Cometary Basics

Michael F. A’Hearn

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Why Study Comets?

The Independent - 1997 Mar 22

Medieval copy of Josephus

Giotto di Buondoni - Scrovegni ChapelBayeux Tapestry

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More Reasons Why

C/Lee 1999 H1 - L. Sanino

C/Bennett 1970 II

C/Ikeya-Seki

C/Hale-Bopp

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Origin of Comets

• Slow accretion everywhere outside the ice line• Beyond Neptune - grow to classical KBOs, some KBOs

captured as Plutinos - – collisions break off pieces, gravitational scattering

hands off to giant planets successively to make Jupiter-family SP comets

– Others are perturbed to the scattered disk which may feed both Jupiter-family comets and the Oort cloud (from opposite edges of the disk) cf. Fernández et al.

• Inside Neptune - some build the giant planets (rapidly), others ejected to interstellar space (particularly in vicinity of Jupiter and Saturn), others ejected to Oort cloud, where subsequent perturbations produce dynamically new comets & thence LP comets & thence Halley-type SP comets

• All Discussed in Monday’s talks

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Cometary Record of Protoplanetary Disk

• Many (but not all) comets formed small– No gravitational heating– Probably no radioactive heating (but some would argue against this)

• Comets formed far from sun– No solar heating

• Thus, ices reflect T,P conditions in protoplanetary disk

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Major Goal of Cometary Studies

• Use observations of comets to constrain models for formation of the planetary system

• BUT!!!

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Cometary Evolution

• Each perihelion passage heats the outer layer, modifying ices– Depth of evolved layer very uncertain

• Each perihelion passage leads to mass loss– Total 1/2 to 5 m depending on q and – unknown fraction of the modified layer - could be nearly all of it

• Comets in Oort cloud are irradiated by galactic cosmic rays - break every chemical bond in outer 10 meters over 4.5x109 yrs– Leads to very unusual photometric behavior on inbound portion of first entry to planetary region

– Entire modified layer is lost on first approach

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Nuclear Models

Note the question marks!!!Can we exclude any of these models yet?

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Evolutionary Models

Benkhoff-Huebner model has density increasing monotonically from surface to 10s of meters. Prialnik-Mekler model has a dense layer of water ice at surface with lower density material below. Ice layer near surface may vary diurnally.

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What We Need to Know

• Nuclear Physical Properties– Density, strength, porosity– Heterogeneity

• Evolutionary (onion-skin) layering• Primordial cometesimal variations

• Nuclear composition– Chemical composition of ices

• Variation with depth or location?

– Mineralogical and chemical composition of refractories

– Scale of mixing among ices and refractories

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Context – Comets Unknown

• Mass – few data– Density and Surface Gravity uncertainty still large

– Measurement from DI (next talk) implies density ~0.4 g/cm3

• Is it typical? Non-gravitational acceleration models suggests it is typical

• Strength– Tensile strength < 103dyn/cm2 at km scale (<102 Pa)

– Upper limit from DI ==> < 10 kPa, but likely << 10 kPa

• Stratification– Know only irradiated mantle on new comets

– Ice to rock ratio unknown

– Layering clear from DI

• Shape– 4 comets with good images - very different shapes

• Photometric Properties – Very dark, grey to “pink” in color

– No mineral absorptions yet detected (except water ice on P/Tempel 1)

• Coma dust and rocks very uncertain– Very detailed models fit many species

– Biggest advance is dust particles returned by Stardust from P/Wild 2)

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Differences Among Nuclei

Stardust Team L. Soderblom

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are needed to see this picture.

