GETEMME—a mission to explore the Martian satellites and the fundamentals of solar system physics

GETEMME (Gravity, Einstein??s Theory, and Exploration of the Martian Moons?? Environment), a mission which is being proposed in ESA??s Cosmic Vision program, shall be launched for Mars on a Soyuz Fregat in 2020. The spacecraft will initially rendezvous with Phobos and Deimos in order to carry out a comprehensive mapping and characterization of the two satellites and to deploy passive Laser retro-reflectors on their surfaces. In the second stage of the mission, the spacecraft will be transferred into a lower 1500-km Mars orbit, to carry out routine Laser range measurements to the reflectors on Phobos and Deimos. Also, asynchronous two-way Laser ranging measurements between the spacecraft and stations of the ILRS (International Laser Ranging Service) on Earth are foreseen. An onboard accelerometer will ensure a high accuracy for the spacecraft orbit determination. The inversion of all range and accelerometer data will allow us to determine or improve dramatically on a host of dynamic parameters of the Martian satellite system. From the complex motion and rotation of Phobos and Deimos we will obtain clues on internal structures and the origins of the satellites. Also, crucial data on the time-varying gravity field of Mars related to climate variation and internal structure will be obtained. Ranging measurements will also be essential to improve on several parameters in fundamental physics, such as the Post-Newtonian parameter ?? as well as time-rate changes of the gravitational constant and the Lense-Thirring effect. Measurements by GETEMME will firmly embed Mars and its satellites into the Solar System reference frame.     

Orbital history of the Martian satellites with inferences on their origin

Recent Viking results indicate the Martian satellites are composed of carbonaceous chondritic material, suggesting that Phobos and Deimos were once asteroids captured by Mars. On the other hand, the low eccentricities and inclinations of their orbits on the equator of Mars argue against that hypothesis. This paper presents detailed calculations of the tidal evolution of Phobos and Deimos, considering dissipation in both Mars and its satellites simultaneously and using a new method applicable for any value of the eccentricity. In particular, including precession of the satellites' orbits indicates that they have always remained close to their Laplacian plane, so that the orbital planes of Phobos and Deimos switched from near the Martian orbital plane to the Martian equator once the perturbations due to the planetary oblateness dominated the solar perturbations, as they do presently. The results show that Deimos has been little affected by tides, but several billion (10⁹) years ago, Phobos was in a highly eccentric orbit lying near the common plane of the solar system. This outcome is obtained for very reasonable values of dissipation inside Mars and inside Phobos. Implications for the origin of the Martian satellites are discussed.     

Search for Phobos and Deimos gas/dust tori using in situ observations from Mars Global Surveyor MAG/ER

More than 490 elliptical aerobraking and science phasing orbits made by Mars Global Surveyor (MGS) in 1997 and 1998 provide unprecedented coverage of the solar wind in the vicinity of the orbits of the martian moons Phobos and Deimos. We have performed a comprehensive survey of magnetic field perturbations in the solar wind to search for possible signatures of solar wind interaction with dust or gas escaping from the moons. A total of 1246 solar wind disturbance events were identified and their distribution was examined relative to Phobos, the Phobos orbit, and the Deimos orbit. We find that the spatial distribution of solar wind perturbations does not increase near or downstream of Phobos, Phobos’ orbit, or Deimos’ orbit, which would have been expected if there is significant outgassing or dust escape from the martian moons. Of the 1246 magnetic field perturbation events found in the MGS data set, 11 events were found within 2000 km of the Phobos orbit, while three events were found within 2000 km of the Deimos orbit. These events were analyzed in detail and found to likely have other causes than outgassing/dust escape from the martian moons. Thus we conclude that the amount of gas/dust escaping the martian moons is not significant enough to induce detectable magnetic field perturbations in the solar wind. In essence we have not found any clear evidence in the MGS magnetic field data for outgassing or dust escape from the martian moons.     

