Scenarios for the formation of Chasma Boreale, Mars

The martian polar caps feature large chasmata and smaller trough systems which have no counterpart in terrestrial ice sheets. Chasma Boreale cuts about 500 km into the western part of the north-polar cap, is up to 100 km wide and up to 2 km deep. One possible formation mechanism is by a temporary heat source under the ice due to tectono-thermal or volcanic activity, which melts the ice from below. It is demonstrated by model simulations that this process is feasible, a moderately increased heat flux of 0.5-1 W m⁻², sustained over at least tens of thousands of years, producing a topographic depression which resembles the real chasma. Associated meltwater discharge rates are small (), but can exceed 10 km³ a⁻¹ if a stronger heat flux of 10 W m⁻² is assumed. Local ice-flow velocities during the process of chasma formation can exceed 1 m a⁻¹ at the head and scarps of the chasma. However, if the thermal anomaly shuts down, glacial flow quickly decreases, so that the chasma can stay open for an indefinite amount of time without an ongoing, sustaining process under the climate conditions of the most recent millions of years.     

Evolution of the north-polar cap of Mars: a modelling study

Celestial-mechanical computations show that, even stronger than for Earth, Mars is subject to Milankovi? cycles, that is, quasi-periodic variations of the orbital parameters obliquity, eccentricity and precession. Consequently, solar insolation varies on time-scales of 10⁴-10⁵ years. It has long been supposed that this entails climatic cycles like the terrestrial glacial-interglacial cycles. This hypothesis is supported by the light-dark layered deposits of the north- and south-polar caps indicating a strongly varying dust content of the ice due to varying climate conditions in the past. This study aims at simulating the dynamic and thermodynamic evolution of the north-polar cap (NPC) of Mars with the ice-sheet model SICOPOLIS. The boundary conditions of surface accumulation, ablation and temperature are derived directly from the solar-insolation history by applying the newly developed model MAIC. We consider steady-state scenarios under present climate conditions as well as transient scenarios over climatic cycles. It is found that the NPC is most likely not in steady state with the present climate. The topography of the NPC is mainly controlled by the history of the surface mass balance. Ice flow, which is of the order of , plays only a minor role. In order to build up the present cap during the last five million years of relatively low obliquities, a present accumulation rate of water equiv a⁻¹ is required. Computed basal temperatures are far below pressure melting for all simulations and all times.     

A global solution for the Mars static and seasonal gravity, Mars orientation, Phobos and Deimos masses, and Mars ephemeris

With the collection of six years of MGS tracking data and three years of Mars Odyssey tracking data, there has been a continual improvement in the JPL Mars gravity field determination. This includes the measurement of the seasonal changes in the gravity coefficients (e.g., , , , , , ) caused by the mass exchange between the polar ice caps and atmosphere. This paper describes the latest gravity field MGS95J to degree and order 95. The improvement comes from additional tracking data and the adoption of a more complete Mars orientation model with nutation, instead of the IAU 2000 model. Free wobble of the Mars' spin axis, i.e. polar motion, has been constrained to be less than 10 mas by looking at the temporal history of and . A strong annual signature is observed in , and this is a mixture of polar motion and ice mass redistribution. The Love number solution with a subset of Odyssey tracking data is consistent with the previous liquid outer core determination from MGS tracking data [Yoder et al., 2003. Science 300, 299-303], giving a combined solution of k₂=0.152±0.009 using MGS and Odyssey tracking data. The solutions for the masses of the Mars' moons show consistency between MGS, Odyssey, and Viking data sets; Phobos GM=(7.16±0.005)×¹⁰⁻⁴ km³/s² and Deimos GM=(0.98±0.07)×¹⁰⁻⁴ km³/s². Average MGS orbit errors, determined from differences in the overlaps of orbit solutions, have been reduced to 10-cm in the radial direction and 1.5 m along the spacecraft velocity and normal to the orbit plane. Hence, the ranging to the MGS and Odyssey spacecraft has resulted in position measurements of the Mars system center-of-mass relative to the Earth to an accuracy of one meter, greatly reducing the Mars ephemeris errors by several orders of magnitude, and providing mass estimates for Asteroids 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta, and 324 Bamberga.     

Constraints on the composition of the martian south polar cap from gravity and topography

The polar caps of Mars have long been acknowledged to be composed of unknown proportions of water ice, solid CO₂ (dry ice), and dust. Gravity and topography data are here analyzed over the southern cap to place constraints on its density, and hence composition. Using a localized spectral analysis combined with a lithospheric flexure model of ice cap loading, the best fit density of the volatile-rich south polar layered deposits is found to be 1271 kg m⁻³ with 1-σ limits of 1166 and 1391 kg m⁻³. The best fit elastic thickness of this geologically young deposit is 140 km, though any value greater than 102 km can fit the observations. The best fit density implies that about 55% dry ice by volume could be sequestered in these deposits if they were completely dust free. Alternatively, if these deposits were completely free of solid CO₂, the dust content would be constrained to lie between about 14 and 28% by volume. The bulk thermal conductivity of the polar cap is not significantly affected by these maximum allowable concentrations of dust. However, even if a moderate quantity of solid CO₂ were present as horizontal layers, the bulk thermal conductivity of the polar cap would be significantly reduced. Reasonable estimates of the present day heat flow of Mars predict that dry ice beneath the thicker portions of the south polar cap would have melted. Depending on the quantity of solid CO₂ in these deposits today, it is even possible that water ice could melt where the cap is thickest. If independent estimates for either the dust or CO₂ content of the south polar cap could be obtained, and if radar sounding data could determine whether this polar cap is presently experiencing basal melting or not, it would be possible to use these observations to place tight constraints on the present day heat flow of Mars.     

