Evidence for Liquid Water on

CometsRob Sheldon,

Richard Hoover

SPIE SanDiego

July 31, 2005

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. ]

Fred Whipple’s 1950 “Dirty Snowball” Model

1906-2004

But equilibrium temperature of bodies in the inner solar system is > 273 K !To keep the comet from melting, we apply some refrigeration principles: - white color (high albedo) - sublimation cooling - low spin rate - dust insulation (porous)But this condition is unstable!

Temperature Regulation

If heat transport overwhelms refrigeration the pressure goes up, or should“black goo” / melt liquid plug reduce the permeability, then the lid goes on the pressure cooker. At high enough pressure, meltwater forms. This clogs more crust, permitting higher pressures, less sublimation cooling, higher heat transport in (waterlogged attic insulation)= more melting. E.g., Positive feedback (Yelle 2004)

And the Rayleigh-Taylor instability can trigger a large increase in heat transport.

Rayleigh-Taylor Instability Contours

Slow Rotator,T=10.4hr Fast Rotator, T=5h T=5hr w/cosine insolation

(4/3 DG – 2) = gravity at equator (x-axis) Sun at left

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.

Stretched 10X y-axis scale below line3min=1hr

stablestable stable

Spin contribution to feedback• 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.)

Concrete 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 crust at the poles (no RT). This makes the RT “spread out” across the surface.

• 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.

A Comet’s LifeIce

Liquid

Vapor

Spin Axis

Spin Flip

a) b) c)

d) e) f) g)Cement

Issues before s/c era

1. Birth:1. Density of comets2. Albedo-Area3. Kuiper Belt vs Oort4. Aphelion vs

Perihelion

2. Life:1. Spin rate2. Shape aspect ratio3. Brightness vs radial

distance

4. Active area, jets

5. New vs. Old comets

6. Outbursts

7. Tail Shedding

3. Death:1. Earth crossing asteroids

2. Fireballs vs chondrites

3. Tidal Force Breakup

Issues after s/c visits to P/Halley (& P/Borrelly & P/Wild-2)

1. Albedo: .02-.03 darker than soot!

2. Shape: very prolate!

3. Dust distribution across limb, size.

4. Small active area Jets: dayside, geyser-like

5. Temperature: 300-400K

6. Pinnacles, cliffs, craters, patterned ground

7. Deep Impact raised only dust

1.1 Comet Density

• Brownlee particles collected in the stratosphere thought to be from comets.

• Comets are thought to have a density 1/10 that of water?

1.2 Albedo

• Before the spacecraft era, 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! So much bigger too. And hotter.

1.4 Aphelion vs Perihelion• Why is

there a gap both for q < 1 and q> 3?

• And nothing near hyberbolic?

Comets, 1981

1.4 Apogees in Theory & Life

Comets, 1981

Theory

Weighted Observations

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. Collisions don’t seem to explain it, nor were there any convenient solar flares.

• Tempel-1 had 2 outbursts during the week before collision.

2.7 Tail Shedding

• Shouldn’t they occur at every sector crossing? Why so infrequent then?

Comets, 1981

3.2 Tensile Strength

• Do fireballs determine the tensile strength of comets? Comets, 1981

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

Courtesy Stardust

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

4.4 Geysers

Giotto

DS-1 Stardust

Deep Impact

4.6 Pinnacles

Stardust

41 hours

Outbursts

4.7 Deep Impact

50ms frames

What, No Water?

Spin Axis Estimate

1.7 deg motion of crater3.0 deg motion of “rim”

+

Insolation vs Axis

• Putative pole

• Impact site

• Pole remains in sunlight, as does impact site

+

x

Evacuation of 9P/Tempel-1

• From the wet comet=critical period calculation, T=41hr, D=20kg/m3. That’s really fluffy snow! And completely inconsistent with cratering data.

• But that assumes uniform density. If the comet has vapor pockets, then RT instability still operates.

g

If pristine comet has D=200 kg/m3, we estimate 90% of the interior is vapor, 10% pristine.

Conclusions• Comets are in an unstable thermal equilibrium as

they enter the inner solar system. We believe many factors contribute to their spontaneous phase change from sublimation cooled to “wet” comets.

• Wet comet theory explains many unsolved puzzles of cometary dynamics.

• Deep Impact results seem to show no water, and we argue why this result is still consistent with the wet comet theory.

• Therefore the major objection to life on comets, the absence of water, appears less defensible.

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

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

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

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?