The origin of the Martian moons revisited

The origin of the Martian moons, Phobos and Deimos, is still an open issue: either they are asteroids captured by Mars or they formed in situ from a circum-Mars debris disk. The capture scenario mainly relies on the remote-sensing observations of their surfaces, which suggest that the moon material is similar to outer-belt asteroid material. This scenario, however, requires high tidal dissipation rates inside the moons to account for their current orbits around Mars. Although the in situ formation scenarios have not been studied in great details, no observational constraints argue against them. Little attention has been paid to the internal structure of the moons, yet it is pertinent for explaining their origin. The low density of the moons indicates that their interior contains significant amounts of porous material and/or water ice. The porous content is estimated to be in the range of 30?C60% of the volume for both moons. This high porosity enhances the tidal dissipation rate but not sufficiently to meet the requirement of the capture scenario. On the other hand, a large porosity is a natural consequence of re-accretion of debris at Mars?? orbit, thus providing support to the in situ formation scenarios. The low density also allows for abundant water ice inside the moons, which might significantly increase the tidal dissipation rate in their interiors, possibly to a sufficient level for the capture scenario. Precise measurements of the rotation and gravity field of the moons are needed to tightly constrain their internal structure in order to help answering the question of the origin.     

A deep search for Martian dust rings and inner moons using the Hubble Space Telescope

It has long been suspected that Mars might be encircled by two faint rings, one originating from each of its moons Phobos and Deimos. Meteoroid impacts into these moons should release clouds of dust that quickly spread out to become rings; similar dust rings have been associated with several small inner moons of the gas giants. On May 28, 2001 Mars’ hypothetical ring plane appeared edge-on to Earth within weeks of its opposition, providing the best Earth-based opportunity to detect these rings in several decades. Using the Wide Field/Planetary Camera 2 (WFPC2) on the Hubble Space Telescope, we obtained a set of deep exposures off the east and west limbs of Mars to search for these hypothetical rings. No rings were detected. This result limits normal optical depths to ∼3×10⁻⁸ for the Phobos ring and ∼10⁻⁷ for the Deimos ring. These limits fall at the low end of prior dynamical predictions and a factor of 1000 below previous observational limits. However, our limit for the Deimos ring is more tentative because of large uncertainties about this ring's expected shape, size and orientation. Our data set is also sensitive to small, previously undetected inner moons. No moons were detected down to a radius limit of 75–125 m. Longitudinal coverage of the region near and between Phobos and Deimos is 40–80% complete. We conclude by describing a promising opportunity for further Martian ring viewing in December 2007.     

Project of the mission to Phobos

This article provides the main scientific objectives and characteristics of the Phobos-Soil project, intended to fly to the Martian satellite Phobos, deliver its soil samples to the Earth, as well as explore Phobos, Mars, and the Martian environment with onboard scientific instruments. We give the basic parameters of the ballistic scenario of the mission, spacecraft, and some scientific problems to be solved with the help of the scientific instruments installed on the spacecraft.     

Spectral heterogeneity on Phobos and Deimos: HiRISE observations and comparisons to Mars Pathfinder results

The High-Resolution Imaging Science Experiment (HiRISE) onboard Mars Reconnaissance Orbiter (MRO) has been used to observe Phobos and Deimos at spatial scales of around 6 and 20 m/px, respectively. HiRISE (McEwen et al., JGR, 112, CiteID E05S02, DOI: 10.1029/2005JE002605, 2007) has provided, for the first time, high-resolution colour images of the surfaces of the Martian moons. When processed, by the production of colour ratio images for example, the data show considerable small-scale heterogeneity, which might be attributable to fresh impacts exposing different materials otherwise largely hidden by a homogenous regolith. The bluer material that is draped over the south-eastern rim of the largest crater on Phobos, Stickney, has been perforated by an impact to reveal redder material and must therefore be relatively thin. A fresh impact with dark crater rays has been identified. Previously identified mass-wasting features in Stickney and Limtoc craters stand out strongly in colour. The interior deposits in Stickney appear more inhomogeneous than previously suspected. Several other local colour variations are also evident.Deimos is more uniform in colour but does show some small-scale inhomogeneity. The bright “streamers” (Thomas et al., Icarus, 123, 536–556,1996) are relatively blue. One crater to the south-west of Voltaire and its surroundings appear quite strongly reddened with respect to the rest of the surface. The reddening of the surroundings may be the result of ejecta from this impact.The spectral gradients at optical wavelengths observed for both Phobos and Deimos are quantitatively in good agreement with those found by unresolved photometric observations made by the Imager for Mars Pathfinder (IMP; Thomas et al., JGR, 104, 9055–9068, 1999). The spectral gradients of the blue and red units on Phobos bracket the results from IMP.     

