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Can Comets Contain Water? A “Wet” Comet Theory Rob Sheldon NSSTC October 29, 2004 Harold’s Bane 1066 AD Her forðferde Eaduuard king. Harold eorl feng to ðam rice heold hit .xl. wucena. ænne dæg. her com Willelm gewann ængla land. her on ðison geare barn Cristes cyrice. [her atiwede cometa .xiii. kalendæ MAI. ] Outline • I. Paradigm Shifts • II. The Standard Comet Model • Birth, Life & Death • Problems: Shape, Spinrate, Coma, Dust… • III. The Spinning Comet Model • Birth, Life & Death • Answers: Shape, Spinrate, Coma, Dust… • IV. Hoyle’s Hypothesis I. Paradigms Thomas Kuhn 1922-1996 Why are some models more equal? • Why do some models change rapidly—e.g cosmology and vacuum energy, whereas others change with great resistance? – Investment of time, energy, resources – Investment of reputation, position, privilege – Investment of philosophy, worldview • What about the standard scientific method? – Science = Data + Hypothesis + Testing : Repeat Thomas Kuhn’s Paradigm Shift • Problem: No great discoveries occur this way, only mundane “normal” science with accepted models. • Vienna circle, Positivism, Russell & Wittgenstein – Everyone should think like a physicist. Carnap’s request. • Kuhn presented a hypothesis called “paradigm shift” – Postmodernists love it. Kantians love it. Sociologists love it. Scientists hate it. the Sokal “hoax” • Why discuss sociology in a physics seminar? Because the comet paradigm is in trouble. When is there a paradigm shift? • Kuhn: NOT just when there is a better theory. Copernicus vs Ptolemy • Several conditions coalesce: – – – – Discrepancies with existing model grow too numerous Metaphysical shifts in thinking New theory appeals to next generation Old generation dies off • We are seeing many fields today, not just cometary/space physics, exhibit all these signatures. Watch biology closely. Today we discuss comets. II. Fred Whipple’s 1950 “Dirty Snowball” Model 1906-2004 Birth • Oort cloud. Volatile rich. Coalescence from primordial nebula. Carbon rich. (Why?!) Loose, weakly bound gravitational objects 1-100km in size. Possible 26Al heating may have caused partial melting. Cosmic ray transformed outer cm-thick crust. Black goo / burnt toast. • Tensile strength of interior estimated 1-10 kPa. (Tidal stress breakup, fireballs) In comparison plaster of paris has a tensile strength 0.6 MPa, ice around 1.6 MPa. Comets are 200x weaker than solid ice! Life • Orbit is deflected from circular to elliptical • As comet approaches the “snow line” at 5AU it begins to vaporize and form a tail. Several tons/s loss of mass. • The tail grows as it nears the sun, produces dust & plasma tails, and dynamic effects due to jets and outbursts. • May break up at any point in orbit. • On receding from the sun, the tails shrink and the comet becomes “stellar” beyond 5AU. • May get trapped or deflected by Jupiter. Death • Volatiles are lost and comet looks asteroidal • Crust of non-volatile material gets too thick mimicking the loss of volatiles. • Comet fragments (tidal forces, spin rate?). • Comet interacts with Jupiter and is either ejected, or trapped. • Comet collides with another body, fireballs (spectacular Shoemaker-Levy-9 collision) • Comet leaves on a hyperbolic orbit Issues before s/c era 1. Birth: 1. 2. 3. 4. 4. 5. 6. 7. Active area, jets New vs. Old comets Outbursts Tail Shedding Density of comets Albedo-Area Kuiper Belt vs Oort Aphelion vs 3. Death: Perihelion 1. Earth crossing asteroids 2. Life: 1. Spin rate 2. Fireballs vs chondrites 2. Shape aspect ratio 3. Tidal Force Breakup 3. Brightness vs radial distance Issues after s/c visits to P/Halley (& P/Borrelly & P/Wild-2) 1. 2. 3. 4. 5. 6. Albedo: .02-.03 darker than soot! Shape: very prolate! Dust distribution across limb, size. Small active area Jets: dayside, geyser-like Temperature: 300-400K Pinnacles, cliffs, craters, patterned ground 1.1 Comet Density There seems to be more zodiacal dust than can be accounted for with comets? (Albedo determines size) Comets are thought to have a density 1/10 that of water? 1.2 Albedo • Before spacecraft, astronomers only knew the product of albedo & area. Comets were thought to have albedo in the .3-.7 range, like most asteroids. This made comets seem much smaller than was actually correct. They turned out to be blacker than soot. • Why do short-period comets have such low albedo? • Do new comets have the same albedo? 