1. How Does Juno See Beneath Jupiter's Thick Clouds?
Jupiter is covered entirely by thick clouds. Telescopes and normal cameras can only see the tops of these clouds. To know what happens underneath, scientists must use things other than visible light. Juno was launched on August 5, 2011, and entered orbit around Jupiter on July 4, 2016 [1]. The main tools for this article are a microwave radiometer that sees through clouds, gravity science that measures mass distribution, and a magnetometer that measures magnetic fields. Juno also has JIRAM, which takes infrared pictures of polar auroras, and JunoCam, a visible light camera used to study cloud movements [1]. With these tools, scientists first looked at the polar storms, which surprised them the most.
2. Why Does Jupiter Have Eight Storms at the North Pole and Five at the South?
Juno flew over both poles of Jupiter and took pictures of the storm arrangements. At the north pole, there is one central storm surrounded by eight others. At the south pole, one central storm is surrounded by five. This was reported in the scientific journal Nature on March 8, 2018 [2]. Some news reports count the central storm in the total, saying there are nine storms in the north and six in the south. This is just a different way of counting the same arrangement.
Researchers were said to be surprised when they saw the first images, noting they had never seen such a thing. Some researchers had expected shapes like the hexagon found at Saturn's north pole [2]. According to NASA, infrared observations show that these storm arrangements are very stable and stay in the same place [3]. This means the storms are not just temporary shapes but seem to stay in the same location for a long time. This raises the question of how far down the striped patterns seen on the cloud tops go.
3. How Deep Do Jupiter's Striped Winds Reach?
The stripes on Jupiter are bands of strong winds blowing east and west. The question was whether these winds only happen near the top of the clouds or if they go deeper. Measurements of gravity by Juno showed that these banded winds reach about 3,000 km (about 1,864 miles) down from the cloud tops [4]. This distance is similar to the length of the Japanese archipelago from north to south.
The mass of the part of Jupiter where the wind moves is about 1% of Jupiter's total mass [4]. In comparison, Earth's atmosphere is less than one-millionth of Earth's mass. When comparing these ratios, the part of Jupiter moved by wind is more than 10,000 times larger than Earth's atmosphere. This means a thick layer under the clouds is being moved as a whole.
4. How Can Scientists Measure Jupiter's Pull on Juno From Over 600 Million km Away?
As the spacecraft gets close to Jupiter, Jupiter's mass distribution pulls on it, causing its speed to change slightly. This change appears as a shift in the radio waves reaching Earth [4]. The measured speed change was about 0.01 mm per second. This is a speed change that would take 100 seconds to move 1 mm. NASA's Deep Space Network antennas caught this tiny change from Earth, which is more than 400 million miles (about 640 million km) away [3]. By studying the difference in gravity between the northern and southern hemispheres, scientists can read traces of wind flow. Small shifts in the arrangement of mass provided clues about the depth of the winds.
The microwave radiometer can look down about 150 km (about 93 miles) under the clouds, to a depth where the pressure is about 33 atmospheres [6]. It can also measure the depth of storms. Some storms reach about 100 km (about 62 miles) down from the cloud tops, while the Great Red Spot reaches more than 350 km (about 217 miles). Estimates combining multiple measurements suggest the Great Red Spot may go down about 500 km (about 311 miles) [3].
5. How Much Water Is on Jupiter?
In 1995, the Galileo probe entered Jupiter's clouds and measured water. The result was only about one-tenth of what was expected [6]. The amount of water is an important clue for understanding how Jupiter formed, so this low amount remained a mystery.
Juno studied the area near the equator more deeply. Results reported in Nature Astronomy in February 2020 showed that water near the equator is about 0.25% of the molecules, which is about three times the amount found in the Sun [6]. It is explained that the Galileo probe may have just happened to enter a dry spot. The microwave radiometer can see about 150 km (about 93 miles) under the clouds, and it was found that water mixes differently in different places, and the deep atmosphere changes with latitude [6]. This value is for the equator and may not represent all of Jupiter.
Models based on precise gravity measurements suggest that Jupiter's core is larger than expected. Heavy materials may not be gathered only in the center but spread into the surrounding hydrogen, creating a "dilute core" [5]. However, this is a model result that depends heavily on the choice of equation of state and is not a fixed conclusion.
6. How Does Jupiter's Magnetic Field Change Over Time?
Jupiter has a strong magnetic field. Juno's magnetometer reported in Nature Astronomy in May 2019 that this magnetic field changes over time from inside the planet. For the first time, this change was clearly detected on a celestial body other than Earth [7]. Most of this change is caused by a strong magnetic field area near the equator, called the "Great Blue Spot" [7]. It is thought that winds in the deep atmosphere under the clouds cause this change [7]. The idea that winds move the interior connects the depth of the winds in section 3 with the changing magnetic field in this section. However, this is a view on the cause and is not a confirmed conclusion.
7. How Small Is the Speed Change Juno Measured, and How Are the Storms Arranged?
To understand the tiny speed change Juno measured, try moving the tip of a pencil along the 1 mm mark on a ruler for 100 seconds. This very slow movement, which looks almost stopped, is a guide to the size of the 0.01 mm per second change measured by Juno.
Place one 1-yen coin (or a similar small coin) on a table. Try placing other identical coins around it, touching the center coin with no gaps. You can fit six coins around the center one. Since the north pole has eight storms, a model using equal-sized circles touching the center cannot reproduce Jupiter's storm arrangement. This is because the actual storms are not necessarily the same size.