about this fascinating Saturnian satellite, and what we can expect to find out.
A Most Mysterious Cosmic Snowball
If you were cruising past Saturn on an interplanetary sabbatical, you might get lucky enough to spy a gleaming white pearl, nestled in the gas giant's dusty E-ring 241,000km out from the planet itself. Whip out your space-grade binoculars for a closer look. If the object is perfectly round, 500km across, and enveloped in a halo of icy grains, then congratulations, my friend, you've found Enceladus.
Covered in a thick ice sheet that reflects nearly 100% of incoming sunlight, the tiny Saturnian moon of Enceladus is one of the brightest objects in our solar system. Enceladus first caught the eye of astronomer William Herschel in 1789, but little was known about the moon until Voyagers 1 and 2 sailed past Saturn in the early 1980s. In recent years we've gotten to know Enceladus much better thanks to NASA's Cassini probe, whose high-resolution images of the moon reveal a motley mixture of cratered glaciers, icy fissures and frozen plains. If the gravity were a bit stronger, it'd be a cross country skier's paradise.
Enceladus, captured at a distance of 112,000km by the Cassini spacecraft on 27 July 2015. Like many moons, Enceladus' northern terrain is covered by impact craters. Land to the south is dominated by fractures and long, linear features. Image Credit: NASA/JPL-Caltech/SSI
Enceladus' diverse and largely crater-free surface suggests that the moon is geologically active today. That means it may have a hot planetary core, and while any radiogenic heat left over from the moon's formation should have dissipated long ago, scientists speculate that Enceladus is tidally heated through gravitational interactions with Saturn and the moon Dione. (Tidal forces are also thought to power Jupiter's eruptive moon, Io, and perhaps Europa). The case for an active interior was strengthened when Cassini flew past Enceladus in 2005 and discovered tremendous plumes of icy material spewing out of the moon's south pole into space.
There are over 100 of these south polar plumes, hurling icy grains through the moon's thin atmosphere and off into space at approximately 1287km per hour. The plumes create a halo of fine mist that supplies Saturn's E-ring, a wide and diffuse disk of ice and dust. Quite literally, Enceladus is being devoured by Saturn. Early on, the moon's plumes were likened to jets from a high-pressure hose, but a recent analysis suggests that "curtain eruptions," similar to Hawaii's famed volcanic fissures, may be a more accurate description for the material spewing out of miles-long cracks in the moon's surface.
Collage of Cassini images showing long, tendril-like features emanating from Enceladus' south polar geysers and feeding Saturn's ring system. a) and c) are computer-enhanced versions of raw Cassini images, while b) and d) are computer-generated images produced by tracing the trajectory of icy particles from 36 of the most active geysers. Image Credit: NASA/JPL-Caltech/SSI
The location of Enceladus' icy plumes in relation to hot spots detected by Cassini suggests that the eruptions are drawing material up from a liquid reservoir deep beneath the surface. Gravity data published in Science last year confirmed the presence of a subsurface sea at Enceladus' south pole, beginning roughly 35km beneath the ice sheet and extending out to at least 50 degrees latitude. Just last month, scientists determined that Enceladus' ocean wraps all the way around the planet's hot, rocky interior. The smoking gun was a slight wobble in the moon's orbit around Saturn, an anomaly, researchers say, that's simply "too large for a rigid connection between [ice] shell and core."
The structure of Enceladus' interior. Image Credit: NASA/JPL
A globe-spanning ocean is exciting enough, but it gets even better: We've tasted this moon's plume water, and it's surprisingly Earth-like. Samples collected by previous Cassini flybys revealed water vapour, simple organic compounds, salts, and ammonia. Earlier this year, Carnegie Science's Christopher Glein used this data to construct a chemical model of Enceladus' ocean. His work suggests that the ocean's seawater is alkaline, with a pH between 11 and 12.
While that's basic by Earth standards -- our oceans have an average pH of 8.1 -- it's not necessarily too basic. There are alkaline lakes on Earth that harbour diverse microbial communities. What's more, the ocean's high pH is best explained by serpentinization, a geochemical process that produces molecular hydrogen (H2). H2, in turn, is an energy source that supports microbes living in Earth's crust today. This metabolic strategy could be as old as life itself, perhaps originating in deep sea vents billions of years ago.
Conceptual model of Enceladus' south pole plumes. Image Credit: NASA / JPL
"The deduced pH is completely compatible with life as we know it; indeed, life on Earth may have begun under similar conditions," Glein and his co-authors write. "These considerations provide major motivation for future missions to explore Enceladus as a habitable world, whether past or present."
A Life Finder Mission?
The apparent Earthliness of Enceladus' ocean places the moon on a very short shortlist of promising candidates for alien life in our solar system. As Cornell astronomer Jonathan Lunine put it at the inaugural ceremony of the Carl Sagan Institute this spring, "Everything that we measure about Enceladus tilts in the direction of habitability."
