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T he enigm a of com et nuclei
- or: Keep the frontiers open ! (D. MЖhlmann, DLR Institut fЭr Planetenforschung, Berlin)
dir k.moehlmann@dlr .de
2n d P hilae Scie nce Wor ksh op , Helsin ki, FM I, 0 4.0 9. 06
1


T he VEGA-2 image of the nuc le us of 1P/Halle y

Modern comet research has begun with:

d =(100-600)µm

Histor ically, this is the first ima ge of contours and structures of a comet nucleus (made here at IKI)
at M arc h 9, 1986; dis ta nce - 8030 k m

Original image

Co ntrast enhanced

1st proof: Co met nuclei are sin gle solid bod ies
Folie 2 > Vortrag > A utor
Dokum ent nam e > 23. 11. 200 4


PUMA-1 dust mass spectro meter onboa rd VEGA 1
(Coo pe rat io n M PI He ide l be rg/ IKI )

Re sults of * PUMA/P IA: CHONs indicate co mple x orga nic constit ue nts * ,,pristine " che mical co mpositio n
Folie 3 > Vortrag > A utor
Dokum ent nam e > 23. 11. 200 4


P1/Halley, March 13, 1986 Image credit: Giotto team

Giotto has confirmed t hat comet nuclei are very dark solid bodies with a specific surface structure. About 10% of t he sur face are ,,active" (i.e. outgassing)
Folie 4 > Vortrag > A utor
Dokum ent nam e > 23. 11. 200 4


Co me t 19/P Borre lly (Dee p Space Missio n de r NAS A) Only 6 % of the surface are ,,active " Large scale surface fe ature s, smoot h te rrains
I mage cre dit: NASA
Folie 5 > Vortrag > A utor
Dokum ent nam e > 23. 11. 200 4


Comet 81 P/Wild 2 Cratered surface ?

Bright spots

No relation between jets and surface features

Sharp st r uct ures

Image cred it : NAS A Stardust miss ion
Folie 6 > Vortrag > A utor
Dokum ent nam e > 23. 11. 200 4


Folie 7 > Vortrag > A utor
Dokum ent nam e > 23. 11. 200 4


d =(100-600)µm

I mage s cre dit: NASA
Folie 8 > Vortrag > A utor
Dokum ent nam e > 23. 11. 200 4


d =(100-600)µm

Geo log ical featu res * Lar ge , smoo th su rfaces * Ro u nd featu res (= cr ate rs? ) * Str ippe d ter r ain (o ld ) * Scar p s * Ev iden ce of laye r s Over all Sh ape * Effe ctive rad iu s 3.0±0.3 km * Max-min d iame ter s 7.6 and 4.8 km b u t ver y un cer tain * We ll-map ped su rface is mo stly in 3 lar ge mo re-or -less flat are as

Te mpe l 1 Parame ters I mage cre dit: NASA, Dee pI mpact Mean r ad ius: 3.0 ± 0.1 km Diame ter r ange : 5.0 - 7.5 km Gr av ity: 0.024 - 0.030 cm /s2 Ar e a: 119 km2 Range of grav itational heights: 0.73 km Mean Den sity: 0.3 ± 0 .2 g m /cm3
Folie 9 > Vortrag > A utor
Dokum ent nam e > 23. 11. 200 4


Ve r y pr on ou n ced in at le ast top 0.5 km So me po ssib ly g lo b al in exten t At le ast two type s o f layer s iden tifie d : "th ick" and "th in " Mo rp ho log y sug gests th at laye rs pr ob ably d iffer in ph ysical char acter istic Or ig in of laye r in g pr ob le matic: pr imor dial accretion ? su b seque nt (ther mal) p ro cessing ? e jecta dep osits o n mu ch larger pre cu r so r ob je ct ? Image cr ed it: DI S cien ce team So me (b ut mu ch mo re sub tle) eviden ce o f "layer in g " exists in Wild 2 and Bor relly image s

Tempel 1 ( Veverka, DP S, 2006): Layering

Smooth terrains
Two se par ate are as of e xtensive smoo th terr ain Ap p e ar un cr atered re lative ly recen t! Ve r y smo oth un ifo r m in te xture Ou tline s are elon gated and flo w-like Occur in top og raph ically low areas Be tter imaged featu re sho ws sug gestio n s o f flow feature s (i.e. sub -p ar alle l, d ar ker mar king s?) Th is fe ature is abo ut 3 km lo ng , 1 km wide and at le ast 0.02 km th ick En d s in abr up t scar p (abo ut 10-20 m h igh )
Folie 1 0 > V ortrag > Aut or
Dokum ent nam e > 23. 11. 200 4


Co me t dus t :
R ad i an ce (W / [m sr µm])

