Reconceiling the orbital and physical properties of the martian moons

Reconceiling the orbital and physical properties of the martian moons

Authors:

Ronnet et al

Abstract:

The origin of Phobos and Deimos is still an open question. Currently, none of the three proposed scenarios for their origin (intact capture of two distinct outer solar system small bodies, co-accretion with Mars, and accretion within an impact-generated disk) is able to reconcile their orbital and physical properties. Here, we investigate the expected mineralogical composition and size of the grains from which the moons once accreted assuming they formed within an impact-generated accretion disk. A comparison of our results with the present day spectral properties of the moons allows us to conclude that their building blocks cannot originate from a magma phase, thus preventing their formation in the innermost part of the disk. Instead, gas-to-solid condensation of the building blocks in the outer part of an extended gaseous disk is found as a possible formation mechanism as it does allow reproducing both the spectral and physical properties of the moons. Such a scenario may finally reconcile their orbital and physical properties alleviating the need to invoke an unlikely capture scenario to explain their physical properties.

Did the Martian Moons Form in a Theia-like Impact?

Scientists believe that Phobos, one of Mars' two moons, is destined to be torn to shreds and form a ring around the Red Planet, and if so, it might be a fitting callback to how it got there in the first place. The mystery of the origins of the moons may have been solved by a new study that gives weight to the theory a huge collision between Mars and an ancient protoplanet resulted in the creation of Phobos and Deimos, as well as several other now-missing moons.

Two main hypotheses are debated regarding the birth of the Martian moons. One suggests that Phobos and Deimos were originally asteroids that became trapped in orbit about Mars. But a study, conducted by researchers in France, Belgium and Japan, suggests that the shape and orientation of their orbits, as well as the fine, grainy composition of the moons, makes that scenario unlikely.

There's a second, more explosive theory that Mars collided with a protoplanet about a third of its size between 4 and 4.5 billion years ago. The debris thrown up by that colossal impact formed a disk around the planet, which, over the course of millions of years, led to the formation of Phobos, Deimos, and other moons which no longer exist.

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Could Phobos' Destruction Give Mars Saturn-like Rings?

Mars may one day have rings similar to Saturn's famous halo, new research suggests.

In a few tens of millions of years, the Red Planet may completely crush its innermost moon, Phobos, and form a ring of rocky debris, according to the new work. Phobos is moving closer to Mars every year, meaning the planet's gravitational pull on the satellite is increasing. Some scientists have theorized that Phobos will eventually collide with Mars, but the new research suggests that the small moon may not last that long.

"The main factor affecting whether Phobos will crash into Mars or break apart is its strength," Tushar Mittal, a graduate student at the University of California, Berkeley and one of the authors of the new research paper, told Space.com by email. "If Phobos is too weak to withstand increasing tidal stresses, then we expect it to break apart."

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Phobos is Already Dying

The long, shallow grooves lining the surface of Phobos are likely early signs of the structural failure that will ultimately destroy this moon of Mars.

Orbiting a mere 3,700 miles (6,000 kilometers) above the surface of Mars, Phobos is closer to its planet than any other moon in the solar system. Mars' gravity is drawing in Phobos, the larger of its two moons, by about 6.6 feet (2 meters) every hundred years. Scientists expect the moon to be pulled apart in 30 to 50 million years.

"We think that Phobos has already started to fail, and the first sign of this failure is the production of these grooves," said Terry Hurford of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

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Phobos & Deimos Could Have Formed From Theia-like Impact (Mars' Iron Theia???)

Formation of Phobos and Deimos via a giant impact

Authors:

Citron et al

Abstract:

Although the two moons of Mars, Phobos and Deimos, have long been thought to be captured asteroids, recent observations of their compositions and orbits suggest that they may have formed from debris generated by one or more giant impacts of bodies with ∼0.01× target mass. Recent studies have both analytically estimated debris produced by giant impacts on Mars and numerically examined the evolution of circum-Mars debris disks. We perform a numerical study (Smoothed Particle Hydrodynamics simulation) of debris retention from giant impacts onto Mars, particularly in relation to a Borealis-scale giant impact (E∼3×1029E∼3×1029 J) capable of producing the Borealis basin. We find that a Borealis-scale impact is capable of producing a disk of mass ∼5×10205×1020 kg (∼1–4% of the impactor mass), sufficient debris to form at least one of the martian moons according to recent numerical studies of martian debris disk evolution. While a Borealis-scale impact may generate sufficient debris to form both Phobos and Deimos, further studies of the debris disk evolution are necessary. Our results can serve as inputs for future studies of martian debris disk evolution.

