A Waterworld Around GJ1214

by Paul Gilster on February 22, 2012

I love the way Zachory Berta (Harvard-Smithsonian Center for Astrophysics) describes his studies of the transiting super-Earth GJ1214b. Referring to his team’s analysis of the planet’s atmosphere, Berta says “We’re using Hubble to measure the infrared color of sunset on this world.” And indeed they have done just this, discovering a spectrum that is featureless over a wide range of wavelengths, allowing them to deduce that the planet’s atmosphere is thick and steamy. The conclusion most consistent with the data is a dense atmosphere of water vapor.

Discovered in 2009 by the MEarth project, GJ1214b has a radius 2.7 times Earth’s and a mass 6.5 times that of our planet. It’s proven to be a great catch, because its host star, an M-dwarf in the constellation Ophiuchus, offers up a large 1.4 percent transit depth — this refers to the fractional change in brightness as the planet transits its star. Transiting gas giants, for example, usually have transit depths somewhere around 1 percent, while the largest transit depth yet recorded belongs to HD 189733b, a ‘hot Jupiter’ in Vulpecula that shows a depth of approximately 3 percent.

Image: This artist’s impression shows how the super-Earth around the nearby star GJ1214 may look. Discovered by the MEarth project and investigated further by the HARPS spectrograph on ESO’s 3.6-metre telescope at La Silla, GJ1214b has now been the subject of close analysis using the Hubble Space Telescope, allowing researchers to learn not only about the nature of its atmosphere but also its internal composition. Credit: ESO/L. Calçada.

GJ1214b is also near enough (13 parsecs, or about 40 light years) that Berta’s follow-up studies on its atmosphere using the Hubble Space Telescope’s WFC3 instrument have proven fruitful. What the researchers are looking at is the tiny fraction of the star’s light that passes through its upper atmosphere before reaching us. The variations of the transit depth as a function of wavelength comprise the planet’s transmission spectrum, which the team is able to use to examine the mean molecular weight of the planet’s atmosphere. Earlier work on GJ1214b’s atmosphere had been unable to distinguish between water in the atmosphere and a worldwide haze, but the new work takes us a good way farther and into the planet’s internal structure.

The reason: A dense atmosphere rules out models that explain the radius of GJ1214b by the presence of a low-density gas layer. The researchers take this to its logical conclusion:

Such a constraint on GJ1214b’s upper atmosphere serves as a boundary condition for models of bulk composition and structure of the rest of the planet. It suggests GJ1214b contains a substantial fraction of water throughout the interior of the planet in order to obviate the need for a completely H/He- or H2-dominated envelope to explain the planet’s large radius. A high bulk volatile content would point to GJ1214b forming beyond the snow line and migrating inward, although any such statements about GJ1214b’s past are subject to large uncertainties in the atmospheric mass loss history…

If you examine the density of this planet (known because we have good data on its mass and size), you get about 2 grams per cubic centimeter, a figure that suggests GJ1214b has much more water than the Earth and a good deal less rock (Earth’s average density is 5.5 g/cm3). We can deduce a fascinating internal structure, one in which, as Berta says, “The high temperatures and high pressures would form exotic materials like ‘hot ice’ or ‘superfluid water’ – substances that are completely alien to our everyday experience.”

We wind up with a waterworld enveloped in a thick atmosphere, one orbiting its primary every 38 hours at a distance of some 2 million kilometers, with an estimated temperature of 230 degrees Celsius. I was delighted to see that Berta and colleagues have investigated the possibility of exomoons around this planet, looking for “shallow transit-shaped dimmings or brightenings offset from the planet’s transit light curve.” The team believes no moon would survive further than 8 planetary radii from GJ1214b, and indeed they find no evidence for Ganymede-size moons or greater, but the paper is quick to note that:

Due to the many possible configurations of transiting exomoons and the large gaps in our WFC3 light curve, our non-detection of moons does not by itself place strict limits on the presence of exo-moons around GJ1214b.

All of which is true, but what a pleasure to see the hunt for exomoons continuing to heat up.

The paper is Berta et al., “The Flat Transmission Spectrum of the Super-Earth GJ1214b from Wide Field Camera 3 on the Hubble Space Telescope,” accepted for publication in The Astrophysical Journal (preprint).