1.4 Aphelion vs Perihelion Why is there a gap both for q < 1 and q> 3? And hardly anything hyperbolic? Comets, 1981 1.4 Observational bias? Comets, 1981 1.4 Apogees in Theory & Life Comets, 1981 1.4 Oort Cloud Simulations Comets, 1981 2.1 Spin by jets • Why do comets spin slower than asteroids? • Why do comets all spin much slower than breakup? than RT? Comets, 1981 2.2 Spin from Stellar obs. CCD camera observations at large distance “stellar” lightcurves for prolate objects ApJ 1988 2.3 Activity • The dust follows a 1/r4 law, but gas doesn’t? • Post<>Pre-perihelion? Comets, 1981 2.4 Active area & jets Why is post-perihelion different from pre (both in absolute and r dependence)? • Why was Kohoutek so disappointing? (Methane ice, or CO ice, etc. • Some phase transition occurred, but no one is sure what. Skylab, H-corona, 1973 2.5 New vs Old Comets New are dustier, but old are supposed to lose their volatiles! If gas/dust ratios are fixed, why aren’t they the same? Hale-Bopp, 1997 2.6 Outbursts? • What would cause 8 order of magnitude changes in brightness P/Schwassmann-Wachmann? Collisions? But then how does the comet survive? • Halley had a 300-fold increase in brightness in 1991, while at 14.3AU. Why? • Collisions don’t seem to explain it, nor were there any convenient solar flares. • Wallis suggests surface freezing, followed by compression of liquid water erupting from a fissure. 2.7 Tail Shedding • Shouldn’t they occur at every sector crossing? Why so infrequent then? Comets, 1981 3.2 Tensile Strength Comets, 198 Do fireballs determine the tensile strength of comets? Comets, 1981 3.3 Tidal Breakup? Comets, 1981 4.1 Black Prolate P/Halley • Blacker than the coma behind it! • Jets! • Prolate • Not outgassing • 400K • Little dust Courtesy Giotto 4.2 Prolate Shape (P/Borrelly) Courtesy Deep Space 1 Prolate P/Wild-2 Stereo pairs showing top panel with large projection out of the frame; middle panel with deep canyon; bottom panel with high pinnacles in the “crater” at the bottom. Courtesy Stardust Courtesy Stardust Prolate Wild-2 4.4 Geysers Giotto DS-1 Stardust Stardust 4.6 Pinnacles Stardust III. Spinning Comet Model Rayleigh-Taylor Instability Contours (4/3p DG – w2)r = gravity at equator (x-axis) Sun at lef Slow Rotator,T=10.4hr Fast Rotator, T=5h stable T=5hr w/cosine insolation stable g=1hr stable Stretched 10X y-axis scale below line 3min 3min Therefore mean temperature has a phase transition at critical Tc. This initiates a positive feedback sequence. Below Tc heat is pumped out on night side, above Tc heat pumped in. Temperature The Whipple model has gas sublimation cooling the permeable crust. Should either “black goo” or melt liquid plug the pores and slow the permeability, then the lid goes on the pressure cooker: melting clogs more crust, permitting higher pressures, less sublimation cooling, more heat transport in (waterlogged attic insulation)= more melting. Positive feedback for melting. Both liquid and vapor are candidates for RT-instability, but water greatly increases heat transport because of heat capacity. Spin • Melted dirty snow will segregate, dust drops “down” to to the equatorial surface. This thickens the crust, reduces the gas flow, and permits higher pressure and hence more liquid. Another feedback. • Dust has higher density than water/ice, so migration to equator will slow the rotation rate of the comet. When it drops below 1/Tc, it immediately refreezes. Thus RT drives a comet to Tc. • Liquid acts as a nutation damper, eliminating precession, giving higher spin in 1-axis, which promotes RT. A positive feedback. • Differentiation lowers the density of the interior, which enhances RT (lowers Tc.) Crust • Dust at the surface reduces the albedo, both by color and roughness. This increases the temperature and heat flow into the comet. Crust “dries out” in original shape. • Meter thick rigid crust develops which can support observed vertical landscape. • Crust at equator may be cooler (due to R-T) than crus at the poles (no RT). This makes the RT “spread out” across the surface. (Need self-consistent modelling!) • As water “leaks out” vapor pockets form in equatorial belt (geysers). • Collapse/explosion