Evolution of the water regime of Phobos
Abstract
An improved model of the evolution of the water regime of Phobos is presented. The central feature of this model is a time-dependent solar insolation that is influenced both by the increasing solar power output over geologic time and by the obliquity and eccentricity cycles of Mars and Phobos which vary over time scales of 105 to 106 years. A one-dimensional model is used to calculate temperatures, water fluxes, and ice depths over geologic time. Results are obtained at various latitudes for two assumed cases of porosity and pore size and for three putative values of the mass fraction of water initially allocated to Phobos. Results are obtained for one model that assumes a homogeneous distribution of water ice and for a second model that assumes that water ice is driven toward the surface by the internal thermal gradient near the poles. Results for the (more likely) surface-concentrated model indicate that ice may be found from 270 to 740 m at the equator and from 20 to 60 m at 80° lat depending on porosity and pore size, and subject to limits implied by assumptions of the initial mass fraction of free ice. A two-dimensional model is used to compute temperatures, heat and vapor fluxes, and ice removal/deposition rates for a two-dimensional grid over one obliquity cycle assuming that ice is distributed uniformly throughout Phobos. The heat and vapor fluxes and the ice removal/deposition rates are integrated over time to obtain the water loss at the surface, the lateral heat and vapor transport toward the poles, and the change in ice concentration at interior points. The results show that a relatively large amount of vapor is produced within 1 km of the surface, mostly at lower latitudes. Some of the vapor is transported to the surface where it is lost, and the remainder is transported to greater depths, where it is condensed at much lower rates over a much larger volume. For the large pore size/porosity case, we estimate that the current H2O loss rate is ∼3 g/sec. The possible effects of the radiative contributions from Mars, the shadowing by Mars, and the use of an ellipsoidal shape for Phobos are discussed.
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Cited by (29)
Phobos, one of the Martian moonlets, has been the focus of long-standing scientific investigation, particularly regarding its origin: whether it was formed by a giant impact or a captured asteroid. In this study, we investigate the long-term internal evolution of Phobos over 4.3 billion years using a 3-dimensional thermal diffusion and water transport model, named ASTRA. The model newly implements reflected visible light and infrared radiation from Mars, orbital evolution of Phobos, water vapor adsorption on rock grain surfaces, and suppressed permeability due to the surface dust layer in addition to the previous study, allowing us to simulate different grain sizes of 1, 10, 100, and 1000 μm, adsorption coefficients of 1 and 10 kg m−3, and initial water contents of 0.1, 1 or 10 wt%.
Our simulation results revealed that without the water vapor adsorption effect, the water inside Phobos would be lost over several billion years for an initial water content of 0.1 wt%. However, when water vapor adsorption was considered, scenarios emerge in which Phobos could retain water to the present day. In the case of a grain size of around 100 μm, Phobos could still continuously release water flux of 10−4–10−3 g s−1 for an initial water content of 0.1 wt%, and 10−2–1 g s−1 for an initial water content of >1 wt%. Furthermore, our research shows the possibility of subsurface condensed water ice in deep high latitude regions and the formation of a gas torus by escaping water-related molecules. If the future MMX mission can measure the water-related ions near the gas torus of Phobos with much >104 cm−2 s−1, the origin of Phobos is most likely the captured asteroid with an initial water content of >1%. For further detailed analysis, our results emphasize the importance of exploring surface soil parameters through soil sample return by the MMX mission.
Signature of Phobos’ interior structure in its gravity field and libration
2019, IcarusCitation Excerpt :The latter is considered by the authors to be more likely than the former. For both scenarios, Fanale and Salvail (1990) predict that, million years after Phobos’ formation, one should find the ice buried under Phobos’ regolith, deeper at the equator (at the kilometer level) than at the pole (a the tens meter level). The water at the surface would have escaped due to solar insolation and replaced by interstitial void in the surface layer (increasing the porosity just below the surface).