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{ 42 comments }

ljk February 22, 2012 at 10:45

Here is the link to the Harvard CfA press release on this topic:

http://www.cfa.harvard.edu/news/2012/pr201204.html

Includes larger versions of the artwork in this piece.

tom baty February 22, 2012 at 12:23

Great and very interesting article. I am sure that in future years we will have additional info and data about this planet. We are being to have better and better research into the charactarisics of individual exoplanets and are beginning to get confirmation that there is going to be wide range of varied worlds out there. Interesting indeed!!!

Tom

philw1776 February 22, 2012 at 13:19

Once again I am so thankful to be around during this unprecedented and unexpected era of extrasolar planetary discovery. The fact that the administration and NASA have failed to fund any of several proposed post Kepler extrasolar focused missions (e.g. TPF) is a travesty.

andy February 22, 2012 at 14:12

On the other hand, the paper does note that if the planet has high-altitude clouds or hazes, the observations are still consistent with a hydrogen-rich atmosphere. These results don’t really resolve much about the nature of GJ 1214b.

Joy February 22, 2012 at 15:17

This pressure cooker temperature waterworld is not a great environment for organic compounds. But the bulk composition probably matches many other worlds with surface temperatures from 0 to 40 degrees C. And what worlds they will be! In brief, worlds like this would have 7x the surface area of Earth, but a similar surface gravity. Ocean depths more like 10,000 km than Earth’s 10 km. Utterly alien, but completely hospitable to life. Perhaps such a world could develop sentients sized as large as 100 metres. But they would not be building any radio beacons or starships.

Daniel Suggs February 22, 2012 at 18:31

Just going with the information available, can we come up with an approximate gravity for this planet? Joy mentioned “worlds like this would have 7x the surface area of Earth, but a similar surface gravity”, does that mean 1g?
These are indeed exciting times!

Doug M. February 22, 2012 at 18:36

“Utterly alien, but completely hospitable to life.”

There is the slight complicating factor that a 10,000 km deep ocean would consist, to a first approximation, of pure distilled water with no solutes.

Life is water with stuff in it. You need the water, but you also need the stuff.

Doug M.

Tony D February 22, 2012 at 19:30

Very, very interesting. You know, this post and the previous post re Alpha/Beta Centauri got me thinking: what are the chances of us ever discovering a world EXACTLY like ours? A world where a future Earth explorer could simply throw off their helmet and breath in and out without passing out. I think we’ll be extremely lucky to find worlds with hydrogen or even “Pandora” nitrogen atmospheres. Not Earth’s twin by any means, but still exciting stuff!

bigdan201 February 22, 2012 at 19:40

This is a boon to exoplanet research, even if its a pressure cooker.

@Joy: indeed, I feel optimistic about waterworlds. The large surface area and widely available water could be home to floating colonies. Pure waterworlds may be more challenging for inhabitation than those with scattered archipelagos, but they would be appealing targets for colonization.

Considering the vast diversity of life in earths oceans, I can only imagine what sort of biosphere an alien waterworld could develop…

Brasidas February 22, 2012 at 22:11

Is it possible for a an advanced technological society to develop on a water world? For example, could the inhabitants learn to make and control fire? If not, could they ever master the technolgies needed for an advanced society (e.g., glass blowing, smelting and casting metals, using chemical propellants for rockets, etc.)? Could they master chemistry? Is it possible to contain and study chemical reactions underwater? I can’t imigine trying to do an underwater titration, let alone anything more advanced.

Is it practical for underwater creatures to develop tool use? There are examples of tool use by land-dwelling creatures here on Earth. Are there any examples for purely underwater species? Does the viscosity and drag of water make tool use impractical? Does the need for hydrodynamically efficient body types make the development of appendages that can manipulate tools unlikely?

Would an underwater environment create special burdens due to its 3-dimensional nature? Would it be impossibel to develop agriculture or aninal husbandry? On land, you can use fences and walls to keep out the creatures that will eat your crops and cattle, and to keep your flock or herd in. Since everything can enter and exit up as well as to the four points of the compass, does that render agriculture impratical to develop?

If so, does this mean that we can rule out water worlds as sources of technologically advanced ETI? If so, does this impact the Drake Equation by taking all water worlds out of it?

Joy February 23, 2012 at 2:19

“There is the slight complicating factor that a 10,000 km deep ocean would consist, to a first approximation, of pure distilled water with no solutes.
Life is water with stuff in it. You need the water, but you also need the stuff.
Doug M.”