The Orbits of the Martian Moons

Martian satellite orbits and ephemerides

Authors:

Jacobson et al

Abstract:

We discuss the general characteristics of the orbits of the Martian satellites, Phobos and Deimos. We provide a concise review of the various descriptions of the orbits by both analytical theories and direct numerical integrations of their equations of motion. After summarizing the observational data used to determine the orbits, we discuss the results of our latest orbits obtained from a least squares fit to the data.

Phobos and Mars Courtesy of Mars Express

Martian Moons' Cartography

Phobos and Deimos cartography

Authors:

Wählisch et al

Abstract:

This paper presents an overview of developments in the cartography of the Martian moons Phobos and Deimos from the early satellite images to recent maps based on data from past and on-going Mars missions, especially Viking and Mars Express. We describe in detail the definition and use of special projections for these irregular-shaped bodies. New controlled Phobos mosaics and a topographic atlas in a scale of 1: 50,000 derived from images of the High Resolution Stereo Camera/Super Resolution Channel (HRSC/SRC) camera on Mars Express are presented.

Is Phobos' Giant Crater Stickney 4.2 Billion Years old?

The age of Phobos and its largest crater, Stickney

Authors:

Schmedemann et al

Abstract:

We derived crater production functions and chronology functions of Phobos for two scenarios, which likely represent the end-members of its dynamical evolution. Case A assumes that Phobos has been in its current orbit about Mars since its formation. Case B assumes a recent capture of Phobos and the impact history of an average Main Belt Asteroid. We determined the age of an average surface to the west of the Stickney crater and of the interior of the Stickney crater. The results indicate i) the formation or major collision of Phobos about 4.3 Ga (Case A) or 3.5 Ga (Case B) ago, ii) the Stickney crater is about 4.2 Ga (Case A) or 2.6 Ga (Case B) old and iii) grooves probably formed between 3.1–3.8 Ga (Case A) or 44–340 Ma (Case B). Thus, Stickney seems to be older than the investigated grooves on Phobos.

Phobos Surface Geology and Geomorphology

The surface geology and geomorphology of Phobos

Authors:

Basilevsky et al

Abstract:

The martian moon Phobos is 26 × 22.8 × 18.2 km in size, and the major landforms on its surface are craters and grooves. We analyzed the visible craters on the surface of Phobos where ~1300 craters ≥200 m in diameter, ~70 craters ≥1 km, and ~30 craters ≥2 km are identified; Stickney, the largest crater on Phobos, is about 8 km in diameter. Most craters are undoubtedly of impact origin although some small craters may be pits formed by drainage of regolith into subsurface fractures. The presence of the observed impact crater population implies that the upper hundreds of meters to a few kilometers of Phobos are heavily fractured. Using the available digital terrain model of Phobos (the dynamic version), the 24 craters larger than 2 km in diameter have been subdivided into three morphologic classes on the basis of their prominence; they are characterized by the following values of d/D ratios and maximum steepness of their inner slopes: greater than 0.1 and greater than 20o, 9 craters; 0.05–0.1 and 10–20o, 7 craters; and less than 0.05 and less than 10o, 8 craters. This subpopulation of Phobos craters has a considerably larger number of craters with shallowly sloping walls compared to lunar highland craters; this may be due to several factors including the very small surface gravity of Phobos.

Most craters on Phobos are bowl-shaped, some with a complex morphology in their interiors, including concentric, flat-bottomed and with central-mounds. The size of these craters with complex morphology is indicative of layering in the target material, both regolith covering bedrock and layers within the regolith. The thickness of the regolith estimated by different techniques varies from ~5 to 100 m. Layering within the regolith does not appear to be continuous, but more lens-like. The regolith of Phobos obviously accumulated by direct crater ejecta deposition and through the return of the ejecta high-velocity fraction that escaped to near-Mars space during the impact events. The Phobos regolith may be deficient in the less than 300 μm size fraction and contain martian material with concentrations ~250 ppm in the upper 0.5 m, and 1–2 orders of magnitude lower at greater depth. Downslope movement of material is revealed by downslope-trending albedo streaks and mounds on the floors and slopes of craters hundreds of meters to kilometers in size, commonly on crater inner slopes and sometimes on the outer slopes of crater rims. The albedo streaks are probably traces of geologically recent talus and avalanche emplacement. The mounds are interpreted to be landslide deposits. The different degrees of mound morphologic sharpness may be considered as an indication of their different age.

