1. 27 NOVEMBER 2007 CLASS #23 Astronomy 340 Fall 2007

2. Research Experience for Undergraduates 10-12 weeks over the summer $3500-$4000 in salary plus travel, housing Applications typically due in late Dec – early Feb  Transcript  Statement of purpose/interest  Letters of recommendation http://www.nsf.gov/crssprgm/reu/list_result.cfm?unitid=5045

3. Review & Announcements Titan  Describe Titan’s atmosphere and possible source of methane Other Moons  Compare and contrast the properties of the 4 Galilean satellites  What are the salient features of Saturn’s moon Enceladus? Rings  What’s the Roche limit? How is it significant?  Compare and contrast the ring systems of the gas giants  Size distribution? Composition? Dynamics?

4. Pluto - basics Discovery  1930 – Clyde Tombaugh (Lowell Obs)  Explain Neptune’s orbit? Important Dates  1976  CH 4 ice, first estimate of diameter via albedo vs apparent brightness  1978  6.4 day variation in brightness  discovery of Charon

5. Pluto’s orbit

6. HST view of Pluto

7. Pluto Composition Spectroscopy – CH 4, N 2, CO, H 2 O ices Varied surface features  Compositional difference  Polar caps brighter  Darker equatorial  hydrocarbons? Ice  Tenuous atmosphere from sublimation, but does it refreeze at 50 AU?

8. Surface Composition - spectroscopy

9. Atmosphere – how do you detect/measure Pluto’s atmosphere?

10. Occultation

11. Atmosphere Detection via occultation  Structure seen in “kinks” in ingress and egress  variation over the years (is Pluto’s atmosphere expanding?) Composition  primarily N 2 Pressure  few μbar

12. Pluto’s Primary Moon Charon  Discovered as appendage to Pluto

13. Pluto’s Primary Moon Charon  Discovered as appendage to Pluto  Orbit  highly inclined  Orbital/rotation axis lie ecliptic  System seen edge-on twice in 248 year orbit  Size (via occultation)  Mass ratio = 0.12 (Moon/Earth ~ 0.01)  D charon = ~ 1200 km (Pluto ~2300 km)

14. Views of Pluto-Charon

15. Giant Impact Origin? Canup 2005 Science 307 546 Need to explain mass ratio/orbit Collisions – similar to our moon Numerical simulation show its possible!  Gravity  Compressional heating  Expansional cooling  Shock dissipation  20000 – 120000 particles  Composition  Mg 3 Si 2 O 5 (OH) 4  Various mixtures of water ice (40-50%) and rock

16. Canup – simulations of Pluto encounter

17. Canup – SPH simulation including gravity, heating, cooling, shock dissipation Ratio of impactor to total mass Composition Ratio of impact to escape velocity Spin period b’ = impact parameter J = final angular momentum

18. Pluto’s New Moons

20. Orbits in Pluto-Charon system

21. Pluto’s Moons Charon  Semi-major axis = 19570 km  P = 6.3872 days  D = 1205 km Nix  A = 48700 km  P = 25.5 days  D = 40 km Hydra  A = 64800 km  P = 38.2 days  D = 160 km Collisional origin? What is the typical impact velocity of objects in the Kuiper Belt? What is the escape velocity for impact ejecta in the Pluto system? What implications can you draw from this?

22. Pluto system formation

23. New Horizons (http://pluto.jhuapl.edu) Timeline  Jan 2006 – launch  Feb 2007 – jupiter encounter  Mar 2007 – June 2015 – “interplanetary cruise”  Jul 2015 – Pluto/Charon encounter Science Objectives  Map surface composition of Pluto and Charon  Geology  Atmosphere – composition and escape rate  Surface temperatures  Similar studies of Kuiper Belt object

25. Triton – composition & hemispheres

26. Triton Stern & McKinnon 2000 AJ 119 945 Only large moon with retrograde orbit Synchronously rotating (like our Moon)  has two distinct hemispheres  Leading side much more heavily cratered High resurfacing rate (like Io, Europa)  Impact population from Kuiper belt  Lots of small impactors (< 1km)  Surface age ~ 100 Myr  volume resurface rate as high as Io, Europa  Geological/tectonic activity – possibly driven by tidal capture