When will Neptune get oceans?

Yes, and a strange question that is. But actually not an unrealistic question. Neptune, now the farthest true planet from the Sun (after Pluto’s demotion), is namesake to the ancient god of the oceans. Neptune is shrouded in layers of icy, dense air laced with enough methane to color the world azure blue… hence the world’s maritime label. But despite its pelagic associations, Neptune has no oceans… though it might have had, with a slightly different composition. And it’s very possible that Neptune will one day be covered in a deep, wine-black sea of cool, liquid water.

neptune_voy2

Neptune is a big world, about 49,530 km in diameter, but despite being sometimes referred to as a lesser gas-giant, it’s not a colossus like Jupiter. It’s not even mostly gas. The sphere we see from outside is only the top of a deep, thick atmosphere. About 20% of Neptune’s apparent diameter is air, mostly hydrogen and helium, but with a fair tincture of methane and ammonia, and some more complex organics at trace levels. As one dives into Neptune the outer air starts off very cold. The planet is 30 AU from the Sun, after all. But inside Neptune the heat rises with depth… along with pressure. At a depth where the air is about as dense as what you’re breathing, the temperature is about -200° C, or ~70 Kelvins… but by the time things warm up to room temperature – around 300 Kelvins – you’d be crushed by over 50 atmospheres of pressure. One of those deep-diving suits from the old movies would be required to survive (along with oxygen tanks, of course).

Below that, things get weird. I said there’s no ocean on Neptune, but there is something like an ocean. As pressure climbs during your dive into Neptune, the atmosphere would gradually thicken into something very much like a liquid. True liquid water can’t exist anywhere inside Neptune, because there is too much hydrogen. The hydrogen molecules shove their way in amongst individual water molecules so much that water can’t fully condense into a true liquid. But at the crushing pressures of Neptune’s interior the thick, hot H2O, H2 and He gas compresses to water-like densities… except it’s not liquid water, and the temperature is over 2000° C. Those temperatures are hot enough to melt iron, and are far too hot for any organic compounds to survive. It is decidedly not an abode for life (as we know it, I must add with geeky circumspection).

But what if all that dense quasi-fluid were cooler? Neptune generates 2.6 times more energy in its core than it receives from the Sun, mostly from radioactive decay in its rocky heart. Really, “core” is an odd word to use, because this rocky kernel is more massive than four Earths. For all we know it might have an adorable little metallic iron core all its own, about the size of ours. In any event, heat from below combined with the thin solar light at 30 AU keep Neptune’s water from condensing as rain. But… if Neptune were colder at its cloud tops, and cooler within, its water could drop out to form a massive ocean realm.

What would such a realm be like? Half the planet’s apparent diameter is currently a hot, ionic quasi-liquid right now, but cooled that material would condense into an ocean thousands of kilometers deep. As a comparison, the Mariana Trench is the deepest part of our ocean, at 10 km. Neptune’s ocean would be hundreds of times deeper, and much stranger. If Neptune had a water ocean, its floor wouldn’t be rock, it would be solid ice. Heavy ice… ice VII, or X, or XI, or some other weird crystalline water-lattice that can survive at ten million atmospheres of pressure. Heat would rise from below, from the imprisoned mass of searing rock in Neptune’s center. Volcanic heat from the core mass would bubble up, melting the icy oceanic crust, and inseminating the deep ocean with precious minerals. Could life exist in such an alien environment?

Maybe. If Neptune could cool enough that it supports a liquid ocean, it might also give birth to life. Only when its massive, world-girdling ocean cools to become normal water, instead of a seething ionic mantle, will the conditions be even remotely similar to what conventional life requires. The big question, of course, is… can that happen?

The answer appears to be a qualified yes. Eventually, when its interior furnaces wind down a bit, and its atmosphere can cool enough, seas become possible on Neptune. The only problem… and it’s a fairly big problem, from our perspective… is that before that can happen the Sun has to go away. Specifically, the Sun must live out its long, long life and ultimately wither into a fading white dwarf. The post-solar white dwarf must, in turn, fade from blazing incandescence to a dim sputter. At that point, tens of billions of years from now, Neptune will cool enough to bear oceans. But not until.

