and oriGin of PhoBos .
P. Rosenblatt1, S. Le Maistre1, A. Rivoldini1, N. Rambaux2,3, V. Lainey3, J. Castillo-Rogez4, C. Le Poncin-Lafitte5, L.I. Gurvits6,7, V. Dehant1, and J.C. Marty8. 1Royal Observatory of Belgium, Brussels, Belgium; 2 Université Pierre et Marie Curie, Paris, France; 3Institut de Mécanique Céleste et de Calcul des Ephémérides/Observatoire de Paris, Paris, France; 4Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA; 5Système de Référence Temps et Espace/Observatoire de Paris, Paris, France; 6Joint Institute for VLBI in Europe, Dwingeloo, The Netherlands; 7Department of Astrodynamics and Space Systems, Delft University of Technology, Delft, The Netherlands; 8CNES/GRGS, Toulouse, France. Contact: rosenb@oma.be
Introduction:
The Phobos-Soil mission, due to launch in October-November 2011, aims primarily to perform in-situ analyses of the soil of Phobos and bring samples back to earth for further investigation. The main scientific objective of this mission is to determine the origin of this small celestial body, still an open question. Key constraints on the physical processes prevailing at the origin come from a better knowledge of the bulk properties and deep internal structure. here, we propose a geodesy experiment which consists of precise measurements of the gravity field and rotation of Phobos, which are direct con- straints on these properties. the proposed experiment will also contribute to improving our knowledge of the Martian system and of fundamental physics. The Phobos-Soil radio-tracking facilities will be used in order to precisely reconstruct the trajectory of the spacecraft when orbiting Mars at close distances to Phobos, as well as the rotation variations and the orbit of Phobos once the spacecraft has landed on its surface. this Phobos Geodesy experiment (PGe) is proposed in the context of the Guest investiga- tor Program call, released by the space research institute of the russian academy of sciences (iKi) and the european space agency (esa). the proposed experiment aims to support and to extend the synergies with and objectives of the PRIDE-Phobos ex- periment (probing Phobos’ interior and testing fundamental physics [1]) and presented at this conference [7].
Scientific rationale:
the origin of the Martian moons, Phobos and deimos, is still an open issue: they may be (a) asteroids captured by Mars, (b) remnants left over from Mars’ formation, or (c) formed in-situ from a circum-Mars debris disk [e.g. 2]. The capture scenario mainly relies on optical remote sensing observations of their surface, which suggest that their material is similar to that suggested for outer-main belt asteroids. However, the dy- namical evolution of an asteroid captured from a heliocentric orbit that can explain the current orbits of the moons remain to be properly modeled. on the other hand, in-situ formation is more prone to account for the current moon orbits and is not in contradic- tion with the surface composition inferred from remote sensing data. some of the data collected recently by the Mars Express (MEX) spacecraft emphasize the importance of exploring the internal structure and origin in a self-consistent manner, opening new paths of investigation. In particular, the MEX data have allowed a precise determina- tion of the density of Phobos, reviving the interest in obtaining a better understanding of its interior [2]. On the one hand, Phobos’ interior can demonstrate a high porosity fraction, which raises a renewed interest for the in-situ formation scenarios. on the other hand, the density can also be explained with a water-rich Phobos’ interior, which calls for a fresh look at the capture scenario [3]. Indeed, water ice is expected to in- crease significantly the rate of tidal dissipation inside Phobos, promoting faster orbital evolution than previously considered. however, a detailed modeling remains to be carried out in order to assess the modalities of capture and post-capture evolution.
density alone cannot provide tight constraints on Phobos’ interior because the water ice content also depends on the porosity content [2]. More data about the interior are thus needed in order to constrain the fractions of porosity and water ice and their rela- tive distribution inside Phobos.
the Mars express images of Phobos have also been used to determine the amplitude of the forced libration in longitude (or periodic variations of the spin rate) as 1.24° +/- 0.15° [4]. Since the error bar includes the expected value of 1.1° obtained from the shape of Phobos in the case of homogeneous mass distribution [4], this measurement is not accurate enough to highlight departures from homogeneity across the satel- lite. still, this value within its error bar indicates a slightly heterogeneous interior, as
suggested by recent models of internal mass distribution [5]. The same models also show that the values of the principal moments of inertia of Phobos are sensitive to the amount and distribution of porosity and water ice in its interior. this implies that precise measurement of these moments of inertia may provide tighter constraints on the interior structure of Phobos (i.e. porosity versus water-ice content) [5]. As the mo- ments of inertia are related to the forced libration amplitude and to the second-order coefficients of the non-spherical part of the gravity field of Phobos [6], precise meas- urements of these parameters will lead to a precise determination of the moments of inertia of Phobos.