Through the geologic analysis of the MRO HiRISE color images of Stickney crater and its vicinity, we documented the distribution and mutual relations of red and blue units of the surface material of Phobos. We conclude that the red and blue “primary” materials may form relatively large blocks comprising the interior of Phobos. Crater ejecta and downslope movement of material redeposit these materials, forming secondary and tertiary derivatives of these color material units and their mixtures.

The grooves on Phobos are typically 100–200 m wide and several kilometers long and can be mapped in several intersecting systems (families) with approximately the same groove orientations within each family. They often crisscross relatively large craters, including crater rims, showing continuity with no gaps. Groove systems often intersect each other showing no lateral offsets at the intersections. At least one of groove families extends along a longitude for about 130o and this should have implications for groove formation mechanisms. Grooves similar to those on Phobos are seen on other small bodies: Eros, Lutetia and Vesta. Three different mechanisms of formation of Phobos grooves are discussed: 1) grooves as fractures/faults, 2) grooves as tracks of rolling and bouncing boulders, and 3) grooves as chains of craters formed by ejecta from impact craters on Mars. The mechanism(s) of groove formation require additional studies.

We conclude that the surface of Phobos is an arena for a variety of geologic processes. The leading role belongs to impact cratering with associated target destruction, material ejection from the crater and often from Phobos, and subsequent deposition partly with temporary residence in near-martian space. Shaking by impacts and surface stirring by day-night temperature changes cause granular surface material to move down along-slope driven by very low, but nevertheless efficient, surface gravity. A sample return mission is crucially important for a better understanding of the geological processes operating on Phobos. In addition to Phobos material, a returned sample will probably contain pieces of material from Mars. A series of outstanding questions to guide future exploration is listed.

Is Phobos' Interior Homogenous?

The Phobos Geodetic Control Point Network and Rotation Model

Authors:

Oberst et al

Abstract:

A new global control point network was derived for Phobos, based on SRC (Mars Express), Phobos-2, and Viking Orbiter image data. We derive 3-D Cartesian coordinates for 813 control points as well as improved pointing data for 202 SRC and Viking images in the Phobos-fixed coordinate system. The point accuracies vary from 4.5 m on the Phobos nearside, to up to 67.0 m on the farside, where we rely on Viking images (average point accuracy: 13.7 m). From tracking of the control points we detect a librational motion synchronous to the Phobos orbital period and measure a libration amplitude of 1.09°, in agreement with predictions from shape information assuming a uniform interior. This suggests that the interior of Phobos is homogeneous – but small local mass anomalies, e.g., associated with crater Stickney, cannot be ruled out. Our new control point network has a higher number of data points and a higher point accuracy than previous data products and will be an important basis for accurate shape models and maps.

The Origin of Phobos' Grooves

Character and origin of Phobos’ grooves

Authors:

Murray et al

Abstract:

Phobos’ parallel grooves, which are such a striking feature of its surface, have attracted interest since their discovery on Viking images in 1976 ( Veverka and Duxbury, 1977), but their origin is still in dispute today. The great increase in knowledge of Phobos’ surface features effected by images from the High Resolution Stereo Camera (HRSC) onboard the E.S.A. Mars Express spacecraft has clearly demonstrated that only one hypothesis can seriously be upheld: that the grooves are chains of secondary impacts resulting from primary impact events on Mars ( Murray and Iliffe, 2011). But even this hypothesis has recently been questioned from a ballistic standpoint ( Ramsley and Head, 2013) mainly using estimated data for the grooves themselves. In the present paper we present summaries of extensive new measurements of groove sizes, pit separations, groove family parameters and geographical distribution. We also re-examine their unique characteristics, and present a review of past ideas. But we concentrate on extending and refining the work of Ramsley & Head using the new measurements, and the much-improved geodetic data from Mars Express (Willner et al., this issue). We find that the total mass of Mars ejecta required to form all the observed grooves on Phobos is between 2.0 × 109 and 2.7 × 1010 kg, and that the total mass of impact ejecta from all Mars craters between 19 and 384 km diameter likely to hit Phobos (in its present orbit) at sufficient velocity to form all the grooves is one or two orders of magnitude greater than this. The larger available ejecta mass from Mars is due to several factors, including the fact that Phobos is known to have orbited Mars at a greater distance at the time when the grooves were formed. A new rigorous celestial mechanical analysis of the transfer of ejecta from Mars to Phobos is presented, including N-body calculations that model not only the gravitational interaction of Mars, but also of Phobos and up to several hundred individual ejecta particles. This allows us to model the possible source areas on Mars of all observed groove families, and to conclude that more than one groove family may have been formed by different batches of ejecta from the same Mars impact event, and that the total number of Mars impact events responsible for creating the grooves may have been less than 10. The N-body calculations also allow us to model the complex gravitational interactions between Mars ejecta particles, and thus to assess the contrast in groove morphometry between those families centred on the sub-Mars and anti-Mars hemispheres.

