11.1: Exploration

Observations from Earth

Spacecraft

Figure 11.2 : Jupiter. The Cassini spacecraft imaged Jupiter on its way to Saturn in 2012. The giant storm system called the Great Red Spot is visible to the lower right. The dark spot to the lower left is the shadow of Jupiter’s moon Europa. (credit: modification of work by NASA/JPL)

Table 11.1: Abundances in the Outer Solar System

Type of Material	Name	Approximate % (by Mass)
Gas	Hydrogen (H2)	75
Gas	Helium (He)	24
Ice	Water (H2O)	0.6
Ice	Methane (CH4)	0.4
Ice	Ammonia (NH3)	0.1
Rock	Magnesium (Mg), iron (Fe), silicon (Si)	0.3

In the outer solar system, gases dominate the two largest planets, Jupiter and Saturn, hence their nickname “gas giants.” Uranus and Neptune are sometimes called “ice giants” because their interiors contain far more of the “ice” component than their larger cousins. The chemistry for all four giant planet atmospheres is dominated by hydrogen. This hydrogen caused the chemistry of the outer solar system to become reducing, meaning that other elements tend to combine with hydrogen first. In the early solar system, most of the oxygen combined with hydrogen to make H₂O and was thus unavailable to form the kinds of oxidized compounds with other elements that are more familiar to us in the inner solar system (such as CO₂). As a result, the compounds detected in the atmosphere of the giant planets are mostly hydrogen-based gases such as methane (CH₄) and ammonia (NH₃), or more complex hydrocarbons (combinations of hydrogen and carbon) such as ethane (C₂H₆) and acetylene (C₂H₂).

Jupiter is the largest planet in the solar system; all of the other planets could fit inside Jupiter. One of its major atmospheric features is the Great Red Spot – cyclonic in nature and twice Earth’s diameter. The Great Red Spot was first seen by Galileo in 1610 and has been seen to change shape and color over the past 400 years.

Link to Learning

The Jet Propulsion Laboratory has a nice video called Voyager: The Grand Tour that describes the Voyager mission and what it found.

Making Connections: Engineering and Space Science: Teaching an Old Spacecraft New Tricks

By the time Voyager 2 arrived at Neptune in 1989, 12 years after its launch, the spacecraft was beginning to show signs of old age. The arm on which the camera and other instruments were located was “arthritic”: it could no longer move easily in all directions. The communications system was “hard of hearing”: part of its radio receiver had stopped working. The “brains” had significant “memory loss”: some of the onboard computer memory had failed. And the whole spacecraft was beginning to run out of energy: its generators had begun showing serious signs of wear.

To make things even more of a challenge, Voyager’s mission at Neptune was in many ways the most difficult of all four flybys. For example, since sunlight at Neptune is 900 times weaker than at Earth, the onboard camera had to take much longer exposures in this light-starved environment. This was a nontrivial requirement, given that the spacecraft was hurtling by Neptune at ten times the speed of a rifle bullet.

The solution was to swivel the camera backward at exactly the rate that would compensate for the forward motion of the spacecraft. Engineers had to preprogram the ship’s computer to execute an incredibly complex series of maneuvers for each image. The beautiful Voyager images of Neptune are a testament to the ingenuity of spacecraft engineers.

The sheer distance of the craft from its controllers on Earth was yet another challenge. Voyager 2 received instructions and sent back its data via on-board radio transmitter. The distance from Earth to Neptune is about 4.8 billion kilometers. Over this vast distance, the power that reached us from Voyager 2 at Neptune was approximately10^–16 watts, or 20 billion times less power than it takes to operate a digital watch. Thirty-eight different antennas on four continents were used by NASA to collect the faint signals from the spacecraft and decode the precious information about Neptune that they contained.

Galileo

The Galileo spacecraft was launched toward Jupiter in 1989 and arrived in 1995. Galileo began its investigations by deploying an entry probe into Jupiter, for the first direct studies of the planet’s outer atmospheric layers.