Ah, but you have a couple of Earth masses of dry mass at the core. Therefore more hydrothermal activity than Earth. The transport of heat and minerals is going to be a challenge to model, but some sort of convection would take place. The ocean would not be distilled water.

@Daniel Suggs – Yes within the error bars, 1 g surface gravity for the hot waterworld. Uranus and Neptune don’t have proper surfaces, but they also have circa 1 g “surface” gravity, a common finding for low density planets. But note the gravity wells are deeper, so it costs a lot more delta V to get off planet.

@Brasidas – Yes. As long as our metric for intelligence is tool use, the cetaceans can not be considered intelligent. Yet people who have had the opportunity to spend a lot of time with them have a very high opinion for the cognitive skills of cetaceans. But they will never broadcast a SETI signal or build spaceships.

If the Kepler stats hold up, and the peak of the planetary mass distribution is in the Neptune – SuperEarth classes, waterworlds may be much more common than partially wetted dwarf worlds like Earth. Possibly a significant impact on the Drake equation.

andy February 23, 2012 at 3:30

Ocean depths more like 10,000 km than Earth’s 10 km

Not going to happen, the base of the ocean would freeze into high-pressure ice long before the ocean depth got to 10000 km. You’re probably going to end up with an ocean of depth ~100 km above an ice mantle.

I personally am sceptical about the potential of such worlds to have life. The problem is getting high enough concentrations of various chemicals, particularly when the only means of getting material from the rocky interior to the ocean is via solid-state convection through the ice mantle.

kzb February 23, 2012 at 8:24

I think octopi use tools? Also cephalopods have built-in visual display units which could be used for symbolic language. They can also predict which football (soccer) team is going to win (apparently) !

Martin J Sallberg February 23, 2012 at 8:55

There may be different “molecular goldilocks” att different temperature intervals. As for intelligence and technology, it is possible that technology simply develops in a different order. It is, for instance, possible to study Brownian motion underwater. Underwater intelligences would seem to be technologically stuck for a long while, but statistically improving their odds of successful innovation as their reasoning ability and knowledge objectivizes, until they discover the physics needed to build technology despite their environment. Brownian motion could be the basis for some form of non-locality technology, which could be used to build starships.

Tom W February 23, 2012 at 9:42

I’m just amazed we can work out whats in an atmosphere of a planet 40 Light years away. At this rate of progress its hard to to imagine a scenario in which we don’t come across something Earth-like in size, mass, atmosphere etc in the coming years!

ljk February 23, 2012 at 12:02

Europa’s global ocean of liquid H2O, which contains twice the amount of water that exists on Earth, may be up to sixty miles deep. However, due to the moon’s lesser mass (it is about as big as Luna), pressure at the bottom of that alien ocean is about the same as at the bottom of the Mariannas Trench; heavy, but not as bad if Europa were Earth-sized.

Ganymede and Callisto may also have their own subsurface oceans, along with Titan and who knows where else in our Sol system. I wonder if these kinds of worlds are far more common in the Milky Way galaxy and beyond than Earth-type planets?

Eniac February 23, 2012 at 18:04

@Brasidas

If so, does this mean that we can rule out water worlds as sources of technologically advanced ETI?

I think this idea is too often and too quickly ruled out, reflexively. Here some thoughts on various aspects:

Energy
For most of our technological history, we have relied almost entirely on human and animal power, as well as wind and water. All of these are available underwater, although wind and water would be one and the same. No reason here to not get to a level of about the human 1800′s, before the steam engine was invented.

Fire
This is more of problem. A major use of fire on Earth is for warmth, which is not really applicable underwater, where the temperature varies much less than on land. However, fire is also used for smelting metals, where it is more difficult to replace or do without.

Materials
Until recently, most materials were derived from plants or animals, which works under water just as well. An important exception is metals. It is plausible that waterworlds will not or much later progress beyond precious metals that can be picked up in pure form, as gold and silver can be on Earth.

Overall, I do not think there are serious barriers for progress until our stage around 200 years ago. Personally, I think that intelligent beings that have come this far will not be easily stopped. They will find ways to work under water that we can not even think of, and will not miss the things they really cannot do, if any.

