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Astronomy and Astrophysics Library Nicolas Thomas An Introduction to Comets Post-Rosetta Perspectives Astronomy and Astrophysics Library SeriesEditors MartinA.Barstow,DepartmentofPhysicsandAstronomy,UniversityofLeicester, Leicester,UK AndreasBurkert,UniversityObservatoryMunich,Munich,Germany AthenaCoustenis,LESIA,Paris-MeudonObservatory,Meudon,France RobertoGilmozzi,EuropeanSouthernObservatory(ESO),Garching,Germany GeorgesMeynet,GenevaObservatory,Versoix,Switzerland ShinMineshige,DepartmentofAstronomy,KyotoUniversity,Kyoto,Japan IanRobson,TheUKAstronomyTechnologyCentre,Edinburgh,UK PeterSchneider,Argelander-InstitutfürAstronomie,Bonn,Germany Steven N. Shore, Dipartimento di Fisica “Enrico Fermi”, Università di Pisa, Pisa, Italy Virginia Trimble, Department of Physics & Astronomy, University of California, Irvine,CA,USA DerekWard-Thompson,SchoolofPhysicalSciencesandComputing,Universityof CentralLancashire,Preston,UK Moreinformationaboutthisseriesathttp://www.springer.com/series/848 Nicolas Thomas An Introduction to Comets Post-Rosetta Perspectives NicolasThomas SpaceResearchandPlanetarySciencesDivision PhysicsInstitute UniversityofBern Bern,Switzerland ISSN0941-7834 ISSN2196-9698 (electronic) AstronomyandAstrophysicsLibrary ISBN978-3-030-50573-8 ISBN978-3-030-50574-5 (eBook) https://doi.org/10.1007/978-3-030-50574-5 ©SpringerNatureSwitzerlandAG2020 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpartofthe materialisconcerned,specificallytherightsoftranslation,reprinting,reuseofillustrations,recitation, broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,andtransmissionorinformation storageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilarmethodology nowknownorhereafterdeveloped. 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Coverillustration:Collageofanartist’simpressionofacometseeninthesky,superposedonaprecise3D shapemodelofthenucleusofcomet67P/Churyumov-GerasimenkowiththeRosettaspacecraft(notto scale)orbiting(ESA,RhiannonThomas).Thegraphicisintendedtosymbolizethegaininknowledge (fromnaked-eyeimpressionstoprecisemeasurements)acquiredusingmoderntechnology. ThisSpringerimprintispublishedbytheregisteredcompanySpringerNatureSwitzerlandAG. Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland For BLT, .... even if it’s not what you originally intended! Preface (Motivation and Scope) Thehistoricalsignificanceoftheirregularappearanceandmotionofbrightcomets acrosstheskyhasoftenbeenreferredtoinscientificliterature,andtherecanbeno doubt that our ancestors would have been mightily impressed by celestial objects similar to the naked-eye comets such as C/1996 B2 (Hyakutake) and C/1995 O1 (Hale–Bopp)thathavebeenseeninrecenttimes.Whilethismightbesufficientonits own to justify detailed scientific investigation of comets, the possibility that they might be relics from the Solar System formation process presents a more scientifi- callyexcitingreasonforstudyingtheseobjects. Figure1showstheremarkableimageofHLTauri(HLTau),ayoungTTauristar, acquired with the Atacama Large Millimeter Array (ALMA). The image shows material intheformofadiscsurroundingtheparent star.Withinthediscthereare rings or gaps that have almost certainly been produced by the formation of proto- planets(e.g.Clery2018;Pérezetal.2019).Thegravitationalfieldofaproto-planet withinthediscattractsmaterialfromitsvicinityclearingoutaringaroundthestar. Whatisnoticeable,however,isthateventhoughproto-planetshavealreadyformed inthedisc,thereisstillalargeamountofmaterialinthediscwhichhasnotyetbeen accretedontotheproto-planets.Furthermore,thismaterialmaybemaskingsmaller objectswhicharegrowingbutarenotyetlargeenoughtocleararing. Our Solar System shows that once the system has fully evolved the regions between the major planets are essentially void. In our Solar System, sometime betweenthestageillustratedbytheHLTauimageandtoday,thematerialbetween theplanetswasremoved.Muchofthatmaterialmusthaveimpactedotherobjectsin our system, but some of it almost certainly did not. Close, rather than impacting, encounterswiththeplanetsandproto-planetswouldhaveresultedinsignificantorbit modification placing the objects on more eccentric orbits with larger aphelion distances. This process implies that objects that had yet to reach planetary size escapedthevicinityofthelargerproto-planetsandcouldpotentiallyhavesurvived throughtothepresentday.Planetarysystemevolutioncodesarenowabletoexplore thisinalittlemoredetail(e.g.Fig.2).Theyshowthatforsystemswithalowinitial amount of solid mass, only 30% of the material ends up in planets larger than one vii viii Preface(MotivationandScope) Fig.1 TheprotoplanetarydiscofHLTauri(HLTau)observedatsub-millimetrewavelengthsby theAtacamaLargeMillimeterArray(ALMA).ImageCredit:ALMA(ESO/NAOJ/NRAO),NSF Earth mass, and hence, there is considerably more material residing in other reser- voirs.Furthermore,asfirst Edgeworthandthen Kuiper deduced (Edgeworth1949; Kuiper1951),planetaryformationprocessesdidnotendattheorbitofNeptune.The orbitalperiodofasmallobjectaroundacentralstaris rffiffiffiffiffiffiffiffi a 3 P¼2π s ð1Þ GM wherea isthesemi-majoraxis,andGMisthegeopotentialofthestarwhichtakesa s value of 1.32712 (cid:2) 1020 m3 s(cid:3)2 for our Sun. Assuming