Strange
weather on the icy dwarf planet Eris could be causing changes that scientists
are now seeing at the methane-ice surface of this distant object in our solar
system.
Eris is the
largest known solar-system object beyond the orbit of Neptune. It is larger than
Pluto, with a diameter of ranging somewhere between about 1,490 miles and
1,860 miles (2,400 km and 3,000 km).
A team of researchers
examined data on Eris
collected from the MMT Observatory in Arizona. They specifically looked at
concentrations of methane ice based on light-reflection and absorption
information.
Their
results show possibly nitrogen ice mixed in with the methane ice covering Eris' surface. And the relative amount of nitrogen ice increases with depth into the
ice, they found.
Here's what
the researchers think is happening:
Currently,
Eris is at its farthest distance to the sun, called aphelion, along its about
560-year orbit, meaning the planet is nearly 100 astronomical units (AU) from
the sun, or about 9 billion miles (14 billion km). Along its orbit, Eris sweeps as
close as 38 AU to the sun when at perihelion.
Due to
Eris' tilt, a different hemisphere faces the sun when at perihelion and
aphelion.
On the
sunlit hemisphere or pole at perihelion, lots of
sublimation would have occurred to turn nitrogen into gas. (Sublimation is
the process of ice turning to gas while skipping the liquid phase.) This gas would
build up in the atmosphere — likely a very thin one around such a small object
— to increase the pressure and drive winds toward the shaded pole.
Nitrogen
ice turns into a gas at cooler temperatures and so there would be more nitrogen
gas in the atmosphere compared with methane. Then, at the shaded pole, also
called the winter hemisphere, the gases would condense into snow-like or
dew-like material that would fall onto Eris' surface as nitrogen ice.
As Eris
moved closer and closer to the sun, this same process would occur for methane,
with methane sublimating on the sunlit hemisphere and falling as methane ice on
the shaded pole.
As the winds
continued to blow from the sunlit side to the shaded hemisphere (as summer
progressed on the sunlit side), nitrogen would be depleted. And so relatively more
methane would whoosh over to the shaded side and fall as methane ice.
The pole
the researchers observed had been shaded for years while Eris was at
perihelion. That same pole is now facing the sun and has been for decades to
centuries, the researchers say.
"The
hemisphere in darkness then, is the hemisphere we are seeing now at aphelion
today," researcher Stephen Tegler, an astrophysicist at Northern Arizona University, told SPACE.com. "In other words, we may be observing
today a signature of the winds that condensed out during the last perihelion
passage."
This
apparent weather could explain why the researchers found more nitrogen deeper
below the surface of Eris, which would've been deposited earlier in the season.
The
research will be detailed in an upcoming issue of the journal Icarus.
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