Lecture 7

1. Lecture 7 Astro 1001 6/13/07

2. Jovian Planets Overview • Much larger than terrestrial planets • Jupiter is over 300x more massive than Earth • About 1/5 of the density of Earth • We know a lot about the planets now • Pioneer and Voyager visits in the 70s • Recently, Galileo and Cassini visits

3. Composition • Jupiter and Saturn are almost entirely hydrogen and helium • Jupiter is often called a “failed star” • Uranus and Neptune are smaller, contain much less hydrogen and helium • Made up of hydrogen compounds • Why are they different sizes?

4. Planets and Pillows • Difference between densities in Neptune and Uranus vs Saturn and Jupiter should now be obvious • Size is not necessarily an indication of mass • More mass compresses things more, increasing density but not planetary radius

5. Rotation Rates • Jovian planets rotate very quickly • Difficult to measure • Fast rotation makes the planets bulge • Saturn’s equator is about 10% wider than its poles • Extra equatorial material keeps moons and rings aligned with bulge

6. The Interior • We can say things about the interior of jovian planets because of experiments and computer simulations • Interior is very dense and hot • Galileo dropped probe that only survived for 200 km

7. The Other Jovian Planets • Saturn is very similar to Jupiter • Uranus and Neptune don’t have metallic or liquid layers, although their cores might be liquid

8. Internal Heat • Jupiter emits about twice as much energy as it gets from the Sun • Cannot be accounted for by accretion, differentiation • Heat is probably coming from gravitational contraction • Neptune is a bit mysterious

9. Weather and Clouds • Jovian planets are typically colorful due to their clouds • We see lots of methane, ammonia, water bands • Jupiter has similar layers to what the Earth has • Neptune and Uranus have different behaviors and can form methane snow

10. Color and Winds • Trace amounts of chemicals produce the colors and Saturn and Jupiter • Saturn has more subdued colors because its clouds are lower • Methane is responsible for the bluishness of Uranus and Neptune • Stripes on Jupiter due to Coriolis effect • Great Red Spot is a giant hurricane

11. Magnetic Fields • The jovian planets all have magnetic fields • Jupiter’s magnetic field is 20,000 stronger than Earth’s • Jupiter has very spectacular aurora • Causes atmospheres on the moons • Neptune and Uranus have odd magnetic fields in that they aren’t aligned with the poles

12. Moons • All of the jovian planets have moons (and rings) • Jupiter has over 60 moons • Ganymede and Titan are larger than Mercury • Lots of ice • Larger moons probably formed near the planets • Smaller moons are probably captured asteroids

13. Io • Most volcanically active world in the solar system • Shoots some of its volcanic materials into space • Tidal heating is why Io is so active

14. Europa • Covered by water ice • Interior might be water or convecting ice • Magnetic field data indicates its probably liquid water • Might be heat sources beneath the surface

15. Group Work • Europa is a fascination moon that has a fairly strong chance for supporting life. However, NASA no longer has any plans to visit it with a probe. Why do you think there are no plans to visit Europa?

16. Ganymede and Callisto • Ganymede • Largest moon in the solar system • Has both young and old features • Features probably erased due to liquid water welling up • Callisto • Heavily cratered • No significant internal heat

17. Titan • Second largest moon in the solar system • Atmosphere is very thick • Mostly nitrogen • No oxygen, but lots of hydrogen compounds • Created by the surface sublimation or evaporating • Recently explored with the Huygens probe

18. Triton • Coldest world in the solar system • Almost certainly captured • Probably had past geological activity

19. Rings • Made up of countless small particles • Particles are icy in nature • Might be thinnest known astronomical structure • Rings have gaps • Might be because of shepherd moons • Might be because of orbital resonances

20. Other Ring Systems • Other ring systems are much darker, smaller • Always go about the planet’s equator • Uranus’s rings are slightly tilted, elliptical

21. How Did the Rings Form? • Large planets have enough gravity to rip things apart that get too close • Difficult to explain why this would frequently happen • Rings were formed from leftover material, but are resupplied from the moons of the planets

22. Asteroids • Small rocky bodies • Discovered only about 200 years ago • Took 50 years to discover the first 10 • 150,000 known asteroids now • Ceres is the largest, under 1000km in diameter • Total mass is probably less than that of the moon

23. Asteroids Continued • Shape depends on mass • Thousands of asteroids have been analyzed through spectroscopy • Made up of metal and rock, with perhaps a big of ice • Asteroids vary greatly in density

24. The Asteroid Belt • Vast majority of asteroid are in the asteroid belt • Individual asteroids are separated by millions of kilometers • Jupiter’s gravity causes two groups of asteroids called Trojan Asteroids

25. Meteorites • Meteors are just particles coming through the atmosphere • Usually around pea sized objects • Meteorites actually hit the ground • Can tell use a great deal about how the solar system formed

26. Types of Meteorites • Primitive • Very old (4.6 billion years old) • Are made up of stone, or a combination of stone and carbon compounds • Processed • Were once a part of another object • Can be made of metals, or rock

27. Comets • Comets formed outside of the frost line • Thus, are made up of lots of ice • Comets do not race across the sky • Vast majority of comets do not have tails and never get anywhere close to Earth • Some comets are knocked into the inner solar system by various sources of gravity

28. Composition • Made up of chunks of ice mixed with rocky dust and complex chemicals • “Dirty snowballs” • Starting to get more and more details about comets • Deep Impact • Stardust

29. Structure of a Comet • Nucleus is the actual icy core • Typically about 20km across • Might have a dusty atmosphere called a Coma • Two tails • Plasma tail consists of the gasses escaping the comet • Dust tail is made up of dust sized particles

30. Where Do Comets Come From? • Left over material was flung far out into the solar system by the gravity from the jovian planets • This lead to the Oort Cloud • Slightly beyond the solar system, remnants stayed put and formed the Kuiper Belt

31. Big Icy Things • Lots of Pluto type objects in the Kuiper belt • Perhaps 1000s of km in diameter • “Xena” (Eris) is a good example • Not really comets • These objects are very cold, but might have atmospheres • Will be visited by a probe in a decade or so

32. Collisions • In 1994, a comet whacked into Jupiter • Each fragment had the energy of a million H-bombs • A massive collision is probably (at least partly) responsible for the extinction of the dinosaurs • A layer of irridium was the initial evidence • Later found the Chicxulub crater

33. Do We Have to Worry? • We witnessed a probably collision in 1908 • Had the force of several atomic bombs • No more than 40 meters across • The asteroid Apophis will pass very close to Earth soon