We often think about how thin Earth’s atmosphere is, imagining our planet as an apple, with the atmosphere no thicker than the skin of the fruit. That vast blue sky can seem all but infinite, but the great bulk of it is within sixteen kilometers of the surface, always thinning as we climb toward space. Now a presentation by graduate student Laura Schaefer (Harvard-Smithsonian Center for Astrophysics) at the 225th meeting of the American Astronomical Society in Seattle points out that, like the atmosphere, water is also a tiny fraction of what makes up our planet.
A small enough fraction, in fact, that although water does cover seventy percent of the Earth’s surface, it makes up only about a tenth of one percent of the overall bulk of a world that is predominantly rock and iron. Dimitar Sasselov (CfA), co-author of the paper on this work, thinks of Earth’s oceans as a film as thin as fog on a bathroom mirror. But we’ve seen recently that water isn’t strictly a surface phenomenon. The Earth’s mantle, in fact, holds several oceans of water pulled underground by plate tectonics and subduction of the ocean seafloor.
What Schaefer presented at the AAS is a report on her computer simulations of the planet-wide recycling that keeps Earth’s oceans from disappearing. Volcanic outgassing from the mantle, primarily at the mid-ocean ridges, keeps water returning to the surface even as subduction returns water to the mantle. The cycle maintains the oceans over aeons. The question for the researchers was whether similar cycles occur on super-Earths, and how long it would take an ocean to form after the cooling of a planet’s crust during its formation period.
The results are encouraging for those hoping to find stable oceans on super-Earths. Planets two to four times Earth’s mass turn out to be better at maintaining their oceans than Earth itself. Super-Earth oceans can persist for ten billion years unless destroyed by a red giant primary star as it nears the end of its life. The largest planet in these simulations — five times Earth’s mass — took a billion years to develop its ocean in the first place, however, the result of a thicker crust and lithosphere and the resultant delay in volcanic outgassing.
Image: This artist’s depiction shows a gas giant planet rising over the horizon of an alien waterworld. New research shows that oceans on super-Earths, once established, can last for billions of years. Credit: David A. Aguilar (CfA).
We have nothing to compare the timeframe of life’s development on Earth with, having no data on life elsewhere. But if we took our model as the norm, says this CfA news release, we would be wise to look for life on older super-Earths, those perhaps a billion years older than the Earth, given the lag time in getting those oceans into play. Sasselov notes:
“It takes time to develop the chemical processes for life on a global scale, and time for life to change a planet’s atmosphere. So, it takes time for life to become detectable.”
My own guess is that once we do develop the ability to study exoplanet atmospheres on the level of Earth-sized worlds, we’ll run into surprises on this front as well, depending on how typical the experience of getting life started on Earth really was. In any case, screening for older planets as the best targets for complex life seems like a rational procedure, but especially with super-Earths for whom surface water may be a slow-developing resource.
The paper is Schaefer and Sasselov, “Persistence of oceans on Earth-like planets,” American Astronomical Society, AAS Meeting #225, #406.04 (abstract).
Obvious to Centauri Dreams readers, but not so obvious to the general public, M~R^3. That Earth density planet with M=2E only has R=1.26E. Only just inside Kepler’s “Super Earth” bin. I do concur with the previous articles on this site that most of Kepler’s super Earths are probably mini Neptunes. Still it is interesting that someone is working on the geophysics of large rocky planets with liquid surface water, as no doubt they do exist.
I wonder if that could help avoid the problems of extended luminous pre-main sequence evolution in lower mass stars?
There is a big hint in the Kepler data that these 2-4RE planets are the norm
and most solar systems have at least one. Could one extrapolate the above studies to get an indication of how DEEP, these Alien oceans are. On Earth some simple organism seem to adapt to the CHALLENGER DEEP. What about on a ocean that is 30 Km deep? what kind of architecture would enable more complex creatures to arise. Put another way, jellyfish are simple creatures and they inhabit the deepest of our oceans, could they evolve beyond their apparently limited body plan given a different enviroment. Of course the drivers for evolution would have to be present, Competition and Mutations, we know our oceans haven’t produced anything like J. Camerons creatures in his ABYSS film, but who can say what an alien ocean will spawn.
Oceans are all very well, but it’d only take a bit more water from the mantle to sink all the land. Super-Earths might be Super-Oceans. Hopefully that modelling becomes a paper with a broader audience than an AAS meeting.
On a tangential point, the abstract notes that more ocean is sequestered over time on smaller planets – not necessarily a bad thing as a planet’s star brightens. Desert planets are more stable against insolation than Ocean planets – Earth might lose water more gradually than oft predicted and transition smoothly to being a still habitable desert planet as the Sun heads towards the end of its Main Sequence life-span.
