NASA’s Juno mission, led by SwRI’s Dr. Scott Bolton, is rewriting what scientists thought they knew about Jupiter speciﬁcally, and gas giants in general. The Juno spacecraft has been in orbit around Jupiter since July 2016, passing within 3,000 miles of the equatorial cloudtops.
"We knew going in that Jupiter would throw us some curves,” said Bolton, Juno principal investigator. “But now that we are here we are ﬁnding that Jupiter can throw the heat, as well as knuckleballs and sliders. There is so much going on here that we didn’t expect, that we have had to take a step back and begin to rethink of this as a whole new Jupiter.”
Juno is in a polar orbit around Jupiter, and the majority of each orbit is spent well away from the gas giant. But once every 53 days, its trajectory approaches Jupiter from above its north pole, where it begins a two-hour transit (from pole to pole) ﬂying north to south with its eight science instruments collecting data and its JunoCam public outreach camera snapping pictures. Downloading the six megabytes of data collected during the transit can take 1.5 days.
The solar-powered spacecraft’s payload is designed to study Jupiter’s interior structure, atmosphere, and magnetosphere. Two instruments developed and led by SwRI are working in concert to study Jupiter’s auroras, the greatest light show in the solar system. The Jovian Auroral Distributions Experiment (JADE) is a set of sensors detecting the electrons and ions associated with Jupiter’s auroras. The Ultraviolet Imaging Spectrograph (UVS) examines the auroras in UV light to study Jupiter’s upper atmosphere and the particles that collide with it. Scientists expected to ﬁnd similarities to Earth’s auroras, but Jovian auroral processes are proving puzzling.
“Although many of the observations have terrestrial analogs, it appears that diﬀerent processes are at work creating the auroras,” said SwRI’s Dr. Phil Valek, JADE instrument lead.
“With JADE we’ve observed plasmas upwelling from the upper atmosphere to help populate Jupiter’s magnetosphere. However, the energetic particles associated with Jovian auroras are very diﬀerent from those that power the most intense auroral emissions at Earth.”
Also surprising, Jupiter’s signature bands disappear near its poles. JunoCam images show a chaotic scene of enormous swirling storms towering above a bluish backdrop. Since the ﬁrst observations of these belts and zones many decades ago, scientists have wondered how far beneath the gas giant’s swirling façade these features persist. Juno’s microwave sounding instrument reveals that topical weather phenomena extend deep below the cloud tops, to pressures of 100 bars, 100 times Earth’s air pressure at sea level.
“However, there’s a north-south asymmetry. The depths of the bands are distributed unequally,” Bolton said. “We’ve observed a narrow, ammonia-rich plume at the equator. It resembles a deeper, wider version of the air currents that rise from Earth’s equator and generate the trade winds.”
Juno is mapping Jupiter’s gravitational and magnetic ﬁelds to better understand the planet’s interior structure and measure the mass of a core. Scientists think a dynamo — a rotating, convecting, electrically conducting ﬂuid in a planet’s outer core — is the mechanism for generating the planetary magnetic ﬁelds.
“Juno’s gravity ﬁeld measurements diﬀer signiﬁcantly from what we expected, which has implications for the distribution of heavy elements in the interior, including
the existence and mass of Jupiter’s core,” Bolton said.
The magnitude of the observed magnetic ﬁeld was 7.766 Gauss, signiﬁcantly stronger than expected. But the real surprise was the dramatic spatial variation in the ﬁeld, which was signiﬁcantly higher than expected in some locations, and markedly lower in others.
"We characterized the ﬁeld to estimate the depth of the dynamo region, suggesting that it may occur in a molecular hydrogen layer above the pressure-induced transition to the metallic state.”
One of the central questions Juno was designed to answer was what was at the giant planet’s core. Most scientists expected to ﬁnd either a small, compact core or no core at all. Based on gravity experiment data, scientists have coined the phrase “fuzzy core” to describe what the Juno data may mean.
“There may be a little compact core but most think there may be layers there,” Bolton said. “There seems to be a fuzzy core that may be much larger than anybody had anticipated. There may be a core there, but it’s very big and it may be partially dissolved.”
The Juno mission also has a campaign to inspire and motivate the connections between science, art, and music. The JunoCam instrument has been incredibly successful engaging the public with Juno’s mission, where amateur astronomers are the driving force behind the incredible images of Jupiter.
Juno is the second mission developed under NASA’s New Frontiers Program. The ﬁrst was the SwRI-led New Horizons mission, which provided the ﬁrst historic look at the Pluto system in July 2015 and is now on its way to a new target in the Kuiper Belt. NASA’s Jet Propulsion Laboratory in Pasadena, Calif., manages the Juno mission for the principal investigator, SwRI’s Bolton. Lockheed Martin of Denver built the spacecraft. The Italian Space Agency contributed an infrared spectrometer instrument and a portion of the radio science experiment.