ABSTRACT

Title of Dissertation PHYSICAL PROPERTIES OF COMETARY NUCLEI

Yanga Rolando Fernandez Do ctor of Philosophy

Dissertation directed by Professor Michael F AHearn

Department of Astronomy

I present results on the physical and thermal prop erties of six cometary nuclei

This is a signicant increase in the numb er of nuclei for which physical information

is available I have used imaging of the thermal continuum at midinfrared and radio

wavelengths and of the scattered solar continuum at optical wavelengths to study

the eective radius reectivity rotation state and temp erature of these ob jects

Traditionally the nucleus has b een dicult to observe owing to an obscuring

or extreme faintness I have taken advantage of new midinfrared array detectors to

observe more than were p ossible b efore I have also codevelop ed a technique

to separate the coma and nucleus from a image I develop ed a simple mo del

of the thermal b ehavior of a cometary nucleus to help interpret the thermal ux

measurements the mo del is an extension to the Standard Thermal Mo del for aster

oids We have enough nuclei now to see the rst demarcations of the cometary

region on an alb edodiameter plot I make a comparison of the cometary nuclei

with outer Solar System small b o dies and nearEarth All of the cometary

nuclei studied in this thesis are dark with geometric alb edos b elow and have

eective diameters of around to km except for comet HaleBopp C O

which is in the next order of magnitude higher I give an extensive discussion of

the nuclear characteristics of comets HaleBopp and PEncke the two comets for

which I have large datasets

PHYSICAL PROPERTIES OF COMETARY NUCLEI

by

Yanga Rolando Fernandez

Dissertation submitted to the Faculty of the Graduate Scho ol of the

University of Maryland College Park in partial fulllment

of the requirements for the degree of

Do ctor of Philosophy

Advisory Committee

Professor Michael F AHearn Chair

Do ctor Alan P Boss

Professor Paul D Feldman

Do ctor Lucy A McFadden

Professor Virginia Trimble

Professor Richard J Walker

c

Copyright by

Yanga Rolando Fernandez

PREFACE

Sections of this thesis have already b een published in scientic p eerreviewed jour

nals and conference pro ceedings A discussion of app eared in

Planetary and Space Science in volume pages A treatment of

is currently under review by Icarus An overview of comets Temp el

Tuttle Wild and Utsunomiya will app ear in the up coming b o ok Cometary Nuclei

in Space and Time edited by M F AHearn and published by the Astronomical So

ciety of the Pacic which is based on the IAU collo quium held in Nanjing China

in May of A pap er on comet HaleBopp app eared in Icarus in July vol

ume pages A discussion of the imagepro cessing technique that I call

the comatting metho d app ears in a pap er rstauthored by my coinvestigator

Dr C M Lisse published in Icarus in July volume pages ii

DEDICATION

To the two constants of my Universe Mom and Dad iii

ACKNOWLEDGEMENTS

This thesis is formally credited to one p erson but it is not completed without the

assistance of many other p eople of course Mike AHearn has guided me through the

bulk of my graduate career and to him I owe many thanks not only for mentoring

me through the scientic asp ects of my apprenticeship but through the so ciologi

cal and p olitical ones as well The rst scientic talk I heard him give was at the

Donn Symp osium in Charlottesville VA in Novemb er of Until then I had

b een waing ab out deciding on a concentration for my thesis but after hearing him

sp eak ab out his database pap er and seeing his enthusiasm ab out these new break

throughs that nally revealed themselves after studying seven dozen comets I

really started to understand some of the excitement and allure of studying these

buggers So thank you so much Mike for all your help

Casey Lisse has b een my other close collab orator in this scientic journey I am

grateful for his quick mind innite patience with my questions and wacky humor

And of course who can forget the Red Pen I will always lo ok fondly back on these

very scientically pro ductive years with him

I also owe a deep debt of thanks to Dennis Wellnitz who is such an exp ert on

things mechanical and electronic The smashing success of the HaleBopp o ccul

tation adventure in a trashdump in upstate Utah would not have happ ened iv

without his exp ertise and quick thinking at the critical moments Yet again I think

how fortunate I am to have entered Mikes Comet Group just when all these terric

p eople were there to help me out

I am very grateful to Lucy McFadden for allowing me to start work on an

pro ject in and nally allowing me to realize that I really did like planetary

science much more than studying clusters of galaxies And she has continued to b e