The interior of the Martian moon Phobos has not been precisely determined yet, in spite of space missions sent at close distance to the body. The current measurements (imagery, astrometric, etc.) lead to controversial conclusions about the level of heterogeneity inside Phobos. Yet, the inside mass distribution is a signature of the conditions prevailing at its formation as well as of its evolution. Here, we study possible heterogenous mass distributions based on internal models built from surface observables and available bulk density and shape measurements. We identify four different families of mass distribution involving rocky material, (macro-)porosity and ice. Mass heterogeneities correspond to either an excess of porosity or a compaction of material under Stickney crater, or a deficit of porosity in the upper layers of Phobos, or a concentration of ice either in depth inside Phobos or in shallow layers. We then discretize the shape of Phobos using 500m-length cubes to fit its volume and total mass. We compute the possible distribution of these cubes for each family of internal mass heterogeneity model. We deduce the possible values of the principal moments of inertia as well as of the geodetic observables such as the libration amplitude and the gravity field anomalies (up to degree and order 10) associated with these models. A comparison of these computed observables between the different heterogeneity models and with their expected homogenous mass distribution values allow us to quantify the possible heterogeneity degree within Phobos. The computed heterogeneity observables can depart by tens of percent from the homogenous values. The most striking departures are from the interior model with mass excess (less porosity) or deficit (more porosity) beneath Stickney crater. In turn, measurements of libration amplitude and low degree coefficients of the gravity field at a precision better than 5% can allow to identify such kinds of heterogeneities mainly located beneath Stickney. Mass excess or deficit can therefore be also distinguished, which is of importance to identify whether Phobos was already porous or monolithic before the formation of Stickney. The current measurements of libration amplitude and degree-two gravity coefficient are quite uncertain, but seem to reject models with higher porosity under Stickney, favoring a pre-impact porous body. The icy models depart less from the homogenous signatures, hence requiring more precise measurements of the geodetic observables and of the shape of Phobos. The improvement of these measurements by the Mars Moon Explorer mission for instance could thus allow for better constraining our model of Phobos’ interior and bring further constraints for its formation.
Phobos MRO/CRISM visible and near-infrared (0.5–2.5 μm) spectral modeling
2018, Planetary and Space ScienceThis paper focuses on the spectral modeling of the surface of Phobos in the wavelength range between 0.5 and 2.5 μm. We exploit the Phobos Mars Reconnaissance Orbiter/Compact Reconnaissance Imaging Spectrometer for Mars (MRO/CRISM) dataset and extend the study area presented by Fraeman et al. (2012) including spectra from nearly the entire surface observed. Without a priori selection of surface locations we use the unsupervised K-means partitioning algorithm developed by Marzo et al. (2006) to investigate the spectral variability across Phobos surface. The statistical partitioning identifies seven clusters. We investigate the compositional information contained within the average spectra of four clusters using the radiative transfer model of Shkuratov et al. (1999). We use optical constants of Tagish Lake meteorite (TL), from Roush (2003), and pyroxene glass (PM80), from Jaeger et al. (1994) and Dorschner et al. (1995), as previously suggested by Pajola et al. (2013) as inputs for the calculations. The model results show good agreement in slope when compared to the averages of the CRISM spectral clusters. In particular, the best fitting model of the cluster with the steepest spectral slope yields relative abundances that are equal to those of Pajola et al. (2013), i.e. 20% PM80 and 80% TL, but grain sizes that are 12 μm smaller for PM80 and 4 μm smaller for TL (the grain sizes are 11 μm for PM80 and 20 μm for TL in Pajola et al. (2013), respectively). This modest discrepancy may arise from the fact that the areas observed by CRISM and those analyzed in Pajola et al. (2013) are on opposite locations on Phobos and are characterized by different morphological and weathering settings. Instead, as the clusters spectral slopes decrease, the best fits obtained show trends related to both relative abundance and grain size that is not observed for the cluster with the steepest spectral slope. With a decrease in slope there is general increase of relative percentage of PM80 from 12% to 18% and the associated decrease of TL from 88% to 82%. Simultaneously the PM80 grain sizes decrease from 9 to 5 μm and TL grain sizes increase from 13 to 16 μm. The best fitting models show relative abundances and grain sizes that partially overlap. This supports the hypothesis that from a compositional perspective the transition between the highest and lowest slopes on Phobos is subtle, and it is characterized by a smooth change of relative abundances and grain sizes, instead of a distinct dichotomy between the areas.
