This is Geology Bites with Oliver Strimpel.
Why is there water on Earth? We asked this question because
one might naively expect water to be too volatile to be
retained by the hot early Earth. Rather, water should have been
confined to the outer solar system, beyond the so-called
snow line, where the distance from the sun is such that water
can freeze. In the solar system.
That line runs between the orbits of Mars and Jupiter.
Lindy Elkins Tanton's research has focused on how to rest your
planet's form and evolve. She has a particular interest in
where the Earth's water came from and is a major proponent of
one of the competing theories that attempt to answer this
question. She's a professor in the School
of Earth and Space Exploration at Arizona State University.
Lindy Elkins Tansen Welcome to Geology Bytes.
Thank you so much, Oliver. It's a pleasure to be here.
Thanks for inviting me. Did I summarize the problem of
the origin of the Earth's water correctly?
Yeah, this is just how people think about it with the snow
line. The old view of water in the
planets, in the rocky planets and water in the Earth is that
the planets formed from a planetary nebula that consisted
of gas and dust orbiting around our young sun while the sun was
forming. And high temperatures and the
solar wind from the active young sun evaporated the volatiles
from the inner parts of that planetary disk and blew them
outwards past this so-called snow line.
And so the inner terrestrial planets like the Earth formed
dry with just rock and metal. But there's a new view.
First of all, we've learned that that planetary nebula only
lasted a few million years, and then it dispersed.
We can see it in exoplanets, and we can measure it in certain
ways in our own solar. System.
So the planets had to have formed very rapidly because
after that there's no more material available to form a
planet. So even though volatiles were
relatively depleted from the inner solar system by those very
processes of solar, wind and high.
Temperature minerals still. Formed with water trapped within
them in crystal voids or as part of the crystal lattices
themselves, and fast forming planetary cores of metal and
rock in the outer solar system. Grew large enough to
gravitationally gather their volatile envelopes before the
planetary nebula dispersed, where the littler planets and
the inner solar system never got big enough to gather that water.
So they look very different. But it turns out the materials
that made those cores are not very different, because a lot of
material migrated regularly both inward and outward while the
planets were forming. And so that idea that the inner
ones were just dry and the outer ones were wet is really almost
the product of what the solar system looks like today and not
the actual process by which things form. 2/3 of the Earth's
surface is covered by oceans. It certainly looks as if there's
a lot of water on Earth, but actually, what proportion of the
Earth is water, and is it mainly in the oceans?
We are such surface creatures. We really feel that all the
water is here and we're standing in it.
But the interior of the Earth still contains quite a lot of
water. And again, it's mostly water
that's either just an oxygen and a hydrogen and a hydroxyl ion
instead of a full water molecule, and in some cases a
full water molecule that are trapped in crystal voids or part
of crystal lattices. So they almost seem like they're
part of the solid Earth, but inside the Earth.
The estimate range is from 2 to 10 times as much water as we
have fear on the surface. So there's actually more water
inside the Earth than there is outside.
Okay, can you summarize the theories that have been put
forward to explain the origin of Earth's water?
There's a. Range.
Of course, there's always a range.
It's not just. Like one thing or exactly?
Another thing, the way these theories go, but the simplest
way to describe it is. One end of the idea.
Is that Earth's water came from cometary?
Impacts that the earth formed. Really dry, and then it was
later wet impactors like comets that brought water to the Earth.
And then the other end member concept is that although comets
have definitely hit the Earth in the past, water was delivered to
the Earth in the very rocky building blocks of the Earth
itself. Found inside.
Those rocky material crystals. And so the earth actually formed
with water from its original rocky material.
So what observations can we make to help distinguish among the
different theories? One thing is detailed analysis
of meteorites and asteroids and comets by audacious space
missions that have measured particles of comets and also
observations from the Earth. We now know a lot more about
what comets are made of, even on the isotopic level.
So what we can do is we can compare the isotopes of certain
elements as they're found on the Earth versus the ones that are
found in comets or in meteorites that are parts of asteroids or
in asteroids themselves. And when you look at deuterium
to hydrogen, those two hydrogen isotopes, or nitrogen 14 to
nitrogen 15, those nitrogen isotopes and also noble gas
compositions, it's been shown that cometary water contributed
1% or less of the Earth's water. Some people say perhaps up to
15%, but really a very low amount.
So water really can't have come from comets.
It had to have come from the water that's bound up in rocky
asteroidal material like meteorites.
When you talked about the isotopic measurements, you
talked about nitrogen. Does that actually bear directly
on where the hydrogen and oxygen in the water came from as well?
Yeah. We think of comets mainly as ice
balls, balls of ice, and we think of them as water ice, but
they're about half ice. And that ice is not just water
ice, but it's also things like ammonia ice that contains a lot
of nitrogen. And the other half of comet
materials, broadly speaking, are organic materials.
And so things that are made of carbon and nitrogen and hydrogen
and then other elements as well. And so when you add the water
from comets, you actually are forced to also add those other
ices and the organic material as well.
And so you can compare those, the whole volatile inventory of
comets to what we find on our Earth.
