15 December 2022

A distant sea

Enceladus (pronounced en-SELL-a-dus) is one of the moons of Saturn.  It's quite small, just three hundred miles in diameter -- one-seventh of the diameter of Earth's moon.  Despite its modest size, it's one of the more interesting objects in our solar system, and likely to be the target of intensive study in the future.

The surface of Enceladus consists of ice -- ordinary water ice just like the ice cubes in your freezer.  But beneath the ice is an ocean of liquid water covering the whole moon, an ocean we estimate to be between fifteen and forty miles deep, far deeper than any ocean on Earth.

The water ocean remains liquid, even though Saturn and its moons are ten times further from the Sun than Earth is, because Enceladus is geologically active.  Tidal and other gravitational effects from Saturn and its other moons constantly stress tiny Enceladus, which generates heat -- quite a lot of heat.  We believe that on the ocean floor there are thermal vents regularly releasing molten magma into the water, just as we see in some places on the ocean floors of Earth.  This provides the warmth that prevents the ocean from freezing.  The outer layer of surface ice, which is what we see from outside, is just the upper few miles of the ocean which are frozen because they are directly exposed to the near-absolute-zero temperature of space so far from the Sun.

The ice layer is thinnest at Enceladus's south pole, probably only one to three miles thick.  From the south polar region, plumes of water from the ocean below are frequently vented into space through cracks in the ice.  In 2005 the Cassini spacecraft was flown through these plumes several times and was able to analyze their composition.  They -- and by implication the ocean beneath the ice -- are rich in trace materials necessary for life as we know it, such as nitrogen, phosphorus, carbon dioxide, and sodium chloride.  The analysis also detected complex organic molecules which implied that amino acids may exist in the ocean.  This is not, in itself, evidence of life there, but it tells us that Enceladus's ocean is an unusually favorable environment for the emergence of life, perhaps the most favorable in the solar system after Earth itself.

Our failure to find any sign of life on Mars has been a disappointment, but Mars is, after all, an arid desert.  Life on Earth originated in the oceans, and didn't emerge onto the land until relatively late in our planet's history.  The warm and chemically-rich ocean of Enceladus is, realistically, a more promising place to look.

Unfortunately, doing so presents frustrating difficulties.  A future probe could perhaps fly through the water plumes again and do more sophisticated analysis, but this is unlikely to provide real proof of the existence of life, even if there is any.  To really settle the question would require a probe which could land on Enceladus and release some kind of combined digging machine and submersible, which would burrow down through the ice (one to three miles thick even at its thinnest, remember), explore the ocean below, and then report back to the lander on the surface, which would transmit the information back to Earth.  But this would really be pushing our technology to the utmost limits of its capabilities.

Nevertheless, someday we will be able to do it.  The results would tell us much about life in the universe more broadly.  If there are two life-bearing bodies (Earth and Enceladus) in a single solar system, this implies that life in the universe generally is abundant.  If we find large, multicellular forms there -- animals and plants or similar -- this would imply that such complex life is also abundant elsewhere.  If Enceladus turns out to be lifeless, that would tell us that even if all the necessary chemicals and an energy source are present, life is not inevitable, and may be rare to nonexistent in most of the universe.  There is perhaps no more vital goal for our space science than the future exploration of this distant sea.


Blogger Lady M said...

Very interesting - perhaps someday we will know more.

15 December, 2022 11:38  
Blogger VoenixRising said...

I seriously doubt I will live long enough to see this, but then again, when I was a child I never thought we'd see the outer planets—much less Pluto up close and personal either.

15 December, 2022 12:28  
Blogger Ami said...

Fascinating. I wish I were going to live another hundred years or so to find out what's there.

As a child I always wanted to go to space. I went through a phase in my early teens where I fantasized about aliens offering me a free trip to wherever they were headed next. Oh, not serious fantasy, more like making up stories. I had a steady diet of sci fi thanks to my grandfather and my dad. Something I still love to this day.

You never know when something you read or hear will spark inspiration... hence the reason I always pass on stuff like this to 'my' kids. They're big dreamers, the elementary set... they haven't internalized the lesson that some things just aren't possible.

15 December, 2022 12:54  
Blogger Mary said...

Wish I was going to be around to see what may be out there. But then I think of Carl Sagan who probably wanted to know more than anyone, but alas, not to be.

15 December, 2022 16:11  
Blogger Richard said...

Enceladus is so interesting! I had no idea of what size it is. I hope the people who will be here will continue to investigate. I have so many questions! Oh well, they will be standing on our shoulders. Mortality really sucks.

