Does Pluto have an ocean? The idea seems a little strange, but ever since New
Horizons visited Pluto in 2015 there's been growing evidence for a subsurface
ocean on this distant dwarf planet. And now two new studies explain not only how
Pluto could have formed its ocean, but maintain it for billions of years to
come. Welcome back to Launch Pad, I'm Christian Ready, your friendly
neighborhood astronomer. The idea of oceans in the outer solar system isn't
new. Several moons of Jupiter, Saturn, and Neptune are suspected of having oceans
beneath their icy exteriors. Evidence includes giant cracks or fissures in
their water-ice surfaces as well as geysers of nitrogen and water erupting
as cryovolcanoes. These features require internal heat; enough to keep the water
underneath the ice warm enough to form an ocean. At -240 degrees Celsius,
Pluto is far too cold to maintain liquid water on its surface. But the New Horizons
spacecraft returned images of long troughs called fossae. These fossae are
thought to be tectonic fractures due to the presence of internal heat that would
occasionally warm up and break apart the ice. Then there's the giant heart-shaped
region called Tombaugh Reggio. The left half is a basin called Sputnik Planitia.
Sputnik is thought to have been created in a large comet impact about four
billion years ago. The impact gouged out a 1300 by 900 kilometer basin that runs
several kilometers deep. Over the next several millennia, the basin was buried
underneath miles of carbon monoxide, nitrogen, and methane ices. But it's
Sputnik's location on Pluto that has astronomers thinking that there's
probably an ocean down there. Sputnik is centered on Pluto's equator - that
suggests that there must be some extra mass there. To see what I mean, take a
look at this video of a rotating ball of water onboard the International Space
Station. As the water rotates, low-density air molecules contract toward the axis
of rotation while the greater density water bulges out at the equator. When you
add some tea leaves and some orange peel, you get a
concentration of mass which bulges its way toward the equator where the ball is
rotating the fastest. Not only is this a great demonstration of the conservation
of angular momentum, but it's a really awesome way to make tea. So if there's
extra mass underneath Sputnik then it should have rolled its way around
towards Pluto's equator. But Sputnik is also lined up opposite the side of Pluto
that faces Charon. That puts Sputnik on the tidal axis, or an imaginary line
between Pluto and Charon. Charon's gravity tugs strongest along this tidal
axis, so it makes sense that over time Pluto would have rolled itself to align
its extra mass up along this axis. But how does an impact basin get extra mass?
If anything, impacts tend to remove mass. Pluto's interior is thought to consist of
silicates mixed with water surrounded by a shell of water ice and nitrogen ice.
There's also plenty of organic compounds, tholins, and volatiles such as methane and
ammonia. Sputnik is covered in nitrogen ice so an early idea was that maybe
there was an excessive buildup of nitrogen here. However, nitrogen is such
low density that it turns out can't provide enough mass to roll pluto around
to its current orientation. But liquid water is much more dense than nitrogen.
Perhaps it flowed from deep inside Pluto's core as the interior rebounded
after the impact. That would create an ocean, but keeping it liquid for the last
four billion years requires plenty of heat. Moons such as Europa, Titan, and
Enceladus are warmed by tidal heating as they orbit their host planets. Tidal
forces fluctuate - growing stronger, then weaker, then stronger again. This applies
mechanical heat to their interiors. But Pluto is tidally locked to Charon so
there aren't any tidal fluctuations to continuously heat Pluto's interior. But
radioactive decay would keep water in Pluto's core liquid. But if that water
were to flow upward as a result of an impact then the heat is going to flow
upward with it and eventually escape to space. The result is that the ocean would
freeze over in just a few hundred million years. But two new studies were
just published this week that showed how Pluto can in fact
maintain a liquid water ocean to this very day. We're gonna learn about them in
a moment but first I want to thank my Patreon supporters for helping to make
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Anyway, if you'd like to help support Launch Pad Astronomy and keep this
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head on over to my patreon page. I'll have a link here and in the description
below. A team led by Dr. Cristina Dalle Ore detected ammonia in Virgil Fossae, a
series of ice-filled fractures in Pluto's dark-red Cthulhu region next to
Sputnik. The water and ammonia ice were likely deposited there via cryovolcanoes.
Ammonia lowers waters freezing temperature, turning it into a kind of
antifreeze. But ammonia is also very delicate and
breaks apart easily when exposed to sunlight or charged particles from the
solar wind. That means that the ammonia found in Virgil Fossae must have been
deposited there fairly recently; within the last 1 million years or so.
That means that Pluto could have an ammonia water ocean underneath its
surface ice. But this introduces a new problem. The sputnik Basin is covered in
hills of nitrogen ice that range from three to ten kilometers in height. If the
water is directly underneath the ice, that would transfer just enough heat to
get those hills flowing around as glaciers. Over time, say a few million
years or so, those glaciers should more or less grind each other down and the
basin would level out. But a new study by Dr. Shunichi Kamata and his team
showed that a layer of clathrates could explain the long-term survival of
Pluto's ocean as well as the rolling hills and valleys on the ice above.
Clathrates are a mixture of gas and water ice molecules. In the case of Pluto,
that gas is probably methane that was either delivered during a comet impact
or bubbled up from the interior afterward. Such a clathrate layer would
act as a thermal insulator keeping Pluto's ocean liquid and it's ice shell
cold. Kamata and his team tested this idea out
in computer simulations. They found that without clathrates, Pluto's ocean would
freeze over in less than a billion years. But with the thin clathrate layer,
Pluto not only keeps its ocean liquid since it formed four billion years ago,
but continues to stay liquid for billions of years in the future. These
results are encouraging and it's amazing how much information has been pulled out
from the New Horizons flyby in 2015. But to really know if there is extra mass
under Sputnik, and indeed if there's an ocean, we would need to send a dedicated
orbiter to Pluto. Such a spacecraft could measure Pluto's gravity field in great
detail and map out where the mass is distributed around the planet. It could
even use radar to probe the nitrogen ice layers at Sputnik as well as the ocean
and even the gas layer if it exists. In fact, the upcoming Europa clipper mission
will use these same techniques at Europa. Right now, scientists are working on a
proposal to NASA to send a dedicated orbiter to Pluto. If accepted, it could
launch in 2030 which would be the 100th anniversary of Pluto's discovery. Oceans
appear to be everywhere in the outer solar system. Life requires liquid water,
heat, and organic molecules in order to evolve and thrive.
Pluto certainly has plenty of heat and organic molecules; the question is
whether or not its ocean would really be habitable? It would certainly be a very
cold ocean so any life that does evolve would have to be adapted to an extremely
cold environment. But you never know! It's interesting to think that there could be
an ocean-bearing, maybe even a life-bearing world in the Kuiper belt.
But Pluto is hardly the only such world. Dwarf planets Eris, Haumea, and
Makemake - even Quaoar are and Sedna may host
subsurface oceans! So much of what we now know about Pluto, and what we're
continuing to learn about this dwarf planet is thanks to the New Horizons
flyby in 2015. That same spacecraft would then fly past another Kuiper belt object
just three and a half years later I have a video about the New Horizon's science
results of its flyby of 2014 MU69, Ultima Thule, so make sure you check that
video out there when we're done over here. And if you'd like to join me on
this journey through this amazing universe of ours,
please make sure that you subscribe and ring that notification bell so that you
don't miss out on any new videos. Until next time, stay curious my friends.