Revisiting Decades-Old Voyager 2 Data, Scientists Find One More Secret
The ice giant Uranus appears to be losing a bit of its atmosphere to space, perhaps siphoned away by the planet's magnetic field.
Eight and a half
years into its grand tour of the solar system,NASA's
Voyager 2spacecraft was
ready for another encounter. It was Jan. 24, 1986, and soon it would meet the mysterious
seventh planet, icy-cold Uranus.Over the next
few hours, Voyager 2 flew within 50,600 miles (81,433 kilometers) of Uranus' cloud
tops, collecting data that revealedtwo new rings, 11 new moonsand temperatures below minus 353 degrees
Fahrenheit (minus 214 degrees Celsius). The dataset is still the only up-close measurements
we have ever made of the planet.Three decades
later, scientists reinspecting that data found one more secret.Unbeknownst to
the entire space physics community, 34 years ago Voyager 2 flew through a plasmoid,
a giant magnetic bubble that may have been whisking Uranus' atmosphere out to space.
The finding, reported inGeophysical Research Letters, raises new questions about the planet's
one-of-a-kind magnetic environment.A Wobbly Magnetic OddballPlanetary atmospheres
all over the solar system are leaking into space. Hydrogen springs from Venus to
join the solar wind, the continuous stream of particles escaping the Sun. Jupiter
and Saturn eject globs of their electrically-charged air.Even Earth's atmosphere leaks. (Don't worry, it will stick around for
another billion years or so.)The effects are
tiny on human timescales, but given long enough, atmospheric escape can fundamentally
alter a planet's fate. For a case in point, look at Mars."Mars used
to be a wet planet with a thick atmosphere," said Gina DiBraccio, space physicist
at NASA's Goddard Space Flight Center and project scientist for the Mars Atmosphere
and Volatile Evolution, or MAVEN mission. "It evolved over time" - 4 billion
years of leakage to space - "to become the dry planet we see today."Atmospheric escape
is driven by a planet's magnetic field, which can both help and hinder the process.
Scientists believe magnetic fields can protect a planet, fending off the atmosphere-stripping
blasts of the solar wind. But they can also create opportunities for escape, like
the giant globs cut loose from Saturn and Jupiter when magnetic field lines become
tangled. Either way, to understand how atmospheres change, scientists pay close
attention to magnetism.That's one more
reason Uranus is such a mystery. Voyager 2's 1986 flyby revealed just how magnetically
weird the planet is."The structure,
the way that it moves ... ," DiBraccio said, "Uranus is really on its own."Unlike any other
planet in our solar system, Uranus spins almost perfectly on its side - like a pig
on a spit roast - completing a barrel roll once every 17 hours. Its magnetic field
axis points 60 degrees away from that spin axis, so as the planet spins, its magnetosphere
- the space carved out by its magnetic field - wobbles like a poorly thrown football.
Scientists still don't know how to model it.This oddity drew
DiBraccio and her coauthor Dan Gershman, a fellow Goddard space physicist, to the
project. Both were part of a team working out plans for a new mission to the "ice
giants" Uranus and Neptune, and they were looking for mysteries to solve.Uranus' strange
magnetic field, last measured more than 30 years ago, seemed like a good place to
start.So they downloaded
Voyager 2's magnetometer readings, which monitored the strength and direction of
the magnetic fields near Uranus as the spacecraft flew by. With no idea what they'd
find, they zoomed in closer than previous studies, plotting a new datapoint every
1.92 seconds. Smooth lines gave way to jagged spikes and dips. And that's when they
saw it: a tiny zigzag with a big story."Do you think
that could be ... a plasmoid?" Gershman asked DiBraccio, catching sight of the
squiggle.Little known at
the time of Voyager 2's flyby, plasmoids have since become recognized as an important
way planets lose mass. These giant bubbles of plasma, or electrified gas, pinch
off from the end of a planet's magnetotail - the part of its magnetic field blown
back by the Sun like a windsock. With enough time, escaping plasmoids can drain
the ions from a planet's atmosphere, fundamentally changing its composition.They had been
observed at Earth and other planets, but no one had detected plasmoids at Uranus
- yet.DiBraccio ran
the data through her processing pipeline, and the results came back clean.
"I think it definitely is," she said.The Bubble EscapesThe plasmoid DiBraccio
and Gershman found occupied a mere 60 seconds of Voyager 2's 45-hour-long flight
by Uranus. It appeared as a quick up-down blip in the magnetometer data. "But
if you plotted it in 3D, it would look like a cylinder," Gershman said.Comparing their
results to plasmoids observed at Jupiter, Saturn and Mercury, they estimated a cylindrical
shape at least 127,000 miles (204,000 kilometers) long, and up to roughly 250,000
miles (400,000 kilometers) across. Like all planetary plasmoids, it was full of
charged particles - mostly ionized hydrogen, the authors believe.?Readings from
inside the plasmoid - as Voyager 2 flew through it - hinted at its origins. Whereas
some plasmoids have a twisted internal magnetic field, DiBraccio and Gershman observed
smooth, closed magnetic loops. Such loop-like plasmoids are typically formed as
a spinning planet flings bits of its atmosphere to space. "Centrifugal forces
take over, and the plasmoid pinches off," Gershman said. According to their
estimates, plasmoids like that one could account for between 15% and 55% of atmospheric
mass loss at Uranus, a greater proportion than either Jupiter or Saturn. It may
well be the dominant way Uranus sheds its atmosphere to space.How has plasmoid
escape changed Uranus over time? With only one set of observations, it's hard to
say."Imagine
if one spacecraft just flew through this room and tried to characterize the entire
Earth," DiBraccio said. "Obviously it's not going to show you anything
about what the Sahara or Antarctica is like."But the findings
help focus new questions about the planet. The remaining mystery is part of the
draw. "It's why I love planetary science," DiBraccio said. "You're
always going somewhere you don't really know."The twin Voyager
spacecraft were built by and continue to be operated by NASA's Jet Propulsion Laboratory.
JPL is a division of Caltech in Pasadena. The Voyager missions are a part of the
NASA Heliophysics System Observatory, sponsored by the Heliophysics Division of
the Science Mission Directorate in Washington.For more information
about the Voyager spacecraft, visit:https://www.nasa.gov/voyagerhttps://voyager.jpl.nasa.gov