Sky & Telescope - December 2020 - 38

The Chemistry of Life

T LIFE AFLOAT A phytoplankton bloom (green) swirls in the dark waters
around the Swedish island Gotland, in the Baltic Sea. Plankton is one of
countless lifeforms that incorporate carbon and depend on water.

38

DECEMBER 2 02 0 * SK Y & TELESCOPE

Losing Carbon
However, this method also leaves us with a problem: We have
way too much carbon. Based on our chemical accounting,
carbon should survive in the inner regions of the protoplanetary disk fairly well, and the relative amounts of elemental
carbon to silicon in our planet's rocks should be a factor
of 4. Instead, Earth's ratio of carbon to silicon is less than
0.001 - that is a factor of 4,000 off. We should be sitting
on a carbon-dominated rock, not a silicate Earth with a tiny
amount of carbon. What went wrong?
We think that at 1 a.u. in the disk, temperatures weren't
hot enough to destroy carbon pebbles prior to their assembly
into larger bodies. So all the carbon found in refractory or
rocky form in space would make its way into forming terrestrial worlds. Yet based on Earth, this cannot be the case. Was
there a time during the evolution of the solar system that the
temperature was high enough to destroy most of these carbon
rocks and not the silicate ones? If such a phase existed, then
the main building blocks that combined to make Earth must
have begun assembling during that time.
The temperature range we need is between 425K and
1200K. Based on observations of nascent planetary systems,
we know that about 1 million years after the birth of the
Sun, the disk at 1 a.u. would have been colder than that. So
we are forced to look earlier, during the first 1 million years
of evolution. During this stage, pebbles are actively forming
from interstellar stardust in the disk. However, the Sun is still
being assembled as it continues to accrete material from the
cloud of molecular gas that served as its nursery. During this
protostar phase, the Sun is more luminous than when the
natal envelope later disappears, and the temperature of the
surrounding material is warmer. If the main building blocks
of Earth formed this early, within the first million years, then
we have solved our problem: We can make a silicate Earth
with essentially zero carbon.
However, this solution leaves us with yet more problems:
We need some carbon, not zero, and we need the amount not
to change drastically over time. The process of sublimation is
sort of a one-way street - it irreversibly puts all the carbon
into gaseous form. We need to keep some of it in rock form.
And in the protoplanetary disk, pebble-size particles are not
just sitting in place waiting to form a new planet: Differential forces cause them to drift inwards towards the highest
pressure point in the disk. The point of highest pressure is the
inner edge of the disk, right next to the star (inside today's
orbit of Mercury). So even if we made carbon-poor rocks in
the first million years, pebble drift would provide a continuous
supply of carbon- and silicate-rich rocks from the outer solar
system to the forming pre-Earth, upsetting the balance later.
We have to either stop or slow this supply within 1 million
years, when the disk is still hot. How can that be done? The
easiest way is to add a second pressure maximum in the disk
- something with a big effect that will reverse the pressure
gradient at a distance from the young Sun that is beyond
Earth's orbit, such that drifting pebbles will just pile-up at

USGS / N ASA / L A NDSAT 7

have been no ices, not even water. As tiny particles here built
up into pebbles and then larger, kilometer-size rocks and,
ultimately, into entire worlds, the planets made would have
been dry, rocky bodies.
Congratulations: We have just proved that Earth is made
of rocks.
This seems trite, but we didn't do it from any assumptions based on what we see around us. Instead, we combined
basic chemical principles with knowledge astronomers have
gleaned about the conditions that exist in regions of incipient
planets, all rooted in observations of the solid and gaseous
chemical species in faraway planet-forming places.



Sky & Telescope - December 2020

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