Download Can Comets Contain Water? A "Wet"

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