So if we can say that the other volatiles have the isotopic
signature of comets, then we can infer that the water must have
come along for the ride. Yeah, that's right.
Or conversely, if none of the other measurements fit, then
cometary water is not the main source.
Perhaps you've already indicated your preference, but which
theory do you favour and why? I came at this not so much to
solve the problem of Earth's water, but instead looking at
the processes by which rocky planets form from giant impacts
of material in early solar systems.
And those giant impacts, as they creep into the planet, deliver
enough kinetic energy that they melt the target body as it's
growing. And the target body goes through
this phase called a magma ocean. And so my interest was how can
we use our quite extensive knowledge of how magma
solidifies and how volatiles interact with magma to predict
and understand how rocky planets form from that starting point of
a magma ocean? And it had been thought the
magma oceans were necessarily fully dry, that the water was
driven off by heat. And again, it's because I think
that some of our scientific intuition comes from our simple
experiences at one atmosphere, pressure and Earth gravity and
the kind of conditions we normally live in where if you,
for example, put your clothes in a hot dryer, it drives off the
water. And that's true, but also the
thing that we forget is that water is actually incredibly
sticky and you never drive it all off.
And so is the amount of water that is retained enough to do
what we need to make a rocky planet.
And so my work showed that if you build a rocky planet with
rocky material that contains just 100 parts per million of
water bound up in crystals, just the way we find it in meteorites
and in asteroids, many of them are in fact wetter than 100
parts per million. But if you process that through
a magma ocean, a lot of water is released into an early steam
atmosphere that's free to later cool and collapse into an ocean,
but then a lot is actually retained inside in the magma
because the magma won't give up all the water.
And. So it turns out that just
starting with just 100 parts per million of water produces.
A nice wet. Planet one with water inside,
similar to what the Earth has today and with a deep water
ocean. In fact, the problem becomes
getting rid of the surface water, not making more.
And So what that work indicated that really confirmed my
opinions about this whole field is that.
Any rocky planet. Born anywhere in our universe at
a material that has just 100 parts per million of water will
be born wet. And therefore if it's in a
habitable zone, it will be born habitable.
And so when you think about it that way, requiring comets to
strike the surface later, which is a stochastic process that you
can't count on, sounds a little bit like special pleading when
you know that the planets going to be wet from the beginning.
The problem is not delivering water to a planet, the problem
is getting rid of it. These planets are born with so
much water. Did the water in the oceans then
have to be extracted from the magma ocean, or do we think it
just came out of the steam atmosphere?
That steam atmosphere is a bit vulnerable.
A hot steam atmosphere is expansive.
It rises up from the planet's surface farther than a cold
atmosphere like what we have today.
And the early young sun was very active and so a lot of that
steam could have been lost to solar wind, but not all of it
before it. Cooled, in fact.
This is one of the ideas of the difference between Venus and the
Earth. Just that distance from the sun
could have meant that Venus's atmosphere was stripped, but the
the Earth retained more of its atmosphere to allow it to
collapse into an ocean. And so I think that the physics
and chemistry would indicate that some steam atmospheres may
be stripped, and then the planet needs to start over with
volcanism, releasing water from the inside, and then others will
collapse and make something of an ocean before they're
stripped. Is that the main mechanism then
for getting the water from the inside to the outside then via
volcanism? That's right.
That's part of why people care so much about plate tectonics.
When we think about habitability, that the way that
we understand that carbon is controlled on our planet and
water is refreshed into the atmosphere.
If the problem then becomes how to get rid of the excess water,
so to speak, what do we think happened to make it disappear?
I think that the primary mechanism would be stripping of
that inflated hot steam atmosphere by the young sun.
But another way to remove water from the surface is through
ongoing impacts. For example, on the Earth, our
last really giant impact had to have been the one that formed
our moon. But impacts?
Didn't just end then. And in fact, the moon forming
impact or impacts of that scale would create a trail of debris
in the Earth's orbit, which the Earth and the Moon would
necessarily sweep up again over time.
And so those impacts, plus other objects flung in from the outer
solar system or out from the inner solar system during those
early chaotic times before the planets completely cleared their
orbits, all of those impacts would have removed some amount
of atmosphere and water from the surface.
Although it's been shown also that that removal is not as
efficient as our surface bound brains might think.
They remove some, but not everything.
As you already mentioned, water plays a critical role in plate
tectonics as the presence of even minute quantities of water
in a rock has a very big effect on its strength and melting
temperature. This in turn effects then what
happens to plate boundaries, especially at subduction zones
where water is released by dehydration reactions in the
down growing plate. And as you should have
mentioned, that's the source of the volcanism that we now see
above, subduction zones in volcanic arcs.
Can we envisage how the Earth might have evolved if it had
much less water? And you mentioned Venus, Is that
a case in point? That's certainly one of the
predominant views in the scientific community, that Venus
is just dry on the inside. Also, as you point out, even a
very small amount of water, again in those crystals within
the crystals not wandering around as a free fluid, reduces
the viscosity tremendously. And that the thing to think
about here is that the interior of the Earth is moving
centimeters to meters per year, so very, very slowly, and it
moves through the deformation of solid crystals.
