The internal structure and the origin of Phobos

1. ROYAL OBSERVATORYOF BELGIUM ROYAL OBSERVATORYOF BELGIUM The internal structure and the origin of Phobos Rosenblatt P., Rivoldini, A., Le Maistre S., and V. Dehant The first Moscow Solar System Symposium (1MS3) Session 2: Phobos (science) October 12th 2010 – Moscow, Russia

2. Overview Origin of Phobos and Deimos:An open issue ! Model of Phobos’ interior: Link to the origin

3. ‘Puzzling’ Phobos Contradictory clues about the origin ! Alternative scenarioIn Situ formation Main argument:Current moon orbits cannot be accounted by capture. Asteroid formed away from Mars andthen captured by MarsMain argument:Carbonaceouschondrite composition Phobos MEX/HRSC image

4. Martian moon composition from ViS/NIR reflectance spectra Featureless reddened spectrum:Composition and/or Space Weathering? Rivkin et al. (2002) ISM/Phobos-2 spectrafrom Murchie and Erard (1996) Lack of 3 µm absorption band indicates anhydrous surface Phobos vs low-albedo meteorite spectra Phobos/Deimos vs low-albedo asteroid 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?

5. 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

6. In situ formation • 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 • 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. • 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).

7. ‘Puzzling’ Phobos Contradictory clues about the origin ! • Alternative scenarioIn Situ formation • Main argument:Current moon orbits Unlikely capture Additional argument:A silicate composition cannot be excluded. • Capture scenario:Main argument:ViS/NIR spectra Carbonaceous • But: • No carbonaceousmeteorite spectral analog yet found. Phobos MEX/HRSC image Can bulk density providea key constraint to origin?

8. Bulk density of Phobos Density of low-albedo asteroids Densitry of low-albedo mateorite • 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.

9. Porosity estimates inside Phobos from its density Bulk porosity Macro-porosity b = 1-ρphobos/ρg m = 1-ρphobos/ρb rb rg b m • 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. • 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

10. High porosity and capture scenario b = 1-ρphobos/ρg rg b High porosity does not solve for the problem of orbital evolution after capture. • 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.

11. High porosity and in-situ formation b = 1-ρphobos/ρg rg b High porosity strongly supports scenario of re-accretion of debrisin Mars’ orbit. • 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.

12. A mixture of porosity and water ice inside Phobos. With: ρrock = density of rocky materialρice = density of water iceρph = density of Phobosfice = mass fraction of water ice b = Phobos bulk porosity Unknows ρrock & fice while one observable ρph ρrock b fice • 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.

13. Internal mass distribution and geodetic parameters • The 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.

14. 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.

15. 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-poroussilicate Green cubes: Porous silicate Blue cubes: Water ice Representative pattern of possible internal mass distribution inside Phobos

16. 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)

17. 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).

18. 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-poroussilicate Green cubes: Porous silicate Blue 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

19. 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) Porosity = 10% Rocky material density = 2.1 g/cm3 (carbonaceous 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.

20. Modeled Phobos’ interior structure: Varying porosity content 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) Porosity: 10% 30% 40% Water ice: 23% 7% 0% • 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.

21. 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.

22. How to improve geodetic parameters of Phobos: Geodesy experiment using radio-tracking data of Phobos-Grunt. Credit IKI Credit ESA • 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) • Very close flyby (distance < 50 km)to improve the current C20 value usingthe Mars Radio-science (MaRS) experiment (opportunity in 2012). • Challenge: Precise Orbit Determination (POD) of spacecraft from the radio-tracking data needs to use dedicated orbitography software.

23. 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.