Composition of Phobos and Deimos Remains Uncertain

Composition of Surface Materials on the Moons of Mars

Authors:

Pieters et al

Abstract:

The two small asteroid-like bodies orbiting Mars, Phobos and Deimos, are low albedo and exhibit similar visible to near-infrared spectra. Determining the origin of these moons is closely tied to determining their composition. From available spectroscopic data Phobos exhibits two distinct types of materials across its surface, and data from both Mars Express and Mars Reconnaissance Orbiter have provided additional details about the properties of these materials and their spatial relation to one another. Although no prominent diagnostic absorptions have been detected, systematic weak features are seen in some data. An extensive regolith is observed to have developed on both moons with characteristics that may be unique due to their special environment in Mars orbit. Understanding the character and evolution of the regolith of Phobos and Deimos is central to interpreting the moons’ physical and optical properties. The cumulative data available for compositional analyses across the surface of Phobos and Deimos, however, remain incomplete in scope and character and ambiguous in interpretation. Consequently the composition of the moons of Mars remains uncertain.

Phobos may not be a Captured Asteroid

Phobos: Observed bulk properties

Authors:

Pätzold et al

Abstract:

This work is a review of the mass determinations of the Mars moon Phobos by spacecraft close flybys, by solving for the Martian gravity field and by the analysis of secular orbit perturbations. The absolute value and accuracy is sensitive on the knowledge and accuracy of the Phobos ephemeris, of the spacecraft orbit, other perturbing forces acting on the spacecraft and the resolution of the Martian gravity field besides the measurement accuracy of the radio tracking data. The mass value and its error improved from spacecraft mission to mission or from the modern analysis of “old” tracking data but these solutions depend on the accuracy of the ephemeris at the time of observation. The mass value seems to settle within the range of GMPh=(7.11 +/−0.09)∙10−4 km3s-2 which covers almost all mass values from close flybys and “distant” encounters within its 3-σ error (1.5%). Using the volume value determined from MEX HRSC imaging, the bulk density is (1873 +/−31) kg/m3 (3-σ error or 1.7%), a low value which suggests that Phobos is either highly porous, is composed partially of light material or both. The determination of the gravity coefficients C20 and C22 from the Mars Express 2010 close flybydoes not allow to draw conclusion on the internal structure. The large errors do not distinguish whether Phobos is homogeneous or not. In view of theories of the Phobos' origin, one possibility is that Phobos is not a captured asteroid but accreted from a debris disk in Mars orbit as a second generation solar system object.

A Geo Information System for Phobos

The Phobos information system

Authors:

Karachevtseva et al

Abstract:

We have developed a Geo-information system (GIS) for Phobos, based on data from the Mars Express and Viking Orbiter missions, which includes orthoimages, global maps, terrain- and gravity field models, all referenced to the Phobos coordinate system. The data are conveniently stored in the ArcGIS software system, which provides an environment for mapping and which allows us to carry out joint data analysis and miscellaneous data cross-comparisons. We have compiled catalogs of Phobos craters using manual and automated techniques, which includes about 5500 and 6400 craters correspondingly. While crater numbers are biased by available image data resolution and illumination, we estimate that our catalog of manually detected craters contains all Phobos craters with diameters D>250 m which is a total of 1072 and catalog of automated detected craters are complete for craters D>400 m (360 craters). Statistical analysis of these large craters reveals a surplus of craters on the anti-Mars hemisphere, whereas differences in crater abundance between leading and trailing hemisphere cannot be confirmed. This in contrast to previous papers, where no such asymmetry was found (Schemdemann et al., 2013). But we cannot rule out remaining biases due to resolution, viewing angles or illumination effects. Using digital terrain model (DTM) derived from photogrammetry image processing we estimate depths of 25 craters larger than 2 km. We also have compiled catalogs of lineaments, and boulders. In particular, we mapped 546 individual grooves or crater chains, which extend in length from 0.3–16.2 km. We identified and determined the sizes and locations of 1379 boulders near crater Stickney. Cross-comparisons of gravity field models against distribution patterns of grooves and boulders are currently under way and may shed light on their possible origins. Finally, we have developed a Geo-portal, which allows the science community to conveniently search for, analyze, and download data of interest from our system. Additionally we provide access to color electronic maps (e-maps) with support for layers based on Phobos geodatabase and ArcGIS tools.