Eruptions from Enceladus' south pole (left) modelled (right) as curtains that stretch along fractures. Image Credit: NASA/JPL-Caltech/SSI/PSI
That's why Lunine and colleagues recently proposed the Enceladus Life Finder. The proposed Life Finder mission, which I wrote about last spring, would be a lightweight, solar-powered probe, designed to sail through Enceladus' south pole plumes, collect samples, and use state-of-the-art mass spectrometers to hunt for amino acids, cell membrane components, and other biosignatures. Analysis of the carbon and hydrogen isotopes present in Enceladus' seawater would could offer further evidence of metabolic reactions. Within weeks of collecting samples, such a mission could definitively tell us whether Enceladus has life.
If it were up to me, it'd be the next mission on our list. But there's a big solar system to explore and limited funds with which to do so. NASA has already committed to a Europa mission in the 2020s; New Horizons is well on its way to the Kuiper Belt and we'd love for it to continue doing great science. The NASA Discovery Program's latest shortlist of low-cost planetary exploration missions highlighted other intriguing targets, including a crazy metallic asteroid, the Trojan asteroids orbiting Jupiter, and our long-neglected nearest neighbour Venus.
Enceladus didn't made the latest cut, but depending on the data Cassini collects this week, the tiny moon could get another shot.
What's Happening Today
At approximately 4:00am today, Cassini made its deepest dive yet into Enceladus' south pole plume, sailing 30km over the the moon's surface. It was moving at a vigorous clip -- some 19,000 km/h -- which meant the flyby was over in a fraction of a second. But that's enough time for Cassini to collect the tiny sample of ocean water needed to answer some of the most pressing questions about this enigmatic moon.
Plumes from Enceladus' geysers are illuminated by Saturnshine. Image Credit: NASA/JPL-Caltech, Space Science Institute.
As Linda Spilker, a Cassini project scientist at the Jet Propulsion Laboratory explained in a teleconference on Monday, today's flyby had three main science goals. First and foremost, using Cassini's neutral mass spectrometer (INMS) to seek out the molecular hydrogen we suspect is present. "The measurement of H2 is a key component of the energetic basis of habitability on icy worlds," Glein told Gizmodo in an email, noting that its discovery would "feed into the objectives of next-generation missions, including the Enceladus Life Finder and sample return." In addition to representing a viable energy source, Spilker notes that molecular hydrogen would "provide an independent line of evidence for hydrothermal activity taking place" at Enceladus' seafloor.
But in order for Enceladus to be truly habitable, other ingredients need to be present besides hydrogen. The second objective of the flyby was to use Cassini's cosmic dust analyser (CDA) to characterise the broader suite of chemicals present in the moon's seawater. We've already sniffed simple organic molecules like methane, but by sampling closer to the surface, Spilker says, "we might find new organics that we haven't seen previously or are just at the limit of our detection."
Five Saturnian moons are present in this Cassini portrait (Enceladus shines brightly at the center.) Can you spot them all? Image Credit: NASA/JPL-Caltech/Space Science Institute
Finally, today's flyby will hopefully settle the debate over the nature of the plumes themselves -- whether they're discrete, high pressure jets, or curtain-like eruptions, a distinction which Spilker says will have "major implications for how long Enceladus might have been active." Although the moon's south pole will be facing away from the Sun at the time of the flyby, Cassini will also be snapping tons of images using Saturnshine, sunlight scattered off the local gas giant's atmosphere. We're expecting a phenomenal view of the plumes, backlit against Saturn and its rings.
We're not going to find hard evidence of life on this flyby. But we're taking an important step in that direction. As Niebur puts it, Cassini will "tell us about the characteristics of that ocean and whether life on Enceladus is a remote possibility."
As for how advanced life might be on Enceladus, should it exist? We're almost certainly talking microbes. But, Spilker syas, "if we use an analogy to Earth's oceans and kinds of life living near hydrothermal vents, you could perhaps have a very diverse community."
Even if giant alien cephalopods are out of the question, discovering any life on Enceladus would be Earth-shattering. "The importance the discovery life would generate is that life exists somewhere else," Cassini project manager Earl Maize said. "And if it arose twice -- in one solar system -- the implications for how probable and frequently it arises in the universe as a whole are profound."
Iess et al. 2014. The gravity field and interior structure of Enceladus. Science. [Link.]
Spitale et al. 2015. Curtain eruptions from Enceladus' south-polar terrain. Nature. [Link.]
Thomas et al. 2015. Enceladus' measured physical libration requires a global subsurface ocean. Icarus. [arXiv preprint]
Glein et al. 2015. The pH of Enceladus' ocean. Geochimica et Cosmochimica Acta. [arXiv preprint]
Top: Enceladus, looking magnificent suspended over Saturn's rings. Image Credit: NASA / JPL