Jupiter’s upper atmosphere has puzzled astronomers for decades. This outer jacket of gas around the giant planet is hundreds of degrees warmer than expected. Now, scientists think they may have figured out why. That bonus heat could come from Jupiter’s intense auroras. These are Jupiter’s version of Earth’s northern and southern lights.
Jupiter orbits about 778 million kilometers (483 million miles) from the sun. At that distance, sunlight is incredibly feeble. It amounts to less than 4 percent of the energy per square meter (10.8 square feet) that hits Earth’s atmosphere. So the region several hundred kilometers (miles) above the planet’s cloud tops should be about –73° Celsius (–99° Fahrenheit). But Jupiter’s upper atmosphere is actually about 426 °C (799 °F).
Scientists first noticed this mismatch more than 40 years ago. Since then, researchers have come up with several possible explanations. For instance, gas churning far below could generate the warmth. But recent observations now trace the heat to Jupiter’s auroras. The observations come from a team led by James O’Donoghue. He’s a planetary scientist in Sagamihara, Japan. He works at the JAXA Institute of Space and Astronautical Science.
His team observed Jupiter with the 10-meter (33-foot) Keck II telescope. It sits atop Hawaii’s dormant Mauna Kea volcano. O’Donoghue and his colleagues peered at Jupiter on one night each in 2016 and 2017. They were looking for infrared light from the planet. One particular wavelength of this glow is produced by positively charged hydrogen molecules (H3+). Those ions are created when particles of solar wind hit Jupiter’s atmosphere. (The solar wind is a stream of charged particles that flows out from the sun.) Such high-speed collisions set the planet’s auroras aglow.
The intensity of the H3+ infrared light revealed how hot it gets high above Jupiter’s cloud tops. The polar regions where auroras form likely reach some 725 °C (1337 °F). The temperature falls to about 325 °C (617 °F) near the equator. The researchers share those findings in the August 5 Nature. The gradual drop-off in temperature away from the poles suggests that Jupiter’s auroras heat the upper atmosphere. Winds then spread that warmth from the poles.
O’Donoghue’s team may have caught this heat transfer in action. In January 2017, the researchers observed Jupiter during a strong solar flare. This burst of energy from the sun caused an intense aurora. At the same time, Jupiter showed a broad swath of warm gases south of the aurora. This could have been a wave of warmth rolling southward. “It was pure luck that we captured this potential heat-shedding event,” says O’Donoghue.
These observations “are close to a ‘smoking gun’” for explaining Jupiter’s mysterious heat, says Tommi Koskinen. He was not involved in the new study. But he is a planetary scientist at the University of Arizona in Tucson. The next step is to understand just how Jupiter’s auroras create and share heat, he says. Those processes can then be added to computer models of Jupiter’s atmosphere.