an observer's guide to jupiter

Basic Information

Jupiter is the fifth planet from the Sun -- its orbit lies between Saturn and the Asteroid Belt, which separates it from Mars. At 142,984 km (88,846 miles) in diameter, Jupiter is the largest planet in the solar system. The other eight planets combined add up to less than 69% of Jupiter's volume. It would take 1,408 Earths to fill a sphere the size of Jupiter. However, Jupiter is not a sphere. It has a very rapid rotation - a day on Jupiter is only 9 hours, 55 minutes long - that flattens the planet so that its equatorial diameter is actually 7% larger than its polar diameter. This squashed shape is obvious even in small telescopes.

It can be argued that Jupiter is the most rewarding planet to observe with a small telescope. Factors in Jupiter's favor are:

• It can be observed every year -- not just during favorable oppositions like Mars
• It presents the largest planetary disc in the telescope except for Venus within a few days of inferior conjunction
• Its changing cloud features and rapid rotation rate give us an ever-changing show
• The Galilean moons display a constant series of transits, eclipses, and occultations

Entry point of the Galileo Probe

The Jovian atmosphere is made up of roughly the same mix of elements as found in the Sun. Molecular hydrogen makes up more than 92%, with helium accounting for most of the remainder (more than 7%). All the other elements add up to less than 0.3% and consist primarily of methane (0.1%), water (0.1%), ammonia (0.02%), ethane (0.0002%), and assorted hydrocarbons. Because of the similar chemical makeup, it had been proposed that Jupiter is actually a failed star. However, at 0.001 solar masses, it is about 80 times too small to support nuclear fusion at its core.

As with all "gas giants" (Jupiter, Saturn, Uranus, and Neptune), when we observe Jupiter, we are only seeing the top of its cloud layers. It is believed that Jupiter's upper atmosphere contains three distinct cloud layers: a topmost layer of ammonia ice, a middle layer of ammonium hydrosulfide, and a thick final layer of water and ice. It was hoped that the Galileo probe would confirm this structure, but it actually penetrated the atmosphere in one of the most cloud-free regions of the planet. Only a trace of the middle cloud layer was found.

Structure

The structure of a gas giant planet, like Jupiter, is completely foreign to those of us who live on rocky planets like the Earth. The Earth has a clear demarcation between the atmosphere and the surface. Jupiter, on the other hand, has no "surface" to speak of at all. From the cloud tops that we see, to the innermost rocky core, it appears that the density of material in Jupiter increases smoothly -- from gas, through liquid, to solid. The exact details of the interior are not fully understood, but the current "best guess" is as follows:

It is believed that there is a very small rocky core, composed of elements and compounds very much like those found on Earth. While this core may be as large as 30 Earth-masses, it is tiny in proportion to the size of Jupiter. The tremendous gravitational pressures at the core, about 70 million times that of sea-level on Earth, compress this material to about twice the density of lead.

The majority of Jupiter's mass is contained in the next layer up, although the transition from the rocky core is likely a gradual one. This layer is composed of liquid metallic hydrogen -- a material that can only exist at pressures above 3.9 million atmospheres. These pressures rip the electrons from the hydrogen atoms, converting it to an electrically conductive liquid. It is this layer that powers Jupiter's immense magnetic field. It is possible that this layer also contains small amounts of helium and crystalline ammonia or methane.

Interior structure of the Earth and Jupiter.
Both planets are scaled to the same size for comparison.

Above the liquid metallic hydrogen layer, reduced pressures allow hydrogen and helium to exist as normal liquids, and at the very highest levels, as gases. Tiny amounts of water, methane, ammonia, and other compounds are present in these layers. At the very top of the gaseous hydrogen layer are the clouds which form the "surface" of Jupiter we see from Earth. It is believed that there are three cloud layers, but only the topmost layer of ammonia ice clouds are visible.

Subtle chemical processes in the ammonia ice clouds, most likely involving sulfur, are responsible for the different color clouds that we see, and the colors seem to be tied to the cloud's altitude. Bluish clouds are the lowest, and are only seen through holes in the higher layers. Browns and whites make up the middle, and reddish colors are the highest.

Magnetic Field and Io

The rapidly spinning, electrically conductive, liquid metallic hydrogen region in Jupiter's core gives rise to the largest and most complex magnetic field in the solar system. At the cloud tops, it is 10 times stronger than the Earth's and extends more than 7 million km (4.3 million miles) into space. In fact, Jupiter's magnetic field is the largest structure in the solar system. Viewed from the Earth, it would appear larger than the full moon if it were visible.

Jupiter's inner satellites, Amalthea, Io, Europa, and Ganymede, all orbit within the most intense portion of the field and are affected by it, especially Io. Io's motion through Jupiter's magnetic field creates an electrical charge that reaches as much as 400,000 volts between Io's north and south poles and dissipates more than 1 trillion watts! In certain orbital positions, up to 5 million amps of electric current flows between Io and Jupiter's ionosphere.

Its close proximity to Jupiter causes Io to experience extreme tidal forces that bend its surface as much as 100 meters each orbit. This bending and stretching generates enough heat to make Io the most volcanically active body in the solar system. Sulfur and other heavy ions spewed into space by Io's volcanoes interact with Jupiter's magnetic field to produce a ring of charged particles that follow Io's orbit. Called the Io Plasma Torus, its highly energetic charged particles make it an extremely high radiation area which is lethal to both manned and unmanned space vehicles. Io, in turn, is being constantly eroded by collisions with these particles as it orbits Jupiter.