@Joy
Agree that it’s good to remember this – and I think that while still in rocky planet territory, planets’ masses will scale even more quickly than r^3 because matter in the core becomes more and more compressed.
So if we doubled earth’s mass the increase in radius may increase by significantly less than 26%
On the other hand, it may have much deeper atmosphere
Atmospheric density is likely to increase sharply with planetary mass, because of the exponential decrease of gas loss with escape velocity. That said, neither a dense atmosphere nor the absence of land should be assumed to inhibit the development of life. Life most likely evolved under water (regardless of atmosphere), and biochemistry is quite insensitive to pressure (water or atmosphere).
Both photosynthesis and land-life are way down the evolutionary tree from life’s beginnings. Even if an atmosphere was 10000 Earth atmospheres strong, and blocked all light, the conditions could still easily be right for life to develop as long as there is liquid water at a suitable temperature. An ocean, or, perhaps, suspended droplets if the atmosphere is dense enough.
On a water world with three times the mass of Earth and an ocean of, say, 30 km average depth, what would the pressure be like at the bottom? What form might the water take? What form of life might be possible under that much pressure?
The more we look, the more questions there are.
The abstract only seems to consider mantle water and recycling. However I thought that the Earth’s oceans would be lost by the increasing luminosity of the sun that will result in photolysis of the oceans and eventually even boiling away. It isn’t clear to me why that is still not the case with super-Earths. The shallower oceans predicted seem to make this effect even more important for these worlds.
from abstract: “Although ocean mass on these planets increases with time, the oceans remain much shallower than for smaller planets, consistent with previous studies”
Do these models have any predictions for the formation of continents and whether they will stay dry, rather than water covered? One impact of higher gravity on these worlds might be that evolving walking land dwelling forms might be harder than on earth, assuming the fish-like forms were similar to terrestrial ones. They would also not reach the huge sizes we have seen on Earth, remaining smaller to compensate, while the ocean forms could develop just as on Earth.
Hi Paul,
Have you ever seen this “Exoplanets: Searching for Earth 2.0? from a newspaper in South Australia?
http://media.adelaidenow.com.au/nnd/exoplanets/desktop.html (sorry forgot to include the link)
Quite impressive.
Thanks
These new model results combined with the latest studies of extrasolar planet properties that suggest that planets with masses less than about 6 times that of the Earth tend to have Earth-like compositions, and the potential long term habitability of extrasolar planets a bit larger than the Earth is looking good.
http://www.drewexmachina.com/2015/01/03/the-composition-of-super-earths/
@Danangel January 5, 2015 at 20:38
The physicist in me just can’t resist this calculation:
“On a water world with three times the mass of Earth and an ocean of, say, 30 km average depth, what would the pressure be like at the bottom?”
A world with three times the mass of the Earth and an Earth-like composition would have a radius around 1.4 times that of the Earth. As a result, it would have a surface gravity about 1.5 times that of Earth. Since water pressure on the Earth increases by 1 bar for about every 10 meters of depth, the pressure at the bottom of a 30 km deep ocean on this hypothetical world would be around 4,500 bars or so.
“What form might the water take?”
That all depends on the temperature. But if we assume it has a temperature comparable to the deep oceans of the Earth, it would be a couple of degrees Celsius, give or take. At that temperature, pure water (at least) would still be liquid (and I suspect salt water would as well). If the temperature were a dozen or so degrees colder, pure water would turn into Ice V which I think is a bit more dense that pure water at that pressure (i.e. it would not float like normal ice). If the pressure were around 40% higher (i.e. an ocean depth of about 42 km or more), pure water would form Ice VI which is also a bit more dense that pure water at that pressure as well. I have no idea how water’s phase diagram is affected by the presence of various salts at those pressures.
“What form of life might be possible under that much pressure?”
Who knows but it’s a good question.
Our oceans are ~5km deep and our tallest base-to-peak mountains (Mauna Kea) are <10km high so I'm worried that, depending on the variance of water delivered to other earth-like planets (1km) and yet not deep enough to engulf all land mass. IE: if we had had 2-3 times the water delivery, maybe we would have no land left. Of all planets that have oceans, maybe most of them are completely water covered (which might make evolution of machine-wielding beings rare). WOuld appreciate a pointer to any simulations papers addressing this.
