1. General Astronomy Moons Many slides are taken from lectures by Dr David Wood, San Antonio College

2. Inner Solar System Mercury and Venus have 0 moons Earth has 1 moon Mars has 2 moons

3. The Moon of Earth

4. Comparing Earth & Moon EarthMoon Water Volatiles Differentiated Atmosphere Metallic core Composition Plate Tectonics Magnetic Field Temperature Life Yes Rock & Metal Yes Moderate Yes No Yes No Rock only No Large extremes No

5. Surface Features Highlands –Oldest surface on Moon (> 3.9 Gyr) –Lighter in color –Less dense –Original crust Maria –Younger surfaces (3.4 ± 0.4 Gyr) –Lower in elevation –Composition of Moon’s mantle not regolith –Associated with basins –Common on near side, absent on far side Impact Craters form Regolith Ice at pole (?) Near side vs. Far side –Near side is thinner than far side –Basins all found on near side except Orientale –Far side is more heavily cratered

6. Surface Features

7. Interior Radius is 1738 km Crust –Regolith is a few tens of meters thick –30 km (nearside) –100 km (farside) Lithosphere is 600 – 800 km deep Asthenosphere is 800 – 1700 km deep Core?? Small if it exists at all How do we know all this? –Apollo astronauts left seismic sensors –Moon rings like a bell –Moonquakes (mostly from impacts but some are tidal in origin) –Network turned off in late 1970s

8. The Man on the Moon 9 Apollo missions ventured to the Moon, 6 missions (12 men) landed on the surface July 20, 1969…Neil Armstrong becomes first man to set foot on the Moon –“Houston, Tranquility Base. The Eagle has landed.” –“One small step for a man, one giant leap for mankind.” April 11 – 17, 1970…Apollo 13 saga delays Moon launches/landings for a year December 17, 1972…Eugene Cernan is last person to leave the Moon Have not returned since 1972

9. The Moons of Mars “…two satellites, which revolve about Mars, whereof the innermost is distant from the center of the primary planet exactly three of its diameters, and the outermost five…” Jonathan Swift, in Gulliver’s Travels, 1726 Mars’ two moons were discovered 151 years later by Asaph Hall in 1871 Many astronomers of that era (including Kepler) felt that there was a simple numerical sequence: Venus = 0, Earth = 1, Mars = ?, Jupiter = 4

10. Moons of Mars Phobos General Properties Radius = 28 x 20 km Orbital period = 0.319 days Orbital radius = 9380 km The large crater is Stickney which has a diameter half as large as Phobos. Phobos orbital period (7.7 hours) is less than the Martian day (24.6 hrs); an observer would see Phobos rise in the west and set in the east every 11 hours.

11. Phobos

12. Moons of Mars Deimos General Properties Radius = 16 x 12 km Orbital period = 1.263 days Orbital radius = 23460 km Not as many craters as on Phobos. The image looks softened – most likely by a coating of thick dust.

13. Deimos This image was taken by the Viking spacecraft at a distance of 18 miles above the surface. The smallest objects are about 15 feet across.

14. Outer Solar System All of the outer planets have large numbers of satellites We are discovering more and more as technology improves

15. Moons of Jupiter General Properties –Galilean Satellites Planet-size (> 1500 km) Visible to naked eye if outside Jupiter’s glow Generally rock & ice composition Resonance and tidal heating Callisto does not participate in resonance Density decreases with distance –Medium-sized moons 300 km – 1500 km Spherical and natural Rock & ice composition Synchronous rotation –Small Moons < 300 km Potato-shaped and captured Usually rocky Inclined, eccentric, retrograde orbits

16. Io –General Properties Radius = 1821 km Orbital period = 1.769 days Slightly larger than Earth’s Moon “Pepperoni pizza” crust analogy Heavy dose of X-ray radiation from Jupiter –Volcanically active Tidal heating Pele & Loki plumes Umbrella shape of geysers due to low surface gravity Sulfur ash and “lava” bury all impact craters –Resonance Europa & Ganymede pull on one side; Jupiter pulls on the other 1:2:4 resonance requires Europa & Ganymede to line up with Io every 4 orbits

17. Io

18. Europa –General Properties Radius = 1565 km Orbital period = 3.551 days Slightly smaller than Earth’s Moon Heavy dose of X-ray radiation from Jupiter –Resurfacing Few impact craters if any; large cracks Blocks of ice appear to be shifted or overturned Tidal heating produces internal ocean of liquid water? Best chance for possibility of life –Resonance Io & Jupiter pull on one side; Ganymede pulls on the other 1:2:4 resonance requires Io & Ganymede to line up with Europa every 2 orbits