Belts, Zones and Winds

Jupiter's ammonia clouds are arranged into a series of light and dark bands ranging from about 45° north latitude down to 45° south latitude. The light areas, which are called zones, are regions where convection from heat lower down in the planet is causing the atmosphere to rise. The risen atmosphere then cools and sinks back into deeper regions causing darker areas referred to as belts.

Jupiter's wind system is also tied to the belts and zones visible in the cloud tops. The prevailing winds in the equatorial zone are eastward -- in the same direction that Jupiter rotates. The Galileo Probe measured a wind velocity of 700 km/hour (435 mph) near the north edge of the equatorial zone and found that this wind speed is constant with depth. In the belts to the north and south, the winds are westward -- the opposite direction from the equatorial zone. The wind direction reverses and the velocity lessens in each successive band until the winds die out completely beyond 45° latitude. The visible band structure ends at this latitude as well.

Central Meridian Systems

Cloud features on Jupiter, in the presence of convection currents and the fierce winds described above, may drift in longitude across the planet. The measurement of longitude on Jupiter is complicated by the lack of any fixed feature on which to base 0° longitude. In addition, it was noticed by early observers that features near the equator of Jupiter move at a more rapid rate than those in the temperate or polar regions.

In order to permit longitude measurements, two systems of arbitrary longitude with different rotation rates were created. System I is used for all markings within 10° of the equator. System I has a standardized rotation period of 9 hours 50 minutes 30.003 seconds, or 877.9° of rotation in 24 hours. System II applies to all areas between 10° latitude and the poles. It has a rotation period of 9 hours 55 minutes 40.632 seconds, or 870.27° in 24 hours. There is a third system, System III, which is the rate of rotation of Jupiter's core. It is generally of interest only to radio astronomers since the period of radio bursts from Jupiter is tied to this system.

By comparing the rotation period of a given feature with the rotation period of the system it is located in, fairly precise measurements of its drift over time can be made. To measure the exact longitude of any feature it is necessary to look up the current longitude of the central meridian -- the imaginary line joining the poles that bisects Jupiter into east and west halves. This data can be found in the Astronomical Almanac or simply refresh this page to update the table below.

Cloud Features

There are several features that are commonly observed along the edges of zones and belts. What follows is the standardized nomenclature for describing these features.

	Garlands or Wisps - Faint narrow streaks extending from the edge of a belt into the adjacent brighter zone. May go in either direction and may bend back to the belt.
	Festoons - Thin bands of dark cloud material connecting belts across the intervening zone. Often difficult to observe.
	Ovals - Light oval or circular areas that may appear on belts or zones. Often some of the brightest features. Most commonly seen in the South Tropical Zone, South Temperate Zone and South Temperate Belt.
	White Spots - Well defined light areas much smaller than ovals, and usually smaller than satellite shadows. Most commonly seen in the North and South Equatorial Belts.
	Rifts - Light, sharply defined linear features that frequently extend to both edges of a dark belt so that the two adjacent zones are connected.
	Notches or Bays - Sharply defined semicircular indentations along the straight edge of dark belts.
	Projections - Dark or light irregularities along the edges of belts.
	Knots - Thickenings seen in narrow belts.
	Dark Concentrations - Dark, ill-defined areas within belts.
	Rafts - Horizontal linear markings usually seen in the North Tropical Zone, just separated from the North Equatorial Belt.
	Bars - Short, thick segments usually found along the edge of belts. May be reddish and are larger than rafts.
	Linear Markings - Fine grayish or reddish lines found in belts.

Great Red Spot

Jupiter's Great Red Spot (GRS) is a massive anti-cyclone or high-pressure storm that has persisted for at least 100 years, although there is some evidence that it may have been observed as early as 1664. The nearest analogy on Earth would be a hurricane. Located at about 22° south latitude, the GRS rotates counterclockwise with a rotational period of about 6 days.

The GRS is large enough to fit roughly three Earths within it -- it is 14,000 km (8,669 miles) south to north and has a variable east-west width of 20,000 to 40,000 km (12,427 to 24,855 miles). The top of the GRS is about 8 km (5 miles) higher than the other clouds on Jupiter.

The size and persistence of the GRS are due to several factors:

• The Coriolis effect that causes the rotation of storms on Earth is greatly increased under Jupiter's more rapid rotation rate
• Jupiter's internal heat provides more energy for its weather than the Sun does on Earth
• There are no land masses on Jupiter, which tend to dissipate storms on Earth
• Strong storms on Jupiter tend to absorb weaker ones, increasing the size of the larger storm

The GRS has changed considerably in size and color over time and has drifted significantly in longitude. For example, the GRS moved 1046° in longitude just between 1831 and 1838 -- equal to almost three complete circuits of the planet. It is also not entirely clear how long the GRS will last. In 2001, it was less than half as large as it was in 1900.

Filters

Color filters often aid in seeing various cloud details on Jupiter. What follows is a list of Kodak Wratten filter numbers, general colors, and features best observed with each.