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Astronomy 340 Fall 2005

This review explores the atmosphere, rings, and geology of Pluto, as well as its moons and the New Horizons mission. Topics include Titan's atmosphere, the Roche limit, comparison of ring systems in gas giants, and the composition of Pluto's surface and atmosphere. It also discusses the discovery and characteristics of Pluto's primary moon Charon and the possible giant impact origin of the Pluto-Charon system. The article concludes with an overview of the New Horizons mission and its science objectives.

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Astronomy 340 Fall 2005

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  1. Astronomy 340Fall 2005 22 November 2005 Class #23

  2. Review & Announcements • Titan • Describe Titan’s atmosphere • Rings • What’s the Roche limit? How is it significant? • Compare and contrast the ring systems of the gas giants • Size distribution? Composition? Dynamics?

  3. Pluto - basics • Discovery • 1930 – Clyde Tombaugh (Lowell Obs) • Explain Neptune’s orbit? • Important Dates • 1976  CH4 ice, first estimate of diameter via albedo vs apparent brightness • 1978  6.4 day variation in brightness  discovery of Charon

  4. Pluto’s orbit

  5. HST view of Pluto

  6. Pluto Composition • Spectroscopy – CH4, N2, CO, H2O ices • Varied surface features • Compositional difference • Polar caps brighter • Darker equatorial  hydrocarbons? • Ice • Tenuous atmosphere from sublimation, but does it refreeze at 50 AU?

  7. Surface Composition - spectroscopy

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

  9. Occultation

  10. Atmosphere • Detection via occultation • Structure seen in “kinks” in ingress and egress  variation over the years (is Pluto’s atmosphere expanding?) • Composition  primarily N2 • Pressure  few μbar

  11. Pluto’s Primary Moon Charon • Discovered as appendage to Pluto

  12. 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) • Dcharon = ~ 1200 km (Pluto ~2300 km)

  13. Views of Pluto-Charon

  14. 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 • Mg3Si2O5(OH)4 • Various mixtures of water ice (40-50%) and rock

  15. Canup – simulations of Pluto encounter

  16. 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

  17. Pluto’s New Moons • S/2005 P1, S/2005 P2 • Discovered via HST • Separation ~ 27,000 km, diameters 64, 200 km) • Don’t know much else about them!!

  18. Orbits in Pluto-Charon system

  19. 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

  20. Triton – composition & hemispheres

  21. TritonStern & 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

  22. Cratering Density

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