Tidal rigidity of phobos

The Martian satellites Phobos and Deimos move along nearly circular coplanar, stable orbits and have created surfaces older than ～ 10⁹ years. The accretion hypothesis suggests that their primordial orbits were also very regular. However, tides raised on Mars and Phobos can substantially alter the semimajor axis a of Phobos' orbit over time. The effect of the Martian tidal torque alone on Phobos' orbit implies that the primordial e was ～0.1 to 0.2 about 4.6 × 10⁹ years ago if the present observed e = 0.015 is naively interpreted as a tidally damped remnant. Significant tidal friction in Phobos reduces the time scale for Phobos to achieve a crossing orbit with Deimos to less than 10⁹ years and permits the primodial e to approach unity. The consequences of orbital intersections cannot easily be resolved by assuming either a catastrophic origin for both satellites (namely, that both are fragments of a common parent body fractured by an impact) or that they were captured sequentially by Mars. Either hypothesis is difficult to accept, given that Deimos' orbit, which is only slightly affected by tides, is now so regular. An alternative scenario is proposed in this paper in which the observed e of Phobos results from several gravitational resonance excitations within the last 10⁹ years, assuming tidal friction in Phobos has had only a small effect on its orbit. In facr, both the primordial e and the inclination i may have been much smaller than presently observed. The constraints imposed on tidal friction in Phobos by both the apparent age of Phobos' surface (> 10⁹yrs) and the above scenario can be satisfied only of μQ > 10¹²dynes/cm². Since the Q factor is ～10², the rigidity μ > 10¹⁰dynes/cm². Thus Phobos should have substantial internal strength.     

A plasma flow velocity boundary at Mars from the disappearance of electron plasma oscillations

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express (MEX) spacecraft is capable of measuring ionospheric electron density by the use of two main methods: remote radar sounding and from the excitation of local plasma oscillations. The frequency of the locally excited electron plasma oscillations is used to measure the local electron density. However, plasma oscillations are not observed when the plasma flow velocity is higher than about 160 km/s, which occurs mainly in the solar wind and magnetosheath. As a consequence, in many passes, there is a sudden disappearance of the plasma oscillations as the spacecraft enters into the magnetosheath. This fact allows us to identify a flow velocity boundary on the dayside, between the ionosphere of Mars and the shocked solar wind. This paper summarizes the results of the measurement of 552 orbits mostly over a period from August 4, 2005 to August 17, 2007. The boundary points found using MARSIS have been verified by measurements from the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) Electron Spectrometer (ELS) instrument on Mars Express. The average position of the flow velocity boundary is compared to flow velocity simulations computed using hybrid model and other boundaries. The boundary altitude is slightly lower than the magnetic pile-up boundary determined using Phobos 2 and Mars Global Surveyor (MGS) crossings, but it is in good agreement with the induced magnetospheric boundary determined by ASPERA-3. Investigation of the effect of the crustal magnetic field revealed that the flow velocity boundary is raised at the locations with strong crustal magnetic fields.     

Mars: Satellite origin and angular momentum

The origin of Phobos and Deimos is considered with a view to accounting for the existence of very small satellites with circular orbits in the Martian equatorial plane, and simultaneously for the suspected angular momentum deficiency of the Mars system. All models considered failed to satisfy at least one requirement, and the problem is considered more puzzling than is at first apparent. The Martian angular momentum deficiency, if physically significant, may be unrelated to the present satellites' origin, but might relate to a large ancient satellite, long ago destroyed. Accretion onto Mars of large amounts of asteroidal dust brought in by Poynting-Robertson drag may have some bearing on the angular momentum problem.     

Libration celestial mechanics experiment

The exploration of planet moons and minor bodies (Avduevskii et al., 1996) is a basic task for comprehending the nature of the processes occurring in our Solar System. Knowing the current state of the moons, we can better describe their past and look into the future. This knowledge is important, first of all, for understanding the origin of the Solar System. Interest in the Martian moon Phobos has been displayed during recent decades. The interest is caused by some questions to which there have been no answers up until now (Sagdeev et al., 1988; 1989). For example, there is a question regarding the origin of the moon: whether it is an asteroid captured by Mars’ gravitational field or it is an accumulated body in the Martian orbit. In connection with this, it is interesting to conduct studies aimed at answering this question. If Phobos appears to be an asteroid, then investigations regarding the chemical and isotopic compositions of the moon as the primary matter of the Solar System as well as its evolution are of great interest.     

Correction of the ionospheric distortion on the MARSIS surface sounding echoes

Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express (MEX) spacecraft has made numerous measurements of the Martian surface and subsurface. However, all of these measurements are distorted by the ionosphere and must be compensated before any analysis. We have developed a technique to compensate for the ionospheric distortions. This technique provides a powerful tool to derive the total electron content (TEC) and other higher-order terms of the limited expansion of the plasma dispersion function that are related to overall shape of the electron column profile. The derived parameters are fitted by using a Chapman model to derive ionospheric parameters like n₀, electron density primary peak (maximum for solar zenith angle (SZA) equal 0), and the neutral height scale H.