There are two ways of looking at this news. From a purely selfish point of view, I’m quite cross that I’ll never be able to see what titans of the deep Neptune’s olympian waters might birth. But from a larger perspective I’m encouraged. Even when the Sun goes dark, it will be possible for life to spawn… or transplant itself and survive… in our solar system. The details will differ from what we’re familiar with today, on Earth, but water in the dim astronomical future will still require fins, and streamlining, to move through, and the animals of any Neptunian ocean of 100 billion years from now will still need to be sleek and smooth and tapered. I don’t have to see them for myself, because I can use chemistry, thermodynamics, geology, and physics to construct a probabilistic prognostication. Or two, or ten… There are many ways the future of Neptune may work out, but among them are highly realistic ways that lead to life, and complexity, and possibly thought… in a future so deep our own biosphere will be an impossibly ancient memory from a time older than the Big Bang is to us.

Now, show me that in your Book of Revelations.

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~ by Planetologist on March 3, 2009.

10 Responses to “When will Neptune get oceans?”

  1. Ten billion years is plenty of time to grow tool manipulators and learn math.

  2. Sounds like a great premise for a sad sci fi short story.

    “Hey! We’re just starting to develop science! Just in time to realize that our planet is just about to run out of energy!”

  3. Good question, Ty. I was wondering that as well. It would be a shame for the energy to peter out just as the little Neptunians first extended their little appendages.

    Actually, I also wonder wether the general hostility of the surrounding oceans wouldn’t restrict them to isolated, seperately evolved colonies located around the different vents. Furthermore, if life evolved here in such a manner, how did it become so widespread? This has always made me think that the mud puddle theories were the more likely candidate for life’s origins. Not that I am qualified for anything more than uninformed speculation. I need to find a good, recent book on seafloor vents and life.

    • The calculations aren’t straightforward, mainly because of complexities like the effects of convection in the silicate Neptunian core, and current disputes over whether most of Neptune’s interior heat is radiogenic. A portion of the interior heat might be due to ongoing gravitational compaction, relict accretionary heat, and perhaps even dark matter interactions in the core (although I have my doubts about that last one). Still, one can make a back-of-envelope calculation that puts some boundaries around how much heat is available. The silicate core of Neptune is about the size of the Earth, but (probably) contains about 10 times the mass of Earth. Assuming half the radiogenic isotope concentration as the bulk Earth has – the silicate material out there is a bit less enriched in U and Th, but a bit more enriched in K – Neptune’s silicate core has roughly five times as much interior-derived thermal luminosity as does the entire Earth at present.

      Most of that heat – again, assuming it’s all radiogenic – comes from decay of U-238, U-235, Th-232 and K-40. The mass of Th-232 alone, inside Neptune, gives out as much energy as 1.5 whole Earths, and it has a half-life of 14 billion years. Adding everything together, Neptune’s interior should be shedding an appreciable amount of heat for tens of billions of years to come. I don’t have firmer numbers yet, but this is something I’m working on as part of another manuscript, so eventually I’ll be able to answer more quantitatively. 🙂 Stay tuned on this bat-channel for more.

  4. How long would that planetary core energy last? Long enough for complex life to evolve?

  5. You have a small typo I think.

    “warm up to room temperature – around 300° C” should be “around 300° K” or “around 22° C”

  6. Will there be enough energy around at that point to base an ecosystem on?

    • That’s something I’m working on, actually. The only proximal energy source would be geologic, in the form of radiogenic heat from the interior… but as long as the silicate core is hot enough to erupt mineral-rich solutions into ocean water, there might be enough energy to drive primary production. Hydrothermal sea floor vents are the closest analogy Earth provides for that kind of biosphere.

  7. Some useful references:

    Lodders K. and Fegley B. (1998) The Planetary Scientist’s Companion. Oxford University Press, 371 pp.

    Wiktorowicz S. J. and Ingersoll A. P. (2007) Liquid water oceans in ice giants. Icarus 186, 436–447.

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