Alex Tolley February 23, 2012 at 18:54

@ Joy
you have a couple of Earth masses of dry mass at the core. Therefore more hydrothermal activity than Earth. The transport of heat and minerals is going to be a challenge to model, but some sort of convection would take place. The ocean would not be distilled water.

If, as the authors suggest, this is an icy world that migrated in from a higher orbit, then the core might well be frozen. There would be no thermal vents. The temperature profile might be stable with ice covering a rocky core, an icy ocean wit a possibly warm surface. A much larger, warm, non ice covered version of Europa?

I could still see solutes in the water column. However even if they were sparse, life might well sequester them, so that the main source would be locked into living organisms.

A possibly fascinating ecology might be expected on such worlds.

philw1776 February 23, 2012 at 21:38

I know it’s the “in thing” here to bash the rare Earth hypothethis but based on the extrasolar data we have now * it looks as if nature favors Neptunes and Super Earths that would mostly be water worlds. Not partially watered planets with dry surfaces. I’m not optimistic about sub-ocean thermal vents providing enough “contaminants” to the massive distilled water for these places to thrive with life.

* caveat being that this may be bogus as we only observe the extrasolar planets we CAN observe with our tools. Earth analogs remain at the very fringe of detectability.

amphiox February 24, 2012 at 1:06

There are examples of tool use by land-dwelling creatures here on Earth. Are there any examples for purely underwater species?

Bottlenose dolphins have used sponges to protect their snouts when foraging for food among sharp corals. If you count air as a tool, dolphins and several other cetacean species have created fishing nets out of bubbles to corral schools of fish. Octopi have been observed using discarded coconut shells are protective shelters (which they carry with them, and hide under when needed). Sea otters will use a stone as an anvil for cracking shellfish open.

amphiox February 24, 2012 at 1:15

Overall, I do not think there are serious barriers for progress until our stage around 200 years ago. Personally, I think that intelligent beings that have come this far will not be easily stopped.

If there is even a little bit of land on the putative waterworld, then it is easy to envision an aquatic intelligent species developing fire-free, metal free technology based on domestic animal husbandry getting to a level where they could attempt colonization and exploration of the land, at least for short intervals, (say by domesticating an amphibious species for a beast of burden, and carrying a water supply, if they need to breathe in water, in organic material containers).

Or, in a wholly ocean world, with no land, an intelligent species reaching that same level would be fully capable of exploring the water-atmosphere interface, and technologically exploring the properties of the gases in their atmosphere. They could collect these gases in organic material bladders and bring them down underwater for study, for instance, or they could build floating platforms with organic materials to stage experiments on (and they would not necessarily have to get up on the platforms entirely themselves – one could envision them swimming by the platform, and reaching out of the water with whatever tool-manipulating appendage they have, to do work on the platform).

Once a species gets to that point, and assuming their planet’s atmosphere can support combustion, it seems fairly inevitable that they will discover fire. And with fire, again assuming their are actually metals on their planet, metallurgy would follow quickly behind (or if not, perhaps a technology based on ceramics?)

Fire and metallurgy might be “late” discoveries in such a species’ technological evolution, rather than “early” as it was with humans. But early or late, once they got them, a high technology level at or above that of current human civilization seems readily achievable.

Tim Whitworth February 24, 2012 at 8:55

Water conducts sound so well that such creatures may well have worldwide communications at once. Using sound to confuse creatures is likely a useful adaption for hunting / fighting, and applying intelligence is probably even more useful. Such a society might well have writing, mathematics, agriculture, calendar, a reasonable understanding of evolutionary biology with practical application, some chemistry, an entire science of signals based on what’s biologically rather than mechanically possible, and presumable many other things. If the oceans are too deep, they may be unable to get rock or mineral, and they may have no concept of the stars.

ljk February 24, 2012 at 11:45

Here is proof that cetaceans will evolve to develop interstellar technology:

http://ricksternbach.com/voyfound.jpg

kalish February 24, 2012 at 13:48

I agree with the idea it would be a pure water word without enough nutritive stuff into. On the other side melting materials seems really easy, on a deep ocean, there might be an alliage, for example water, that is solid at one pressure and liquid at a lower pressure (or higher why not). Thus you just have to take some of this alliage, and soar a little, then you can model it when it is not too much melt, and not too much solid, and then go back to higher pressure wher your friends live, and kill them with your brand new arrow. octopus are really smart creatures, and they have a lot of limbs to use tools, 8.

