Applying what we’ve learned from exoplanets to the Earth’s formation
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Known to whom? Can you produce a survey to back this up that isn't biased by questioning tourists from only one planet?
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Sadly, the only other opinion I can find says simply "mostly harmless".
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Don’t we have evidence that Mars started with a lot of water and dried over something more like a billion years (long after this water-forming dynamic would have been at play)? My understanding has been that Mars’ drying was an inevitable result of its low gravity and/or lack of magnetic field (with the exact contribution of each being somewhat in debate). That Mars would be dry by now almost regardless how much water it started out with.
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Can you produce a survey showing that there’s anyone else to survey?
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There is some argument over the size and length of Mars' "Noachian" era. Mars likely did start with water but how quickly that water reacted with the rocks to form the iron-rich crust, was sequestered as brines, and was broken and swept away on the Solar wind is a matter of debate, as is the contribution of Mars' equivalent to Milankovitch Cycles.
What we really need is a lot of good samples from Mars to help us sort out the details. And that is part of the rationale behind the sample-return mission proposals.
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No, actually.
We have evidence that Mars HAD water, but the volume of that water has always been in doubt. Recent surveys by the rovers suggest the water it had was exceptionally limited, and what ancient bodies of water it did have were quite small and shallow compared to Earth.
A Mars covered in water with no difference in elevation of the land would have seen the depth of that water probably measured at a few meters at most. If the land on Earth were all of the same elevation, there would be about 2700 meters of water covering the land on Earth.
So yes, Mars had water. It just never had very much of it for very long compared to Earth.
As for Mars' drying out, that's mostly attributed to a very weak magnetic field, which does little to mitigate the solar winds that carry away the water vapor at a far greater rate there than it does here on Earth.
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Earth's water content is very marginal compared to many rocky moons and to the postulated compositions of many exoplanets. Despite appearances, with its vast oceans, Earth seems to teeter on the brink of dessication with its meager 0.2% water by mass compared to worlds with 30% and more. If the Earth had even slightly greater water volume however, say 0.5% or 1% there would be only a tiny fraction of today's land mass. And worse, if water by mass were only a few percent, pressures at the sea floor would create exotic ices that would block the water rock interface creating a subsequent paucity of chemical reactants and nutrients in the water column. A major problem of habitability of "water worlds".
So, while Mars is described as an example of a dry world with negative implications for life, Earth is really close to also being "dry". -0.2% and there would be no oceans. That's a very delicate balance.
So, while Mars is described as an example of a dry world with negative implications for life, Earth is really close to also being "dry". -0.2% and there would be no oceans. That's a very delicate balance.
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This really puts tight constraints on the Drake equation "mean number of planets that could support life per star with planets" variable.
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I'm finding it hard to imagine a planet that can lose its water, while retaining enough hydrogen to make more. The difference in volatilities is enormous.
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Of all of the proposed sources of Earth's water (comets, primordial, etc.), this hypothesis makes the most intuitive sense of any I have seen to date.
A very interesting and sensible proposal for the source of Earth's water.
Thanks for the write-up, Dr. John.
A very interesting and sensible proposal for the source of Earth's water.
Thanks for the write-up, Dr. John.
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Yeah, bad things like that might happen if the oceans were around 100 km deep. Jupiter's moons may escape that fate due to their lower gravity and therefore lower pressure at depth.
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Wouldn’t vulcanism and plate tectonics have a tendency to create holes through such ices, with lava and gasses then getting access directly to water? Then anything that settled out would then settle on top of the ice, able to continue interacting with the water column?
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There was a previous posting on Ars that talked about research showing that the amount of water absorbed into Earth's mantle is about three times more than what's in the oceans. Much of that is related to movement via plate tectonics, some of it is mineralogy. That's not to say, though, that the amount of water in the mantle approaches anything close to saturation. If that's so, there may be a fairly wide latitude of available water content that would allow at least a respectable amount of surface water to accumulate.
I would be interested to see how the water content of Mars and Venus compares in their internal minerals.
I would be interested to see how the water content of Mars and Venus compares in their internal minerals.
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Let’s not forget that the solar wind itself actively creates water, as it’s a source of energetic hydrogen and oxygen is the third-most most abundant element in the universe. Hell, the earth itself is 46% oxygen (30.1% by mass).
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This seems promising from a habitability perspective. Instead of the amount of surface water either being more or less chance or maybe a product of plate tectonics sucking down a bunch of it to be chemically bound in the mantle rock, it's tied to the planet's size and original hydrogen atmosphere. That might suggest that similar sized and mass planets might have similar hydrogen (and thus water) allotments - at least at the start (Mars and Venus have obviously lost much of whatever water they once had for other reasons).
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Mars is also quite a bit closer to water's snow line, and so could have received significant contributions from icier building blocks.
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Venus would be another candidate for this process, but it has hardly any water. It's naturally hotter, though, and its hideous CO2 greenhouse effect makes liquid water impossible on its surface. It also doesn't have a magnetosphere, so the solar wind slams directly into its atmosphere and breaks up its molecules. The free hydrogen can then be lost to space. It also doesn't appear to have the subduction of tectonic plates that the Earth does, which may be why it has far more CO2 than Earth. The subduction exposes new rock to soak up CO2. That, and it didn't have a Carboniferous Era to turn CO2 into coal! The Earth is looking more special all the time.
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But the Carboniferous comment is kind of moot as far as Earth’s specialness, isn’t it? That happened looooong after complex life evolved.
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So how much deeper would the Marianas trench need to be to create ice at the bottom due to pressure? You make it sound like only something like 10% more gravity would do the trick. Or is it even lower? Is 1G the upper bound for 1% water worlds? And this also means high gravity planets with 2G surface gravity would be really really wet, but also icy on the bottoms of their oceans. Unless of course the higher gravity leads to higher heat generation in the core.
