The first Moscow Solar System Symposium (1MS3) Session 2: Phobos (science)

October 12th 2010 – Moscow, Russia

The internal structure and the origin of Phobos

Rosenblatt P., Rivoldini, A., Le Maistre S., and V. Dehant

ROYAL OBSERVATORYOF BELGIUM

ROYAL OBSERVATORYOF BELGIUM

Overview

• Origin of Phobos and Deimos:

An open issue !

• Model of Phobos’ interior: Link to the origin

‘Puzzling’ Phobos

Asteroid formed away from Mars andthen captured by Mars

Main argument:Carbonaceouschondrite composition

Alternative scenarioIn Situ formation

Main argument:Current moon orbits cannot be accounted by capture.

Contradictory clues about the origin !

MEX/HRSC image

Phobos

Martian moon composition from ViS/NIR reflectance spectra

Phobos/Deimos spectra best matched by D/T-type asteroid spectra: carbonaceous material. Featureless reddened spectra: signature of space weathering, which can mask the the true composition of Phobos as derived from the ViS/NIR spectra. No match with D-type meteorite analog spectra has been found so far (Vernazza et al., 2010) Phobos’ TIR spectra show better match with spectra of silicate material (Giuranna et al., 2010)

Ambiguity on composition: either carbonaceous or silicate material or unknown material?

Featureless reddened spectrum:Composition and/or Space Weathering?

ISM/Phobos-2 spectrafrom Murchie and Erard (1996)

Phobos vs low-albedo meteorite spectra Phobos/Deimos vs low-albedo asteroid spectra

Lack of 3 µm absorption band indicates anhydrous surface

Rivkin et al. (2002)

Is capture possible ?• Asteroid capture requires tight constraint given Mars’ mass and

possible initial velocity of the captured asteroid (Burns, 1992).

• Asteroid capture has to explain how changing the orbit after capture (ecliptic & elliptical orbit) into the current near-circular & near-equatorial orbits of both moons

• Model of tidal evolution of the orbit show that such orbital changes are not possible for Deimos and would require unlikely high tidal rate dissipation inside Phobos (~100 x Mars’ tidal dissipation, Mignard, 1981).

• Circularization by drag effect in the primitive Martian atmosphere or planetary nebula (Sasaki, 1989).

• Alternative scenario: In-situ formation

In situ formation

Re-accretion of impact or collision ejecta/debris blasted into Mars’ orbit (Craddock, 1994) It may explain the current orbits. It can be reconciliated with carbonaceous composition if the impactor body had a carbonaceous composition.

Remnants of a larger early moon captured by Mars, then destroyed by Mars’ tidal forces (Singer, 2007): It may explain the current near-equatorial orbits It can be ‘reconciliated’ with carbonaceous composition if the early moon had also a carbonaceous composition

Co-accretion with Mars (Peale, 2007) How to explain a carbonaceous composition for the two moons ?

So far, all these models have not been studied in detail and appear to be somewhat‘ad hoc’ but no observational evidence argue against them (at least two of them).

‘Puzzling’ Phobos

Alternative scenarioIn Situ formation

•Main argument:Current moon orbits Unlikely capture

Additional argument:A silicate composition cannot be excluded.

Contradictory clues about the origin !

MEX/HRSC image

Can bulk density providea key constraint to origin?

Capture scenario:

Main argument:ViS/NIR spectra Carbonaceous

But: •No carbonaceousmeteorite spectral analog yet found.

Phobos

Densitry of low-albedo mateorite

Density of low-albedo asteroids

Phobos has a low density of 1.87 +/- 0.03 g/cm3 (Andert et al., 2010)

Low-albedo asteroid have also a low density compared to their chondritic material analog. This has been interpreted as large space of voids (macro-porosity) in their

interior.

Low density of Phobos interpreted as substantial porosity in its interior.

Bulk density of Phobos

b

m

m = 1-ρphobos/ρb

Macro-porosity

g

b = 1-ρphobos/ρg

b

Bulk porosityPorosity estimates inside Phobos from its density

Formation of large craters such as Stickney requires high porosity (Britt et al., 2002) Replacing high porosity estimate inside Phobos into the context of its origin

Bulk density of material analog, ρb, provide macro-porosity m (large space of voids).

