#subsurface ocean

This rumpled fabric at the corner looks like evidence of ongoing tectonic activity.

Ask Ethan: How Can Worlds That Never Get Above Freezing Have Liquid Water?

“I was reading about Saturn's moon Enceladus, and how scientists believe it has liquid water oceans beneath its water-ice crust. And yet I also read that the warmest surface temperatures are -90 celsius. How can this moon have liquid water? [...] At such cold temperatures and low pressures it seems Enceladus can have water ice and water gas but not liquid. What am I missing?”

Here on Earth, water can easily exist in all three phases of matter: solid, liquid, and gas. The reason for this is simple: Earth has the right range of temperatures and pressures to experience not just the common solid and gas phases, but the liquid water phase, too. In the outer Solar System, worlds like Europa, Enceladus, and Pluto are too far from the Sun to ever reach surface temperatures high enough to create a liquid phase; it seems that water is a no-go. But there must be subsurface oceans on these worlds! Not only is there geological evidence of an ocean beneath a thick layer of ice, but on some worlds, like Enceladus, we can actually see large plumes of liquid water ejected hundreds of kilometers above the surface, like some sort of planet-scale geyser. While the increased pressure from the ice plays a role, it isn’t enough on its own; there must be other factors, too.

Pluto’s moon Charon is the best sidekick a dwarf planet could hope for: unwavering in its loyalty, content to be a minor character in somebody else’s narrative. But two years after the New Horizons flyby, the largest of Pluto’s five moons is finally getting some well-deserved time in the spotlight. New research suggests that Charon’s storied history includes tectonic activity, cryovolcanism, and perhaps, a globe-spanning ocean.

TODAY IN PHILOSOPHY OF HISTORY

Are subsurface ocean worlds Plato’s cave? 

The allegory of the cave is Plato’s great thought experiment in metaphysics, epistemology, and ethics, and subsequent philosophy has produced countless variations on Plato’s theme, which is the distinction between appearance and reality. Reading Plato against the grain, I take the allegory of the cave from a naturalistic perspective and explore how certain environments approximate or fail to approximate the dilemma of Plato’s cave, and what this entails for our knowledge of the universe we inhabit.

Quora:             https://philosophyofhistory.quora.com/ 

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Astronomers have just found the best evidence yet of an entire ocean in an exceedingly unlikely place—the dwarf planet Pluto, in the dark hinterlands of the solar system. There, nitrogen and other “volatile” gases freeze solid in the cryogenic conditions, and water turns to rock-hard ice. For decades scientists have theorized how that ice might act as an insulator, preserving vestiges of warmth and moisture deep within Pluto and other objects so far from the Sun. But there was not enough data to confirm such wild speculations.

All that changed when NASA’s New Horizons mission flew by Pluto last year. Amid the dwarf planet’s many wonders, the brightest and most striking feature the probe saw was a 1,600-kilometer-wide heart-shaped plain sprawled across the distant world’s surface. The heart is dubbed “Tombaugh Regio” after the world’s discoverer, American astronomer Clyde Tombaugh. Fissures and fractures around Tombaugh Regio and other parts of the planet suggested a subsurface layer of watery slush might be slowly solidifying, breaking up the surface as it expands like ice cubes in a freezer—but other, drier possibilities could also explain such cracks. Now, however, two studies published Wednesday in Nature are strengthening the case that Pluto’s icy heart contains a warmer, wetter inner world.

“If we’re right, oceans in the outer solar system are common, and other objects of similar size to Pluto there probably also have subsurface oceans,” says Francis Nimmo, a lead author of one of the studies and planetary scientist at the University of California in Santa Cruz.

The icy Saturn moon Dione appears to have an underground ocean of liquid water, just like two of its more famous neighbors, a new study suggests.

This huge ocean is probably buried about 60 miles (100 kilometers) beneath Dione's icy shell, according to the study. Intriguingly, Dione's putative ocean is likely in contact with the moon's rocky core, team members said.

"The contact between the ocean and the rocky core is crucial," study co-author Attilio Rivoldini, of the Royal Observatory of Belgium in Brussels, said in a statement. "Rock-water interactions provide key nutrients and a source of energy, both being essential ingredients for life."

If the researchers are correct, 700-mile-wide (1,120 kilometers) Dione would be the third Saturn moon known to harbor a subsurface ocean, after giant Titan and geyser-spouting Enceladus. Astronomers think the Jupiter moons Europa, Callisto and Ganymede also have buried oceans, and recent research indicates Pluto might as well.

