An image of Uranus — snapped moments before the Voyager 2 spacecraft flew within 81,433 kilometers (50,600 miles) of the gas giant 34 years ago — was recently found to suggest the spacecraft flew through a plasmoid, which is a magnetic bubble that may account for up to 55% of the atmospheric loss on Uranus, as tiny portions of gas slowly leak into the depths of interplanetary space, reports NASA.
Voyager 2 moved through Uranus' magnetic bubble
Planetary atmospheres are leaking into space all over our solar system. Hydrogen springs rising from the planet Venus join the solar wind, a constantly-flowing stream of particles from the Sun. Saturn and Jupiter fling massive globs of electrically-charged air into the darkness of space. Even Earth's precocious atmosphere leaks into space (but won't totally leave the surface for another billion years ago, says NASA).
This is hard to notice on human timescales, but as millennia turn into billions of years, the fates of planets hang in the balance. Take Mars, for example.
"Mars used to be a wet planet with a thick atmosphere," said Space Physicist Gina DiBraccio of NASA's Goddard Space Flight Center, who is also a project scientist for the Mars Atmosphere and Volatile Evolution — or MAVEN — mission. "It evolved over time," losing atmosphere to space for 4 billion years "to become the dry planet we see today."
Atmospheric leakage ebbs and flows according to the activity of a planet's magnetic field. Scientists think magnetic fields can protect a planet, shielding it from blasts of solar wind that strip a planet of its atmosphere. However, they also create opportunities for gas to escape in giant globs that cut loose from the gas giants Jupiter and Saturn — when magnetic field lines knot up.
This is another reason why Uranus is so mysterious. In 1986, Voyager 2 made its flyby, unveiling what a strange magnetic case Uranus is.
"The structure, the way that it moves...," DiBraccio said, "Uranus is really on its own."
Uranus spins almost perfectly on its side — like a jump-rope — completing a spin every 17 hours. Its magnetic field axis is 60 degrees out of alignment with that spin axis, which means the space moved into magnetic lines by the planet's magnetosphere wobbles around like a poorly-thrown football, said NASA.
This attracted DiBraccio and her coauthor Dan Gershman, another Goddard space physicist, to the project. Both have collaborated on a team planning for future missions to the 'ice giants' Uranus and Neptune. The strange magnetic field of Uranus — last monitored more than 30 years go — seemed an apt place to begin their work.
They downloaded Voyager 2's magnetometer readings — which measures the direction and strength of magnetic fields near Uranus during the spacecraft's flyby. Without knowing what to look for, they zoomed in closer on earlier studies, plotting new data points every 1.92 seconds, and that's when the smooth lines turned into jagged spikes and dips — a short zigzag with a long history.
The plasmoid bubble of Uranus
The plasmoid Gershman and DiBraccio found filled only 60 seconds of Voyager 2's 45-hour Uranus flyby. Although it looked like a quick up-down blip in the magnetometer's data, "if you plotted it in 3D, it would look like a cylinder," said Gershman. The loop-like shape of the plasmoid suggested it had formed while Uranus flung bits of its atmosphere into space, according to NASA.
"Centrifugal forces take over, and the plasmoid pinches off," added Gershman. According to the NASA scientists' estimates, plasmoids like this one might account for somewhere between 15% and 55% of lost atmospheric mass to Uranus — a greater proportion to its total mass than that of Saturn, or even Jupiter. With a suite of new NASA missions potentially on the horizon, it's interesting to note that data monitored with technology more than 30 years behind the times can still astound modern-day scientists, and show us more about the evolution of planets in our solar system.
Editor's Note: An earlier version of this article suggested that more than half (55%) of Uranus' atmosphere could have been lost due to plasmoids. This is an incorrect interpretation. Current estimates suggest that Uranus loses roughly 0.02 kilograms per second. Per Voyager 2's data, plasmoids could potentially account for 55% of that loss — a very small fraction of the planet's total atmospheric mass. The title, dek, and introduction of this article have since been changed to reflect the correct interpretation. IE regrets this error.