The probe plunged at a shallow angle into Jupiter’s atmosphere, traveling at a speed of 50 kilometers per second—that’s fast enough to fly from New York to San Francisco in 100 seconds! This was the highest speed at which any probe has so far entered the atmosphere of a planet, and it put great demands on the heat shield protecting it. The high entry speed was a result of acceleration by the strong gravitational attraction of Jupiter.

Atmospheric friction slowed the probe within 2 minutes, producing temperatures at the front of its heat shield as high as 15,000 °C. As the probe’s speed dropped to 2500 kilometers per hour, the remains of the glowing heat shield were jettisoned, and a parachute was deployed to lower the instrumented probe spacecraft more gently into the atmosphere (Figure 11.3). The data from the probe instruments were relayed to Earth via the main Galileo spacecraft.

Figure 11.3 : Galileo Probe Falling into Jupiter and Juno Image of Jupiter's South Pole. (a) This artist’s depiction shows the Galileo probe descending into the clouds via parachute just after the protective heat shield separated. The probe made its measurements of Jupiter’s atmosphere on December 7, 1995. (b) This Juno image, taken in 2017 from about 100,000 kilometers above the cloudtops, shows the south polar region of Jupiter with its dramatic complex of storms and clouds. The enhanced-color image was processed for NASA/JPL by citizen scientist John Landino. (credit a: modification of work by NASA/Ames Research Center; credit b: modification of work by NASA/JPL-Caltech/SwRI/MSSS/John Landino)

The probe continued to operate for an hour, descending 200 kilometers into the atmosphere. A few minutes later the polyester parachute melted, and within a few hours the main aluminum and titanium structure of the probe vaporized to become a part of Jupiter itself. About 2 hours after receipt of the final probe data, the main spacecraft fired its retro-rockets so it could be captured into orbit around the planet, where its primary objectives were to study Jupiter’s large and often puzzling moons.

The Voyager and Galileo missions to Jupiter were primarily designed to study the moons and the atmosphere of the planet. The next NASA mission, an orbiter called Juno, arrived at Jupiter in July 2016. In order to meet its objectives of studying the jovian magnetosphere, it has a very elongated (eccentric) 55-day orbit, that takes it from 4 thousand kilometers above the cloud tops out to 76 thousand kilometers. The orbit takes the craft over Jupiter’s poles, giving us remarkable close-ups of the polar regions (previous spacecraft viewed the planet from lower latitudes).

Juno was originally designed without a camera, but fortunately scientists rectified this omission, adding a simple downward-looking color camera to use during close passes by Jupiter. Recognizing the value of such images, both scientific and artistic, it was decided to post the raw images and encourage “citizen scientists” to process them. The product has been many dramatic, brightly colored views of Jupiter, such as Figure 11.3.

 

Lightning and thunderstorms have been observed in Jupiter’s upper atmosphere.

Jupiter has some other interesting features, including:

• It gives off more heat than it receives.
• It has a small, dense rocky core surrounded by hydrogen and some helium even though it is primarily a Gas Giant planet. Some speculate the core is solid hydrogen.
• It is a natural radio source, like the Sun and other stars.
• It has aurora events, much like Earth.

Jupiter at a Glance

Characteristics to Compare

Atmosphere

• Hydrogen (H ₂ ), Helium (H _e ), Methane (CH ₄ ), Ammonia (NH ₃ ), Water (H ₂ O)

Rotation

• Short rotation — 9. 8 hours

Revolution

• Long revolution — 11. 86 years

Rings

• One thin ring detected by the Voyager 2 spacecraft

Magnetic Field

• A very strong magnetic field; 20,000 times that of Earth’s

The planet Juptier via NASA Cassini SpacecraftPublic Domain | Image courtesy of NASA.

Jupiter’s Great Red Spot via NASA Voyager 2Public Domain | Image courtesy of NASA.

Jupiter’s Great Red Spot with turbulent cloud tops via NASA Galileo SpacecraftPublic Domain | Image courtesy of NASA.

Lightning in Jupiter’s upper atmospherePublic Domain | Image courtesy of NASA.

Attributions

https://phys.libretexts.org/Bookshel...Planet_Jupiter 

https://phys.libretexts.org/Bookshel..._Outer_Planets