National Weather Service United States Department of Commerce

NASA Jupiter Image with Great Red Spot lower center     Jupiter is the fifth closest planet to our Sun and is the first planet beyond the relatively small, inner four, rocky planets.  It is the first of the four "gas giant" planets in proximity to the Sun.  Jupiter has 300 times the mass of Earth, but is less dense.  It is by far the largest planet in our solar system and has 2 1/2 times the mass of all the solar system's planets put together.  Jupiter has 63 known satellites and like Saturn, there is a large number of very small satellites orbiting Jupiter from about seven million to 13 million miles away.  In addition, the tiny satellites are all similar in structure, suggesting that they are pieces from a parent body.  Jupiter's average distance from the Sun is 480 million miles and takes nearly 12 years to make one revolution.  Like the rest of the gas giants, Jupiter has a ring, albeit small and flat.  Its rotation is the fastest of all solar system planets, rotating once on its axis every 10 hours.  This means at the equator, Jupiter is moving at 22,000 mph, compared with 1,000 mph for the Earth.  See what this does to Jupiter's weather below.  (For the curious, the small object to the lower left of Jupiter in the photograph above is Ganymede, one of its four large inner moons).

Atmosphere and Weather:   Jupiter's extremely dense and relatively dry atmosphere is composed of a mixture of hydrogen, helium and much smaller amounts of methane and ammonia.  The same mixture of elements which made Jupiter also made the Sun.  It is reasonable to assume, that under more extreme conditions, Jupiter could have evolved into a double-star companion to our Sun.  However, Jupiter would have had to become at least 80 times more massive to become a star.    

The atmosphere is probably a few hundred miles in depth, pulled toward the surface by the intense gravity.  Closer to the surface, the gases become more dense, and likely turn into a compound of slurry.  Pioneer's 10 and 11 found evidence that the planet itself is composed almost entirely of liquid hydrogen and that there likely is no real interface between the atmosphere and surface.  Jupiter's rocky core lies well below the "surface" and is very hot (around 36,000 degrees F.) due to gravitational compression (compression is a heating process).  But Jupiter is much too small and cool to ignite nuclear fusion reactions which are required to become a star.

As mentioned above, Jupiter's extremely fast rotation flattens the globe at the poles and drives extremely changeable weather patterns in the clouds which envelope the planet.  The clouds are likely made of ammonia ice crystals, changing to ammonia droplets further down.  It is estimated that the temperature of the cloud tops are about -280 degrees F.  Overall, Jupiter's average temperature is -238 degrees F.  Since Jupiter is only tilted slightly more then 3 degrees on its axis, seasonal fluctuations are minimal.

Jupiter is basically a turbulent, stormy, whirlpool of wind, banded with variable belts and a giant "Red  Spot."  This giant Red Spot is an oval shaped, counter-clockwise moving storm and is four times larger than our Earth.  The storm is by far the largest of similar ovals found on other parts of Jupiter and the other gas giants.  Jupiter's wind appears to be driven by internal heat rather than from solar insolation.  A probe dropped by the Galileo spacecraft late in 1995 provided evidence of wind speeds of more than 400 mph and some lightning.  

(Data is from NASA Goddard)

Average distance from Sun 482,300,000 miles
Perihelion 459,100,000 miles
Aphelion 506,300,000 miles
Sidereal Rotation 9.925 Earth hours
Length of Day 9.925 Earth hours
Sidereal Revolution 11.87 Earth years
Diameter at Equator 88,650 miles (largest planet)
Tilt of axis 3.13 degrees
Moons 79 known
Atmosphere Hydrogen (90%), Helium (10%), trace amounts of methane and ammonia
Discoverer Unknown
Discovery Date Prehistoric



Average distance from Sun:  Average distance from the center of a planet to the center of the Sun. 
Perihelion:  The point in a planet's orbit closest to the Sun.
Aphelion:  The point in a planet's orbit furthest from the Sun. 
Sidereal Rotation:  The time for a body to complete one rotation on its axis relative to the fixed stars such as our Sun.  Earth's sidereal rotation is 23 hours, 57 minutes.
Length of Day:  The average time for the Sun to move from the Noon position in the sky at a point on the equator back to the same position.  Earth's length of day = 24 hours
Sidereal Revolution:  The time it takes to make one complete revolution around the Sun.
Axis tilt:  Imagining that a body's orbital plane is perfectly horizontal, the axis tilt is the amount of tilt of the body's equator relative to the body's orbital plane.  Earth is tilted an average of 23.45 degrees on its axis.

A side note:  Beginning on July 16, 1994, 21 large fragments of the comet Shoemaker-Levy 9 bombarded Jupiter over a six day period.  The fragments impacted the planet in a systematic order, one after the other at 134,000 mph.  This provided a pyrotechnic show of unbelievable proportions.  The impact of the comet's fragments released massive plumes of gas into Jupiter's atmosphere, emitting huge fireballs and leaving scarring behind.  One of the largest fragments impacted Jupiter with a force of 6 million megatons of TNT and produced a plume about  1,500 miles high and 5,000 miles wide.  It left a dark discoloration larger than Earth.  The top image to the left shows an impact from fragment "G" on Jupiter.  This picture was  taken by Peter McGregor at the Mount Stromlo and Siding Observatories on July 18, 1994. 

The bottom image displays residual scarring from comet fragments "G" "D" and "L", taken by Dan Burton at the Texas A&M observatory on July 20, 1994.  The dark discoloration at the lower left is from fragments "G" and "D".  The lower right impact is from fragment "L".