In the context of the Phobos-Soil mission, we propose an additional geodetic experi- ment using the radio-tracking data of the spacecraft in order to get a precise view on the bulk internal structure of Phobos.
Measurements and goals of the Phobos Geodesy Experiment
the proposed geodesy experiment relies on the ranging and doppler tracking data performed with the onboard coherent transponder (2-way ranging and Doppler) and the onboard Ultra-Stable-Oscillator (USO, 1-way Doppler) [7]. These measurements will provide radial distance and velocity along the line-of-sight between the spacecraft and Earth-based deep space tracking or VLBI (Very Long Baseline Interferometry) sta- tions. the Uso also offers the opportunity to perform VlBi tracking of the spacecraft, affording additional spacecraft position measurements in the plane-of-sky [8].
Determining the gravity field of Phobos: Before landing on Phobos, the Phobos-Soil spacecraft will orbit Mars in a Quasi-Synchronous-Orbit (QuSyO) with Phobos. During this mission phase, scheduled to last one month, the spacecraft will remain at very close distances to Phobos (45-55 km), and will serve as a sensor of its gravity field.
Radio-tracking data will enable precise reconstruction of the orbital perturbations of the spacecraft. This Precise Orbit Determination (POD) process consists of fitting a dynamical model of the spacecraft motion to the available tracking data in order to estimate the gravity field coefficients, in particular the second-order ones, which will be used for a precise determination of the principal moments of inertia of Phobos. all deep space and VlBi tracking data will be used in order to separate robustly the gravity field signal from the perturbations induced by the maneuvers required to maintain the QuSyO. The tracking data will be processed with the dedicated orbitography software called GINS [13].
Monitoring the rotational motion of Phobos: Once landed on Phobos, the radio-track- ing data of the spacecraft will be used to monitor the rotational motion of Phobos. a complete model of Phobos’ rotational motion will then be fit to these tracking measure- ments in order to determine the amplitudes of the physical librations in longitude and in latitude at short and long periods [9, 10]. Preliminary simulations, using the GINS software, have shown that the radio-tracking data will allow determining the amplitude of the short-period librations with a precision of 0.1% after a few weeks of data acquisi- tion (and better than 0.01% after a few months) [9]. Merging the radio tracking data with the star-tracker data will improve the determination of the librations as foreseen by the celestial mechanics experiment [11]. Along with the gravity field, the libration ampli- tudes will allow measuring the principal moments of inertia of Phobos at the single-digit percent level, which is required to constrain tightly the internal mass distribution inside the moon [3].
Monitoring the orbital motion of Phobos: The radio-tracking data of the landed space- craft also contain information on the fine variations of the orbital motion of Phobos. The doppler measurements are well suited to detect these variations, which could not be measured precisely with astrometry data so far. the gain of precision expected on the reconstructed orbit of Phobos will allow sensing the fine variations of the gravity field of Mars (e.g. due to the seasonal co2 mass exchange between the atmosphere and polar caps). it will allow determining the time variations of Mars’ even zonal harmonics with a precision of a few percent [12], which have been poorly constrained so far by tracking data of spacecraft orbiting Mars on low-altitude polar orbits [e.g. 13]. It will also lead to an improved determination of the relativistic parameter β by a factor of 2.5 for tracking data acquired over one year. It will enable new tests of fundamental physics along with the detection of potential variations of the universal gravitational constant, as proposed by the PRIDE-Phobos experiment [1]. As the radio-tracking data contain information on the full motion of Phobos, the star-tracker measurements will be help- ful to decouple the orbital motion from the rotational one. the tracking data will be processed with the noe software dedicated to precise computation of natural satellite ephemerides.
An “integrated” geodesy experiment: another important part of the proposed experi- ment will be based on the merging of the radio-tracking data of both QuSyO and land- ing phases of the mission. the goal of this global inversion of the tracking data is to improve the determination of physical parameters, like the gravity field of Phobos,
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which influence both spacecraft’s and Phobos’ orbital motions. This inversion scheme will follow the one using the natural satellite astrometry data together with the space- craft tracking data proposed in the context of the European project ESPaCE [14]. The Phobos Geodesy experiment will also provide a preview of what a space mission dedi- cated to the Mars geodesy, like the GETEMME mission [15], would be able to deliver.
the results of the Phobos Geodesy experiment will support the interpretation of other experiments targeting the interior (e.g. using the seismometer, MUss) and will comple- ment surface observations, helping to answer the question of the origin of this small body.
