Mohawk Valley Astronomical Society

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Beneath a Watchful Eye

by Perry Pezzolanella, MVAS

Jupiter is the largest planet with a diameter of 88,846 miles and can hold up to 1300 Earths. It is king in almost everything including storms large enough to swallow Earth, lightning that can span a continent, towering thunderclouds hundreds of miles high, and storms that can rage for centuries. Jupiter’s size is a factor, but why is it so stormy and colorful?

Jupiter is completely covered with clouds with no solid surface. The clouds range in color from red to white, blue, yellow, and brown. These colors are due to the chemistry of the atmosphere, which is composed of hydrogen, helium, ammonia, methane, sulfur, phosphorus, arsenic, and other compounds. The bluish clouds are the lowest and warmest; they are composed mainly of water. The brownish and yellowish clouds are higher and cooler; they are composed of ammonium hydrosulfide. The white clouds are the highest and coldest; they are composed of fine ammonia ice crystals. The tops of the clouds are as cold as –255ºF because Jupiter is 500 million miles from the Sun. At an altitude of 60 miles below the cloud tops, the temperature rises to 105ºF and 600 miles below the cloud tops the temperature soars to nearly 4000ºF. The increasing heat and pressure lower down make hydrogen gas act like a liquid and eventually like metal. There is a rocky core probably no larger than Earth and the pressure is so great that the core is still gradually collapsing, giving off huge amounts of heat. The heat rises into the cooler cloud layers and stirs them up into a riot of color and storms. Jupiter’s rapid rotation of 9 hours, 50 minutes stretches the clouds into belts that wrap around the entire planet. The brighter belts, properly called zones, are areas of rising air that condense out to form high ammonia ice clouds. The cooled air then begins to sink into the nearby belts. As the air sinks, it heats up and dries out to reveal a region of lower, warmer, darker clouds that appear as brownish belts. Jet stream winds over 400 miles per hour blow in opposite directions causing swirls and eddies in the clouds.

The Great Red Spot is the largest storm in the solar system, twice as large as Earth. It has a hurricane-like circulation that revolves in six days and is an area of rising air from deep within Jupiter. Phosphorus compounds are dredged up from the depths that give the spot its reddish tint. The Great Red Spot, which has raged since the invention of the telescope and probably longer, is like a huge bump in the atmosphere, with its stormy clouds being some of the highest and coldest observed on Jupiter. It persists because it resides between two opposing jet streams that keep it spinning. There is no solid surface beneath it to create friction and tear it apart, plus there is a steady source of heat from Jupiter’s core to feed it. Two smaller red spots have formed in recent years and Red Junior has persisted, but Baby Red, along with several other white spots, were devoured by the Great Red Spot when they wandered too close. This process may keep the Great Red Spot from shrinking and dissipating over time.

Jupiter has a thin sheet of very dusty rings and 64 known moons, most of which are probably captured asteroids, but four of them are large enough to be considered worlds in their own right: Io, Europa, Ganymede, and Callisto. They are known as the Galilean moons after their discoverer Galileo Galilei. There is no stranger moon than Io, which is slightly larger than Earth’s Moon at 2250 miles in diameter. Voyager 1 discovered at least eight active volcanoes on Io that stain it a wild range of colors including red, orange, yellow, white, brown, and black, which gives it the appearance of a bad pizza. These are huge volcanoes with plumes towering up to 300 miles and blistering hot flows of molten sulfur at 3000ºF, twice as hot as Earth’s lava! It is the sulfur in various states of melting that give Io its colors. The powerful flexing of Io by Jupiter and the other three Galilean moons creates surface tides as high as 300 feet and heats the interior to incredibly high temperatures. Volcanic activity is so intense that Io may have turned itself inside out several times in its lifetime! Tvashtar Catena is a beautiful area near Io’s north pole that is active with towering plumes up to 300 miles high, fiery geysers, brilliant sulfur flows, a fire pit, fissures, and a mesa all stained in the vibrant yellows of sulfur. The New Horizons spacecraft confirmed that Tvashtar was still active with a huge plume on the limb and the fiery glow of lava on the night side when it flew by in February 2007 on its way to Pluto. Io is so fascinating that a fully dedicate budget-minded mission known as the Io Volcano Observer is being proposed for launch in 2015 with arrival at Io in 2021 to monitor the violently changing volcanic activity.