a great teacher through my graduate career Thanks Lucy

Lastly on the scientic front I have to thank b oth NASA and NSF for there

consistent nancial supp ort throughout these last years the telescop e allo cation

committees and stas of IRTF KPNO CTIO ESO VLA Lowell Observatory and

MKO for there helpful service and p ositive attitude Mike Ressler Bill Homann

and Ulli Kau for granting us p ermission to play with their instruments Without

all of these p eople I would not have b een so successful in gaining the necessary data

to complete this thesis Among other things Ive learned you can have o o dles of fun

with only oxygen in your lungs formtting foam is a go dsend theres nothing

like jicama and asparagus salad to help you gure out how to drive a stickshift

Now planetary science has b een go o d to me if I had not b een studying comets

I would not have b een able to travel all over the world and exp erience more of our

planet than I had imagined It makes you realize how much of Earths culture and

nature is still left to explore Twentyeight trips in grad scho ol and I have only

scratched the surface from strolling along Victoria Harb or among the bright lights

of Hong Kong to bathing in a giant waterfall in Waipio Valley on the Big Island to

b eing the highest human for miles around at the tip of Mt Lassen to watching the v

rainb ows app ear and reapp ear on a misty summers day in Sto ckholm I could go

on for a while Unfortunately now I have no choice but to submit to this wanderlust

Ive picked up and sp end the rest of my life traveling the no oks and crannies of our

planet

One rep ercussion of all this scamp ering around has b een to instill in me a great

sense of appreciation for how lucky I am Though the physical distance b etween me

and that subsistence farmer in central Chile that family selling trinkets in Nogales

and those p o or homeless blokes near Constitution Avenue may b e small what an

incredibly fortunate p erson I am to b e able to go to my warm house my wellsup

plied gro cery my steadily paying job We are all just a few small but critical events

away from any of these predicaments

And now for the other imp ortant p eople of my time here Thanks to Arunav

Kartik and Laura I wouldnt have b een able to nish this tome without all of their

supp ort and understanding through this crabby and grumpy time esp ecially near

the end Mes amis your friendships mean more to me than you imagine The mid

to late s would have b een quite a bit more dicult to slog through without

you all Lets all nd enough inner p eace and gumption to make the XXIst century

b etter than the XXth AK Thank you for keeping my head on straight KS Thank

you for keeping me honest LW Thank you for giving me selfcondence

Continuing with the imp ortant p eople thanks Don for among other things

bringing ice cream and going sledding thanks Leslie for among other things playing

racquetball and nding my bike thanks Amy for among other things going to see

ob eat movies with me thanks Doug for among other things making me stay loyal vi

to the Padres thanks Steve for among other things having practical knowledge

in a institute full of geeks thanks Pete for among other things endlessly quoting

