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Comet Activity and Composition

Comet Activity and Composition. K. Meech Ast 734 Seminar 8/30/04. Dirty Snowballs. Inactivity to Activity. Sublimation of gases Drags dust from nucleus Gravity low Most dust escapes Solar radiation pressure  coma  tail Photodissociation of gas. Ionization  gas tail

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Comet Activity and Composition

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  1. Comet Activity and Composition K. Meech Ast 734 Seminar 8/30/04

  2. Dirty Snowballs

  3. Inactivity to Activity • Sublimation of gases • Drags dust from nucleus • Gravity low • Most dust escapes • Solar radiation pressure  coma  tail • Photodissociation of gas • Ionization  gas tail • Activity develops • Km-scale nucleus • Coma ~ 105 km • Tail ~ 106-107 km

  4. Dust Coma Development 01/19/99 r=3.1AU 04/06/98 r=4.3 AU 07/15/99 r=2.2 AU q-620 dy; Afr = 14 cm q-350 dy; Afr = 31 cm q-150 dy; Afr = 105 cm 08/22/00 r=2.6 AU 09/30/00 r=2.8 AU 11/12/01 r=4.4 AU q+210 dy; Afr = 87 cm q+300 dy; Afr = 83 cm q+590 dy; Afr = 13 cm

  5. Who Cares? “extragalactic student” “extragalactic astronomers observing a comet”

  6. Cosmic Solar System History Earth in the Hadean Oceans & rocks form ~4.4 Gy ago >4.6 Gy ISM dark cloud Planetesimals condense Planets accrete Form few x100 million years Late planetary bombardment Comets, asteroids bring water & Organics to Earth The Archean Epoch Oldest life on Earth 3.5-3.8 Gy ago

  7. The Oort Cloud • 17th century physics: Brahe, Kepler & Newton • Eorbit = -m/2a • Distribution of 1/aoriginal • 22 long-period comets • Strongly peaked  • Source 50,000-150,000 AU • Contains 1011 comets • Width very narrow • Fading Problem • “Volatile Frosting” • Different chemistry Oort, J. (1950) B.A.N. 408, 91-110. Oort J. H. & M. Schmidt (1951) B.A.N. 419, 259-270

  8. The Modern Oort Cloud • Outer Oort Cloud 15,000-105 AU • Stellar perturbations > 104 AU • Inner Oort Cloud 2000-15,000 • Galactic Tides • Dynamically inert 50-2000 AU • Kuiper Belt 35-50 AU • Stable, dynamically active • Classical, 3:2, scattered • Dynamically new • 1/aorig < 100x10-6 AU-1 • Long Period P > 200 yr • Short Period P < 200 yr • Halley family – Oort cloud origin • Jupiter family – KBO origin • Centaurs transition objects

  9. The Evidence for Fading • Different types of evidence • Really bright comets are all long-period • Distant comets  narrow tails (large dust)  volatile gases • New comets tend to split more frequently (more volatiles) • Non-gravitational motion (jets) • Problems • Non uniform data sets • Non-linear detectors Great Comet 1577 Morehouse 1908 III Halley 1910 Delavan 1914

  10. SP Comets 3.4-14.5 AU

  11. Comet Activity Levels  Trends

  12. Evidence for Differences • Dots = All SP obs • Squares = Halley • Triangles = DN comet

  13. Sublimation of Volatiles? • Delsemme’s original work: albedo too high • Water-activity out beyond Jupiter

  14. Water Ice Physics • Phase I: P < 2700 atm • Ih – hexagonal • Ic – cubic (low T, low P phase) • High P forms: II to XIV • Amorphous Tcond< 100K • Traps gases • Clathrates • Mechanical trapping in cages

  15. Comet Formation 100K 64K 31K 0 10 100 AU

  16. Low Temperature Condensation • Ices in comets condensed T< 100K • Amorphous form • Trapped other gases • Amounts depend on r • Release of gases • 137K amorphous  crystalline phase change • Annealing (30-35K) • Sublimation 160-180K CH4 N2 Ar CO • Gas release at large distances: controlled by Water

  17. Heat Transfer in Comets • Conduction low • Depends on porosity (unknown) • Radiation • Gas phase conduction (recondensation) • Sintering • Changes the conductivity • Volatile re-distribution • Insulating layers

  18. The Halley Outburst • Gas Laden amorphous ice model • Heat from perihelion penetrates to ice layer • Exothermic transformation (137K) • Released gases build up pressure  outburst

  19. Chiron’s Behavior • Amorphous ice model • 60% dust • 40% amorphous ice • 0.1% trapped CO • Matches observations • Density < 0.4 g/cm3 • Mass loss rates & dust • CO fluxes match obs • Tsurface matches obs • Activity sporadic  not refreshing surface

  20. Hale Bopp • Active at large r • Discovered 7.2 AU (1995) • Pre-discovery image 13.0 AU (1993) • Dynamically young • Large CO fluxes seen • Molecules of different volatilities appear at similar times

  21. Thermal models: Comet Hale Bopp • Amorphous ice crystallization model • Porosity 0.65 • 4% by mass trapped CO

  22. Activity at Larger r? • Distance for T ~ 137K • Beginning near 10 AU • Mechanisms at r > 10 AU • Solid volatiles (e.g. CO, CO2) sublimation • Annealing C/2003 A2 Gleason q = 11.43 AU 1/a = 42 x 10-6 AU-1

  23. KBO1996 TO66 – Activity? • Orbit • Q = 48.6, q = 38.5 • q: 5/3/1910 Q: 2/1/2054 • Lightcurve period • 1997: 2 peak 6.25 +/- 0.03 hr, Dm = 0.12 mag • 1998: single peak, Dm = 0.33 • Consistent with activity • Blue colors • Vary with rotation in 1999

  24. Observations • Subaru 8m + Suprime Cam • 8x12 K CCD mosaic • 0.2”/pixel, 0.25o FOV • Target Selection • 15 blue-neutral objects • Select smallest r = 33.8 AU • 1997QJ4: V-R = 0.296 (Plutino) • r = 33.8 AU, Hv=7.5 (rad = 80 km) • October 3, 4 2002 UT • Nt 1 phot, Nt 2 clouds • Sensitivity • S/N = 3, V=28 12000s

  25. Composite Image • Single exp, 400 sec

  26. Composite Image • Single exp, 400 sec • 12000s sum

  27. Composite Image • Single exp, 400 sec • 12000s sum (zoomed 80”)

  28. Composite Image • Single exp, 400 sec • 12000s sum (zoomed 80”) • Median combined

  29. Composite Image • Single exp, 400 sec • 12000s sum (zoomed 80”) • Median combined • Shift & sum for KBO rate

  30. Composite Image • Single exp, 400 sec • 12000s sum (zoomed 80”) • Median combined • Shift & sum for KBO rate • Median star subtracted

  31. Surface Brightness Result: Q < 0.01 kg/s F = Sopagr2pvQf / 2r2D2vgr Constants: So ,p , r ,D, f Assume: agr = 0.1 mm (max lifted off) pv = 0.04, vgr = 0.1 km/s (CO)

  32. Comet Paradigms • “Comets are the most pristine things in the Solar System” • “Comets tell us about the formation of the Solar System

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