Our estimated ionospheric parameters are in good agreement with Mars Global Surveyor (MGS) radio-occultation data. However, since MARSIS does not have the observation geometry limitations of the radio occultation measurements, our derived parameters extend over a large range of SZA for each MEX orbit.

The first results from our technique have been discussed by Safaeinili et al. [2007, Estimation of the total electron content of the Martian ionosphere using radar sounder surface echoes. Geophys. Res. Lett. 34, L23204, doi:10.1029/2007GL032154].     

An analysis of Martian satellite photographic observations of 1967

Photographic observations of the Martian satellites were made at the opposition of 1967 with the Naval Observatory's 61-inch astrometric reflector. A small partially transparent metallic film filter was used to diminish the light from Mars in order that a measurable image for the planetary disk as well as for the satellites could be obtained. The plates were reduced by the method of plate constants using positions for the faint background stars determined from astrographic field plates. The random mean error of these observations was estimated to be not greater than ±0″.10.The main result of the orbital adjustment is a +2° correction to the zero of mean longitude for Phobos. This confirms the findings of Wilkins (1970) and is compatible with the results of the Mariner 9 observations. The scale of the orbits of both satellites gave accordant values for the mass of Mars and the combined value of 30 99 500 ± 2800 (m.e.) is in good agreement with modern determinations.The mean error for Deimos derived from the residuals after solution is ±0″.11, which agrees well with the observational error and indicates no large systematic error in either the theory or the observations. For Phobos, however, the residual error, ±0″.19, is twice the expected observational error. The implications of this discrepancy are discussed.     

Photometric modelling of the Martian dust rings

There are reasons to expect that Mars is surrounded by a region of dust, similar to rings, originating from the bombardment of Phobos and Deimos by meteroids. Using a simple radiative transfer model, we have investigated the angular distribution and the absolute values of the solar radiance scattered by such a dust region, to the purpose of assessing the possibilities and limitations of future photometric searches after the circummartian dust. Our model values of the number density of the dust grains in the space around Mars and of their size distribution have been derived from the results obtained by other authors. The single-scattering albedo of the dust grains has been deduced from the reflectance spectra of Phobos, taken by the spacecraft Phobos 2. Calculations, carried out for a few phenomenological phase functions, have shown that in the visible the radiance scattered by the rings is well within the detectability range of a modern sensible photometer, so that the prospectives for photometric search for the Martian dust rings are optimistic. Furthermore, our results confirm that the dust region could not be observed by the Viking cameras and this supports o our assumptions regarding the optical properties of the circummartian grains.     

Simultaneous measurements of Martian plasma boundaries by Rosetta and Mars Express

We present the first two-spacecraft near-simultaneous observations of the Martian bow shock (BS), magnetic pileup boundary (MPB) and photo-electron boundary (PEB) obtained by the plasma instruments onboard Rosetta and Mars Express during the Rosetta Mars flyby on February 25, 2007. Our observations are compared with shape models for the BS and MPB derived from previous statistical studies. The MPB is found at its expected position but the BS for this event is found significantly closer to the planet than expected for the rather slow and moderately dense solar wind. Cross-calibration of the density measurements on the two spacecraft gives a density profile through the magnetosheath, indicating an increasing solar wind flux during the Rosetta passage which is consistent with the multiple BS crossings at the Rosetta exit.     

The equilibrium figures of Phobos and other small bodies

The shape of a close planetary satellite is distorted from a self-gravitating sphere into a triaxial ellipsoid maintained by tidal and centrifugal forces. Using the family of Roche ellipsoids calculated by Chandrasekhar, it should be possible in some cases to determine the density of an inner satellite by an accurate measurement of its shape alone. The equilibrium figure of Phobos is expected to be the most extreme of any satellite. The shape of Phobos as observed by Mariner 9 approaches but appears not to be a Roche ellipsoid, although the uncertainties of measurement remain too large to exclude the possibility. In any case, Phobos is so small that even the low mechanical strength of an impact-compressed regolith is sufficient to maintain substantial departures from the equipotential figure. If larger close satellites, particularly Amalthea, are found to be Roche ellipsoids, their densities can be estimated immediately from the data presented.Asteroids of size comparable to Phobos and Deimos appear to have more irregular shapes than the Martian satellites. This may reflect the absence of a deep regolith on those asteroids due to the low effective escape velocity for impact ejecta. For Phobos and Deimos, on the other hand, ejecta will tend to remain in orbit about Mars until swept up again by the satellite, contributing to a deeper equilibrium layer of debris.     