The Simp ons thanks Alison for among other things endlessly quoting Star

Wars

Where would we b e without the department sta shielding us from the three

to ed sloth that is the university administration To John T Mary Ann and Maggie

thanks for your protection Where would I b e if I hadnt babysat the PDS machine

during SL impact week To Anne thanks for that job years ago and for your

humor in the meantime

Where would I b e if I were not able to go to Giant at oclo ck in the morning

and buy PopTarts What a country

Finally thanks to

 Carl Sagan for writing Cosmos

 Halleys Comet for b eing visible right when I was so impressionable may

I see you again when Im ninety

 Voyager for sending back very co ol pictures of Uranus and Neptune vii

TABLE OF CONTENTS

List of Tables xi

List of Figures xii

Cometary Nuclei Their History and Imp ortance

A Brief Rundown

The Role in the Solar System

Motivation

A Description of Chapters

Data Acquisition and Pro cessing

Obtaining Optical Data

Obtaining Infrared Data

Obtaining Radio Data

The Ideal Dataset

Pro cessing the Data Coma Removal

Data Interpretation

Philosophy of Thermal Mo deling

The Energy of a Nucleus

The Augmented Thermal Mo del

Rotation of the Nucleus viii

The Nucleus of Comet HaleBopp

Background

Occultation Measurements

Thermal Emission and Scattered Light Imaging

Summary of HaleBopp Results

The Nucleus of Comet Encke

Background

Observations and Reduction

Analysis

Discussion

Previous Work

Summary of Encke Results

The Nucleus of Comet Hyakutake C B

Background

Thermal Measurements

Optical Measurements

Summary of Hyakutake Results

The Nucleus of Comet Temp elTuttle

Background

Thermal Measurements

Optical Measurements

Summary of Temp elTuttle Results

The Nuclei of Comets Wild and Utsunomiya ix

Background

Utsunomiya

PWild

Summary of This Chapter

Conclusions and The Future

Comets and Their Disguised Relations

Comparing Radii and Alb edos of Cometary Nuclei

Alb edo and Orbital Parameters

A Motivator for the Future

Future Data Rates

References x

LIST OF TABLES

Characteristics of Comet HaleBopp Occultation

Observations of Occultation by Comet HaleBopp

Parameters of the Mo del of Nuclear and Comatic Structure

Constraints on Parameters to Occultation Mo del

Observations of Comet HaleBopp

Observations of Comet Encke

Flux of Comet Encke

Estimated Nuclear or m Magnitudes for Enckes Nucleus

2

Sizes and Alb edos of Cometary Nuclei xi

LIST OF FIGURES

Current Canonical Cometary Nucleus

Main Belt Asteroids Radius Distribution Function

Schematic of MidInfrared Observing

Schematic of Temp erature Map for Slow and FastRotators

Lo cations of Observers for Occultation by HaleBopp

Comparison of Occulter Cometary and Occultee Stellar Sp ectra

Light Curve of Occultation by HaleBopp Team

Light Curve of Occultation by HaleBopp Team

Schematic of Occultation and Light Curve

ad Example Mo del Fits to Occultation Light Curve

eh Example Mo del Fits to Occultation Light Curve

PreOccultation Image of Comet and Star

ComaFitting Metho d Applied to Optical Image of HaleBopp

ComaFitting Metho d Applied to Optical Image of HaleBopp

ComaFitting Metho d Applied to Optical Image of HaleBopp

ComaFitting Metho d Applied to Optical Image of HaleBopp

ComaFitting Metho d Applied to MidInfrared Image of HaleBopp

Rotation Sequence of Comet HaleBopp xii

LIST OF FIGURES contd

CLEAN Contour Map of Comet HaleBopp Microwave Continuum

Radio Continuum Sp ectrum HaleBopp Nucleus Mar

Temp erature Map of HaleBopp Mar

Comet Encke at Microns a and Angstroms b

Radial Proles of Comet Encke in MidIR a and Optical b

Light Curve of Comet Encke Phased by hr

StringLength Metho d Determination of Enckes Rotation Perio d

Optical Phase Behavior of Comet Enckes Nucleus

ISOPHOT Sp ectrophotometry of Encke Dust Coma Plus Nucleus

2

Plots of Encke Dust Temp erature and Nucleus Size



Comet Hyakutake in a wide Field of View

CLEAN Contour Map of C B

Size and Temp erature of C B Nucleus

ComaFitting Metho d Applied to Optical Image of Hyakutake

Rotation Sequence of Comet Hyakutake

Photometry Light Curve of Comet Hyakutakes Photo center

MidInfrared Images of Comet Temp elTuttle

MidInfrared Sp ectrophotometry of Comet Temp elTuttles Nucleus xiii

LIST OF FIGURES contd

ComaFitting Metho d Applied to Optical Image of Temp elTuttle

Optical Phase Behavior of Comet Temp elTuttles Nucleus

MidInfrared Images of Comets Utsunomiya and Wild

ComaFitting Metho d Applied to Comet Utsunomiya

ComaFitting Metho d Applied to Comet PWild

Size and Alb edo of Cometary Nuclei and CometLike Asteroids

Alb edoOrbit Connection of Cometary Nuclei and

CometLike Asteroids Perihelion

Alb edoOrbit Connection of Cometary Nuclei and

CometLike Asteroids Tisserand Invariant

Current Size Distribution of Known Cometary Nuclei

and CometLike Asteroids xiv

Chapter

Cometary Nuclei

Their History and Imp ortance

A Brief Rundown

Most studies of the comet phenomenon fo cus on the coma and tail of the ob ject

usually the most obvious parts that one sees However this thesis presents a study

of the nuclei of several comets which are in general much harder to observe While

much work has b een done to understand the nuclei indirectly by studying the gas

and dust around them I have tried to directly prob e their physical and thermal

prop erties It is only in the last two decades that this has b een observationally and

computationally p ossible the recorded history of the study of comets extends back

a few millenia but for the vast ma jority of that time the very existence of a cohesive

