approaching the internal structure of the nuclei of...

Levasseur-Regourd Alicante, 2007 1

Approaching the internal structure

of the nuclei of comets

Anny-Chantal Levasseur-Regourd

J. Lasue, E. Hadamcik

Univ. Paris VI / Aéronomie [email protected]


Approaching the internal structure

of the nuclei of comets

Outline

1. Motivation

2. Past understanding

3. Present results

4. Future projects

Comet McNaught, January 2007

Tiny hidden nucleus


1. Motivation

Knowing the internal structure of cometsnuclei mandatory to

- understand their formation and evolution

- prepare future missions

In the absence of direct evidence, internalstructure approached through

- estimations of the density

- determination of the properties of dustparticles ejected from the nuclei

- observations of fragmentation andcatastrophic disruption events

Note

Icy comet nuclei small and very fragile

Rosetta artist view (ESA)

LINEAR 1999 S4 disruption (HST)

Mini comae around fragments


Densities

1P/Halley (1985-1986)

VeGa (IKI) and Giotto (ESA)

! Low density dark nucleus

- no detectable perturbations

- in agreement with NGF models

(0.2 to 0.8 g cm-3, Rickman et al. 1987)

! Dust particles, from OPE+DID data

- evolving with nucleus distance (time)

- density ! 0.1 g cm-3 and albedo ! 4%

IonsHalley/Giotto (MPI/ESA)

2. Past understanding

Inner comaInner coma

Levasseur-Regourd

et al. 1999

Keller et al. 1986

Log Nucleus distance (km)


Fragmentation

26P/Grigg-Skjellerup (1992) from Giotto

! Possible detection of a fragment by OPE

! Fragility of cometary nuclei suspected

Remote observations

- Shoemaker-Levy 9 discovery and evolution

(tidal disruption)

- Motion of icy fragments in C/1996 B2 Hyakutake

! Sizes ! 10-100 m, densities 0.1-0.8 g cm-3 (Desvoivres et al. 2000)

- Also C/LINEAR 1999 S4 and 73P/S-W 3 disruptions

IonsS-L 9, HST

LINEAR, HST

Hyakutake, HST

McBride et al. 1997

Yakutake, HST

73P/S-W 3, Spitzer

26P/G-S

Main coma

Secondary

coma


Ions

Remote light scattering observations and simulations

Observations of polarization of solar light scattered by dust,

with laboratory simulations (reduced gravity)

and numerical simulations (e.g. DDA)

! Irregular particles, with size > !

! Presence of fluffy -possibly fractal- aggregates

(clue to the nucleus structure?)

! At least bimodal population, silicates and more absorbing organics

3. Recent results

Pred(")100

80

60

40

20

0

Pola

rizati

on (

%)

150100500

Phase angle (°)

mixtures 50/50 in mass SiO2(40±20)nm+C

SiO2+C (14±5)nm

SiO2+C

(95±20)nm

SiO2

Data points and

corresponding fits

- red (near 670 nm)

- green (near 523 nm)

Levasseur-Regourd et al. 2007


Ions

81P/Wild 2 (2004-2006) and Stardust

! Rather rounded nucleus

! Complex morphology with ‘flat’ craters

! Swarm detected at ! 4000 km (Tuzzolino et al. 2004),

possibly related to dust fragmentation

! Estimated density 0.1-0.8 g m-3 (Davidsson & Guttierez, 06)

Return capsule with impacts on Al foils and tracks in aerogel cells

!Direct evidence for low density dust aggregates

Hölz et al. 2006

Brownlee et al. 2004

3. Recent results


Ions

9P/Tempel 1 (2005) and Deep Impact, Pre-Impact

Worldwide observations ! Aperiodic outbursts (nucleus ‘crumbling’)

High resolution images prior to impact

! Flat layers, scarps, erosion features

! Icy patches, leading to tiny jets at the limb

! Impact craters

Zone d’impact

Farnham et al. 2007

A’Hearn et al. 2005


Ions

9P/Tempel 1 (2005) and Deep Impact, Post-impact

Realistic impact experiment (m = 370 kg v = 10.2 km s-1)

- crater hidden by dust

- speed of fastest dust particles > 300 m s-1

! Density ! 0.35 ± 0.25 g m-3

(Estimated density ! 0.45 g m-3 (Davidsson et al. 2006)

! Super porous, porosity > ! 80 %

! Organics and water ice in subsurface

Zone d’impact

Sunshine et al. 2007

A’Hearn et al. 2005


Rosetta mission

Objectives

Rendezvous (Rosetta orbiter)

Landing on nucleus (Philae lander)

! Characterization of its evolution

from aphelion to perihelion

! Detailed study of the nucleus: composition, mineralogy, structure

Target 67P/Churyumov-Gerasimenko

- a = 3.507 UA, e = 0.632, i = 7.12°

- nucleus ! 3 km x 5 km

- active areas ! 7% at perihelion

- density estimated ! 0.1 to 0.37 g cm-3

Successful launch, 2 March 2004

Philae

4. Future projects

Estimated shape of 67P nucleus, assuming a

prograde (up) or retrograde (bottom) rotation

Lamy et al. 2007


9P/Tempel 1

Mars

Rosetta milestones

2005, Earth swing-by, Tempel 1 impact observations (from .6 AU)

2007, Mars swing-by in February and 2nd Earth swing-by in November

2008, 2867 Steins (! 10 km, E-type) flyby on 5 September 2008

2009, 3rd Earth swing-by in November

2010, 21 Lutetia (! 100 km, M-type) flyby on 10 July 2010

2014, Rendezvous, with Philae landing ! 6 months later

Mars


CONSERT experiment

CONSERT, Cometary Nucleus Sounding by Radiowave Transmission

with parts on both the orbiter and the lander(Kofman et al. 2007)

Objectives: nucleus tomographic sounding from the determination of

its dielectric properties (materials, voids, discontinuities, porosity)

Instrumentation: radio-waves (90 MHz, 8 MHz band-pass) from the

orbiter, with reception and re-emission by Philae

! Information on celerity V and electric field E


Mars

Preparation of Rosetta encounter

Dust coma

Optimisation of the mission from observations, numerical simulations

and laboratory simulations

CONSERT

Optimisation of the science return from

- measurements of the properties of ices-dust samples with relevant

porosities, temperatures, concentrations

- numerical simulations with 3D thermal models of the nucleus

Other proposed missions

ESA & Jaxa, Marco Polo, NEO sample return?

ICAPS for accretion exp. on board ISS?

Philae artist view (ESA)


Mars

Please do not forget comet nuclei in

catastrophic disruption studies

- Small

- Fragile

- Mostly hidden

- Often suffering disruptions

- Underdense

- Possibly built of fluffy particles

- Having suffered impacts

More to be hopefully known in 2014

Thank you

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