a circular orbit, the mean orbitalspeedisthen rffiffiffiffiffiffiffiffi GM v¼ ð2Þ a s where the proportionality to √(1/a) shows that relative speeds in the outer Solar s Systemdecreasewithheliocentricdistance,r .Thisledtoareductioninthenumber h Preface(MotivationandScope) ix Fig.2 Thetotalmassofmaterialinformedplanetarysystemsasafunctionoftheinitialsolidmass in the discs computed from a planetary formation model by C. Mordasini (pers. comm.). The resultingamountofmassintheformedplanetarysystemsisabout30%oftotalsolidmassinthe originaldiscforlowinitialamounts.Thisincreasestoover100%(theplanetscanalsoaccumulate lightgasessuchasHandHe)withincreasinginitialsolidmass of interactions between outer Solar System objects leaving a population of bodies beyondtheorbitofNeptunethatmayhaveexperiencedrelativelylittlechangeover thepast4.6billionyears. It is the fact that comets are active that leads to the suspicion that they were involvedinthisprocess.Thisactivityisproducedbythesublimationofice(mostly water ice) from the surfaces of small, irregular-shaped, solid nuclei. The probable presence of ices (including many that are far more volatile than water ice such as carbonmonoxide)suggeststhattheseobjectshavenotbeensignificantlythermally processedsincetheirformation.OtherobjectsinourSolarSystemhavebeenheated throughgravitationalorcollisionalprocessesandhaveeitherlosttheirsurfaceiceor the ice has been structurally modified (e.g. through melting). In the case of some objects,notablyasteroids,watericemightstillbepresentatdepth,butthisis,atleast partially,thesubjectofspeculation. ThepossibilitythattheSolarSystemstillcontainsremnantsthathavehardlybeen altered since the completion of the planetary formation process is a tantalizing prospect. Understanding the conditions under which Solar System formation began would clearly be a major step in trying to establish the frequency with which planetary systems like our own form. The current drive to determine the x Preface(MotivationandScope) numberandstructuresofplanetarysystemsaroundotherstarshasshownthat,while our Sun is not unique in having planets, the diversity in the distribution of planets withintheothersolarsystemsisfargreaterthanimagined25yearsago.Hence,there remains a need to understand how our specific system formed and evolved. Carl Saganonceremarkedthat“youhavetoknowthepasttounderstandthepresent”and one may invert this by saying that one can get to know something of the past by studyingevolutioninthepresent. Theperturbationofcometorbitsbytheplanetsraisesfurtherquestionsaboutthe significance of comets for planetary evolution. Their motion with respect to the ratheruniform quasi-circular orbits oftheplanets providesameans oftransporting materialoveralargerangeofheliocentricdistances.Althoughthepresentnumbers of comets may be small, Fig. 1 also indicates that there were many more of these objects in the early Solar System. Perturbation followed by impact with accreting planetswasameansofincorporatingobjectsformedatmanydifferentheliocentric distances into the growing proto-planets. The significance of this mass transport is not well established, but its implications are profound. It has been known for 35 years that the main driving volatile in comets when they reach the inner Solar Systemiswaterice.Furthermore,itwasshownduringthedetailedobservationsof comet 1P/Halley in 1985–1986 that the less volatile components contain copious amounts of organic material. This combination of ice, organics, and large relative motion has led to the idea that the Earth obtained most, if not all, of its water and organicsfromcometsandthatitwasthisinfluxofmaterialthatultimatelyledtothe developmentoflife.Thereisnodoubtthatthesurfacesoftheterrestrialplanetshave beenimpactedbycometsmanytimesoverthelifetimeoftheSolarSystem.Itshould also be noted that meteor showers, which are the products of dust ejected from comets, enter the Earth’s atmosphere on a regular basis. But the full significance remainsunclearandprovidesfurthergroundsfordetailedinvestigationofcometary material. Giventheimportanceofcometaryresearch,therehavebeenratherfewbookson the subject and one of the motivations for this work is the absence of good introductory texts. Probably the closest in nature to the concept of this book is the 2010thirdeditionofPhysicsofCometsbyK.S.KrishnaSwamy.Thishasanumber ofexcellentintroductionstovariousaspectsofcometaryphysics.Itisquitefocused on spectroscopy and gas emission, whereas here I have weighted the text more towardsthenucleusandtheinnermostcomaasadirectresultoftheobservationsof comet 67P/Churyumov–Gerasimenko by the European Space Agency’s (ESA) Rosetta spacecraft. Similarly Brandt and Chapman’s book Introduction to Comets from2004(2ndedition)emphasizestheplasmaaspectsofcometsandissomewhat outofdatewithrespecttothenucleus. Although Physics and Chemistry of Comets edited by W.F. Huebner is now nearly 30 years old, much of the text remains relevant. Huebner collected eight chaptersfromexpertsonallaspectsofcometaryresearchandproducedanexcellent summary roughly 4 years after the Giotto encounter with 1P/Halley. On the other hand,itwaswrittenpriortothediscoveryofthefirstKuiper–EdgeworthBeltObject in1992andwellbeforeseveralmajorspacemissionstocomets(includingnotonly