Phobos parallel grooves were first observed on Viking images 38 years ago and since then they have been greatly debated leading to several formation hypotheses. Nevertheless, none of them have been favoured and widely accepted. In this work, we provide a different approach, assuming that Phobos grooves can be the expression of fracture planes, and deriving their spatial distribution and orientation on 3D reconstructions, we point out that any origin related only to craters at Phobos surface should be ruled out, since the majority of the grooves is unrelated to any craters now present at its surface. This raises the intriguing possibility that such grooves, if expression of fracture planes, are remnant features of an ancient parent body from which Phobos could have originated. Such scenario has never been considered for Phobos, though this origin was already proposed for the formation of 433 Eros grooves (Buczkowski, D.L., Barnouin-Jha, O.S., Prockter, L.M. [2008]. Icarus 193, 39). If this idea holds true, the observed groove distribution could be explained as the result of possible major impacts on the larger parent body, which were inherited by the “Phobos shard”.
The surface geology and geomorphology of Phobos
2014, Planetary and Space ScienceCitation Excerpt :This, if true, could indicate the presence of water ice in the subsurface layers of Phobos. Theoretical models suggest that if Phobos contained water ice early in its history, it is possible that this ice-containing material might still remain at depths 270–740 m at the equator, and at tens of meters at higher latitudes (Fanale and Salvail, 1990). However, the observed intensive impact reworking of the Phobos surface leaves little possibility for the occurrence of ice close to the present surface.
The martian moon Phobos is 26 km×22.8 km×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: >0.1 and >20°:9 craters; 0.05–0.1 and 10–20°:7 craters; and <0.05 and <10°: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 <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.
Are Phobos and Deimos the result of a giant impact?
2011, IcarusCitation Excerpt :This would suggest that these objects have a significant amount of pore space. However, this pore space is the direct result of loose aggregation of material, which may have been aided in part by subsequent impact brecciation (Murchie et al., 1991; Fanale and Salvail, 1989, 1990). Such an origin agrees with the libration of Phobos, which indicates that this satellite has a uniform density with depth (Duxbury, 1989).
Despite many efforts an adequate theory describing the origin of Phobos and Deimos has not been realized. In recent years a number of separate observations suggest the possibility that the martian satellites may have been the result of giant impact. Similar to the Earth–Moon system, Mars has too much angular momentum. A planetesimal with 0.02 Mars masses must have collided with that planet early in its history in order for Mars to spin at its current rate (Dones, L., Tremaine, S. [1993]. Science 259, 350–354). Although subject to considerable error, current crater-scaling laws and an analysis of the largest known impact basins on the martian surface suggest that this planetesimal could have formed either the proposed 10,600 by 8500-km-diameter Borealis basin, the 4970-km-diameter Elysium basin, the 4500-km-diameter Daedalia basin or, alternatively, some other basin that is no longer identifiable. It is also probable that this object impacted Mars at a velocity great enough to vaporize rock (>7 km/s), which is necessary to place large amounts of material into orbit. If material vaporized from the collision with the Mars-spinning planetesimal were placed into orbit, an accretion disk would have resulted. It is possible that as material condensed and dissipated beyond the Roche limit forming small, low-mass satellites due to gravity instabilities within the disk. Once the accretion disk dissipated, tidal forces and libration would have pulled these satellites back down toward the martian surface. In this scenario, Phobos and Deimos would have been among the first two satellites to form, and Deimos the only satellite formed—and preserved—beyond synchronous rotation. The low mass of Phobos and Deimos is explained by the possibility that they are composed of loosely aggregated material from the accretion disk, which also implies that they do not contain any volatile elements. Their orbital eccentricity and inclination, which are the most difficult parameters to explain easily with the various capture scenarios, are the natural result of accretion from a circum-planetary disk.