The driving force behind Jupiter's continued "fever" has been found, it turns out to be this!

According to our common understanding, the closer a planet is to the sun, the higher its temperature, and the farther it is from the sun, the lower its temperature. But this does not make sense for Jupiter. Although Jupiter is very far away from the sun, ranking behind the Earth and Mars, the temperature of Jupiter's upper atmosphere can reach 400 degrees Celsius, which can be said to be constantly "feverish".

Schematic diagram of the solar system. Do you know which one is Jupiter? (Image source: veer Gallery)

This anomaly has puzzled scholars for 50 years, but a recent study has finally found the "pushing force behind the scenes" and provided an answer to the confusing "Jupiter energy crisis."

Jupiter Profile

Let’s first get to know Jupiter, which is famous for its huge size.

Jupiter is a giant gaseous planet, 2.5 times the mass of all other planets combined, and is mainly composed of hydrogen and helium. Jupiter has many satellites, among which Io to Callisto can be seen with a telescope in a clear night sky. The following picture is a photo of Jupiter and its three satellites taken by the author using a telescope in Devengro, the Netherlands.

(Image source: provided by the author)

Jupiter has the strongest planetary magnetic field in the solar system, with a magnetic moment about 18,000 times that of the Earth's magnetic field. Compared to the Earth, Jupiter is very far away from the Sun, with the semi-major axis of Jupiter's orbit being 5.204 AU, about five times the distance between the Sun and the Earth.

The "Jupiter Energy Crisis" has troubled people for fifty years

Intuitively, Jupiter, which is farther from the sun, should be colder than the Earth. Based on the amount of solar radiation received, the average temperature of Jupiter's upper atmosphere should be about minus 73 degrees Celsius. However, the actual measured value is higher than 400 degrees Celsius. This unexplained abnormal high temperature is called Jupiter's "energy crisis." The energy crisis facing the Earth is that the available energy is close to exhaustion. Unlike the energy crisis on Earth, Jupiter's energy crisis is "there is energy (heat energy) but no source can be found."

Jupiter's abnormally high temperature has troubled humanity for fifty years. During this period, troubled scholars have proposed many hypotheses about the heating mechanism of this overheated gaseous planet. Some scholars believe that it is caused by Jupiter's super storm, the Great Red Spot, because according to measurements, the Great Red Spot is indeed hotter than the surrounding area.

Scholars speculate that superstorms generate a large number of disturbances in the lower atmosphere, which propagate upward in the form of gravity waves and sound waves and dissipate into heat energy in the upper atmosphere, heating the upper atmosphere. However, these speculations cannot provide a complete description of the mechanism of global heating of Jupiter.

Image of the Great Red Spot heating up (Image credit: NASA)

Spectral radiation intensity indicates that the Great Red Spot is a high-temperature area (Image source: Nature 536,190–192 (2016))

Jupiter's atmosphere is baked by the aurora

Recently, J. O'Donoghue of the University of Leicester led a team to conduct a multi-instrument joint observation of Jupiter. The main result of this observation came from the high spatial resolution measurement of the temperature of Jupiter's atmosphere. The team used three instruments: Keck Observatory in Hawaii - using a near-infrared telescope to observe the emission spectrum of trihydrogen cations to measure temperature; NASA's Juno spacecraft and Japan's JAXA's Hisaki satellite were used to assist in observing the magnetic field and Jupiter's satellite "Io".

The figure below is the observation result of temperature distribution. High temperature distribution appears in the auroral regions in the north and south of Jupiter, while relatively low temperature gradually transitions from the poles to the equator. This shows that Jupiter's polar regions are the main heat source in its upper atmosphere, and the heat in the polar regions can be transmitted across the globe to the equatorial region, heating the entire upper atmosphere of Jupiter.

Temperature distribution of Jupiter (Image source: Nature 596, 54–57 (2021))

This means that the research team has confirmed that Jupiter's abnormally high temperature upper atmosphere is heated by the auroral regions at the North and South Poles (emphasis added), solving the mystery of Jupiter's abnormally high temperature upper atmosphere that has puzzled us for 50 years. This result has now been published in the scientific journal Nature.

'Volcano Moon' Behind Jupiter's Aurora

Why are Jupiter's auroras so strong? We need to ask Io.

Jupiter itself has a special structure. As a huge gas planet, it has the strongest planetary magnetic field in the solar system, and there is a very active satellite in the magnetic field - Io. Io's orbit is an eccentric orbit. The tidal effect in different orbital phases stretches differently, causing friction in the internal structure, triggering active geological activities, and forming hundreds of volcanoes on Io.