I was just thinking about the pressure at the bottom of these oceans which could form ices, now would these higher density ices be easily subducted like water as they would tend more likely to move about as a vicous mass. The other thing is that if these higher gravity worlds formed around a higher ‘early stage’ luminous star such as a low mass red dwarf and the original oceans where lost would the mantle still convect as the covection process is highly dependant on water to aid movement. In effect the water would become trapped in the mmantle when the convection process stalls.
Mind you we could look to Venus to solve that one in that if the convection process stops the heat in the planet due to radiactive decay would build up to the point where massive lava outpourings would occur potential liberating large quanties of water, but I doubt the convection process would restart and would have effectivily seize up.
@Adam
Didn’t the continents form later, such that early on the Earth basically was an ocean planet for a while? You’d have to have oceans so deep that it’s effectively impossible for plate tectonics on a super-earth to pile up enough continental crust forming at plate boundaries to push its cratons above the sea level.
The bit about Super-Earths having a thicker crust is news to me. I always thought the greater, longer-lasting internal heat inside of them would cause more tectonic activity, fragmenting their crusts more thoroughly than on Earth.
@Alex Tolley
It depends on how high the gravity gets. A planet with five times the mass of Earth and 1.5 times its radius would have gravity over twice as strong – that’d be a big problem.
The full paper is up on the arXiv: The persistence of oceans on Earth-like planets: insights from the deep-water cycle
Intriguingly while there’s a delay in the rise of an ocean level on a Super-Earth, it also declines quicker than on an Earth-mass or smaller planet, becoming a Desert Planet within ~5 billion years because so much ocean water is sequestered to the mantle. This process depends on the mantle convection type assumed, so it’ll take refined models – and more hard data – to know just how oceans evolve.
Gilese 667Cc and its ilk have appeared to have risin from the dead, with “Phoenix oceans” MORE like Earth’s than if they were PRIMORDIAL, and; with an in situ NON-biologically formed Oxygen in the atmosphere, that MAY accellerate evolution.
Graham writes:
An impressive newspaper treatment indeed! Thanks for the link, Graham. They’ve done a good job.
Alex Tolley:
Brett:
On a planet with twice the gravity, and organism of half the size could be exactly as mobile as its bigger cousin with identical relative bone mass on Earth. That is, twice the gravity would simply cut the maximum size of organisms in half.
I would not call that a big problem, at all. Consider, also, that the maximum size on Earth has a lower bound in the size of dinosaurs, which tells us that the size of today’s organisms (including ourselves) is constrained by something other than gravity.
I agree with Brett that we should not neglect the effect of interior heating. Presumably, a larger rocky planet generates more interior heat per surface area. It is quite possible that this leads to an increase in deep ocean temperature at some point. In the extreme, a large enough planet could support liquid water even in the absence of a central star. Talk about extending the habitable zone!
@Harry R Ray January 6, 2015 at 10:45
I’m afraid that the potential habitability of GJ 667Cc has been overstated. Since it was discovered using precision radial velocity measurements and the inclination of its orbit to our line of sight is unknown, all we know is that its MINIMUM mass is ~4.1 times that of the Earth. It can be and, in all probability is, larger than this minimum value… possibly much larger. Given an unconstrained orbit inclination, there is only about one chance in three that the actual mass of GJ 667Cc is less than ~6 times that of the Earth and therefore likely to be a terrestrial planet. Since recent work now suggests more massive planets are most likely mini-Neptunes instead, it is more probable that GJ 667Cc is not a rocky planet. *IF* GJ 667Cc beats the odds and is a terrestrial planet after all with a mass between 4 and 6 times that of the Earth, recent work on its spin state suggests that it will experience excessive tidal heating owing to its small, eccentric orbit around its sun. Either way, it is highly improbable that GJ 667Cc is habitable. For a detailed discussion, see the following:
http://www.drewexmachina.com/2014/09/07/habitable-planet-reality-check-gj-667c/
Late comment but for the record;
I am a bit confused here, reading two versions of the story:
One version that larger terrestrial planets (super-earths) have longer-lasting (and/or deeper) oceans, the other that their oceans are not as long-lasting (because of loss to the mantle) and shallower.
Apart from that, I now wonder whether terrestrial planets may show a (gradual or sudden?) transition from sub-earthlike (little ocean and atmosphere, 1.5 Re).
Undoubtedly, the future will tell us, and what a fascinating future that will be.
” Geodynamics and rate of volcanism on massive Earth like planets ” Kite et Al 2008.
A great and readable review of tectonics that says anything and everything can happen but generally plate tectonics can last the entire lifetime of the main sequence parent star of a Super Earth. Good news for habitability , especially for planets with big oceans.