ASTR178 Other Worlds

1. ASTR178Other Worlds A/Prof. Orsola De Marco 9850 4241 orsola.demarco@mq.edu.au

2. Announcements • If you do not make your talk day you risk not getting that side of the assessment. • Observing is on October 6th 7:15PM and 13th 8:15PM. (Sign up in class or on my door: E7A-316). • Assignment 2 posted, due in 17th September. • Moon practical due in 17th September. Sample questions for this lesson • 1-6, 8-11, 13-36,38,41, pages 349-351

3. In last class: gas giants part II • Jupiter and Saturn. • Atmospheres • Inner composition • Magnetic fields • The rings • Introduction to the moons of Jupiter

4. In this class: giants moons • Introduction to the moons of Jupiter • Io • Europa • Ganymede • Callisto • Saturn’s Titan • … and all the other moons …

5. Giant Moons

6. The Galilean satellites • Noticed by Galileo ~1610 • Could be resolved by naked eye were it not for Jupiter’s glare. • Synchronous rotation. • Innermost 3 have orbital periods in a 1:2:4 resonance. Why only 3 not all 4? • Transits and occultations used to measure diameters

7. The Pioneers and the Voyagers satellites measured the moons’ masses – how? • From radii and masses one could infer densities – how?

9. The composition of the satellites • Low density Callisto and Ganymede are likely made of water ice (low density). • Io and Europa have densities similar to rock on Earth. • All but Io have spectra indicative of some water ice on the surface. • The formation of the satellites • Density decreases as you go away from Jupiter, as is the case for the Solar System. • Jupiter formed as a miniature Solar System, where Jupiter itself was a source of heat: rocky materials condense close where it is warmer while rock + ice coalesce further out, where ice exists because it is colder.

10. Io

11. Not quite like the Moon!!!

12. Io’s heat source • Tidal forces from Jupiter + Io’s elliptical orbit. • How much heat is generated? 24 tonnes of TNT exploding every second! • How much heat goes through the surface? 2.5 Watt/square meter – Earth’s: 0.06 Watt/square meter!

13. Volcanos • Plumes 70-280 km high. • Ejected at ~1100-3600 km/h. Compare Earth’s 360 km/h! • More like geysers on Earth – but propelled by Sulfur dioxide.

14. Rapidly cooled sulfur will lead to all the colours • Sulfur dioxide leads to white flakes

15. Aside from geysers there are explosive lava flows.

16. Hotter lava (from IR observations). • Contain magnesium – hotter interior. • 100 km crust floating on a fully magma interior – explain widespread volcanism • 300 active volcanos at any one time. • Enough material to cover Io in 1 m of ejecta in 100 years.

18. Jupiter’s field generates a current across Io (400,000 V!) • This in turn generates a magnetic field on Io.

19. moving magnetic fields induce currents • Magnetised strip in your credit card induces a current in the card reader.

20. Measured field on Io is larger than predicted from above argument - hence Io has additional magnetic field from molten interior. • Smallest body in Solar System with its own field. • Differentiation caused a solid iron and iron sulfide core • Core ½ the size of Io – how do we know?

21. Europa • The smoothest world • The density indicates rock composition… • … but the spectrum is pure ice. – what is a spectrum? • Almost no craters – what does it mean?

22. Cracks probably from geologic activity – stress. • Heat source as for Io, just less so since Europa is further from Jupiter.

23. Cracks may be the result of tidal flexing • Many young areas (smooth) indicate much activity

25. Frozen sea nearGreenland, photo Taken by Francisco Diego from a plane (10,000 m high) Ice on Europataken from similar Altitude

26. A liquid ocean under the crust? • Evidence from rafts and cracks. • Galileo measured magnetic field induced from Jupiter – variable, needs conducting medium – water + minerals – colour on cracks. • 100 km ocean on rocky core on metallic core? • Water + heat = life? • (Galileo was deorbited into Jupiter not to collide with Europa)And the atmosphere? • O2 atmosphere (very very tenuous) from collisions of particles with ice, liberating O (H escapes into space)

27. Ganymede young icy craters … • Older darker • Younger lighter (unlike the Moon!)

28. Old cratered terrain. • Younger (but still 1 billion yr old) lighter terrain. • Fractures – tectonic activity. Heat in this moon till recently. • Ice on the surface, come through cracks? • Strange since Ganymede is very small to have retained heat. • Magnetic field twice as strong as Mercury. Molten interior! • Highly differentiated metallic core – how do we know? • Changes in magnetic field: Layer of liquid water – why? • Source of heat???

29. Callisto • Craters on ice • What is all the “muck”? • Lots of large craters, where are the small ones? • Changing magnetic field – hence liquid water – who keeps it liquid? Ammonia as antifreeze. Still…. • Not differentiated – hence cold. • This CO2 atmosphere

31. Cassini-Huygens explores Saturn and Titan Short animation

32. Saturn’s Titan • Discovered by Huygens in 1665 • Suspected to have an atmosphere in 1900 • Found to have one in 1944 – Kuiper found to have NH3 atmosphere

34. Saturn’s Titan • NH3 in atmosphere broken down such that 95% atmosphere is nitrogen • Surface pressure 1.5 times Earth’s. 10 times more atmosphere. • Temperature 95 K – methane and ethane liquid.

35. Cassini’s flybys mapped Titan in Infrered • Sand dunes parallel to equator made of ice and polymers: wind action • Methane from recent (100 million year) volcanos – also white spots. • Where does the heat come from? Possible periodic reheating of the core due to radioactive decay.

38. The Huygens probe was launched from Cassini in 2005. • It landed safely and broadcast images for 70 minutes. • CH4 chemistry leads to hydrocarbons and the reaction of those with N2 leads to polymers which have a reddish colour – they can be in the air as aerosols or on the ground. Movie

39. Possible liquid methane lakes only detected near the north pole – might be seasonal. • Huygens probe vaporised methane. • Only 5 cm “rainfall” per year • Few craters - erosion

40. Saturn and Titan in the news 10 August 2009 Titan was observed to have tropical clouds forming in this infrared image taken with the 8 m Gemini telescopeTitan’s clouds are likely to make liquid methane rain.

41. Star Trek

42. Titan’s geologic history • A proposal • Solid core, liquid water mantle icy crust formed 4.5 billion years ago. • Methane from volcanos • Methane destroyed between 3.5 and 2 billion years ago. • Radioactive isotopes – reheating of interior, convection transports the heat to the surface, new outgassing of methane (2 billion years ago) • Methane destroyed once again. • 500 million years ago one more reheating episode. Once more outgassing. • In the future methane will go. We are just observing Titan at a special time…

43. The 4 innermost satellites of Jupiter. • All within Io’s orbit. • All prograde motion: they formed with the Galilean moons and J. • There are 55 outer ones, all outside the orbit of Callisto. • These are likely captured (inclined orbits and 48 have retrograde motion). • 23 discovered in 2003 alone! • All non spherical – why?

44. Saturn has Titan and 6 larger satellites (spherical). • They orbit on the Saturn’s equatorial plane, have prograde orbits and are phase locked. • Orbit between 3 and 59 Saturn radii. • They all look different! • It also has 54 small satellites, some of which might be captured others are likely fragments of impacts.

45. Low average density • Mimasis old and cratered and has a HUGE impact crater.

46. Enceladus has a new surface. • Most reflective object in the solar system • Ice eruptions come from the cracks.

47. Eruptions from Enceladus spew particles out that form a thin ring. • Heat source? Dione 2:1 resonance… possible. (Mimas and Tethys are also in a 2:1 ratio, but no heating…)