Eniac February 24, 2012 at 17:16

I’m not optimistic about sub-ocean thermal vents providing enough “contaminants” to the massive distilled water for these places to thrive with life.

I am not sure where this notion of distilled water comes from. Isn’t the water dirty to begin with? What process do you propose by which it is being “distilled”?

Rob Henry February 24, 2012 at 19:10

It seems to me, that in the worst case scenario, the mineral content of a 100km ocean over a 10,000 km ice crust would be limited by the infall of zodiacal dust. As luck would have it Fred Hoyle examined the exact same problem. In his case it was in regards to Venus. He calculated that the entire mass of particulate matter in the Cytherean atmosphere could be explained in terms of a bacteria with the same requirements of Earth life whose growth rate was limited by the minerals in this infall, and which were drifting down under gravity.

Remember that is just a baseline. Life at the bottom of that 100 km ocean sea that lives off detritus might somehow facilitate a mechanism for recycling rate limiting minerals.

Rob Henry February 25, 2012 at 0:08

Eniac, the solvation shell around aqueous charged ions tends to be denser than the surrounding water, and should produce a similar scale height effect for ions to that of atmospheric gasses of different molecular weights. Of cause the need for anion/cation balance and the nebulous nature of a solvation shell complicates things, such that I would also love to see a reference rather than attempt to work it out de novo, but 10,000km sounds sufficient to produce deionised water enriched in organic scum (which its geology would have great difficulty sequestering).

It would be my guess that most ions atop even a 100km ocean would be monovalent.

Eniac February 25, 2012 at 0:21

For example, the abundance ratio of sodium vs. oxygen in the solar system is about half of that in seawater. Which would mean that, in the worst case, our water planet should be half as salty as Earth’s seawater, which is not so bad at all.

So, unless someone can provide a reasonable explanation otherwise, I do not believe this water will be anything close to distilled. It will likely be laden with a lot more minerals than terrestrial fresh water, which has in fact been distilled.

I am not 100% sure, but I think freezing enriches minerals in the liquid phase. If that is the case, would we not expect 100km of water over a 10,000 km ice core to be really briny?

tesh February 25, 2012 at 6:10

What about vulcanism, ala Io, as a source for providing the “dirtying” of the surface ocean? Or, would the bottom ice layer be too thick. I’m thinking as the planet is likely due to experience quite large tidal forces, then vulcanism is likely to be enhanced somewhat.

Eniac February 25, 2012 at 12:31

@Tesh: The water is already dirty when it condenses during planetary formation. It does not need to be dirtied. What we need is for those who talk about distilled water to come up with a mechanism for its purification. Rob has given it a try, but as he says himself, his idea needs a more solid foundation before it could be accepted. Besides, we do not have any evidence the water is pure, so there is no urgent need for a theory that would make it so.

@Rob: I don’t know off-hand what kind of salinity gradient gravity would induce in perfectly still water, but a) the water is not likely to be perfectly still, and b) less minerals near the surface just means more further down.

Rob Henry February 25, 2012 at 17:42

Eniac, I should have mentioned earlier that I was just interested in the surface because that is where photosynthesis and absorption of high energy compounds produced in the atmosphere occurs. Also the chemical potential of every reaction alters with pressure, such that an organism with enzymes tuned to 2 km depth would operate less well on the surface. From what I have seen of our own biosphere, I suspect that the absolute limit of difference in depths that one creature can operate between without coding for multiple sets of enzymes is less than 10km. I thus also feel that even a 100km ocean would produce an even more extreme oceanic biosphere than Earth with much life at the surface, some on the bed, and a lifeless abyss in between.

Your point on the added complication of the bulk flow of water would be particularly appropriate if that world suffered from strong tides.

Nick February 26, 2012 at 1:38

Life requires far more minerals than just sodium. The following elements are considered essential to earth life (reference — http://www.mso.anu.edu.au/~charley%20/papers/ChopraLineweaver8assc%20proceedings.pdf):

bulk elements: H, O, C, N, P, K, S, Na, Mg, Ca, Cl.

trace elements believed to be essential to bacteria, animals and plants alike: B, F, Si, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Se, Mo, I, W. These metal elements in particular are very important as the catalytic cores of enzymes, and it is doubtful that most of the vast number of kinds of chemical reactions which seem to be necessary for even the simplest forms of life could occur without a sufficient concentration of most of these elements in the environment. Indeed, about half of all proteins contain one or more metals.