The study team, led by Mikael Beuthe of the Royal Observatory of Belgium, modeled the icy shells of Dione and Enceladus using gravity data gathered by NASA's Saturn-orbiting Cassini spacecraft during its various flybys of the satellites.

It is only in recent years that the prevalence of subsurface ocean worlds has come to our attention, but we may need to eventually recognize a class of biospheres that consists of subsurface ocean worlds in isolation from a “terrestrial” biosphere. (I place “terrestrial” in scare quotes because this is a geocentric and anthropocentric concept specific to Earth, but its use here ought to be unproblematic as I assume that the reader can make the extrapolation from the particular case of Earth’s landmass biospheres to any landmass biosphere on any planet in contradistinction to any oceanic biosphere on any planet.) Part of what makes these discoveries so interesting is that we are finding potentially “habitable zones” for life as we know it (in contradistinction to Weird Life, i.e., life as we do not know it) far beyond the habitable zone in which something like the terrestrial biosphere can occur (i.e., the circumstellar habitable zone, or CHZ). Thus these discoveries are re-shaping our conception of the habitability of a planetary system.

Recently there has been a lot of attention directed at Saturn’s moon Enceladus, which we now know to have a subsurface ocean, like several of the Jovian moons. (Cf. Does Enceladus support life? 7 key facts and Saturn's Geyser Moon Shines in Close Flyby Views) NASA's Cassini spacecraft has flown past Enceladus a few days ago (28 October 2015), close enough to pass through the plumes of gas and ice that are erupting from Enceladus’ subsurface liquid water ocean, sampling the plumes with the spacecraft’s gas analyzer and dust detector instruments. As this information is analyzed over the coming weeks it will provide new insight into the interior structure of Enceladus and its subsurface ocean.

We also know that Ganymede, a Jovian moon that is the largest moon in the solar system, has a subsurface ocean (cf. NASA detects subsurface ocean on solar system’s largest moon). It has been known for some time that Europa, another Jovian moon, has a subsurface ocean as well. I previously wrote about Europa’s subsurface ocean in Europa’s Cryosphere and Hydrosphere. When we someday possess the technological wherewithal to explore within the many subsurface oceans within our solar system, in addition to the possibility of finding other life, I think that we will discover quite complex internal structures, and that these internal structures will be different in the different subsurface oceans, accordingly whether they are heated by the tidal forces of the gas giants they orbit, by an internal molten core, by radioactive decay, or by some combination of these sources of energy. These structures, will, in turn, be implicated in any life that is found, since we know that on Earth the organic chemistry of microbial life has a close relationship with the inorganic chemistry of the lithosphere, hydrosphere, and cryosphere. With or without life, these subsurface oceans are likely to be geologically complex environments with all manner of internal structure unprecedented in our knowledge of terrestrial ocean environments. 

In a recent post I discussed an origins of life experiment that has produced organic molecules in conditions replicating the extreme cold of extraterrestrial space, noting earlier origins of life experiments that have shown the importance of cold as well as of heat in the formation of organic molecules. These subsurface oceans of Europa, Ganymede, Enceladus, and other moons of our solar system (and perhaps even on rogue planets wandering in interstellar space, perhaps ejected from their planetary system by a gravitational interaction with a passing star) will have strong chemical interactions between their heated core (by whatever means heated) and their extremely cold icy shells. It is in such an environment of complex geological and chemical interactions that one would expect to find the conditions that would independently give rise to the emergence of life.

As a thought experiment we can imagine an alternative history of Earth in which the iced over “snowball Earth” never thawed out, but retained its oceanic biosphere under a layer of ice many kilometers thick. The kind of oceanic biosphere that would have emerged in this context, cut off from any expansion onto land, would look very different from the biosphere that we have on Earth today. One can think of glaciation as the retreat of the biosphere before the expansion of the cryosphere, and the opposite climatological development of warming as the retreat of the cryosphere before the expansion of the biosphere.The two are locked in a relationship that develops over geological time. Another scenario for a thought experiment, taken from another theory of terrestrial glaciation, might involve a nearly entirely glaciated Earth, but with a small “slushy” zone near the equator, where the sun shone on a small patch of open water, with floating icebergs. Here life might have had an opportunity to find its way out onto the ice-covered surface, or even into the skies of our planet. Again, this would have resulted in a very different counterfactual biosphere. One suspects that, given what we now know about the abundance of water in the universe, and the number of planets within the habitable zones of stars, that these scenarios have occurred millions and millions of times over, each time with a slightly different result.