Pre- Impact Coma
0.30

Achieve ments:
* ve ry fine dust ~ 2-5 µm * we ak te nsile stre ngth (< 100 Pa ?) - under discussio * m ass de nsity ~0,4 g/cm
3

Impact Impact + 720

M g, Si, O: Mg-ric h S iO3 and S iO4 (Fos te rite , Sme cti te , Ens tatite ) Al, O, Ca: Al2 O3 ( hibo nite , s pine l) C,H, O, Ca, M g Ca rbona te s (CO3 ) PAHs , HAC? CO2 (i ce and gas )

H2 O
0.20

2

n

0.10

CH-X

* CO2 ~ 7%w rt H2O be fore
HCN
0.00 2.5 3.0 3.5 4.0

and up to 10% w rt H2O imme diate ly afte r im pact

Wavelength (µm)

Cred it: DI S cien ce t eam

Whe re are Fe and S? S ­ probably in Sx , SOx , O CS, CS Fe ­ se que s tered in Fe Ox,? Fe Sx ? (20 - 24 um fe at ure s )

* dram atic incre ase of organic m ate rial prom ptly afte r im pact
2

* organic m ate rials de cay slowe r than H2O or CO2 * HCN and CH3CN found, m any m ore ex pe cte d to follow (H-C bonds) * for the first time: w ate r-ice !
Folie 1 1 > V ortrag > Aut or
Dokum ent nam e > 23. 11. 200 4


5 - 35 um Spitzer Spectra : ( L+0.6 hrs) - ( L-23hrs)

The re are carbo na te s , carbo n-co ntai ning po lycyc lic a ro mat ic hydroca rbo ns , s me ctite (clay), crys tal li ne s ilica te s , me tal s ulf i de s After
SiO3 , Si O4 CO FeS, FeO "Al 2O3" SiO3 , SiO4

Before
3

Ejecta
"Al 2O3"

CO

SiO3 , SiO4
3

"Al 2O3"

PAHs

Ejecta - BB

H2O (DI) CO3 PAHs Fe , C gr ain s CO
3

Cred it: DI S cien ce t eam
Folie 1 2 > V ortrag > Aut or
Dokum ent nam e > 23. 11. 200 4


Relations to the early Solar sy stem O.Yu. Schmidt, V. S. Sa frono v (bot h Moscow, in the 1940s - 1980s): Collisional gro wth (,,accretion" ) of solid preplanetary bodies incl. comet nuclei:
,,So ft accretio n" in the o uter parts o f the early so lar system: -> lo w density, ,,fluffy" bo dies, impact fo rmed surface and internal st ructu re, ,,pristine" co mpo sitio n! VEGA/Gio tto + ... : Co met nuclei are o f lo w density, pristine materials,...

Primordial Rubble Pile Model Weissma n (1986)

F luffy Aggregate Model Donn et al. (1985)

All seems to fit together ,,in principle"

Layered Piles Model Belton et al., 2006
Folie 1 3 > V ortrag > Aut or
Dokum ent nam e > 23. 11. 200 4


But, comet nuclei show indications for a much more complicated origin and evolution:
Surface fe ature s : Laye ri ng a nd f lo w fe ature s may indicate muc h la rge r pre curs o r o bje cts M ine ralogy /c he mis t ry: Clay a nd ca rbo nate s re qui re li quid wate r to make . Crys tall i ne s ilicate s re qui re hot te mpe ra t ure s to form. Orbits : Co me ts have re se rvoi rs (fo rmatio n s ite s ) i n t he oute r s ola r s ys te m, but the re are co me t gro ups mo re ins i de : The ,,clande s tine " co me ts Image cr ed it: DI S cien ce team in t he as te roid be lt e .g. (Je witt, Hs ie h, 2006). Cha lle nge s : Have come ts forme d by acc re tio n o nly, o r may col lis io na l de s truc tio ns of la rge r and wate r/vola ti le -ric h pa re nt bo die s have cont ri bute d too (as in the as te roi d be lt)? Can o utgas s ing of wa te r/ice de bris -bo die s final ly have le d to po ro us or lo w de ns ity co me ts ? Has a hig h- a nd lo w-te mpe rat ure ma te rials mix i ng ove r g re at dis tance s ha ppe ne d i n t he pri mo rdia l sola r syste m ? Did co me t fo rmat io n i n t he early s ola r s ys tem take place at dif ferent s i te s , are there re late d di ffe re nt clas s es of come t nucle i? Be ope n minde d fo r ne w as pe cts /f ro ntie rs ! The pla ne t fo rmi ng proce ss es in the oute r s ola r s ys te m, i ncl. co me ts , a re a ye t ope n is s ue !
Folie 1 4 > V ortrag > Aut or
Dokum ent nam e > 23. 11. 200 4