Volcanic eruptions on Io will form a neutral gas exosphere around it. The gas atoms interact with the electrons and protons in Jupiter's magnetosphere to ionize and form charged particles. These charged particles are accelerated in Jupiter's magnetic field to become high-energy charged particles. Some high-energy charged particles are confined in Jupiter's magnetic field, helping Jupiter to build a radiation belt that is tens of millions times stronger than that of the Earth. The figure below describes the process of Io generating high-energy charged particles in Jupiter's magnetic field.

Io, the plasma ring, and Jupiter's magnetic field (Image source: Wikipedia : Io (moon))

These charged particles travel along the strong magnetic field to Jupiter's polar regions, enter Jupiter's atmosphere, and after settling, produce the strongest aurora in the solar system and release a large amount of energy. These energies are the "energy" source of the abnormally high temperature on Jupiter's surface.

Jupiter's Aurora (Image credit: NASA)

Why are the Earth's poles so cold even though we have the same aurora?

Seeing this, you may have a question - they are also planetary auroras, why don’t the Earth’s auroras have such powerful energy?

The essential difference lies in the different supply of high-energy charged particles. Io's volcanoes and Jupiter's own super-strong magnetic field provide a stable and large supply of high-energy charged particles to Jupiter's magnetosphere, while the Earth's aurora comes from the injection of high-energy charged particles in the solar wind (Editor's note: refers to the supersonic plasma charged particle flow emitted from the upper atmosphere of the sun). At the same time, the Earth's magnetic field is relatively weak, and its ability to transport high-energy charged particles to the poles is not strong, and not a large number of particles are transported to the poles.

Therefore, the intensity of the Earth's aurora is much smaller than that of Jupiter's aurora. It cannot significantly heat the atmosphere and has no significant impact on the Earth's climate. The Earth is still cold at the poles and hot at the equator.

If the Earth's magnetic field becomes 10 times stronger, the intrinsic magnetic field (Editor's note: the magnetic field that spontaneously occurs, maintains and changes inside a planet) can completely cover the lunar orbit. At the same time, volcanic eruptions will begin on the moon, injecting a large amount of charged particles into the Earth's radiation belt. Then, extremely bright auroras will appear at the Earth's poles, and the heating effect will cause the poles to be hotter than the equator.

The Earth's aurora was photographed from the International Space Station (ISS) (Image credit: NASA)

Model + observation, understanding the unknown side of planets

We know very little about planets other than Earth, mainly because they are too far away. We can only try to describe the physical processes occurring on distant planets through the "observation + modeling" method.

The research process described in this article is a good example. In fact, in previous observations, researchers have discovered that Jupiter's aurora region is a high-temperature area, but the model at the time believed that the latitudinal circulation in Jupiter's atmosphere isolated the heat in the polar region and prevented it from spreading to the equatorial region. In contrast, the Great Red Spot is closer to the equator. The joint observations described in this article, using an unprecedented resolution (2 degrees/pixel), found that the heat was not confined to the polar regions, but gradually transitioned from high temperatures at the poles to low temperatures at the equator. This observation result discovered a phenomenon that could not be described by previous models, which actually proposed a direction for improving the model.

On the one hand, as technology develops, instruments can achieve higher and higher resolutions, and probes can fly farther and even land on some planets for in-situ detection. On the other hand, by collecting and assimilating observation data, models can be improved to predict and describe corners that instruments cannot observe. Models and observations can be said to be our two eyes for deep space exploration and understanding more planets. Models guide observations, and observations improve models.

The "Jupiter Energy Crisis" has an answer. In the future, there will be more stars waiting for us to explore their unknown side. What cosmic mysteries have you been bothered by for a long time? Feel free to express your curiosity in the comment area!

References:

[1] O'Donoghue J, Moore L, Bhakyapaibul T, et al. Global upper-atmospheric heating on Jupiter by the polar aurorae[J]. Nature, 2021, 596(7870): 54-57.

[2] O'Donoghue J, Moore L, Stallard TS, et al. Heating of Jupiter's upper atmosphere above the Great Red Spot[J]. Nature, 2016, 536(7615): 190-192.

[3] Lam HA, Achilleos N, Miller S, et al. A baseline spectroscopic study of the infrared auroras of Jupiter[J]. Icarus, 1997, 127(2): 379-393.

[4] Jupiter's Great Red Spot Likely a Massive Heat Source: NASA

[5] Jupiter’s magnetosphere https://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter

Author: Zhang Peijin

Unit: University of Science and Technology of China

The article was first published in Science Park and only represents the author’s views and does not represent the position of Science Park.

Science Academy is the official science popularization micro-platform of the Chinese Academy of Sciences. It is hosted by the Science Communication Bureau of the Chinese Academy of Sciences and operated by the China Science Popularization Expo team. It is committed to in-depth interpretation of the latest scientific research results and scientific voice on hot social events.