And those solid crystals are the same as you might pick up in a
piece of rock on the surface, but under heat and pressure they
can flow. And you can imagine this makes
some kind of intuitive sense. That if.
In their little crystal voids are bits of water, or in their
crystal lattices are water. Those are generally weaker
crystals than ones without, and so they flow more easily.
So just little bits of water inside the solid crystals.
The mineral crystals actually allows the inside of the Earth
to flow more easily, particularly right up underneath
the tectonic plates, and that's where there's an area of
particularly low viscosity that allows that.
Plate tectonic movement, which we don't.
Think is. Happening on Venus, although
that's debated. It could be.
Still happening on Venus. But the idea is that the
interior of Venus in fact is drier and therefore much stiffer
and much less willing to flow. But an interesting question is.
If there was plate tectonics on Venus, how would we really know?
Because the way that we absolutely confirmed it on the
Earth was with seismic networks that were able to locate
earthquakes that conform to the shape of a down going slab under
these volcanic arcs that you refer to.
We have no seismic network on Venus.
We could hope, maybe someday. So just to get back to why Venus
is dry and the Earth is not, is it you think fundamentally
because it's slightly closer to the Sun and therefore it
couldn't retain water in an atmosphere?
That is a good solid hypothesis for why that is.
And I think that's probably where I would go first to look
for an answer. But there's still so much that's
not known about Venus, so much more that we need to learn.
And so I wouldn't venture to say that I'd place money on that
bet, but it's absolutely plausible.
I said in the introduction that the snow line runs between Mars
and Jupiter. So is Mars wet?
Mars is a bit wet at the present time and has a bunch of water
ice on its surface that migrates back and forth.
But there's evidence that it was much wetter in the past and so
there's the thought that. Then in the.
Distant past Mars was as wet and habitable as the Earth, and that
it lost its water over time and became much much drier, which is
where it is is today. The thought for that water loss
is mainly just that Mars is much smaller than the Earth and water
is not as well gravitationally bound to it, and especially if
Mars lost. Its magnetic.
Field early on, which it has. None now.
It's not as protected on Mars as it is on the Earth, and so that
water will be lost over time to the solar wind.
That's one set of ideas. That hangs together well and
might. Explain what we see on Mars
today. As we search for exoplanets,
there's a lot of discussion about the habitable zone,
defined as the band within which liquid water can exist, because
we believe water is essential for life.
But could it be that a typical planet in the habitable zone
might just be dry, even if any water would have been in liquid
form if it was there? Would you think the sort of
mechanisms that you describe for Earth would prevail in any
plausible exoplanetary system? Well, I hate to generalize to
the universe because we know so little about it.
I can only say that if a planet like the Earth was formed around
another star from materials similar to what we have in our
solar system, then it would also be wet.
But the concept of a habitable zone is a very, very simplistic
concept because everything about the habitability of a planet
depends upon its chemistry. In fact, more even than its
distance from its star or how large it is.
In a solar system where for some reason there's a higher
percentage of radiogenic elements than interior, heating
up the planet can keep its surface wet longer and you don't
need to rely on that star distance.
The composition of its atmosphere will entirely
determine the current surface temperatures of that planet.
And if you look at the difference between Venus's
atmosphere and the Earth's atmosphere and how different our
surface conditions are, knowing we could not discriminate
between Venus and the Earth, when looking through telescopes,
you see the composition is everything.
And I would just add that even in planets that are not in the
classical habitable zone, there could be the liquid water just
under the surface of those planets, Either brines or waters
that are heated from radiogenic heating inside.
Or the moons of planets that are not at all in the habitable
zone, but tidal heating from their main planet keeps them
warm enough to habitable. Have we in fact seen evidence of
water on an exoplanet? Yes, traces of.
Water and exoplanetary. Atmospheres have been found.
So we know. There's water out there, There's
water out there and then a. Whole.
Chain of questions need to be asked about what that water
might mean and does it indicate any kind of habitable situation
that we could understand with our comprehension of life.
Unfortunately, it seems that the most likely kind of life
elsewhere in the universe is going to be microbial life
living under the surface of a planet, which is unfortunately
going to be virtually undetectable using today's
technologies. If you were to be in charge of
formulating missions to try and figure out if and where there
might be life, how would you go about it?
Well, my favorite thing to do would be to go to Mars and drill
into the groundwater system and measure what's there.
That's where I would go first. But that's already technology
that we don't have. What about under the surface of
Europa or Enceladus? Or under those icy exteriors.
Sure. Famously great places that
people want to go look for life? Absolutely.
What are you working on at the moment?
I am absolutely focused on the Psyche mission, which is a
mission to a metal asteroid, not a site for searching for life
and no liquid water. I've been working on that since
2011. And we won our NASA competition
in 2017 and we launched in 2023, and we'll get to the asteroid in
2029. And so all of my time is spent
leading this very large team of people on this project, making
sure that our spacecraft stays happy and healthy on its its way
to the asteroid so that we can get our science.
Lindy Elkinstanton, thank you very much.
Thank you very much indeed for this time, Oliver.
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