b o dy in the middle of the coma never mind its prop erties was not known

Though Seneca seems to have had the correct idea in the st century AD for

much of history a comet seen in the sky by the ancients was not even recognized as an

astronomical phenomenon until the th century when Tycho Brahe set an upp er

limit on the comets parallax that put it far from Earth previously comets were

b elieved to b e atmospheric phenomena The comets basic place in the planetary

system moving on parab olae or on ellipses typically crossing the orbits of several

ma jor planets was of course noted by Halley using Newtons thennew universal

gravitation idea through his accurate timing and astrometric prediction of the

return of the comet now b earing his name Aside from most notably work by

Bessel investigations into the physical nature of comets as opp osed to just orbital

or astrometric studies b egan in earnest only in the late th century with detailed

studies of morphology and apparent luminosity and the advent of photography and

then sp ectroscopy

The study of a comets nucleus sp ecically was fraught with uncertainty As

Bobrovniko wrote in reference to comet PHalleys app earance around

the term nucleus has no precise signicance Sometimes the nucleus was

p erfectly starlike without any measurable diameter Sometimes it lo oked like a

small planetary disc Sometimes there was nothing that could b e interpreted as a

nucleus It is questionable whether most observations of the diameter of the nucleus

refer to the real nucleus A pap er by VorontsovVelyaminov gives no less

than seven separate op erational denitions of the nucleus The rampant confusion

of nuclear nomenclature is indicative of the lack of understanding of exactly what is

at the heart of a comet That is not to say that we are fully enlightened now but

in hindsight we can see fundamental misconceptions

The dominant mo del for the comets nucleus for ab out a full century from the

mids to the mids was the sandbank mo del whose tenets were most re

cently championed by Lyttleton The main motivations for p ostulating

the nucleus as an unb ound agglomeration of meteoritic solids and not a monolithic

mo del were a a cometary coma contracts as the comet approaches the b

meteor streams are coincident with cometary orbits c nuclei tend to uctuate in

apparent size and brightness sometimes even disapp earing and d comets are of

ten as much as an arcminute away from predicted ephemeris p ositions even for well

determined orbits The obvious choice to make at least back then was to assume

that there is no one central b o dy in the photo center of the comet but rather just

a cloud of dust grains and that what one observes as the nucleus is just the place

where the optical depth or the concentration of particles is higher The complicated

patterns that emerge in the nearnuclear coma of some of the more active comets

made it attractive to assume that there is just an amorphous cloud of dust grains

deep inside the coma For example the head of comet PHalley during its appari

tion in Bobrovniko showed many centers of brightness with tendrils

and sheets of coma p ointing in multiple directions The mass of the comet would

b e spread out over much of the coma not just in the photo center but all of the

particles in the comet are on indep endent orbits of all more or less the same p erio d

there is no gravitational binding but also they are not tidally disrupted as they

pass close to a planet or the Sun

The literature is full of measurements of the size of the nucleus that range

from a few tens to a few thousand kilometers eg Chamb ers p Vo

rontsovVelyaminov Lyttleton pp Frequently observers would

measure the angular size of whatever resolved disk was at the center of the comet if

any A few published rep orts give values within the same order of magnitude of the

mo dern values ie a few kilometers but the ma jority are similar to the case eg

of a sp ecic comet mentioned by Richter with a diameter lower limit that

is times bigger than the currently accepted value Of course there was also the

problem of a thentotally unknown alb edo and thenundetermined phase eect that

complicated matters The observation of comets transiting the solar disk Finlay

and Elkin Bobrovniko placed upp er limits on the diameter of roughly

to km but in the context of the sandbank mo del this was taken to conrm

the idea that there were several smaller b o dies at the heart of the comet rather than

one single b o dy pro ducing the coma and tail phenomena

This then was the heart of the problem for the sandbank mo del the actual

diameters of cometary nuclei and here I do mean the central monolithic b o dy

are much smaller than was commonly thought a century ago As I will show in later

chapters most comets seem to b e on the order of just a few kilometers in radius

This is not to say that comets do not have multiple sources for the dust and gas we

see for of course there are a couple dozen cometary nuclei that have b een known

to split into pieces some for not obvious reasons Sekanina However

usually the pieces evap orate away or cease activity in short order so that at any

given moment a comets nucleus is usually just a singular ob ject with a radius on

the order of to km This should not b elittle the work of the th and early

th centuries I merely p oint out that in hindsight many conclusions were based on

