Sky & Telescope - December 2020 - 37

cally abundant elements. Silicon has the same number of
bonds, but in space all silicon is locked in rocks, unavailable
to react with other elements and build up compounds. That
is not the case for carbon: About 50% of the carbon is in the
form of volatile compounds such as CO, CO2, and a host of
small organic ices. The remaining carbon is found in macromolecular forms of carbon locked in refractory stardust. This
preponderance of available carbon may explain why we are
carbon-based lifeforms.

G R AIN Y TE X T URE: IRIN A Q IWI / SHU T TERSTOCK .CO M; FOR MING STA R: N ASA
/ ESA / HUBBLE HERITAG E TE A M (STSCI / AUR A)

Ice Marks the Spot
The next step in the story of carbon focuses on the formation
of terrestrial worlds such as our own, a planet made from solids. Planets are born inside disks of gas and stardust, and the
tiny dust particles of silicate and carbonaceous compounds
that stick together to build these worlds are coated with layers
of ice - not just water ice but also CO, CO2, and CH4 and
other organic ices.
Which ices are present depends on how far the grains are
from the forming star. Different carbon compounds have
different levels of volatility. This means that particles that
coagulate at successively greater distances from the Sun will
have different amounts of carbon present and in different
forms.
For example, out at Neptune's current distance from the
Sun, where the planet-forming disk would have been a chilly
20K, bodies essentially wouldn't have lost any carbon to
vaporization: They would contain both rocks and ices with
the same fraction of carbon that is seen in the Sun. Cryovolcanism or other geologic processes can change the molecules
carrying carbon over billions of years in large bodies such as
Pluto, but in smaller bodies, such as comets, we think the
molecules maintain their original form.
Slightly nearer the Sun, around Uranus's orbit, temperatures in the disk would have risen above 20K, and CO ice
would have evaporated into the surrounding gas. Solids that
formed at this point would have incorporated about 70% of
the available carbon. Indeed, astronomers observe some comets that are CO-poor but rich in CO2 and other compounds.
Within Jupiter's orbit at 5 astronomical units, we are inside
the CO2 and CH4 snowlines, the minimum distances from the
Sun where these compounds will exist as ice. These ices would
sublimate into the gas, meaning any carbon taken up by growing rocky-icy bodies in this region would only appear in simple
organic molecules or carbon rocks. Carbon-bearing ices aren't
the only ices in the disk, however; these rocks would still have
substantial amounts of water ice, as well as silicates.
At 1 a.u., we find an even warmer situation. There, the
disk is 200K, and the only carbon available to form Earth is
from the refractory (rocky) forms of carbon - no organic
molecules - combined with the silicate rocks. There would

Organic
An organic molecule is one that contains
at least one hydrogen atom bonded to
a carbon atom. They're called organic
because they're associated with living
organisms.

X SHARPLESS 2-106 A young star blows out hot gas (blue) as it forms.
A ring of dusty gas shapes the outflows into an hourglass. Ridges in the
hot gas arise from interactions with the cooler surrounding medium.
sk yandtelescope.org * DECEMBER 2 02 0

37


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Sky & Telescope - December 2020

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