It would be interesting to see what the error bars are on this simulation, and also how the Nitrogen and other reactions influence things. It's certainly a fascinating piece of information, and not that intuitive in alot of ways.
The timelines are just enormous too... 10 million, 50 million, 200 million? How long this all took, and how insensitive this formation is to initial conditions is also funky to think about.
This paper raises so many fascinating questions for anyone interested in world building.
Just think, Tatooine has only .5% less water than earth! Unless I'm doing my math wrong. LOL!
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I dunno, I agree that the volumetric amount of water on early Mars is poorly constrained but the rocks play a role. We wrote a paper on this in some crappy journal...
The divergent fates of primitive hydrospheric water on Earth and Mars - Nature
Modelling the reactions of water with the crusts of early Earth and Mars sheds light on how water was transported through their crusts to give the surfaces we see today.
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It also formed quicker so according to this model should be more water rich.....
actually, this model may l struggle to arrive at the weakly/moderately siderophile element content of the Earth's mantle (V, Cr, Nb and W in particular), although I may be misreading the assumptions made of regarding metal/silicate equilibration conditions
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danielravennest
Ars Tribunus Angusticlavius
Conversely, the early solar system had a lot of smaller bodies. The ones that formed farther out would have a lot of water, which could be delivered to Earth by impacts. In addition, the young Sun was only about 70% of its current brightness. That would move the "frost line", where ice can survive on small bodies, closer to Earth's current orbit.
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And it should be noted that the frost line isn't a hard and fast boundary. Given what we've learned frmo other systems, there was probably a lot of material moving Sol-ward and outward during the early part of the Solar System's formation.
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Tatooine has plenty of water. It's all just segregated deep in the sand by the sand trout.
Oh, sorry, wrong sci-fi franchise.
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Not to mention temperature.
If the question is what pressure to completely lock out liquid water from interacting with the crust, then we have to assume a pressure high enough that point temperatures of things like thermal vents, underwater volcanoes and ridge lines won't locally melt through the ice layer. Eyeballing a water phase chart, in order for pure water to stay solid at say 700 K, pressure would have to be in the double-digit GPa, somewhere. Pressure at the bottom of the Marianas is something like 0.1 GPa.
Maybe I'm missing something, but seems like we're talking thousands of kilometers depth for a water column to produce pressures in the double digit GPa range.
The 100 km referenced by someone above gets you to 1 GPa, and that will freeze pure water into exotic ices, but only up into the 300 K kind of range. That could potentially cut off the liquid water from the crust on a planet lacking volcanic/plate-tectonic activity, though.
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Also... The USA seems to be approximately the same size of the entire Asian continent in the photo. Does NASA have a special PR office that caters to the MAGA crowd?
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Since the paper that suggested Earth water primarily comes from accretion hydrogen it has been an interesting idea which this model paper amplifies. https://astronomy.com/magazine/2019/04/where-did-earths-water-come-from :
But we are talking about a planet which has the highest water vapor pressure at the current time and which leaks water mostly through the polar regions where the solar wind is concentrated.
The respective water histories and if a strong magnetic field protects against atmosphere loss or accentuates polar loss to an equal loss rate are still open questions, and the individual planetary loss processes makes it hard to do comparisons.
Between Venus, Earth and Mars, it is Earth that currently loses water at the largest rate.
But we are talking about a planet which has the highest water vapor pressure at the current time and which leaks water mostly through the polar regions where the solar wind is concentrated.
The respective water histories and if a strong magnetic field protects against atmosphere loss or accentuates polar loss to an equal loss rate are still open questions, and the individual planetary loss processes makes it hard to do comparisons.
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Like the "Venus lacks plate tectonics" the "water worlds have a habitability problem" with the corollary that Earth is a rare habitable, these things are questioned by research. Water worlds bottom ices may be transporting minerals to the ocean and Venus may have hot spot turnover (explaining the volcanoes et cetera), icy moons may have oceans with or without bottom ices, et cetera. There are worlds that have higher potential habitability than Earth (with slightly larger mass) and worlds that have less, but evolution can be common.
The relevant question for rarity is inherent in evolution itself, since we can expect animal analogues and especially language using such to be rare or unique. Earth is unique, even if Earth habitability analogs may be fairly common (and we don't know that yet).
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I would tend to argue that once you get life on a planet, the potential to evolve broad tool-using may be even more difficult than evolving language. Lots of animals communicate through sound, so it’s not hard to imagine a lineage developing more and more capacity in that regard.
The problem with evolving broad tool-using, IMHO, isn’t just that it seems to be rare to develop a high-enough level of intelligence, but also how easily tool use can be limited by body plan.
Birds like corvids can use tools in pretty sophisticated ways — maybe more sophisticated than any other non-human tool user. But evolving even more sophisticated tool use would be hampered by their body plan, in which the primary extremities available for tool using are the beak and feet.
Dolphins are damn smart and have been seen using some objects as tools, but they have even more profound body plan limitations vis-a-vis tool use.
Meanwhile, primates had these great, dexterous hands already available for tool use, long before they got smart enough to use even primitive tools.
As even more of an aside, the fact that primates had these great hands available long before pre-humans starting making a wide variety of tools, and that many primates had already de-emphasized using their hands for walking (not being full bipeds, but locomoting in ways that they can often keep at least one hand free), makes me wonder about therapod arms. Were any of them doing anything particularly clever with those non-locomoting arms, like maybe primitive tool use? And even if that’s far-fetched, might that have been where tool use eventually evolved, had a big asteroid not come along?
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