incorrect precepts Indeed the main problematical situation in observing cometary

nuclei still remains when the comet is close by the nucleus is shrouded in the

coma but when it is far away and the coma is not so strong the nucleus is faint

and dicult to measure The recent journals contain many estimates of the size of

cometary nuclei but the error bars are usually large and if they are not then many

times they probably should b e

The late s and early s saw the publication of signicant pap ers on

several cometary phenomena the nucleus Whipple the plasma tails Bier

mann the reservoir of longp erio d comets Oort and the source of the

Jupiterfamily comets Edgeworth Kuip er For my immediate purp oses

here Whipples work is the most signicant The nucleus is a single b o dy a con

glomerate of ices combined in a conglomerate with meteoric materials to use

the original wording with ices subliming o due to insolation Quantitative studies

of the sheer magnitude of gas mass in cometary comae and tails at the time indi

cated that a huge reservoir of ice was needed in the comet far more than could

b e supplied by the grains in a sandbank even if the grains did adsorb volatiles on

their passage through space The ejection of material would over time leave an

insulating mantle on the nucleus surface and also measurably push the nucleus in

a reaction force This latter p oint made Whipples mo del sup erior to the sandbank

mo del in that b oth acceleration and deceleration could b e explained by the sense

of rotation of the central b o dy The sandbank mo del used solar radiation pressure

and collisions within the bank to explain acceleration but not deceleration The

idea of a single b o dy for the nucleus was not totally new in eg Wurm

mentions it in the context of the formation of the gas coma

Whipple was the rst to make an extensive analysis of the rotation states of

many cometary nuclei he has given a summary and historical and contex

tual review However his metho d for determining rotation p erio ds based on the

timing of features moving through the coma app ears frequently to give misleading

results Whipple himself states that his metho d either gives exactly the right answer

or something totally sp ecious The photo electric measurement of the brightness of

a comets photo center as a function of time was rst done only in The de

termination of a cometary rotation state is a dicult problem a go o d review of

the pitfalls is given by Belton and it has not b een done satisfactorily even

for the nucleus of comet PHalley a comet visited by several spacecraft I will

elab orate on the metho dology of rotation p erio d determination later

In the mid to lates a series of groundbased exp eriments were p erformed

that gave us size and reectivity information on cometary nuclei for the rst time

Much of my work elab orates on the same principle ie combining the information

from the thermal radiation and reected light of a nucleus The advent of sensitive

germaniumgallium b olometers to detect to m radiation made this metho d

p ossible I will describ e the metho d fully in Chapter The work gave our rst

indication that cometary nuclei are some of the blackest ob jects in the solar system

with geometric alb edos of just a few p ercent Previously the consensus was to

assume a much higher value something comparable to the icy satellites of the outer

Solar System

The study of cometary nuclei received a b o ost in with the data taken by

the otilla of spacecraft that ew by comet PHalley most esp ecially by Giotto

For the rst time ever a resolved image of a nucleus was pro duced and I show

a representation in Fig taken from a review article by Keller which

is the combination of several highresolution images The ybys conrmed many

of our basic suspicions Halleys nucleus is a cohesive b o dy and not a sandbank

its visual geometric alb edo is very low a few p ercent it is approximately prolate

and elongated by ab out there are regions on the surface that are more active

than their neighb ors are these regions pro duce jets similar to what is seen in the

groundbased images an active region is active apparently only on the sunlit side

not on the night side but a go o d fraction of the gas and dust do es not come from

these active regions While the study of Comet PHalley revolutionized cometary

science it of course left many questions still unanswered Most obviously it would

b e wise to obtain similarly detailed closeup data of other nuclei Fortunately this

will probably happ en in the next decade there are several spacecraft missions with

cometary targets scheduled to y in the coming years and we hop e not all of them

will suer from the budget axe or system failure The near future will bring exciting

scientic knowledge to us ab out these denizens of our Solar System

This short history should make it clear how dicult observations of the nucleus

can b e In general if the comet is close to Earth it is also close enough to the

Sun to b e outgassing and the light from the gas and dust coma comp etes with and

often swamps the light from the nucleus On the other hand if the comet is far

from the Sun where it is not outgassing and we have an easier view of the nucleus

the comet is also far from Earth and the nucleus is dicult to observe due to

its faintness Furthermore once the comet is several AU away it b ecomes extremely

dicult to tell the dierence b etween a little bit of comatic ux and no comatic ux