There are also enzymes that while not necessary to the origin of life are prerequisite to rich ecosystems. Among these is nitrogenase, which splits N2 allowing the creation of the single nitrogen needed for proteins and nucleic acids without the presence of single nitrogen compounds like ammonia or urea (also likely to be very scarce in water world oceans) in the environment. All nitrogenases require Fe and S, but the forms that also contain Mo, Co, and/or V are much more efficient.

So the question seems to be whether a sufficient concentration of minerals containing these elements would remain dissolved in the ocean over geological time, or whether some necessary elements would precipitate creating such low concentrations that life (or at least photosynthetic life near the surface) could not be supported.

Interestingly, in earth’s oceans, the ratio of elemental concentrations in life to its concentration in our oceans is highest in the following: C, N, P, K, Mn, Cu, Fe, Zn, Zr, Co, Cr. In particular adding shavings of the latter metals to our oceans causes vast algal blooms. Some parts of earth’s ocean surfaces are so bereft of these metals that such fertilization can increase local ecosystem productivity by orders of magnitude. Wouldn’t the concentrations of these metals be orders of magnitude lower still on most water worlds, making the enzymes necessary for life impossible?

Eniac February 26, 2012 at 12:51

The core of a big planet like that would have to be hot, leading to a temperature gradient like that on Earth. This would lead to convection in any liquid layers, which should serve to thoroughly mix up liquid water of any depth. For the same reason, there should be volcanism at any rock/water or rock/ice interface, with all the interesting consequences for possibilities of life. High pressure ice/water interfaces should be less interesting, but we really do not know that for sure, do we?

It is true that pressure has an impact on chemistry, but the dependence is relatively weak, and I do not think that it necessarily makes for poorer chemistry even at extreme pressures. Different, yes, but with no less possibility for biochemistry.

Rob Henry February 26, 2012 at 16:21

Nick, I was also thinking of those algal blooms, and the following is starting to worry me… Ever since I speculated that the ions atop a 100 km ocean might just be monovalent due to the probable density of the solvation shell of multivalent ions, I have been transfixed by my knowledge that our deep oceans are biological deserts, and the claim that pumping water from near the bed causes so much photosynthetic activity that it would be an efficient way to rapidly sequester carbon. Could it possibly be true that even a 10km ocean (where 10km = actual depth x gravity in g’s) is too deep, and that is the real reason why our continental shelves are so much more fertile?

If this really is true I’m wondering why I have never heard statements such as “if there is life on Europa it should be close to the ocean bed where there are higher concentrations of transition metal ions”. Perhaps the real problem is this – what is the absolute minimum number of elements that any community of Earth life could live (but not thrive) with? What if we give it a few million years to adjust to the poorer environment? Does anyone really have any idea?

And as to Joy’s ocean depth, I note that a substance with a dozen different solid states is hard to prevent from freezing over a wide pressure range, but pure water could go to a 1000km depth if we raise the ambient temperature from 300K (giving 100km) to 600K, and we could go much deeper if we don’t mind it going supercritical. With a high concentration of the right solutes, it would be my guess that we could go a few times deeper at a lesser temperature, especially if you didn’t mind your ocean being periodically punctuated by layers of ice. 10,000km is really stretching it though.

andy February 26, 2012 at 17:34

The issue is that once you have a waterworld you lose a lot of the interfaces that could act to provide locally-enhanced concentrations of various substances, and the resulting chemical potential gradients that let you do interesting things out of equilibrium. Sure there is almost certainly going to be convection in the ice mantle, but you are making the interface between the chemically-rich core and the ocean a lot slower and less energetic.

Of course it might be fun to speculate on a watery moon that gets polluted by a nearby volcanic satellite similar to Io. We could imagine their Rare Earth hypothesis containing a factor along the lines of “nearby Io-like moon to supply essential nutrients to the oceans”…

Eniac February 26, 2012 at 23:32

@Nick

Life requires far more minerals than just sodium.

Well, yes, obviously. I meant what I said about sodium to be equally applicable to all the other solutes, as well.

These metal elements in particular are very important as the catalytic cores of enzymes, and it is doubtful that most of the vast number of kinds of chemical reactions which seem to be necessary for even the simplest forms of life could occur without a sufficient concentration of most of these elements in the environment.