Dust, the Martian Moons & Circummartian Space

Dust at the Martian moons and in the circummartian space

Authors:

Zakharov et al

Abstract:

The paper provides the current understanding of the dust particle dynamics near the surface and in the circummatrian space of the Martian moons based on existing models developed for airless and non-magnetized bodies. In particular we discuss the response of the regolith of the Martian moons to exposure to radiation, the dynamics of charged dust on their surfaces, their plasma environments, the models and indirect observations of their putative dust tori. It is concluded that there is a good theoretical understanding of the behavior of the dynamics of dust particles near the moons Phobos and Deimos. Current models predict dust rings near orbits of the Martian moons based on detailed estimates for the sources and sinks of the dust particles as well as their lifetimes. However, there is no compelling observational evidence for the predicted dust torus around Phobos or Deimos orbits, and there are no observations yet of dust dynamics near their surfaces. Naturally, in order to detect the motion of dust near the surfaces of these moons, and their dust tori we need measurements using a complementary set of sensitive instruments, including impart dust detectors, electric field sensors, and optical cameras in future missions to Mars and its moons.

Phobos' Shape

Phobos' shape and topography models

Authors:

Wilner et al

Abstract:

The global shape and the dynamic environment are fundamental properties of a body. Other properties such as volume, bulk density, and models for the dynamic environment can subsequently be computed based on such models. Stereo-photogrammetric methods were applied to derive a global digital terrain model (DTM) with 100 m/pixel resolution using High Resolution Stereo Camera images of the Mars Express mission and Viking Orbiter images. In a subsequent least-squares fit, coefficients of the spherical harmonic function to degree and order 45 are computed. The dynamic models for Phobos were derived from a polyhedron representation of the DTM. The DTM, spherical harmonic function model, and dynamic models, have been refined and represent Phobos' dynamic and geometric topography with much more detail when compared to Shi et al. (2012) and Willner et al. (2010) models, respectively. The volume of Phobos has been re-determined to be in the order of 5741 km3 with an uncertainty of only 0.6% of the total volume. This reduces the bulk density to 1.86±0.013 g/cm3 in comparison to previous results. Assuming a homogeneous mass distribution a forced libration amplitude for Phobos of 1.14° is computed that is in better agreement with observations by Willner et al. (2010) than previous estimates.

Mars Express' Investigations into the Martian Moons of Phobos and Deimos

Mars Express Investigations of Phobos and Deimos

Authors:

O. Witasse et al (too many again)

Affiliations:

no way.

Abstract:

The Mars Express mission was launched in June 2003 and was inserted into orbit around Mars in December 2003. Its main objective is to study the Mars' subsurface, surface, atmosphere and interaction with the solar wind. A secondary objective is to study the martian moons, in particular the largest one Phobos, thanks to a near polar and elliptical orbit which allows the spacecraft to perform close flybys about every five months. The Mars Express data not only consist of high-resolution 3D color images, but also astrometric images, spectra from 0.18 to 20 μm, radar echoes, Doppler signals from gravity experiments, and ion data. A new view of the moons has emerged from this data set, favoring now the idea that they are not captured asteroids, but rather the result of a re-accretion following a major impact on Mars. This unique set of data is available in the ESA Planetary Science Archive (PSA) and mirror imaged in the NASA Planetary Data System (PDS). This paper presents an overview of the Mars Express Phobos flybys, the specificities of their operations and the scientific achievements.

How Much...

Would people speculate that 1 kg of Phobos is worth returned to Earth?  Serious discussion here if possible.

French Make Agreement For Phobos-Grunt

French space agency CNES and the Russian Space Agency have come to an agreement that will allow CNES to receive soil samples from the Phobos-Grunt mission.

Due to lift off in October, the mission will return samples from the Martian moon Phobos, characterize the physical and chemical properties of the moon in-situ, and study ionization and solar wind effects in the Martian atmosphere.

The French agency is supplying several subsystems for the Phobos-Grunt gas analytic package, which was developed by the Moscow Institute for Space Research IKI. The subsystems are the gas chromatograph, designed by the Latmos lab for the French national science center CNRS, and the tunable diode laser spectrometer (TDLAS).

China is supplying four instruments on a Martian orbiter - Yinghuo-1 - that will be part of the mission, and the European Space Agency is providing support with its deep space radar network.

Let's hope the Galactic Ghoul doesn't eat this one!

PG up there definitely has a Russian/Soviet space probe style to it.