What Can You See in the Night Sky This Week? Jupiter!

16 February 2012 by Anonymous (not verified)

Jupiter is the fifth planet from the Sun and the largest in our solar system - it weighs about two-and-a-half times as much as all the other planets put together! Given its massive size, it seems appropriate that it was named after the Roman king of the gods. This celestial god can be seen from sunset untill just before 10:00pm in the West, writes Tessa Baker.

Jupiter is one of the four gas giant planets (along with Saturn, Uranus and Neptune), which means that it isn't actually a solid body like the Earth. What we see as the surface of Jupiter is really the top layer of its thick, turbulent atmosphere, made mainly of hydrogen and helium. On Earth we know hydrogen as a gas, but the huge pressures in Jupiter's atmosphere cause it to condense into a metallic liquid about 1000km below the apparent surface. This liquid layer forms the bulk of the planet, with only a small solid core right at the centre.

The distinctive stripes of Jupiter's appearance are caused by alternating bands of rising and falling gas. The pale bands are called "zones" and correspond to cold upwellings; their colour is probably due to crystallised ammonia ice. The darker bands, known as "belts" mark falling air. Their varied array of red-brown hues are caused by compounds of sulphur, phosphorous and carbon that change colour when exposed to sunlight.

In addition to these regular stripes, Jupiter's atmosphere is littered with storms. The most famous of these is the Great Red Spot, visible as a oval in the lower half of the planet. This gigantic cyclone has raged for at least 187 years, and possibly as long as 350 years. It is big enough to encompass several Earths (see image)! It rotates counterclockwise every six Earth days and experiences slow changes in colour, along with the rest of Jupiter's bands. In July 2008 astronomers observed a collision between the Great Red Spot and a smaller storm (nicknamed the 'Baby Red Spot'), which was completely destroyed in the interaction.

An image of the Great Red Spot with the Earth (as seen from Apollo 17) shown for size comparison (created by NASA). Swirling turbulence patterns are also visible in Jupiter's atmosphere, along with several smaller storms (white circles).

With a small telescope or good pair of binoculars you should be able to see four moons in orbit around Jupiter - their names are Io, Europa, Ganymede and Callisto. These are the largest of Jupiter's 66 confirmed satellites, all named after mythological characters associated with the god-king. Io, the innermost moon, is violently geologically active - it has over 400 active volcanoes that pump out toxic gases. This is due to an 'orbital resonance': the radii of the orbits of the innermost three moons happen to be in the neat integer ratio 1:2:4. Under these circumstances the moons exert strong gravitational effects on each other, which squash and stretch their cores. Io, being the closest moon to Jupiter, experiences the strongest forces. These forces heat its core, which in turn leads to its extreme volcanic activity.

The toxic gases emitted by volcanoes on Io settle into a thick belt around Jupiter. The gas mix contains ionised atoms of sulphur, oxygen and chlorine ('ionised' means that the atoms are missing some electrons). Radio waves are emitted when the ionised atoms interact with Jupiter's complex magnetic field - meaning that you can 'hear' Jupiter at radio frequencies! The American space organisation NASA has set up a project called Radio Jove where you can listen to these radio emissions from Jupiter: click here to learn more.

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