since there is no set distance known a priori at which one can declare the comatic

activity negligible This Catch problem exists in b oth the infrared and optical

regimes In the radio there is some hop e b ecause there are not enough grains in

the coma to pro duce enough radiation to comp ete with the nucleus However at

these wavelengths the PSF b eam in this case is so large as to make spatial

dierentiation of the coma and nucleus very dicult it is even harder to tell how

much ux is comatic and how much is nuclear Interferometric observations can b e

used to improve the spatial resolution as I will show in Chapter but then one

needs a large nucleus since the wavelengths are so far down on the RayleighJeans

side of the Planck function The fact that our knowledge of cometary nuclei was

almost nonexistent all the way up into the mids dramatically indicates the

diculties in approaching the study of these ob jects

The Role in the Solar System

Origins

In the midth century Kant sp eculated that the nonastrological and non

Figure Current canonical cometary nucleus This is a pro cessed image of the

nucleus of comet PHalley taken by the Giotto spacecraft in March Keller

This image represents our current view of the typical cometary nucleus

anthropic reason for the comets existence was tied to the origin of the Solar System

To this day among the largest unanswered questions in comet science are What

exactly was the role of the comets in the Solar Systems formation and How

is the currentlyobserved group of comets related to the original p opulation The

comets are some of the b est prob es we have for studying Solar System origins since

they are some of the least pro cessed observable ob jects

The story apparently b egins b efore the Solar System was b orn Recent studies

of the bright comets Hyakutake and HaleBopp have indicated an interstellar origin

for the ices based on the isotopic ratios Meier et al a b and unusual

hydro carb on abundances Mumma et al The ices were in the solar nebula as

the gas giants were forming and the comets are remnants from the accretion pro cess

that created the gas giants There is much debate ab out the exact metho d of gas

giant formation gravitational stability Boss or core accretion Pollack et

al but lowsp eed collisions of grains undoubtedly played some role in the

agglomeration of the cometesimals The existence of the ice implies that the comets

we see to day formed in the AU range and b eyond since closer to the Sun they

would not have retained the volatile comp onent

Currently there are four ma jor ideas for the structure of the nucleus as a result

of the formation pro cess Whipples icy conglomerate mo del is the original

Variations on that idea have b een created by Donn who created a fractalized

uy aggregate by Weissman who created a primordial rubble pile of a

cometesimal collection with low tensile strength and by Gomb osi and Houpis

who p ostulated a collection of closelypacked b oulders held together with icy glue

This is by no means exhaustive and extensive reviews of the mo dels of the bulk

structure of cometary nuclei have b een written by eg Donn The main

variations among the mo dels are the density of packing of the cometesimals from

which they formed and the makeup of the icero ck matrix of which they are made

There are apparently testable predictions for the mo dels based on how they suer

collisions and the physics and hydro dynamics of the gas and dust ejection Work

on split comets Sekanina seems to indicate a very low tensile strength

for the b o dies but in general dierentiating b etween the mo dels may have to wait

until we have many very close observations of several nuclei by spacecraft Notable

among the future missions is Deep Impact which will re a missile at a comet and

simulate a meteorite impact and thus allow us to observe crater formation on the

surface

The current domicile of a comet within the Solar System dep ends strongly on

its birthplace Gyr ago According to numerical simulations comets b orn near

Jupiter and Saturn predominantly found themselves either crashing into the Sun or

b eing ejected from the Solar System entirely due to the strong gravitational inuence

of the two largest gas giants A small p ercentage collided with the terrestrial planets

ie and Saturn provided the imp etus for some of the heavy b ombardment

suered by Earth in its early history It should b e noted that even to day it is thought

that a typical shortp erio d comet with a year p erio d and aphelion passing less

than AU from Jupiters orbit can exp ect to survive less than a million years

b efore b eing strongly p erturb ed into the Sun out of the Solar System or into a

nearEarth asteroidlike orbit Wetherill

Then there are the comets b orn near Uranus and Neptune The lower mass of

these gas giants compared to Jupiter and Saturn prohibited them from completely

ejecting the comets into interstellar space However they were apparently very

go o d at p opulating the Weissman Once a comet had b een ung

outward by Uranus or Neptune it would sp end several thousand years barely held

by the Suns gravity and sub ject to signicant p erturbations by passing stars giant

