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Pluto might once have seemed like the last place in the solar system to look for liquid
water,
given that its balmiest days peak around negative two hundred twenty-five degrees Celsius.
But ever since the New Horizons probe flew by it in two thousand fifteen,
scientists have been studying what they think is Pluto’s underground ocean.
Seriously!
They think Pluto has an ocean!
This hypothesis is based on various surface features New Horizons observed four years
ago —
but even as the evidence piled up, it still wasn’t obvious how Pluto had hung on to
this liquid water for so long.
Now, though, researchers think they might have figured it out.
On Monday, in a paper published in Nature Geoscience,
they announced that a layer of gas locked in ice could be protecting Pluto’s underground
water.
If they’re right, this could help us understand how a liquid ocean exists inside the dwarf
planet —
and how other icy bodies might be holding onto hidden oceans of their own.
After the New Horizons flyby, scientists started to find a bunch of signs that Pluto might
have an underground ocean —
including how they saw light reflect off its surface.
Strangely, the light appeared to be bouncing off crystals of ice.
That was weird, because ice can only hold its pretty, snowflake-like structure down
to a certain temperature.
When it gets really cold, ice molecules get all amorphous.
Not only that, but radiation from space also destroys those crystals.
So, scientists had lots of reasons not to expect crystal ice on the surface of Pluto.
But there it was.
Eventually, they figured out that the crystals were likely coming from underground water
that was erupting to the surface.
That might seem far-fetched, but later evidence from other surface features backed up the
idea that Pluto’s ocean was still liquid.
For instance, if it had frozen, Pluto’s surface would have buckled in recognizable
ways.
But there was no trace of that in the images.
Using data from the New Horizons flyby, scientists ultimately hypothesized that Pluto’s hidden
ocean lay under a shell of ice that varied in thickness.
But they couldn’t figure out how a shell like this could exist at the same temperature
that would keep the ocean from freezing.
They modeled lots of different scenarios—maybe the ocean was insulated by a thick surface
layer,
or maybe it was full of molecules like ammonia that act like antifreeze.
There were lots of possibilities, but nothing quite added up and matched the actual features
they saw.
But now, in this new paper, a team of scientists has landed on a hypothesis that seems like
it might work.
They suspect that a layer of gas molecules trapped in water ice is acting as insulation
between the ocean and the outer shell.
A material like this, called a gas hydrate, forms when water molecules act like a cage
that captures gas molecules.
This can happen when liquid water begins to freeze under high pressures and other special
conditions.
If the water has a high concentration of dissolved gas, it can freeze into a gas hydrate instead
of normal ice.
At the border between Pluto’s ocean and its heavy shell of ice, scientists think this
just may have happened.
They tested this hypothesis by modeling two different versions of Pluto over time.
One version had this gas hydrate layer, and it kept its ocean and uneven ice shell as
the dwarf planet evolved.
The other version had no gas hydrate layer, and the ocean froze over in just a few million
years.
That’s because gas hydrates conduct much less heat than regular ice.
Models suggest that it would only take a thin layer of trapped gas—
just several kilometers or so—to preserve temperatures on either side that could explain
both the liquid ocean and the uneven shape of the icy shell.
If this is the case, it might explain how objects way out in the Kuiper Belt could support
oceans, even though they’re billions of kilometers from the Sun.
Even though scientists will be analyzing data from Pluto for a long time, New Horizons has
moved on to new adventures!
Most notably, on the first day of two thousand nineteen, it whizzed past the icy body MU
sixty-nine.
It’s the most distant world a spacecraft has ever visited, and last week,
NASA published the first results from this flyby in the journal Science.
Now, this isn’t the first time the New Horizons team has shared information about MU sixty-nine:
In January, the icy rock made headlines for looking like a couple of weird hamburgers.
But this is the first peer-reviewed paper with the scientists’ analyses.
In their study, researchers tried to get to the bottom of how this oddly-shaped body formed,
by analyzing its shape and geological features.
They think the two lobes formed separately and fell into orbit around each other.
Then, eventually, it seems those two rocks came together and gently ran into each other.
This little fender-bender created the stuck-together hamburger situation, which scientists call
a contact binary.
But it’s not just its shape that makes MU sixty-nine intriguing.
For one, it has a bunch of small pits across its surface.
Some of the larger ones probably came from impacts, but there are also some chains of
smaller, similarly sized holes that scientists think formed differently.
They might have formed when frozen solids turned to gas, or when material collapsed
into spaces under the surface.
At this point, it’s still a bit of a mystery, but it’s one teams will keep looking into.
MU sixty-nine is also, in the words of NASA, “ultra-red.”
It’s not quite as red as Mars, but the entire surface has a red hue.
That’s not totally bizarre, because observations suggest that a lot of Kuiper Belt objects
are reddish, but we don’t know exactly why.
Researchers think the color could come from molecules that break down from light and radiation—
and then combine into heavier molecules that are reddish in color.
There are still a lot of open questions and hypotheses to test, but fortunately, there’s
still more information to come,
since New Horizons will keep sending back data on MU sixty-nine until late summer twenty
twenty.
In the meantime, the spacecraft will also keep zooming through the Kuiper Belt, studying
other objects from a distance.
It may even get assigned another target.
Regardless, scientists expect that the more they learn about MU sixty-nine and Pluto,
the more they’ll understand about how our solar system formed.
Because, in the end, studying weird rocks in space is never just about those rocks.
By exploring these frigid bodies billions of kilometers away, we can learn about things
like how our planets were born, or where the solar system can support water —
lessons from the edge of the solar system that hit very close to home.
Thanks for watching this episode of SciShow Space News, which is produced by Complexly.
We produce over a dozen shows, including PBS’s Origin of Everything.
Join Dr. Danielle Bainbridge as she explores under-told histories and cultural dialogues.
From the history of the laugh track to the history of racial passing, no concept is too
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Check out the Season 2 playlist in the description below!
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