19. Ganymede & Callisto Ganymede –General Properties Radius = 2634 km Orbital period = 7.155 days Largest moon in solar system Larger than Mercury Unusual ice geology Perhaps a liquid water mantle?? Callisto –General Properties Radius = 2403 km Orbital period = 16.689 days 3 rd largest moon in solar system Larger than Mercury Surface saturated with craters Valhalla impact basin is largest impact basin in the solar system

20. Other Jovian Moons Small inner moons –Probably natural moons –10 km – 135 km –Metis, Adrastea, Amalthea, Thebe –1999 J1 Small outer moons (prograde) –Captured in prograde orbits –7 km – 22 km –Substantially inclined orbits –Leda, Himalia, Elara, Lysithea –2000 J11 Small captured outer moons (retrograde) –Captured in retrograde orbits –5 km – 15 km –Inclined and eccentric orbits –Ananke, Carmae, Pasiphae, Sinope –2000 J1 – 2000 J10

21. Moons of Saturn General Properties –Titan Planet-size (> 2500 km) Only large moon of Saturn –Medium-sized moons 10 moons 100 km – 800 km Spherical and natural Rock & ice composition Synchronous rotation –Small Moons < 100 km Potato-shaped and captured Usually rocky Inclined, eccentric, retrograde orbits

22. Titan –General Properties Radius = 2575 km Orbital period = 15.945 days Larger than Mercury –Atmosphere Massive nitrogen & hydrocarbon atmosphere Pressure = 1.5 bars Why does Titan have an atmosphere –Outgassed NH 3 –UV dissociates NH 3 molecule and H escapes –Callisto and Ganymede too warm for NH 3 to condense…none to outgas! –Surface Appears to have methane and ethane oceans May have solid, liquid, and gaseous methane states –Life?? Right ingredients Chemistry too slow?

23. Titan’s Shoreline

24. Ice Volcano on Titan

25. Titan’s Surface

26. Other Saturnian Moons Tethys –Radius = 530 km –Very heavily cratered Enceladus –Radius = 249 km –Bright, lightly cratered; H 2 O volcanoes? Rhea –Radius = 764 km –Significant ice geology (tectonics?) Dione –Radius = 559 km –Lightly cratered; interesting ice geology Mimas –Radius = 199 km –Very heavily cratered (Death Star) Iaepetus –Radius = 718 km –One hemisphere dark; other hemisphere bright

27. Dione

28. Hyperion

29. Enceladus

30. An Ocean Below Enceladus’ Icy Crust? NASA’s Cassini spacecraft has observed plumes of material escaping from Saturn’s small icy moon, Enceladus The plume is mostly water vapor, with tiny ice particles and other gaseous molecules mixed in (e.g. CO 2, CH 4, C 2 H 6 ) The plume supplies ice particles to one of Saturn’s rings Some ice particles contain salt, which may indicate they originate in an ocean deep below the icy crust Image mosaic of Enceladus taken by Cassini, showing individual plumes of gas and ice escaping from the surface. The plumes extend 100’s of km into space from the ~500 km diameter moon. Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider dpsdisc@aas.org - http://dps.aas.org/education/dpsdisc/ - Released 03 December, 2009 dpsdisc@aas.orghttp://dps.aas.org/education/dpsdisc/

31. Plumes may be material escaping through surface cracks from an internal salty ocean or lake Alternatively, ice along cracks may sublime or melt, followed by escape of water vapor and icy particles Many scientists find the salty ocean model most convincing, but others favor combinations of alternative explanations What Process Creates the Plume? Left: Enceladus may have a salty subsurface ocean that releases material to space through cracks in the moon’s icy shell. Right: The walls of icy cracks in the surface may melt or sublime, venting gas and icy particles to space. Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider dpsdisc@aas.org - http://dps.aas.org/education/dpsdisc/ - Released 03 December, 2009 dpsdisc@aas.orghttp://dps.aas.org/education/dpsdisc/

32. The Big Picture Enceladus is surprisingly active for such a small body - likely a consequence of tidal heating Future flybys of Enceladus by Cassini may help to resolve whether Enceladus joins the growing “club” of solar system bodies believed to have oceans If Enceladus has an ocean, then it contains all of the ‘ingredients’ known to be important for life: liquid water, molecular building blocks, and energy Image of Enceladus showing the ‘tiger stripes’ region in the southern hemisphere, where the plumes originate Tiger stripes Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider dpsdisc@aas.org - http://dps.aas.org/education/dpsdisc/ - Released 03 December, 2009 dpsdisc@aas.orghttp://dps.aas.org/education/dpsdisc/

33. Mimas

34. Phoebe

35. Tethys

37. Rhea

38. Moons of Uranus General Properties –Medium-sized moons 236 km – 789 km Miranda is triaxial; others are spherical Icy – rock composition Synchronous rotation –Small Moons < 100 km Inner moons are spherical and natural Outer moons are potato-shaped and captured Usually rocky Inclined, eccentric, retrograde orbits