molecular clouds and the Galactic tides One net eect was to raise the p erihelia

and aphelia distances of these comets and hence keep them out of the inner Solar

3

System Weissman the residents of the Oort Cloud live b etween ab out 

5

AU and  AU from the Sun However the p erturbative sources also tend to

destroy the Oort Cloud over time sending the comets into interstellar space The

existence of an Inner Oort Cloud has b een invoked to resupply the outer cloud since

apparently few outer cloud memb ers could survive Gyr at the edge of the Suns

gravity Duncan et al have done numerical calculations to show that an

inner cloud would b e p opulated by ejected memb ers of the UranusNeptune region

and could help to preserve the outer clouds p opulation

Lastly I will mention the Kuip er Belt originally lled with comets that were

b orn b eyond Neptune With no large planet to shepherd them the planetesimals

remained planetesimals Many of the Kuip er Belt ob jects discovered in the past

seven years reside in a resonance with Neptune as Pluto do es that keep them

safely orbiting over Gyr timescales However Fernandez no relation was one

of the rst to numerically explore the idea that the shortp erio d comets originally

came from this region and recently Levison and Duncan have p erformed

extensive numerical calculations to mo del the currently observed orbital spread of

Jupiter family comets by integrating the orbits of particles in the Kuip er Belt

Classication

I will give here a brief description of the relation b etween cometary dynamics

and nomenclature Historically a comet has either a shortp erio d SP or a long

p erio d LP the dividing line b eing at years An LP comet can either b e new

or old in the Oort sense dep ending on whether or not it is passing for the rst

time through the inner Solar System An SP comet can either b e a memb er of

the Jupiter family JF or Halley family HF JF comets originally come from the

Kuip er b elt HF ones came from the Oort Cloud Both JF and HF comets have

b een p erturb ed by the gas giants into orbits that keep them mostly in the inner

Solar System The usual distinguishing characteristics b etween JF SP and HF SP

comets are the inclination and p erio d In my opinion one can make a case for the

existence of an Encke family of SP comets EF for comets in orbits similar to

the ma jority of nearEarth asteroids NEAs Levison has come up with

a similar categorization but currently this family is p opulated by only known

memb ers Recent observations have found comets residing in the Main Asteroid

Belt Marsden b Lien but these ob jects represent exceptional cases and

are probably caused by colliding asteroids rather than indep endent outgassing so

it is likely that this is not a separate dynamical class of comets

With the publication of a pap er by AHearn et al detailing molecular

gas sp ecies abundances in seven dozen comets we may have entered the era of com

etary taxonomy based on comp ositional dierences instead of just dynamics Such

categorizations are just starting to b e found and understo o d but continuing surveys

of cometary comae and improved remote sensing techniques may allow us to obtain

more accurate determinations of the comp ositional dierences from comet to comet

Evolution

The cometary nuclei have not b een quiet since their formation Numerical con

siderations indicate that comets from anywhere from b oth the Oort Cloud and

Kuip er Belt have undergone some collisional events in the intervening eons Stern

Stern Farinella and Davis an imp ortant question is how many

The observed size and rotation distribution that we measure from the p opulation

of nuclei that has managed to p enetrate the inner Solar System will likely not b e

the same as the original distribution with which the nuclei were b orn However

we would b e able to tell if the nuclei are as collisionally relaxed as the main b elt

asteroids are or if they have not quite reached that stage yet

There are other eects that have altered the comets even those that were in the

deep freeze of the Oort Cloud Cosmic rays have b ombarded the nuclei and aected

the top layer of cometary material although presumably this is blown o on the rst

passage of a comet near the Sun Passing stars and nearby sup ernovae briey warm

the nuclei from their usual K temp erature and hence motivate some chemical

reactions in the ice Some calculations Stern and Shull indicate that at least

once during the previous Gyr have the Oort Cloud nuclei warmed up to K

due to passing stellar or sup ernova radiation which could initiate sublimation of the

more volatile icy comp onents and induce some otherwiseinert chemical reactions

The shortp erio d comet p opulation of course is more evolved than their long

p erio d new in the Oort sense counterparts The aging pro cess is thought to

manifest itself among other ways in the chemical dierentiation of the topmost

layers of the nucleus and the creation and thickening of a mantle Meech

The physical destruction of the comet also contributes eg via splitting or the

blowing o of relatively large fractions of the comets mass during outbursts These

phenomena could aect any observed size distribution and would tend to smear out

the small end of the distribution However currently there is a much more worrisome

