NASA Cassini mission's final moments reveal the reason why Saturn's upper atmosphere is so hot
Before it met its doom, the spacecraft gathered data that puts to rest a long-standing mystery about the sixth planet from our Sun.
After two decades in space, on September 15, 2017, NASA's Cassini spacecraft ended its life dramatically by plunging into Saturn’s atmosphere, disintegrating into nothingness.
But before it sacrificed itself, NASA’s mission to the ringed giant conducted 22 close orbits over a period of around five months, threading through the space between Saturn's main body and the planet's rings.
Dubbed Cassini’s Grand Finale, this final tour of Saturn revealed unprecedented detail about the planet's atmosphere, magnetic field, iconic rings, and opaque clouds. And now, almost three years on, this treasure trove of data have resolved yet another mystery about the planet and its curious climate.
In a new study published Monday in the journal Nature Astronomy, a team of scientists used data from Cassini’s Grand Finale to create a map of temperatures of Saturn’s temperatures.
The findings suggest a resolution to the planet’s energy crisis. Unlike on Earth, Saturn’s ‘energy crisis’ has nothing to do with a limited supply of natural resources. Instead, the crisis on Saturn refers to the inexplicably hot temperatures in the planet’s upper atmosphere.
In the past, scientists had observed strangely hot temperatures in Saturn's upper atmosphere — twice as hot as they should be. The new data support a long-standing hypothesis that the planet’s aurorae may be responsible for driving these extreme temperatures.
Ultimately, the findings could help scientists in their quest to study exoplanets, and understand how they lost their atmosphere.
Saturn's strange climate — For years, scientists have tried to understand why Saturn’s upper atmosphere, known as the planet’s thermosphere, is much hotter than it should be.
Earth’s thermosphere is heated by the Sun. But the further you get away from the Sun in the Solar System, the less the intensity of the star’s radiation. That means objects further in the Solar System receive less heat from the star.
Saturn is the sixth planet from the Sun. It is about 10 times further away from the Sun than Earth. Based on this, the temperature of its upper atmosphere should be around 150-200 Kelvin, or between -190 and -100 degrees Fahrenheit. Freezing, in other words.
The ringed giant has posed an energy crisis in the Solar System.
Instead, the temperatures of Saturn’s upper atmosphere are between 350-600 Kelvin, according to the study. That's the equivalent of a positively boiling 170 to 620 degrees Fahrenheit.
“When I plotted all the temperatures up from pole to pole, I could see that the temperatures and polar regions were not doing what all the models were predicting,” Zarah Brown, a researcher in planetary atmosphere at the University of Arizona’s Lunar and Planetary Laboratory, and lead author of the new study, tells Inverse.
Instead, the results reveal that Saturn’s upper atmosphere is at its hottest in a hot ring at auroral latitudes, where the planet's auroras take place. Similar to Earth’s Northern Lights, auroras take place when the Sun emits charged particles in the form of solar wind, exciting hydrogen atoms in Saturn's ionized upper atmosphere.
The auroras generated by the charged solar wind connect Saturn’s thermosphere to its magnetosphere, a protective bubble of magnetic field that surrounds planets. This process results in electric currents that in turn heat the planet's thermosphere up. The extreme heating is a direct result of solar wind interacting with particles being emitted by Saturn’s moons and rings, the study suggests.
“The heat is coming from these processes associated with the aurora,” Brown says.
“The data supports that by just the fact that there’s sort of this ring of elevated temperatures right near the auroral regions, that is suggestive that that’s where the heat is being deposited.”
A view of Saturn, with its orbiting moon Titan, captured by Cassini.
Other giant planets in the Solar System like Jupiter and Uranus also experience inexplicable heating in their upper atmosphere, although whether the same auroral process is driving their strange temperatures can't be inferred from these data, Brown says.
But they do suggest new avenues for researchers to explore.
“No other giant planet has been studied in this kind of detail, this is really a great starting point,” Brown says.
Cassini wonders — Cassini launched on October 15, 1997, and entered Saturn’s orbit on June 30, 2004. The intrepid satellite was originally slated for a four year mission, which was later extended twice.
During its 20-year mission, the spacecraft conducted nearly 300 orbits of Saturn, and took more than 453,000 images and sent back 635 gigabytes of data.
In April, 2017, Cassini began its final tour of the giant planet, diving further into the approximately 1,200-mile-wide gap between Saturn and its rings at a distance of about 1,000 to 2,500 miles above the planet’s clouds.
The team behind the mission were unsure if the spacecraft could even survive this close encounter with Saturn, and whether some of the material from the planet’s rings would ultimately destroy Cassini. But the risk paid off.
The reason for its dramatic death drop was to avoid crashing into Saturn’s moons and contaminating them, especially with Enceladus and Titan believed to contain some form of life.
“It was a risky procedure, we didn’t know if it would encounter impact,” Brown says.
“The Grand Finale data set is really unprecedented,” she says. “It was designed to give us this very special ability to have this snapshot of Saturn’s atmosphere.”
But even as the spacecraft plunged to its death, it was still sending data back to Earth.
Exploring other worlds — The data not only help us better understand Saturn, but may also be useful in the quest to study exoplanets, and potentially find life outside the Solar System.
Missions such as NASA’s Kepler and the Transiting Exoplanet Survey Satellite (TESS) have led to the discovery of more than 4,000 exoplanets to date.
Most of the exoplanets scientists have found have been giant planets such as super-Earths or mini-Neptunes, and some of them had lost their atmosphere at some point during their existence.
Studying giant planets in our own Solar System may provide scientists with a more detailed model of what these faraway worlds look like than we could otherwise achieve with current technology.
“Understanding the planets in our Solar System is critical to understanding exoplanets in general,” Brown says.
Abstract: Temperatures of the outer planet thermospheres exceed those predicted by solar heating alone by several hundred degrees. Enough energy is deposited at auroral regions to heat the entire thermosphere, but models predict that equatorward distribution is inhibited by strong Coriolis forces and ion drag1,2. A better understanding of auroral energy deposition and circulation are critical to solving this so-called energy crisis. Stellar occultations observed by the Ultraviolet Imaging Spectrograph instrument during the Cassini Grand Finale were designed to map the thermosphere from pole to pole. We analyse these observations, together with earlier observations from 2016 and 2017, to create a two-dimensional map of densities and temperatures in Saturn’s thermosphere as a function of latitude and depth. The observed temperatures at auroral latitudes are cooler and peak at higher altitudes and lower latitudes than predicted by models, leading to a shallower meridional pressure gradient. Under modified geostrophy3, we infer slower westward zonal winds that extend to lower latitudes than predicted, supporting equatorward flow from approximately 70° to 30° latitude in both hemispheres. We also show evidence of atmospheric waves in the data that can contribute to equatorward redistribution of energy through zonal drag.