15 December, 2022 21:46  
Anonymous Reaganite Independent said...

Very interesting, have studied up on Saturn’s moons a bit, the variety is mind blowing. And can see at least three of them from my cheap living room telescope.

15 December, 2022 22:38  
Anonymous NickM said...

I dunno. For me Titan is the money moon. It probs has surface liquid. The Jovian and Saturnian moons really are amongst the most interesting things in the Solar System. Unfortunately they're rather a long way off. That is from me wearing my astrophysics hat... But wearing my chapeau poetique I can also imagine what Saturnrise must look like from somewhere like Enceledus or Titan. Wow!

If you mean "living room telescope" literally then try taking it outside. The problem of sticking a scope through a window from your nice warm house is that the heat difference causes convection currents in the air which distorts images. Alas, you ain't gonna get any good images without getting cold.

16 December, 2022 05:58  
Blogger SickoRicko said...

Thank you for all of this information.

16 December, 2022 08:01  
Blogger Infidel753 said...

Lady M: We will -- it's just a question of how long it takes.

Voenix: Unfortunately getting under that ice is a tougher technical challenge than photographing things. We've seen pretty much every planet and moon close up, but getting beneath the surface is a whole different matter.

Ami: Going on a trip with aliens would be fascinating -- if you were sure they'd bring you back.

I doubt it will take a hundred years to explore the Enceladus ocean, but it could easily be a couple of decades, even if the space agencies make it a priority.

Mary: Sagan would have been fascinated by what we've learned so far. It's very regrettable he died relatively young.

Richard: This is one of the main reasons I hope to see aging and involuntary death defeated -- so as not to miss out on all the discoveries to come.

Reaganite: There's a lot of interest about the outer moons. The old "Goldilocks zone" concept hugely underestimated the factors that can make a world suitable for life.

NickM: At least on the surface, Titan is far too cold for liquid water. There are seas, but they're liquid methane. There's been speculation about a possible underground water ocean, but no confirmation that it exists, much less that it has the kind of favorable organic chemistry we see on Enceladus.

Ricko: Thanks for your post a few days ago, which got me thinking about this.

16 December, 2022 10:11  
Anonymous NickM said...

Well, I for one think a sea of liquid methane would be a wonder to behold. The odd thing about Goldilocks is that to a certain extent Jupiter and Saturn a kinda large enough to sort of have their own Goldilocks zones. Sort of.

16 December, 2022 12:36  
Blogger Infidel753 said...

It has been beheld (I posted a photo here at some point). But in this post I'm mostly looking at the prospects for discovering life.

The problem with the Goldilocks zone concept is that it looked exclusively at the heat a planet would receive from its sun as a measure of its odds of being in the right temperature range for life to develop. But this ignores the fact that so many other factors can influence the temperature of a planet or moon. Venus is only twenty million miles closer to the Sun than Earth is, but it's far too hot for liquid water, for reasons hardly connected to the amount of solar radiation it gets. Jupiter and Saturn don't generate anything like enough radiant heat to warm up their moons, but tidal forces can keep some of them warm enough for liquid water. We'd expect similar situations in other solar systems.

16 December, 2022 19:40  
Anonymous NickM said...

You surprise me Infidel. I'd got the distinct impresssion you weren't that interested in ET. You are very right about Goldilocks. It is so very dependent on such bizarre (and poorly understood) things such as the Venusian greenhouse effect plus ultra or Mars being almost totally atmospherically stripped due to the shut down of its mag-field (why?). It's all a complete bloody mystery not least because we're in a very peculiar situation empirically. We know this planet very well*, our neighbours pretty well and extra-solar planets very poorly. It makes for extremely problematic empirical science - having extraordinarily different levels of data to compare is not easy. At. All.

*I mean that relatively speaking.

17 December, 2022 12:28  
Blogger Infidel753 said...

I think it's enormously unlikely that intelligent life, or alien technological civilizations, exist elsewhere in the universe -- for reasons I've already discussed pretty extensively. I consider it fairly unlikely that life of any kind, even the most primitive, exists beyond Earth. That doesn't mean I find the possibility uninteresting. If I'm wrong and an alien ecosystem exists in the ocean of Enceladus, near enough that we could study it up close in the foreseeable future, that would be one of the most exciting discoveries in the history of science. My belief is that the odds are against it. But there are too many unknowns to be dogmatic one way or the other. Given the favorable conditions on Enceladus, it's very much worth checking out.

17 December, 2022 13:42  

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