of vapor pockets lead to cratering, and eventually to prolate erosion of comet. Activity & Fragmentation • Release of vapor and/or liquid from vapor pockets = geyser. See Yelle (Icarus04) Partial pressures can support liquid water. • When sufficient equatorial erosion has made comet prolate, liquid water facilitates a swap of rotation axes. Old polar regions had been under compression, now find themselves under tension = likely breakup scenario. • Weakest prolate crust is at the poles, where Borrelly had a stable geyser. Accident? Or global melting? • Breakup separation speed depends on aspect ratio and/or vapor pressure, both functions of light intensity. Cartoons Orbital changes • Liquid water with dissolved salt is a plasma. Water moving through a magnetic field is forced to circulate, which produces a diamagnetic field. • Such a field will inflate the magnetopause, reduce reconnection, and produce a non-gravitational, nonconservative frictional force with the solar wind B. • Inbound friction reduces the aphelion, outbound increases it = diffusion of aphelion. • Sector changes in IMF will cause lags between the internal field and external. Acceleration? Objections • Doesn’t the observed slow rotation of short period comets preclude RT-instability? Not if the density is inhomogeneous. But it changes the location. • Isn’t 3 AU a bit far for 270K equilibrium T? RT is a “greenhouse” effect. • Doesn’t Shoemaker-Levy show low tensile “snow”? Not if magnetic stresses from Jupiter’s massive magnetosphere is included. • Doesn’t the “snow line” preclude water out at 5 AU? Freezing is outside-in, so thermal latency is large. Summary • 1.1-4 Water explains why fireballs break up early-I.e. Columbia. Crustal differentiation with water explains albedo. W. explains rapid diffusion of aphelion. • 2.1-7 W. explains slow spinrate, prolate shape, and lightcurves. W. explains asymmetry around perihelion, gas production with wrong radial dependence, and existence of jets. W. explains why new comets (dry, subliming) are brighter than old (wet, crusty). W. explains non-tidal fragmentation. W. may explain rapid brightening by collision (splashing=large surface area). W. may explain reduced tail-shedding. • And the $64,000 question: What about Life? IV. Hoyle’s Hypothesis 1915 - 2001 Panspermia • Diatoms? • Comets are incubators Martians & Europeans • Given that 3 CI (comet-like) meteorites have struck the Earth in last 100 years, they most certainly have struck Mars in the past 3 billion. • So we know what Martians look like: they’re green. • Europeans are red-brown, sort of like Halley. Planetary Protection • “Planetary Protection Matters” J. Rummel, NASA HQ, and L. Billings, SETI (Cospar 8/04) Planetary protection is the term given to the policies and practices that protect other solar system bodies…from terrestrial life, and that protect the Earth from life that may be brought back…. The cost of meeting stringent Category V requirements on a Mars surface sample return mission is estimated at about 5-10% of the entire mission budget. • Genesis category I? Stardust category II? • SpaceNews 9/20/04 “Genesis Mishap Renews Debate About Mars Sample Return”. “Genesis did not have a planetary protection requirement for containment.” Rummel. “Everyone agrees that we must be as careful as possible with the Mars sample,..The question is whether we want to spend billions are tens of billions of dollars to make the risk even more infinitesimal.” Mendell Conclusions • Did life evolve on Earth or get transported? Not sure, but it isn’t coincidence that CI chondrite fossils are prokaryotes. • Does this solve the “origin of life” problem? Probably not. Hoyle the optimist estimated the chances at 1:1040000, give or take a few Planck #s. Assuming every star has an Earth type planet evolving since the big bang with comets zipping around, subtracts ~12 from 40000. Chances are still slim. But Hoyle the pessimist didn’t believe in the Big Bang. Eternity has as many zeroes as needed. • Will Hoyle remain controversial? You betcha. References: • www.panspermia.org • Comets, ed. L. Wilkening, 1981 • Physics and Chemistry of Comets, ed. W.Huebner 1990. • ApJ 1988 Jewett and Meech • Icarus issue on Borelly 2004. • http://stardust.jpl.nasa.gov • R. Hoover et al. SPIE proceedings 5555