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The Humpty-Dumpty Problem

Must use all possible wavelengths and techniques to sort it outAll measurements are of the coma, not the nucleus

Species Observed in Comets

Volatile Atoms

Refractory Atoms

Diatomic Triatomic Polyatomic IonsIsotopic Variants

(Ar) Al C2 (C2H) C2H2 C+ C15N

C Ca CH C3 C2H6 CH+ 13C12C

H Co CN CO2 CH3CN CO+ 13CN

(He) Cr CO CS2 CH3OCHO CO2+ DCN

N Cu CS H2O CH3OH H2O+ H13CN

O Fe NH H2S CH4 H3O+ HC15N

S K NS HCN H2CO HCO+ HDO

Mg OH (HCO) H2CS (N2+) HDO+

Mn S2 HNC HC3N O+

Na SO NH2 HCOOH OH+

Ni OCS HNCO

Si SO2 NH3 C++

Ti NH2CO

V

Mg2SiO4

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Coma ≠ Nucleus

• Biver et al.– Production rates vs.

heliocentric distance (from mm-wave data)

– Ratios of abundances vary dramatically

– Thus ratios at any given time can not be representative of nuclear abundances

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Dormant Comets

• Earlier “best” cases (Oljato, Phaethon, etc.) showed anomalous, possibly cometary properties but were not good analogs in other ways

• Recent examples very likely to be comets– Tisserand invariants <3, including retrograde

objects– Low albedos (all <0.1, most <0.06)– Data from several sources beginning with Harris

et al. 1999, Y. Fernández et al. 2003

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Dormant Comets

Y. Fernández et al. 2002

Convincing evidence for dormant comets!!(at last)

- cometary nuclei NEAs & UAs, T<3 NEAs & UAs, T>3

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Nuclear Sizes

• Fernández, Tancredi, et al. – 1999 A&Ap 352, 327; catalog on web– Wide variety of published data

• Weissman, Lowry, et al. – 2003 LPSC– Recent data plus selected older data

• Lamy et al.– Various papers; chapter in Comets II– HST data plus selected older data

• A’Hearn - unpublished reanalysis of Lamy et al.

• Recent reanalysis of all data by Tancredi et al. (2006)

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Size Distribution

First size distribution, data from literatureJupiter family comets onlySlope +0.54 in magnitude --> slope -0.88 in massAlbedo assumed to not vary with size

J. Fernández et al.

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More Recent Sizes

54 Jupiter-family comets

20

2

4

20

40

80

24 0.20.41

10Cumulative Number

Cumulative Size Distribution

110

Radius - km

Slope = -1.59 +/- 0.03(r > 1.4 km)

Weissman & Lowry, 2003 LPSCSlope -1.59 ± 0.03

Lamy et al., 2003, in Comets IISlope -1.87 ± 0.02

Detailed studies of individual objects show significant disagreement with results of Fernández et al., but not obviously systematic effectsSlopes indicate evolutionary processes other than or in addition to collisions

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More Size Distributions

Data from Lamy et al. re-plotted to highlight region of slope -2.5 for “ecliptic” comets Blue = ecliptic comets Pink = all cometsChiron & SW1 excluded by Lamy et al. & here as being Centaurs

Results sensitive to assumptions! - Cutoff for observational selection at small end - Cutoff for small-number statistics at large end - Binning vs. counting!Whether collisions explain the size distribution is still an open question!Is depletion at small sizes real? Or is it selection? If real, is it primordial or evolutionary?

Slope -2.5

Cumulative Size Distribution

-0.500

0.000

0.500

1.000

1.500

2.000

-1.5 -1 -0.5 0 0.5 1 1.5 2

log( R )

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Dormant Comets

Pink - ecliptic comets (Ecs)Blue - near-isotropic comets (NICs)

Data from Lamy et al. 2003. In Comets II.

Is the dispersion difference real? I.e., is there a wider distribution of albedos among ECs than among NICs?

Are these really dormant comets?

Albedo vs. Size - Dead Comets

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 2 4 6 8 10 12 14 16

Radius [km]

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Dust Trails

• Traditionally discovered in thermal IR• Dominated by large particles released long

prior to observation• Spatial resolution limited by observations at

thermal wavelengths• Recent optical detections provide good spatial

resolution and also provide additional constraints on particle distribution– Ishiguro et al., 2003. Ap.J., 589, L101– Weissman et al., private communication

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Trail from Wild 2

Ishiguro et al., 2003

Detailed spatial profiles important for spacecraft encounters

Estimate 0.5 mg particles

Hazards for all close flybys should be reassessed.

