Pluto may not be categorised
as a planet any more, but it still holds plenty of fascination. For instance,
how did the dwarf planet form, and why is it so different from the planets? By
examining its chemical composition, researchers have come up with a new idea:
Pluto is made of comets.
According to the currently
accepted model, planets are formed by the gradual accretion of smaller objects
- and Pluto, situated right next to the Kuiper Belt asteroid field, has long
been thought to have formed the same way. So that part is nothing new.
But there are similarities
between Pluto and Comet 67P/Churyumov-Gerasimenko that scientists from the
Southwest Research Institute (SwRI) believe may not be coincidental. In
particular, the nitrogen-rich ice in Pluto's Sputnik Planitia. Thanks to the
Pluto probe New Horizons and Rosetta, the space probe sent to study Comet 67P,
we have a new and unprecedented wealth of data about both Pluto and comets.
"We've developed what
we call 'the giant comet' cosmochemical model of Pluto formation," said
geochemist Christopher Glein of the SwRI's Space Science and Engineering
Division.
"We found an intriguing
consistency between the estimated amount of nitrogen inside the glacier and the
amount that would be expected if Pluto was formed by the agglomeration of
roughly a billion comets or other Kuiper Belt objects similar in chemical
composition to 67P, the comet explored by Rosetta."
Nitrogen on Pluto is akin to
methane on Titan, or water on Earth - the key volatile responsible for shaping
the dwarf planet's surface. Because of its low viscosity at Pluto's surface
temperatures, nitrogen is able to flow like glaciers on Earth - eroding the
bedrock and changing the shape of the landscape.
Earth's atmosphere is around
78 percent nitrogen (our temperatures don't get as cold as Pluto, so it remains
gaseous), but Pluto's is about 98 percent. So between the nitrogen ice and the
nitrogen atmosphere, the dwarf planet has an unusually high proportion of it.
Previously, scientists
thought that maybe the nitrogen came from comets that landed on Pluto - but
that model would not account for the sheer amount of it.
In addition to the comet
model, the researchers also investigated a model whereby Pluto formed from very
cold ices with chemical compositions similar to that of the Sun. By examining
these models, they hoped to get a better understanding of Pluto's leaky
atmosphere, to figure out how much nitrogen is escaping into nearby space. They
also needed to reconcile the amount of carbon monoxide in Pluto's atmosphere,
and neither model was able to explain how little there was.
"Our research suggests
that Pluto's initial chemical makeup, inherited from cometary building blocks,
was chemically modified by liquid water, perhaps even in a subsurface
ocean," Glein said.
It's also possible, under
the cometary model, that the missing carbon monoxide is trapped, frozen under the Pluto's surface. Because there are more explanations for the missing carbon
monoxide under the cometary model, it seems more likely than the solar model,
the researchers said.
Of course, this is
hypothetical at this point and, as the researchers put it, this leads to "an
appreciation of many subsequent questions that must be addressed" in
future analyses, such as whether the abundance of nitrogen on Comet 67P is
representative of other comets, and what role liquid water has played in the
evolution of volatiles on Pluto.
"This research builds
upon the fantastic successes of the New Horizons and Rosetta missions to expand
our understanding of the origin and evolution of Pluto," said Glein.
"Using chemistry as a
detective's tool, we are able to trace certain features we see on Pluto today
to formation processes from long ago. This leads to a new appreciation of the
richness of Pluto's 'life story,' which we are only starting to grasp."
The paper has been accepted
for publishing in the journal Icarus, and can be read on arXiv.
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