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The Jovian Planets Dr. Ken Rice Discovering Astronomy S The Jovian Planets • The Jovian planets are the 4 outer solar system planets – Jupiter, Saturn, Uranus, Neptune Jupiter Saturn Uranus Neptune Distance = 19.2 AU Distance = 30.1 AU Mass = 14 MEarth Mass = 17 MEarth Distance = 5.2 AU Distance = 9.54 AU Density = 1.24 Mass = 318 MEarth Mass = 95 MEarth Composition : H Composition : H compounds, rock, H, He compounds, rock, H, He Density = 1.33 g/cm3 Density = 0.71 Composition : Mostly H, He Composition : Mostly H, He g/cm3 Discovering Astronomy S g/cm3 Density = 1.67 g/cm3 Jupiter and Saturn • • Central rocky core ~ 10 Earth masses Atmosphere – Hydrogen and Helium • Both have a very similar size – Saturn much less dense than Jupiter. – If Jupiter were more massive it would get smaller. Discovering Astronomy S Uranus and Neptune • • • Core of hydrogen compounds, rock and metals. Cores are bigger than Jupiter and Saturns because less compressed by the atmosphere. Gaseous hydrogen atmosphere – Pressure not high enough for liquid or metallic hydrogen Discovering Astronomy S Internal heat • • • In terrestrial planets internal heat is important for geological activity In Jovian planets, it is important for weather (they have no solid surface). Jupiter and Saturn both radiate twice as much as they receive from the Sun – Jupiter – contraction releasing gravitational potential energy – Saturn – Helium condense in the upper atmosphere and sinks, releasing energy (differentiation) Discovering Astronomy S Atmospheric structure • Galileo probe plunged into Jupiter’s atmosphere in 1995 – Structure similar to the Earth’s atmosphere – Troposphere, stratosphere, thermosphere • Jupiter and Saturn – 3 clouds layers – Water (H2O), ammonium hydrosulfide (NH4SH), ammonia (NH4). • Uranus and Neptune – 1 cloud layer (methane – CH4) – May be additional layers lower down??? Discovering Astronomy S Coriolis force • The coriolis force is a fictitious force that occurs because the actual ground speed of a rotating sphere varies with latitude – Something on the equator moves faster than something nearer the poles – Something moving from one latitude to another follows an apparently curved path when viewed from the ground. Discovering Astronomy S Storms on Jupiter • • Dynamic weather – strong winds and powerful storms Circulation cells – Hot Equatorial gases expand and move towards poles – Polar air flows towards equator • Jupiter rotates every 10 hours – Strong coriolis force – Produces alternating bands • Alternating colours are different molecules – Reddish ammonium hydrosulfide covers the entire planet at low altitude – Rising air forms white ammonia clouds at high altitudes. • No Seasons Discovering Astronomy S The Great Red Spot • • Giant storm – twice as wide as the Earth Somewhat like a hurricane – High pressure rather than low. • Long lived – Has been observed for more than 300 years! • Smaller storms – relative to the Great Red Spot - are constantly brewing in Jupiter’s atmosphere. Discovering Astronomy S Saturn, Uranus and Neptune • Saturn – Like Jupiter, rapid rotation creates fast eastwest winds – Axis tilt produces some seasonal effects • Neptune – Also banded and has high-pressure (but short lived) storms like the Great Red Spot. – Also has seasons • Uranus – No bands and no storms (very low internal heat) – May have some seasonal storms Discovering Astronomy S Jupiter’s magnetosphere • • The liquid hydrogen in Jupiter’s envelope and its rapid rotation produce a strong magnetic field. Strong magnetic field produces an enormous magnetosphere – Deflects the solar wind about 3 million km before ahead of Jupiter • Gas escaping from Jupiter’s moon Io creates a ring of charged particles – Creates very bright aurorae Discovering Astronomy S Saturn, Uranus and Neptune Discovering Astronomy S Planetary rings • All the Jovian planets have ring systems Discovering Astronomy S Saturn’s rings • Made up of particles – Ranging in size from icy dust grains to boulders • Frequent collisions (every few hours in the densest regions) – Thinnest known astronomical structure – A few tens of metres thick – The orbits are almost perfect circles Discovering Astronomy S Rings and gaps • Moons inside the rings can open gaps – Gravity • Moons outside the rings can also open gaps – Resonances (Kepler’s laws) • Shepherd moons – Two moons can produce a very narrow ring between them Discovering Astronomy S Tidal forces and the Roche limit • If we consider a satellite (moon) orbiting a planet with mass Mpl, there is a difference between the gravitational acceleration of a point at the centre of the satellite, and a point on the surface. A= GM pl d 2 − GM pl (d + r ) 2 ≈ 2GM pl r d3 • The satellite is held together by its own gravity. • If the satellites gravity is not strong enough, it will be torn apart by the tidal force from the planet. • The closest distance at which this will happen is 1 3M pl 3 d > r m Discovering Astronomy S Examples of tidal limits – moons of Saturn distance (km) radius (km) mass (kg) tidal radius (km) Pan 134000 15 4.90E+15 106585.6789 Atlas 138000 14 6.60E+15 90078.19616 Prometheus 139000 46 2.70E+17 85896.64486 Pandora 142000 46 1.35E+17 108222.991 Epimetheus 151000 57 5.60E+17 83461.29844 Janus 151000 89 2.01E+18 85113.33564 Mimas 186000 196 3.80E+19 70361.40387 Enceladus 238000 260 8.40E+19 71650.60737 Tethys 295000 530 7.55E+20 70247.87251 Telesto 295000 15 2.70E+17 28009.7755 Calypso 295000 13 2.70E+17 24275.13877 Dione 377000 560 1.05E+21 66496.36176 Helene 377000 16 2.70E+17 29877.09386 Rhea 527000 765 2.49E+21 68118.94969 Titan 1222000 2575 1.35E+23 60581.34821 Hyperion 1481000 143 1.77E+19 66224.38069 Iapetus 3561000 730 1.88E+21 71385.47724 Phoebe 12952000 110 4.00E+18 83633.25085 Discovering Astronomy S Why do Jovian planets have rings • The rings fall close to the Roche tidal zone – Zone within which bodies held together by gravity (i.e. moons) are ripped apart by the tidal forces from the Jovian planet – Tidal forces result from the different gravitational force on either side of a large body (this is why we have tides). • Ring particles are continually colliding, and being ground down – Pressure from sunlight causes them to eventually spiral into the planet – Must be replenished • Rings particles must come from small moons in the planets’s equatorial plane! – These small moons are small enough to overcome the tidal forces. – Small impacts release small particles from the surface of these moons – Occasional large impacts can destroy these moons completely! • The above also means that all large moons lie outside the Roche tidal zone – There are no large moons within the rings Discovering Astronomy S