References: [1] Linkin et al., 2009, EPSC2009-376; [2] Rosenblatt P., 2011, submitted to A&A Rev.; [3] Castillo-Rogez et al., 2011, EPSC-DPS2011-1673; [4] Willner et al., 2010, EPSL 294, 541-546; [5] Rosenblatt et al., 2010, 1st Moscow Solar system Symp., 29; [6] Borderies and Yo- der, 1990, A&A 233, 235-521; [7] Duev et al., this meeting; [8] Gurvits et al., 2010, 1st Moscow Sol. Syst. Symp., 73; [9] Rambaux et al., EPSC-DPS2011-818; [10] Le Maistre et al., EPSC- DPS2011-1021; [11] Andreev et al., 2010, Sol. Syst. Res., 44, 438-443; [12] Lainey et al., EPSC- DPS2011-952; [13] Marty et al., PSS, 57, 350-363; [14] Thuillot et al., EPSC-DPS2011-1833; [15]
Oberst et al., 2011, submitted to Experimental Astronomy.
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Mass distriBUtion inside PhoBos: a Key oBserVational constraint for the oriGin of PhoBos.
A. Rivoldini1, P. Rosenblatt1, N. Rambaux2,3, and V. Dehant1. 1Royal Observa- tory of Belgium, Brussels, Belgium; 2 Université Pierre et Marie Curie, Paris, France;
3Institut de Mécanique Céleste et de Calcul des Ephémérides/Observatoire de Paris, Paris, France; Contact: rosenb@oma.be
Introduction:
the origin of the Martian moons, Phobos and deimos, is still an open issue. it has been proposed that they formed away from Mars and then captured by Mars gravita- tional attraction [1] or that they formed in-situ from a disk of debris in Mars’ orbit [2].
The capture scenario has, however, major difficulties to account for the current near- circular and near-equatorial orbit of Phobos [1]. Previous works of tidal orbital evolution have shown the critical role of the tidal dissipation inside a satellite to make the capture possible, i.e. Phobos’ interior might have high dissipative properties [3], which would be closer to those of icy material than to those of rocky material [4]. Among the recent observations made by the Mars express spacecraft, those concerning the internal structure of Phobos are particularly pertinent for assessing the scenario of origin [4].
Indeed, the density of Phobos, 1.87 +/- 0.02 g/cm3 [4], is lower than the density of presumed material analogs, suggesting that the interior of this small moon can contain light elements like porosity or water-ice. The former supports in-situ formation while the latter favorizes an asteroid capture scenario [4,5]. Therefore, the assessment of the porosity/water-ice content inside Phobos is a key measurement relevant to the open question about its origin [6].
in this study, we develop models of mass distribution inside Phobos, and use the measured libration of amplitude and density of Phobos to constrain the mass distribu- tion within. We explore the possible internal mass distributions, considering three kinds of material inside Phobos: rock, porous-rock and water-ice. We compute the principal moments of inertia, related to the second-order gravity field coefficients, C20 and c22 and libration amplitude of Phobos, for each of these possible internal mass distribu- tions. Then, we select the distributions that fit the measured libration of amplitude and the density of Phobos within their error bars. For those distributions, we find values of the gravity field coefficients which depart from the expected value of a homogene- ous mass distribution for a large amount of porosity and a low amount of water-ice. In turn, precise measurements of both gravity field coefficients and rotation variations of Phobos may provide new constraints on the origin of this small moon of Mars.
Models of mass distribution inside Phobos:
the proportion and repartition of water ice and rock porosity cannot be determined from the average density alone. Another datum, like the libration amplitude (-1.24 +/- 0.15 degrees [7]), which depends on the principal moments of inertia of Phobos (thus on its internal mass distribution), is likely to provide further constraints. in order to constrain Phobos’ interior structure, we have discretized its volume by a set of cubes (2626), each having an identical volume of 1300mx1300mx1300m. The cubes are made of one of three different materials: water ice (940 kg/m3), porous-rock and non- porous rock. For a given porosity and non-porous rock density the number of cubes of each material is determined from the bulk density of Phobos. our model contains a parameter that controls the size of clusters of identical material within the volume of Phobos. We have calculated the probability density functions for the three principal moments of inertia and for the libration amplitude taking various degrees of porosity, fractions of water ice and rocky material density into account.