Hydrous chondrite 10% porosity Silicate material 25-45% porosity ‘Tagish Lake’ density No fit (due to 40% micro-porosity. Representative of D-type population?).

Considering grain density of material analog, ρg, provide bulk porosity b (i.e. macro + micro porosity).

All material yield high bulk porosity inside Phobos: 25-45% of the volume.

High porosity and capture scenario

High porosity indicates a Rubble Pile internal structure consistent with a re-accretion process likely resulting from catastrophic collisional event(s) in the asteroid history (Richardson et al., 2002).

High porosity is expected to accelerate the orbital tidal evolution (Goldreich and Sari, 2009) but unlikely not so much as needed to reach the current Phobos’ orbit around Mars.

Such porous asteroid could not orbit too close to Mars because large tidal stresses generated by Mars onto this body, would destroy it (not less than ~2 Mars radii, Sharma, 2009). Thus, atmospheric drag could not circularize the initial elliptical orbit of a captured porous asteroid.

g

b = 1-ρphobos/ρg

b

High porosity does not solve for the problem of orbital evolution after capture.

High porosity supports the idea of re-accretion of Martian large impact debris blasted into

orbit (Craddock, 1994) or resulting from catastrophic collisional event in Mars’ orbit.

It does not support the origin as remnants of former larger moon (Singer, 2007), unlike these remnants re-accreted later.

TIR data suggest a silicate material analog to Phobos (see Giuranna et al.), providing additional support to the scenario of re-accretion of large Martian impact debris.

High porosity and in-situ formation

g

b = 1-ρphobos/ρg

b

High porosity strongly supports scenario of re-accretion of debrisin Mars’ orbit.

A mixture of porosity and water ice inside Phobos.

Additional light element as water ice since water ice-rich ‘Phobos’ is expected if it formed at larger distance from the Sun than Mars did.

Porosity between 10 and 50% or water ice between 0 and 35% or a mixture of both can fit the bulk density of Phobos.

Additional geodetic observables are needed such as the forced libration amplitude and the gravity field of Phobos Mass distribution inside Phobos.

icerock

icerockphice

rock

phb f

)(1

b

fice

With:ρrock = density of rocky materialρice = density of water iceρph = density of Phobos fice = mass fraction of water iceb = Phobos bulk porosity

Unknows ρrock & fice while one observable ρph

ρrock

Internal mass distribution and geodetic parametersThe internal mass distribution is related to the values of the principal moments of inertia (A<B<C), which can also be expressed through the following geodetic parameters: The quadrupole gravity coefficients C20 and C22 and the libration θ

Where M is the mass of Phobos, r0 is the mean radius of Phobos and e is the ellipticity of its orbit around Mars.

Models of internal mass distribution provide the possible range for these geodetic parameters.

The comparison between these models and the current Phobos’ measured values would allow to constrain porosity/water ice proportion.

In turn, it would provide additional constraints on the origin of Phobos.

Models of Phobos’ interior

•We discretize the volume of Phobos (as obtained from MEX/HSRC-SRC) into 2626 cubes of equal volume (1300x1300x1300 m3) corresponding to 3 kinds of material of either silicate (3.10 g/cm3), porous silicate (1.35 g/cm3), providing 10% of bulk porosity and water ice (0.94 g/cm3).

• The number of cubes for each material is fixed, given the mass and the porosity and water ice content.

We have randomly distributed these cubes into the volume and computed the associated values of the geodetic parameters (gravity field coefficients and forced libration amplitude). We have run numerous models by varying the porosity (thus the water ice content) as well as the rocky material density.

Modeled internal mass distributionsfor the runs of case 1: ‘Homogeneous Phobos’

Most of the runs providewell-mixed distribution of the 3 kinds of assumed material in the volume of Phobos.

Red cube: Non-porous silicateGreen cubes: Porous silicateBlue cubes: Water ice

Representative pattern of possible internal mass distribution inside Phobos

Modeled geodetic observables for ‘homogeneous’ mass distributions (runs case 1)

Probability density functions of the quadrupole gravity coefficients C20 and C22 and of the libration amplitude θ

Case 1: Random distribution of the cubes. A bulk porosity of 10% is assumed.

