Hello, and welcome to this introduction
to NST (Natural Sciences Tripos),
Earth Sciences. My name is Oliver
Shorttle, and I work between the
Institute of Astronomy
and Department of Earth Sciences at the
University of Cambridge.
And I'm excited to talk to you today
about a recent project we've completed
on 'meteoritic messengers of Earth's
ancient atmosphere',
and what I really like about this
research is how it connects between the
geological record of Earth
and basic questions we might ask about
planets, whether in the solar system, or
around other stars as exoplanets -
and that is about the origin and
evolution of their atmospheres.
So, just to recap, Earth's atmosphere at
the present day is
dominantly nitrogen - the next major
component being oxygen with minor
components of
argon and carbon dioxide. And it's one bar
(the surface pressure of Earth's
atmosphere is one bar) and it's that
detail in particular that we're going to look
at in the remainder of this talk - and
ask the question has Earth's atmosphere
always been one bar? Or, in the past, was
it more massive, or less massive, than it
is today?
Earth's atmospheric composition has also
changed over time, and that's a topic for
another
talk - But today we're going to
look at the amount of it, and how that's
changed.
And this is particularly relevant for
thinking about habitability of planets,
and in particular exoplanets. So what
we're looking at here is a recent
compilation
of roughly Earth-sized exoplanets
that have been discovered,
ordered by distance from Earth,
and, although the the artistic cartoons
here are very compelling,
in fact the details of most of them
are completely unknown.
In particular, whether or not they have
an atmosphere. Yet the presence of an
atmosphere is really critical to their
habitability, and whether they can
sustain liquid water at their surfaces.
And so we really want to understand how
ubiquitous planetary atmospheres are
and, in particular, whether or not a
planet can create an atmosphere -
maybe by volcanic degassing over the
course of its history, or whether it has
to be born with one in order to have an
atmosphere.
And that's what we're going to
investigate by looking at Earth's
history of atmospheric mass.
So, this really becomes a problem of
trying to understand the
past pressure of Earth's atmosphere: what
was the pressure of Earth's surface in the
distant past?
And unfortunately, we have to find very
indirect routes to recover that
information - there have been some
ingenious ones that have been come up
with.
So what we're looking at here is a
picture on the right of a bit of lava
and it's got all these holes in it, and
these holes are 'vesicles'. So these are
formed by pockets of gas in the magma
trying to escape and expanding as the
magma
moves to lower pressure. And it turns out
the size of those vesicles is related to
the pressure of the atmosphere that
the magmas erupted into. So people have
tried to use the sizes of vesicles to
reconstruct the past pressure at Earth's
surface.
Another intriguing method has been to
use fossilized raindrops
that fell on muddy sediments - and to use
the size of those raindrops to estimate
atmospheric pressure, which also
affects the velocity at which rain
falls to the ground.
The method we're going to use today
actually uses micro-meteorites -
so these small fragments of dust
from space enter
Earth's atmosphere at high velocity,
heat up in the upper atmosphere, react
with the atmosphere,
fall to the ground and get incorporated
into the sedimentary record, 
so that we may then - many millions or
billions of years later find them
and ask questions about the past
composition of the atmosphere.
And what we're looking at in this image
is one of these very small
micrometer-sized micro-meteorites.
And in the centre, the light colour, is
the iron - the original composition of
this meteorite which was
iron nickel metal. And what happens is, as it
enters the atmosphere, it heats up and
it reacts with the atmosphere and gets
oxidized, so the iron metal
becomes potentially more oxidized iron.
And the region of the atmosphere we're
sampling, during micro-meteorite entry,
is the mesosphere - so it's this level
above the stratosphere, starting about 60
kilometres height.
So the micro-meteorites enter this this
region of the atmosphere,
and if there's sufficient oxygen around,
may become oxidized so the iron metal
FeO goes to Fe2+, as it reacts
with the oxygen, and that produces this
mineral called Wüstite 
which is this dark grey rim around the
micro-meteorite we're looking at here.
If that goes one step further it can
oxidize yet again and the iron becomes
Fe3+, producing minerals
like magnetite which is what we're
seeing in this micro-meteorite.
Now, the modern Earth surface has about
20%
oxygen. But in the past there's
lots of lines of evidence that show the
oxygen abundance of the surface of the
Earth was much much lower -
a million times or maybe 10 million
times lower than the present
abundance of oxygen on the Earth. So,
superficially, that's going to suggest to
us that if we put a micro-meteorite into
the Earth's ancient atmosphere it isn't
going to get oxidized,
because there isn't the oxygen around.
But what we've shown in this project
is that, although the surface
concentration of oxygen may be very low,
as you get to high altitudes in the
atmosphere, you can actually have
relatively high oxygen
abundances through the breakdown of
water.
And what that means is that if the
atmospheric pressure on the early Earth
was low,
then in the region where micro-meteorites
enter and heat up, they may still be
oxidized.
So if we can go to the geological record,
find a very old rock,
and see evidence that these little
micrometeorites have been oxidized
it's a good good evidence for the early
atmosphere of Earth having been
low pressure. And indeed that is what we
see,
so these two meteorites that we've been
looking at, both of which show evidence
for having been oxidized -
the metal being oxidized by the oxygen
in the atmosphere producing wüstite
or magnetite. These two minerals
are both meteorites,
found by Tompkins at al in the Pilbara
region of Australia,
which are rocks that are 2.7 billion
years old.
So, both of these micro-meteorites show
evidence of iron have been oxidized to
either FeO
or further oxidized still to FeO1.5.
And this is really only understandable
by the atmosphere having been low
pressure -
because at this time, as we've already
said, there's abundant evidence the
atmosphere having been very
low in oxygen at low levels.
So what this research has shown is that
Earth has actually grown its atmosphere
over time and, in fact, the pressure has
increased from maybe
a third of a bar 2.7 billion years ago
up to the one bar atmosphere we have
today.
So, coming back to our exoplanets, this is
really good evidence
that planets can indeed grow their
atmospheres over time
and that these artistic impressions of
what these exoplanets might look like
are perhaps maybe not so far from the
truth - and hopefully, in the next decade
or so, we'll be able to discover in
in detail what these planets might look
like. Thank you.