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Cometary Breakup

• Breakup is a long-established process but usually the largest fragment remains an active comet at later apparitions

• Comet LINEAR 1999 S4 dissipated rapidly, but other comets have dissipated on longer time scales

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Comet LINEAR 1999 S4

C/LINEAR - 99S4: breakup for no apparent reason; Weaver et al. infer most mass in fragments with 500m < diam < 50mPrevious comets - S-L9, tidal breakup ==> tensile strength <103 dyn-cm-2 at km scalesPrevious comets - few % per passage break up in some wayPredominant process - one large fragment and a few smaller fragments (e.g., West, Biela, et al.)Predominant cause and mechanism - lots of speculation but really totally unknown

Bottom line - comets are really fragile, certainly at some scales and probably at all scales

Should landers & other contact missions abandon trying to work at all strengths and assume low (e.g. <104 dyn-cm-2 for all strengths and at all scales?

Weaver et al., 2001 Science 292, 1329

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High-Resolution Spectra

• Optical and IR high-resolution spectra continue to provide valuable insight and also to provoke new puzzles

• Most data are for Oort-cloud comets (long-period and Halley-type; NICs in the terminology of Lamy et al.)

• Missions tend to go to Jupiter-family comets - can we relate the two? Or is a mission to a NIC essential for our understanding?

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High-Resolution Spectroscopy

• Two optical examples– P/de Vico - Cochran & Cochran (not JF)– C/Hyakutake - A’Hearn et al.

• Near-IR example– Survey by Mumma et al. (not JF)

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122P/de Vico

Allows detailed models of fluorescence which in turnallows understanding of physical processes - production, motion, etc.Note huge number of unidentified lines at all wavelengths - new chemistry!!Similar spectra to shorter wavelengths essential to map C3 and its effect on lower resolution spectra

Cochran & Cochran 2002

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122P/de Vico

Cochran & Cochran 2002

No features identified in this segment!!!

We are missing a huge understanding of the chemical composition!

Beginnings of new identifications in some segments, e.g., O lines by Feldman

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C/Hyakutake

Opportunistic observation at small geocentric distanceMany bands of S2

Highest resolution ever for S2

Trot ~ 70K (collisional control)Note also prompt emission from hot OHNew formation scenario in inner comaOCS + S(1D) --> CO + S2

How can missions probe this chemistry?

Is S2 in JF comets? (It is probably in all non-JF comets; if chemical production is right, it should be in all comets)

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C/Hyakutake

For the “real” astronomers, note unidentified feature at 4425-4435ÅDIB at 4430 is wider & diffuseAre they related - gas phase in comets and grain surface phase in ISM?This is one of many unidentified features

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Near-IR Spectroscopy

Example of high-resolution, near-IR spectra obtained by Mumma and collaborators.Some species, with telluric counterparts, can only be measured with large Doppler offsets.All species require careful model of Earth’s atmosphere.

C/Hyakutake - Mumma et al.

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Near-IR Results

Generally similar values except for C/LINEAR (99S4)

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Compare with Older Result

• Two classes of comets– Normal abundance ratios (solid)– Depleted in carbon chain molecules (open) - all

are JF

• Reanalysis in progress• Is Mumma’s depleted comet (C/1999 S4) related

to these depleted comets?

A’Hearn et al. 1995, Icarus, 118, 223

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Near-IR Results

Wide range for CH4 - what does this mean?CO shows comparable range of abundances, but uncorrelated with CH4

Other (less volatile) species show much less variation

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Implications

• Most missions will be to Jupiter-family comets

• Can we generalize chemistry from a mission when our Earth-based ensemble is mostly Oort-cloud comets?