The possible values of the moments of inertia are close to the homogeneous values (red line) The libration is close to the homogeneous value (-1.1°) but does not fit the observed Phobos libration (red curve) of -1.24° +/- 0.15° (from MEX/HRSC-SRC, Willner et al., 2009)

Modeled geodetic observables for the runs of case 2

Probability density functions of the quadrupole gravity coefficients C20 and C22 and of the libration amplitude θ

Case 2: Case 1 + ‘Smoothing’ constraint (i.e. favoring large volumes of same density).

Better accordance with the observed libration for most of the models. The most likely C22 values of those models slightly deviate from the value of a homogeneous Phobos model (red vertical line).

Modeled internal mass distributionsfor the runs of case 1: ‘Inhomogeneous Phobos’

Most of the runs provide a‘Cluster’ distribution of material of same densityin the volume of Phobos.

Red cube: Non-porous silicateGreen cubes: Porous silicateBlue cubes: Water ice

In this series of model Porosity is P=10% and rocky material density is 3.1 g/cm3

What happens if porosity is increased ?What happens if rocky material is decreased?Representative pattern of possible

internal mass distribution inside Phobos

Modeled Phobos’ interior structure: Varying rocky material density

Probability density functions of the quadrupole gravity coefficients C20 and C22 and of the libration amplitude θ

Porosity = 10%Rocky material density = 3.1 g/cm3

(silicate material)

The models don’t show so good agreement with observed libration amplitude (red curve) for lower rocky material density, whatever the applied ‘smoothing’ constraint. It suggests a large density value (silicate?) for the rocky material inside Phobos. Nevertheless, a better precision of a few percent on the libration amplitude value (instead of 15%) is needed to constrain the density of the rocky material inside Phobos from our models.

Porosity = 10%Rocky material density = 2.1 g/cm3

(carbonaceous material)

Modeled Phobos’ interior structure: Varying porosity content

Larger porosity inside Phobos show higher C20 and C22 values than the homogeneous values. MEX/Mars C20 value favorizes high porosity (low water ice) content inside Phobos (Pätzold et al., 2010), suggesting that Phobos may formed in the inner solar system. Nevertheless, a better precision of a few percent (instead of 50%) on the C20 value is needed to constrain the porosity vs water ice content inside Phobos from our Phobos’ interior models.

Porosity: 10% 30% 40%Water ice: 23% 7% 0%

Probability density functions of the quadrupole gravity coefficients C20 and C22

MEX/MaRS C20 value: -0.105 +/- 0.05 from MEX close flybyat distance of 77 km (performed in March 2010)

Summary

The low density of Phobos can be interpreted as large porosity (25% - 45%) in its interior: Loosely consolidated body

Such large porosity indicates that a re-accretion process occurred in the history of Phobos.

Such large macro-porosity does not preclude capture scenario but strongly support in-situ formation: re-accretion of impact debris in Mars’ orbit.

However, the question about origin still needs to determine the true composition of Phobos (Phobos-Grunt mission).

Additional insight about Phobos’ origin can be obtained from probing its interior (porosity vs water ice content), but it requires more precise measurements of libration amplitude and gravity field coefficients, than currently available.

How to improve geodetic parameters of Phobos: Geodesy experiment using radio-tracking data of

Phobos-Grunt.

Very close flyby (distance < 50 km)to improve the current C20 value usingthe Mars Radio-science (MaRS) experiment (opportunity in 2012).

Quasi-synchronous orbit at ~50 km of distance to Phobos (Jan. 2013) in order to determine the C22 coefficient Landed mission phase to improve the libration amplitude θ (2013)

Credit IKI

Credit ESA

Challenge: Precise Orbit Determination (POD) of spacecraft from the radio-tracking data needs to use dedicated orbitography software.

Acknowledgements

This work was financially supported by the Belgian PRODEX program managed by

the European Space Agency in collaboration with the Belgian Federal

Science Policy Office.