Astrometric studies of the results of a new reduction of old photographic observations of the Saturnian System based on the comparison with the modern theories of satellite motion

The paper shows the possibility of increasing the accuracy of the results of photographic observations of Saturn and its moons made in the 1970s and reduced using the old reference star catalogues and semiautomatic measurements. New celestial coordinates of the moons (from the third to the eighth), “satellite minus satellite” relative moon coordinates, and Saturn coordinates by positions of satellites are obtained without measuring its images. The results are stored in the Pulkovo Observatory database on the Solar System bodies and are available online at www.puldb.ru. The efficiency of the reduction method based on digitizing of astronegatives using 21 Mpx Canon digital camera and IZMCCD software is shown. The comparison of new results of old observations with the latest theories of moon motion has revealed a significant increase in satellite positioning accuracy. The investigation of the differences (O–C) of celestial coordinates from satellite positions in their apparent Saturn-centric orbits has revealed a noticeable motion of the differences (O–C) in right ascension depending on their distances from Saturn for all moons.     

The magnetic field near Mars: A comparison between a hybrid model, Mars Global Surveyor and Mars Express observations

Mars Express (MEX) does not carry its own magnetometer which complicates interpretation of ASPERA-3/MEX ion measurements. The direction of the interplanetary magnetic field (IMF) is especially important because it, among other things, determines the direction of the convective electric field and orientation of the cross tail current sheet and tail lobes. In this paper we present a case study to show the properties of the magnetic field near Mars in a quasi-neutral hybrid (QNH) model at the orbits where the Mars Global Surveyor (MGS) has made measurements, present a method to derive the IMF clock angle by comparing fields in a hybrid model and the direction of the magnetic field measured by MGS by deriving the IMF clock angle. We also use H⁺ ring velocity distribution observations upstream of the bow shock measured by the IMA/ASPERA-3 instrument on board MEX spacecraft. These observations are used to indirectly provide the orientation of the IMF. We use a QNH model (HYB-Mars) where ions are modeled as particles while electrons form a mass-less charge neutralizing fluid. We found that the direct MGS and non-direct IMA observations of the orientation magnetic field vectors in non-crustal magnetic field regions are consistent with the global magnetic field draping pattern predicted by the global model.     

Mars geodesy,rotation and gravity

This review provides explanations of how geodesy, rotation and gravity can be addressed using radioscience data of an orbiter around a planet or of the lander on its surface.The planet Mars is the center of the discussion.The information one can get from orbitography and radioscience in general concerns the global static gravitational field, the time variation of the gravitational field induced by mass exchange between the atmosphere and the ice caps, the time variation of the gravitational field induced by the tides, the secular changes in the spacecraft's orbit induced by the little moons of Mars named Phobos and Deimos, the gravity induced by particular targets, the Martian ephemerides, and Mars' rotation and orientation.The paper addresses as well the determination of the geophysical parameters of Mars and, in particular, the state of Mars' core and its size, which is important for understanding the planet's evolution.Indeed, the state and dimension of the core determined from the moment of inertia and nutation depend in turn on the percentage of light elements in the core as well as on the core temperature, which is related to heat transport in the mantle.For example, the radius of the core has implications for possible mantle convection scenarios and, in particular, for the presence of a perovskite phase transition at the bottom of the mantle.This is also important for our understanding of the large volcanic province Tharsis on the surface of Mars.     

Asymmetry of plasma fluxes at Mars. ASPERA-3 observations and hybrid simulations

The asymmetry of fluxes of solar wind and planetary ions is studied by using the ASPERA-3 observations onboard the Mars Express spacecraft in February 2004 to March 2006. Due to the small scale of the Martian magnetosphere and its induced origin, the flow pattern near Mars is sensitive to the directions of the interplanetary magnetic and electric (-V×B) fields. Asymmetry of the magnetic field draping produces an asymmetry in plasma flows in the plane containing the IMF. The crustal magnetic fields on Mars also influence the flow pattern. Scavenging of planetary ions is less efficient in the regions of strong crustal magnetization and therefore the escape fluxes of planetary ions in the southern hemisphere are smaller. The results of the observations are compared to simulations based on a 3D hybrid model with several ion species.