• Orbiters should map the innermost coma chemistry to separate native ices from other sources

Some Useful Concepts

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Optical Depth

• Well known and widely used in astronomy but often ignored in cometary science

• I/I0 = e- where = optical depth• For simple scattering and absorption

• = N • where = extinction cross section

• and N = column density• A typical photon travels = 2/3 before being absorbed or scattered

• We can measure this in the afternoon for DI ejecta

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Scattering Function

• A single particle (grain of dust or whatever) scatters light anisotropically

• The phase function describes the distribution of light with scattering angle, both for microscopic particles and for large bodies like cometary nuclei and asteroids

• Phase function is often approximated by many different simple functions of the scattering angle

• For a single particle, – I = Fsun p f( where p = geometric albedo and f()

is the scattering function– Be careful of confusion between geometric albedo and

Bond albedo - geometric is backscattering and Bond is integrated around the sphere.

– The widely used quantity Af uses Bond albedo as A, although this is not well defined for microscopic particles

• We can measure this in the afternoon also

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Typical Cometary Scattering Function

Scattering Functions

0

5

10

15

20

25

0 50 100 150 200

Phase Angle (180 - scattering)

Phase Function (normalized)

DustNucleus

• See spread sheet for others

Dust from Ney & Merrill for comet West (1976 VI)Nucleus from Lumme & Bowell model

Backup Slides

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Deep Impact

"It [an asteroid] was racing past them at almost thirty miles a second; they had only a few frantic minutes in which to observe it closely. The automatic cameras took dozens of photographs, the navigation radar's returning echoes were carefully recorded for future analysis - and there was just time for a single impact probe. The probe carried no instruments; none could survive a collision at such cosmic speeds. It was merely a small slug of metal, shot out from Discovery on a course which should intersect that of the asteroid.

.....They were aiming at a hundred-foot-diameter target, from a distance of thousands of miles...

Against the darkened portion of the asteroid there was a sudden, dazzling explosion of light. ...”____________________

Arthur C. Clarke, 1968. In 2001: A Space Odyssey. Chapter 18

Deep Impact - - An artificial meteorite impact - 360 kg at 10.2 km/s - Are there pristine ices at depth? - What are the surface material properties?Unlike SL-9 at Jupiter, we will know everything about the impactor so the only unknowns are in the target

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Scientific Objectives

• Primary Scientific Theme– Understand the differences between interior and

surface– Determine basic cometary properties– Search for pristine material below surface

• Secondary Scientific Theme– Distinguish extinction from dormancy

• Additional Science Addressed– Address terrestrial hazard from cometary impacts– Search for heterogeneity at scale of cometesimals– Calibration of cratering record

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Mission Overview

• 2 spacecraft – Smart Impactor + Flyby• Fly together until 1 day before impact

– 6-month Earth-to-comet trajectory• Smart Impactor

– Impactor Targeting Sensor (ITS)• Scale 10 microrad/pixel• Used for active navigation to target site• Images relayed via flyby to Earth for analysis

– Cratering mass (~360 kg at 10.2 km/s)• Excavates ~100-meter crater in few*100 seconds

– Baseline prediction - other outcomes are possible

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Mission Overview (continued)

• Flyby Spacecraft– Diverts to miss by 500 km– Slows down to observe for 800 seconds– Instruments body-mounted – spacecraft rotates to

follow comet during flyby• Instruments on Flyby Spacecraft

– High Resolution Imager (HRI) • CCD imaging at 2 microrad/pixel (0.4 arcsec/pixel)• 1-5 micron long-slit spectroscopy (R>200, 10 microrad/pix)

– Medium Resolution Imager (MRI)• CCD imaging at 10 microrad/pixel• Identical to ITS but with filter wheel added

• Major Earth-based Observing Campaign

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Cometary Dichotomies

• Comets have the most primitive, accessible material in the SS

• Comets must become dormant

• There must be many dormant comets masquerading as NEAs

• We know more chemical and physical details than for other small bodies in the SS

• Abundances in the coma are used to infer ices in the proto-planetary disk

• Comets break apart under small stresses

• We do not know what is hidden below the evolved surface layers

• Is the ice exhausted or sealed in?

• We can not recognize dormant comets among NEAs

• We do not know how to use these details to constrain models of nuclei

• Abundances in the coma differ significantly but in unknown ways from nuclear abundances

• Variation of strength with scale is totally unknown