Jupiter

Because of its enormous size and high albedo (0.51), Jupiter is a bright planet in our skies. It orbits the Sun with a small eccentricity (0.0484) inclined only 1.3° to the ecliptic, at a semi-major axis of 5.2028AU. It completes one sidereal orbit in 11.862 Earth years and the synodic orbital period of 398.88^d implies that Jupiter returns to opposition about one month later each year.

The planet’s rotation period is determined by;

• Following atmospheric features (such as the Great Red Spot)
• By measuring the Doppler shift from the limbs
• And by studying the rotation of the magnetic field

The magnetic field period is used as the body rotation rate. The rotation axis is inclined at 3.7° to the orbital axis and the sidereal rotation period varies from 9^h50^m at the equator, to 9^h55^m at the poles. Jupiter’s rapid rotation results in a large oblateness (0.062).

The angular diameter of Jupiter at opposition is 47″, its radius is 11.19R_⊕ and its mass is 318M_⊕. Its mean density is 1,330kgm^-3 which implies a composition similar to the solar abundances of 75% H, 24% He and 1% all the heavier elements.

The disk of Jupiter shows bands of white, blue, red and yellow clouds, which change their structure with time. The Great Red Spot (first reported in 1664) is a giant storm about 12,000 by 23,000km.

The alternating strips of light and dark are called zones (light) and belts (dark). The zones have a lower temperature than the belts and are therefore higher in the atmosphere. They are the tops of rising high pressure areas, whereas the belts are descending regions of low pressure.

The atmospheric composition is methane (CH₄), ammonia (NH₃), molecular hydrogen (H₂) and helium (He). The Voyager spacecraft also found acetylene (C₂H₂), ethane (C₂H₆), Phosphine (PH₃), water (H₂O), Germane (Ge₂H₄), and confirmed the existence of Carbon Monoxide (CO₂) and hydrogen cyanide (HCN).

The visible clouds at the tops of the zones are most likely ammonia ice as the temperature there is 150K.Below this is a layer of ammonia hydrosulphide (NH₄HS).

Jupiter radiates into space about twice as much energy as it receives from the sun. The internal heat is probably left over from Jupiter’s formation. The total internal excess power is 4 x 10¹⁷W and as the thermal conductivity of liquid hydrogen is not high, Jupiter has retained much of its primordial internal energy.

As the density, temperature and pressure increase towards the centre of the planet, hydrogen will exist in its liquid state. At a pressure of 3 x 10⁶atm, the hydrogen is squeezed into metallic hydrogen. This persists to within 14,000km of the planet’s centre at which point there is a solid core of heavy elements.

Taking the pressure at the core as P_c = (2/3)π G<ρ>²R² = (1.4 x 10^-10)<ρ>²R², then for Jupiter with a radius of 70,000km and an average density of 1,300kgm^-3, P_c ≈ 1.2 x 10¹²Pa = 1.2 x 10⁷atm, about 10 times greater than Earth’s.

The Jovian magnetic field is ~ 4 x 10⁴T at the surface. This arises from a dynamo mechanism in the rapidly rotating liquid core of metallic hydrogen. At wavelengths from 3 to 75cm, the planet radiates non-thermally. This is synchrotron radiation from relativistic electrons spiraling in the Jovian radiation belts, trapped by Jupiter’s magnetic field.

Jupiter has radiation belts similar to the van Allen belts, but extending beyond 3R_J at the magnetic equator. The magnetic axis is inclined 9.6° to the rotation axis. The magnetosphere, if it were visible, and seen head on, would subtend an angle of 2°, four times larger than the moon! It falls into 3 zones: The inner magnetosphere, where the magnetic field generated within the planet dominates and extends out 6R_J. From 6R_J to 50R_J is the middle magnetosphere where equatorial azimuthal currents control the field configuration. Beyond 50R_J, the outer magnetosphere depends on the sunward orientation: The night side having a tail ~400R_J wide and extending several AU.

~ by jamesdow2013 on April 27, 2013.