39. “Large” Uranian Moons Titania –Radius = 789 km –Orbital period = 8.706 days –Once geologically active –Lightly cratered Oberon –Radius = 761 km –Orbital period = 13.463 days –Not very active geologically –Heavily cratered Umbriel –Radius = 585 km –Orbital period = 4.144 days –Not very active geologically –Heavily cratered Ariel –Radius = 579 km –Orbital period = 2.520 days –Once geologically active –Lightly cratered Oberon Umbriel Titania Ariel

40. Miranda –General Properties Radius = 236 km Orbital period = 1.413 days Very small –Surface Tectonically active at one time in its past Three large coronae –Arden Corona (near top) –Elsinore Corona (near bottom) –Inverness Corona (middle) Fault scarps are tens of kilometers tall; takes hours for objects to fall –How were coronae produced? Moon shattered and re-accreted? Tectonic activity and convection? Arden & Elsinore best modeled as diapirs (rising plumes); Inverness best modeled as a sinker

41. Other Uranian Moons Small inner moons –Probably natural moons –10 km – 100 km –Located within Uranian ring system –Some may be shepherd moons –Cordelia, Ophelia, Bianca, Cressida, Puck, Desdemona, Juliet, Portia, Rosalind, Belinda –1986 U10 Small outer moons –Captured in both prograde & retrograde orbits –< 60 km –Caliban, Sycorax, Stephano, Prospero, Setebos –Little known of the properties of these moons –Two more discovered in 2003 Puck

42. Moons of Neptune General Properties –Triton Planet-size (1353 km) Only large moon of Neptune Icy composition –Medium-sized moons 2 moons (Proteus & Nereid) 170 km & 218 km Natural??? Icy composition Synchronous rotation –Small Moons 6 known (Naiad, Thalassa, Despina, Galatea, Larissa) < 100 km Usually icy Located within and near Neptune’s ring system Natural??

43. Neptune and Triton

44. Triton

45. –General Properties Radius = 1353 km Orbital period = 5.875 days Retrograde Will eventually fall within Neptune’s Roche limit where it will be torn apart Largest captured object in solar system –Atmosphere Thin nitrogen atmosphere Why does Titan have an atmosphere –Outgassing NH 3 (?) from currently active volcanoes –UV dissociates NH 3 molecule and H escapes –Surface Volcanically active Large smooth areas are indicative of recent lava flows (composition is NH 3 ) Long trails of volcanic plumes evidence atmosphere

46. The Moons of Pluto Pluto is in a 3:2 resonance with Neptune Charon is an almost identical twin to Triton Are Pluto and Charon escaped moons of Neptune? –Orbits suggest probably not –Triton is captured suggesting that Triton may once have been a Kuiper Belt object Nix and Hydra

47. Sizes

48. For More Information… Press Releases Space.com - 06/14/09 - “Ocean Hidden Inside Saturn's Moon” http://www.space.com/scienceastronomy/090624-enceladus-ocean.html Images Enceladus plume image courtesy NASA/JPL/Space Science Institute/E. Lakdawalla http://antwrp.gsfc.nasa.gov/apod/ap091124.html Europa plume cartoons adapted from image courtesy NASA/JPL http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2944 Enceladus image courtesy NASA/JPL/Space Science Institute http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2032 Source Articles (on-campus login may be required to access journals) Postberg et al., ‘Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus’, Nature, 459, p. 1098, doi: 10.1038/nature08046, 2009. http://www.nature.com/nature/journal/v459/n7250/full/nature08046.html Schneider et al., ‘No sodium in the vapour plumes of Enceladus’, Nature, 459, p. 1102, doi: 10.1038/nature08070, 2009. http://www.nature.com/nature/journal/v459/n7250/full/nature08070.html Porco et al., Science, 311, p.1393, 2006. http://www.sciencemag.org/cgi/content/abstract/311/5766/1393 http://www.sciencemag.org/cgi/content/abstract/311/5766/1393 Nimmo et al., Nature, 447, p.289, 2007. http://www.nature.com/nature/journal/v447/n7142/full/nature05783.html http://www.nature.com/nature/journal/v447/n7142/full/nature05783.html Kieffer et al., Science, 314, p.1764, 2006. http://www.sciencemag.org/cgi/content/abstract/314/5806/1764 http://www.sciencemag.org/cgi/content/abstract/314/5806/1764 Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider dpsdisc@aas.org - http://dps.aas.org/education/dpsdisc/ - Released 03 December, 2009 dpsdisc@aas.orghttp://dps.aas.org/education/dpsdisc/