Europa is slightly smaller than Earth’s Moon at 1950 miles in diameter and one of the most fascinating. It has a surface so smooth that the highest hills are only about 300 feet high, and it looks like a cracked egg. It is also one of the brightest objects in the solar system and this, along with its smoothness, led to speculation that a vast global ocean may lie beneath a thin icy crust. The Galileo spacecraft revealed evidence of an ocean by discovering areas of upheaval and collapse in the crust that may be due to upwelling of warmer pockets of water from below. The darker patches on the crust may be evaporated deposits that percolated through the cracks in the crust, erupted onto the surface, and then vaporized leaving behind a salty residue. Europa is a frigid world at -250ºF, but tidal flexing similar to Io generates heat in the interior to maintain a liquid ocean. This raises an exciting question as to whether there may be life on Europa. Unfortunately, the answer will have to wait until future spacecraft land there, analyze the salt deposits and possibly find a way to penetrate the icy crust to explore the ocean below.

Ganymede is the largest moon in the solar System with a diameter of 3267 miles and is larger than Mercury. It is made of a rock and ice mixture and looks like a large marble of light and dark contrasting features. Denser, dark rock covers the surface in large, straight-edged blocks broken by brighter icy flows that squeezed up beneath the crust through long fissures. The darker surface is the oldest and most of the brighter ice may have been vaporized by countless meteorite collisions. Ganymede also has whitish polar ice caps of thin water frost deposits. These may have formed when fissures released water vapor, which was then carried by Ganymede’s weak magnetic field towards the poles where it condensed. The fissures plus radiation from the Sun interacting with the surface ice may also be responsible for the thin oxygen atmosphere discovered around Ganymede. The craters have dark and light rays, which indicate the crust is layered like sedimentary rock. The Galileo spacecraft also detected the possible existence of a shallow ocean deep beneath the crust, deeper than Europa’s.

Callisto is a crater-ravaged world and is 2980 miles in diameter. There are so many craters that new ones can only form by destroying old ones. A giant impact basin, Valhalla, is nearly 1000 mi, which indicates the ice softened from the heat generated by the impact, then flowed and filled in the crater. All large craters have evidence of softening and flow fills while smaller craters are distinct with no fill indicating that insufficient heat was generated to melt the subsurface ice. There should be a vast area of chaotic terrain on the opposite side of the Valhalla basin where the shock waves would converge, however nothing unusual was seen. This could only mean that something inside Callisto absorbed the shock waves and the only material that could do this would be a liquid, most likely a vast subsurface ocean. This shallow ocean is theorized to lie very deep with Callisto, even deeper than Ganymede’s.

The highly successful Galileo orbiter mission ended in 2003 leaving many unsolved questions in the wake of all its discoveries that future spacecraft will try to answer. Juno will be a polar orbiter mission around Jupiter with launch during August 2011 and arrival on October 19, 2016. It will concentrate mainly on Jupiter; therefore a dedicated orbiter mission involving the moons is still necessary. Europa is the focus with a $4.5-billion dollar flagship mission known as the Europa Jupiter System Mission (EJSM) that will orbit Europa and analyze it with sophisticated equipment. It will also consist of a Ganymede orbiter, which will be developed by the European Space Agency (ESA) and launch independently, but will explore the Jupiter system in tandem with the NASA Europa orbiter during 2026-29. Jupiter is a mini solar system itself and the intense exploration in the decades ahead will be full of surprises and wonder, which will forever add to our knowledge of the King of Planets.