Jupiter

Young Astronomers Blog, Volume 28, Number 18.

If you look up in the night sky over the next few months you might see what looks like a bright star. However, it is not a star, it is the planet Jupiter.

Jupiter is the largest planet in our solar system, at 11 times the width of the Earth, and the first of the gas giants. Jupiter is five times farther from the Sun that the Earth at almost a half billion miles and takes just under 12 years to complete one orbit. For such a large planet, Jupiter rotates quite fast completing one revolution in ten hours.

For Ancient astronomers Jupiter was one of the five “wanderers” that moved against the background of stars. Galileo was the first to observe Jupiter up close in 1610. On January 7^th, Galileo pointed his telescope toward Jupiter. He couldn’t see any detail on the planet, but he observed three small “stars” off to the side. He noticed over the next few nights that the stars appeared to move. He soon observed a 4^th star. He eventually concluded that the stars were moons orbiting Jupiter and published his results in The Starry Messenger (Sidereus Nuncius) in March 1610.

Galileo initially named the moons “the Medician stars” to honor the four Medici brothers including his patron Cosimo II de’ Medici. These names didn’t catch on. Simon Marius, who claimed to have seen the moons before Galileo, and after a suggestion by Johannes Kepler, offered the current names for the four “Galilean” moons of Jupiter, which are Io, Europa, Ganymede, and Callisto, from Greek mythology.

The moons themselves are among the largest in the Solar System and have some interesting characteristics.

• Io is constantly being stretched by Jupiter’s gravity and is covered by sulfur volcanoes.
• Europa has a surface of ice and an underground ocean of liquid water.
• Ganymede, the largest moon in the Solar System, is larger than the planet Mercury. Its surface is divided into older/darker regions and younger/lighter regions.
• Callisto is just a bit smaller than Mercury and is one of the most cratered objects in the Solar System.

As larger telescopes were built, some of the features of Jupiter became evident. Jupiter has striped patterns and a rather distinct “red spot.” The stripes turned out to be clouds at the lowest level of Jupiter’s atmosphere (troposphere) separated by high winds (jet streams).

• Zones are the light bands found at higher altitudes formed by ascending high-pressure gas with clouds of ammonia.
• Belts are dark bands found at lower altitudes formed by descending low-pressure gas with clouds of ammonia and sulfur (ammonium hydrosulfide).
• Water Clouds are found a bit lower in the atmosphere.

The north and south equatorial belts are typically easy to spot even through a small telescope.

The Great Red Spot is a storm (anticyclone) first observed by Giovanni Cassini in 1665 and possibly by Robert Hook a year earlier in 1664. After 1713, it wasn’t found again until around 1831. The Great Red Spot is higher in altitude that most of Jupiter’s clouds and takes six to seven Earth days to complete one rotation. Over the past few years, astronomers have noticed that Jupiter’s Great Red Spot is shrinking! We are not sure what will happen in the future? Will it go away, or will it stabilize? We just don’t know.

Jupiter’s interior is similar to the other gas giants consisting of mostly molecular hydrogen (H₂) with a bit of helium (He). Hydrogen gas is found at the surface. As we go deeper, the pressure builds and the temperature increases, bringing us to a huge area of liquid hydrogen below which is liquid metallic hydrogen. Finally, there might be a small rocky and metallic center. The metallic hydrogen gives Jupiter a huge magnetic field that stretches all the way to the orbit of Saturn.

Jupiter has been called the vacuum cleaner of the Solar System. Its size and gravity capture objects (comets and asteroids) that otherwise would enter the inner part of the Solar System. Back in July 1994, a comet called Shoemaker-Levy 9 headed for Jupiter. As it got closer the comet was ripped apart by Jupiter’s gravity. The pieces slammed into Jupiter creating huge scars on the planet’s surface.

In July 1972, Pioneer 10 became the first spacecraft to pass through the asteroid belt before flying by Jupiter in December 1973. Pioneer 11 followed a year later. Next came Voyager 1 & 2, the two spacecraft that performed a “grand tour” of the outer Solar System in the 1970s and 1980s. Between January and July 1979, Voyagers 1 & 2 flew by Jupiter taking thousands of photographs of the planet and its moons.

The first spacecraft to orbit Jupiter was Galileo. Launched from the Space Shuttle Atlantis in October 1986, it took a long winding route to Jupiter before arriving in 1995. NASA nicknamed the path VEEGA for Venus-Earth-Earth Gravity Assist. Shortly after arriving at Jupiter, Galileo dropped a probe down into Jupiter’s clouds. The probe hit Jupiter’s atmosphere at over one hundred thousand miles per hour and reached a depth of around 150 km before the signal was lost after about an hour. Galileo’s mission ended in 2003 when the spacecraft was sent into the planet and burned up in the Jovian atmosphere.

Because of Jupiter’s size, it has been used by other missions to “slingshot” spacecraft toward the outer part of the solar system.

• Cassini, bound for Saturn, passed by in December 2000.
• New Horizons paid a visit on its way to Pluto in February 2007.

In 2011, the Juno spacecraft was launched toward Jupiter. It arrived on July 4, 2016 and is currently studying the atmosphere and interior of the giant planet. NASA has invited the public to download and enhance Juno images creating a catalog of amazing views of the planet. Just recently Juno turned its camera on Ganymede giving us the first ever look at the moon’s north pole, where it is raining plasma.

Two future Jupiter missions, the JUpiter ICy moons Explorer (JUICE) and the Europa Clipper, are scheduled to launch in the next few years.