In my view, it is true that some metals are probably needed for their greater chemical versatility, but that it is much less clear that any specific metal is needed. For example, the role of iron as oxygen carrier in vertebrate blood is handily played by copper in crustaceans and spiders. And those instances where there aren’t alternative metals used in nature do not prove that there could not be. I believe the use of metals is driven at least as much by their availability as by their distinct chemical characteristics.

Interestingly, in earth’s oceans, the ratio of elemental concentrations in life to its concentration in our oceans is highest in the following: C, N, P, K, Mn, Cu, Fe, Zn, Zr, Co, Cr. In particular adding shavings of the latter metals to our oceans causes vast algal blooms.

This is interesting, and I think it shows that life is constantly bumping against limits of availability. I suspect that this is so not because the elements were sparse to begin with, but rather because they have exhaustively been fixated in biomass. I would not be surprised if a plot of oceanic abundance against crustal or solar system abundance would clearly show some strongly depleted elements which are relatively common in biomass.

Nick February 27, 2012 at 0:03

Rob Henry: “what is the absolute minimum number of elements that any community of Earth life could live (but not thrive) with?”

According to the paper I cited, those would be the 26 elements I listed above.

What I wonder is whether the “hot ice” layer at the bottom of a hot light Neptune ocean would be anything close to pure. Would it contain “hot ices” of other molecules, e.g. methane clathrates? Would it actually be separated from silicates and other kinds of rocks, or would water “hot ice” just tend to be intermingled with the solids of other minerals, as solids of various minerals tend to be on earth? If they are intermingled, then a very strenuous level of volcanic activity might be sufficient to create the kind of mixing needed to have metals dissolved in the oceans.

However, the much higher ocean volume / ocean bottom surface area ratio could still pose a daunting problem for photosynthetic (i.e. ocean surface) life even if the volcanic activity were as vigorous as on Earth. Although this raises another question: if the ocean is very pure, would pure water itself be sufficient to stop photons from reaching depths of hundreds of kilometers to trigger photosynthesis that far down?

Nick February 27, 2012 at 0:09

BTW Rob that’s a great question about Europa. One clue might be to see how pure its surface ice is (i.e. what are the concentrations of metals in it). Presumably we have done some spectroscopy along these lines — anybody know the answer?

Eniac February 27, 2012 at 9:06

@Andy:

Sure there is almost certainly going to be convection in the ice mantle, but you are making the interface between the chemically-rich core and the ocean a lot slower and less energetic.

I am not sure that is true. I think the amount of heat transferred between the core and the surface is determined solely by how much heat is generated in the core. The mantle will need to go to whatever trouble is necessary to let this heat go through, or else it will accumulate until the entire planet is liquid, or gaseous, or plasma. It is quite possible that a thick ice crust has even more interesting things going on it it than a rocky mantle, with all those different phases.

ljk February 27, 2012 at 15:00

Nick said on February 27, 2012 at 0:09:

“BTW Rob that’s a great question about Europa. One clue might be to see how pure its surface ice is (i.e. what are the concentrations of metals in it). Presumably we have done some spectroscopy along these lines — anybody know the answer?”

Those numerous brown lines do not look “pure”. Hopefully they are full of organic material or something even more complex, like dead aquatic type creatures.

ljk February 27, 2012 at 15:03

By the way, I said dead aquatic type creatures in the Europan ice not because I want them to be deceased, but that I assume anything which goes from the moon’s global ocean to its icy surface will not survive the transition.

Then again, it would be VERY interesting if some native Europans DO thrive on the moon’s surface. We probably do not need to drill down to find life from that alien ocean, though eventually we will want to explore that amazing environment directly, too.

Europa should definitely be our next major target of exploration. I am only putting Titan after Europa because it is so much farther away. Of course if society had its priorities straight, we would not need to make such decisions and sacrifices to human curiousity and knowledge.

Abelard Lindsey February 28, 2012 at 2:16

Speaking of Earth-like planets, here’s something else to keep in mind:

http://www.astrobio.net/pressrelease/4590/earth-siblings-can-be-different

The ratios of C/O and Mg/Si appear to be the most significant (and the most variable among stars). The waterworlds and hot Neptunes (which appear to outnumber Earth-sized planets) are only the beginning of the various kinds of planets we will find.

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