problem to overcome namely the small numb er of ob jects ab out which we have a

detailed physical understanding Also the evolution of cometary nuclei is a mostly

theoretical pursuit at the moment b ecause we have not b een able to observe the

decay of a nucleus through multiple passages The most obvious candidate for such

a study Enckes comet has selshly guarded its nuclear secrets until recently

see Chapter and we will have to wait a few more years b efore the eect can b e

observed on that ob ject There may b e some indication that small comets simply

do not exist in great numb ers in the inner Solar System Rickman and that

nuclei disintegrate rapidly once they get b elow some threshhold size However the

observational bias is strong and until we are more condent of sampling most of the

shortp erio d comets we should hold o on any conclusions Future cometdetection

searches or asteroidsearches adapted for comets could help improve the statistics by

at least removing the sky coverage bias that currently prevents us from discovering

many long p erio d ob jects

Motivation

We need detailed studies of more than just a few cometary nuclei if we are ever

to place the nuclei in the correct context of Solar System formation and evolution

Our current knowledge of the nuclei is rather limited so learning basic physical

characteristics such as size shap e reectivity rotation state and thermal b ehavior

represents a ma jor step Spacecraft will b e busy during the next few years studying

a few nuclei in detail but I hop e that we can more rapidly build up a reliable

database of information with groundbased observations

As an indicator of how imp ortant thermal studies of nuclei are as opp osed to just

using optical data I show the cumulative size distribution function of the Main Belt

asteroids in Fig I have used the database of Bowell lo cated on the World Wide

Web at httpasteroidlowelledu to create this graph One can estimate a

radius based on just the optical magnitude by assuming a geometric alb edo in this

case for the asterisks in the graph and for these mainb elt asteroids

2:5

that gives roughly a R size distribution I have considered only the high end

of the distribution where the sampling is at least reasonably complete However if

one lo oks at the actual radii of the ob jects measured via the thermal radiation for

ab out mainb elt asteroids with the IRAS satellite one gets a much dierent

3:5

distribution of R in the more complete end This eect do es not dep end on

the value of the assumed alb edo since changing it would merely slide the p osition

of the asterisks left or right Moreover the slop e is shallower for the b ettersampled

optical case whereas if this alb edo eect were a manifestation of our incomplete

knowledge of the mainb elt one would exp ect a steep er slop e since there would b e

more smaller asteroids known I do not want to argue the actual value of these

slop es my p oint is simply that they are very dierent and that a similar pitfall

could very well o ccur for the cometary nuclei Optical data alone cannot necessarily

guarantee the validity of size distribution information

The fruition of such an endeavour is guaranteed as evidenced by the previous

lyunknown conclusions from the work of AHearn et al I make no claims

that an understanding of the Solar System origins can b e teased out of my study of

a halfdozen ob jects but the revolution in infrared astronomy currently happ ening

will make it technically and observationally feasible to continue studying the small

b o dies of the Solar System and eventually reach the holy grail of comet science

answers to such questions as How do comets t into the birth and evolution of the

Solar System How many times have they collided with each other What accounts

for the dierences in the reectivity the dusttogas ratios the active regions and

the emitted grains Is there any correlation with dynamical age How do the

comets contribute to the interplanetary medium and the dust p opulation of the

Solar System How do es their app earance reect the alterations they have suered

Do es their comp osition reect an interstellar origin for the volatiles

A Description of Chapters

I will rst describ e the metho ds used to study the nuclei and then individually

discuss each nucleus Sp ecically in Chapter I will discuss my reduction metho ds

for this study Chapter will have a description of my interpretation metho ds this

Figure Main Belt asteroids radius distribution function The asterisks repre

sent the distribution using an assumed alb edo and so give a naive radius when

combined with the known absolute visual magnitude The diamonds represent the

IRASderived radii and so they have the alb edo ambiguity removed Note that

the slop es of the two distributions in the large particle wellsampled end are quite

dierent

chapter will explain how I have taken advantage of the new generation of sensitive

midinfrared detector arrays to overcome the problems of nucleus observation that

I mentioned in Section In Chapters through I will discuss the nuclei of

comets HaleBopp Encke Hyakutake and Temp elTuttle resp ectively I will add

some information ab out two other comets with smaller datasets in Chapter

Finally in Chapter I will combine the results of the previous chapters and make

comparisons with other ob jects of the Solar System and try to place these results

within the framework of Solar System formation and evolution