Results:
our results show that the most likely models with a homogeneous matter distribution have libration amplitudes that deviate from the estimated libration amplitude, suggest- ing a Phobos interior mass distribution that deviates from homogeneous distribution.
In order for the models to fit the observed libration amplitude the smoothing param- eter values have to be chosen such that clusters of material of intermediate size are obtained. Models with rocky material density lower than about 2.1 g/cm3 do not fit the libration amplitude whatever the porosity/water ice content and the smoothing param- eter values. however, the precision on the observed libration does not allow for a tight constraint on the porosity/water ice content inside Phobos. from our models we have also computed the c20 gravity field coefficient. The predicted C20 values depart more and more from the value expected for a homogeneous mass distribution when more and more porosity (up to 40%), and equivalently less and less water-ice content (down
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to a few percent of the mass of Phobos), are considered. in turn, it shows that a precise measurement of Phobos’ gravity field could provide additional constraints on its interior and origin.
Summary and perspectives:
A precision of a few percent on the gravity field and the rotation measurements of Phobos are needed to tightly constrain its interior structure. such precise measure- ments are challenging but might be obtained from the Phobos-Grunt spacecraft [8], due to launch in October-November 2011 (arrival date to Phobos in early 2013). The Phobos-Grunt spacecraft will indeed orbit Mars at close distance to Phobos (45-55 km), offering the opportunity to measure the gravity field of Phobos and then will stay at Phobos’ surface, offering the opportunity to measure the fine variations of the spin rate and orientation of the rotation axis of Phobos. our models of Phobos’ interior will be useful for interpreting these future data. for instance, they can be used for modeling of the rotation of Phobos [9] which will be constrained by the Phobos-Grunt observa- tions [10].
References: [1] Burns J.A., in Mars, Univ. Arizona Press, pp. 1283-1301, 1992; [2] Peale S.J., in treatise of geophysics, vol. 10, Elsevier, 465-508, 2007; [3] Mignard F., Mon. Not. R. astr. Soc., 194, pp. 365-379, 1981; [4] Andert T.P. et al., GRL 37, L09202, 2010. [5] Rosenblatt P. submitted to A&A Rev.; [6] Rosenblatt P. et al., this meeting; [7] Willner et al., Earth Planet. Sci. Lett., 294, pp. 541-546, 2010: [8] Rosenblatt P. et al., 1st Moscow Solar System Symposium, Abstract-29, 2010; [9] Rambaux et al., EPSC-DPS2011-818; [10] Le Maistre et al., EPSC-DPS2011-1021.
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PhotoMetric and radioMetric ProPerties of PhoBos reGolith froM data Gathered By the PhoBos Mission
L. V. Ksanfomality, Space Research Institute (IKI), Moscow. Contact:
ksanf@iki.rssi.ru
In the paper the reflectance (spectrophotometric) and thermal (radiometric) proper- ties of Phobos’ regolith are reviewed. The study is based on data gathered in 1989 by means of the KrfM spectrophotometer installed onboard the Phobos spacecraft. ac- cording to the program of the Phobos mission, it had been assumed that the spacecraft would approach the Martian satellite to a distance of roughly 50 m and drift at this alti- tude above the surface for 20 min. The spectrophotometer and radiometer would have to carry out the measurements with a resolution, respectively, of 26 and 50 cm on the surface of Phobos. Unfortunately, 10 days prior to this approach, the spacecraft was lost, and this portion of the program was not carried out. the two tracks investigated prior to the loss of the spacecraft did, however, provide a large volume of data. the analysis of these data suggest certain extremely interesting properties of fine-grained material, or regolith, on Phobos, established mainly in the spectrophotometric experi- ment. Improved spectrophotometry of Phobos in the 300-600 nm band, complemented with data on albedo in the shorter wavelengths of the ultraviolet and near infrared bands are presented. on the basis of the surface properties, Phobos would appear to possess an unambiguously nonhomogeneous composition, which suggests a complex history of development. Judging from the spectrophotometric properties of Phobos regolith discovered in the experiment, its true reflectance properties do not agree very well with the properties of carbonaceous chondrites or yield clear cut analogies to other meteorite materials. The significant inhomogeneity of the properties of regolith seen in the 315-600 nm band is associated with particular topographic features. Two series of spectrophotometric and thermal (radiometric) measurements that were car- ried out by the Phobos spacecraft are presented.