Experimental study of the sublimation of ice through an unconsolidated clay layer: Implications for the stability of ice on Mars and the possible diurnal variations in atmospheric water

We have studied the sublimation of ice and water vapor transport through various thicknesses of clay (<63 μm grain size). We experimentally demonstrate that both adsorption and diffusion strongly affect the transport of water, and that the processes of diffusion and adsorption can be separately quantified once the system comes to a steady state. At shallow depths of clay, water vapor transport is determined by diffusion through both the atmosphere and the clay layer, whereas at greater depth the rate of sublimation of the ice is governed only by diffusion through the clay. Using two different models, we determine the diffusion coefficient for water vapor through unconsolidated clay layer to be 1.08±0.04×¹⁰⁻⁴ and . We also determined the adsorption isotherms for the clay layer, which follow the Langmuir theory at low water vapor pressure (<100 Pa, where a monolayer of water molecules forms on the surface of the clay) and the BET theory at higher pressure (where multiple water layers form). From our analysis of both types of isotherms we determined the adsorption constants to be and c=30±10, respectively, and specific surface areas of 1.10±0.2×¹⁰⁵ and , respectively. Finally, we report a theoretical kinetic model for the simultaneous diffusion and adsorption from which we determine adsorption kinetic constants according to the Langmuir theory of and . If the martian regolith possesses diffusive properties similar to those of the unconsolidated montmorillonite soil we investigated here, it would not represent a significant barrier to the sublimation of subsurface ice. However, at the low subsurface temperatures of high latitude (180 K on average), ice could survive from the last glaciation period (about 300 to 400,000 years ago). Higher subsurface temperatures in the equatorial regions would prevent long-timescale survival of ice in the shallow subsurface. In agreement with previous work, we show that adsorption of water by a clay regolith could provide a significant reservoir of subsurface water and it might account for the purported diurnal cycle in the water content of the atmosphere.     

High outflow channels on Mars indicate Hesperian recharge at low latitudes and the presence of Canyon Lakes

A number of martian outflow channels were carved by discharges from large dilational fault zones. These channels were sourced by groundwater, not surface water, and when observed on high-standing plateaus they provide indicators of elevated paleo-groundwater levels. We identify three outflow channels of Hesperian age that issued from a 750-km-long fault zone extending from Candor Chasma to Ganges Chasma. Two of these channels, Allegheny Vallis and Walla Walla Vallis, have sources >2500 m above the topographic datum, too high to be explained by discharge from a global aquifer that was recharged solely in the south polar region. The indicated groundwater levels likely required regional sources of recharge at low latitudes. The floodwaters that erupted from Ophir Cavus to form Allegheny Vallis encountered two ridges that restricted the flow, forming temporary lakes. The flow probably breached or overtopped these obstructions quickly, catastrophically draining the lakes and carving several scablands. After the last obstacle had been breached, a single main channel formed that captured all subsequent flow. We performed hydrologic analyses of this intermediate phase of the flooding, prior to incision of the channel to its present depth. Using floodwater depths of 30-60 m, we calculated flow velocities of 6-15 m s⁻¹ and discharges in the range of . Locally higher flow velocities and discharges likely occurred when the transient lakes were drained. Variable erosion at the channel and scabland crossing of MOLA pass 10644 suggests that the upper 25-30 m may consist of poorly consolidated surface materials underlain by more cohesive bedrock. We infer that an ice-covered lake with a surface elevation >2500 m probably existed in eastern Candor Chasma because this canyon is intersected by the Ophir Catenae fault system from which Allegheny Vallis and Walla Walla Vallis originated. We introduce a new hydrology concept for Mars in which the groundwater system was augmented by recharge from canyon lakes that were formed when water was released by catastrophic melting of former ice sheets in Tharsis by effusions of flood basalts. This model could help to reconcile the expected presence of a thick cryosphere during the Hesperian with the abundant evidence for groundwater as a source for some of the circum-Chryse outflow channels.     

Thermal evolution of Kuiper belt objects, with implications for cryovolcanism

We investigate the internal thermal evolution of Kuiper belt objects (KBOs), small (radii <1000 km), icy (mean densities ) bodies orbiting beyond Neptune, focusing on Pluto's moon Charon in particular. Our calculations are time-dependent and account for differentiation. We review evidence for ammonia hydrates in the ices of KBOs, and include their effects on the thermal evolution. A key finding is that the production of the first melt, at the melting point of ammonia dihydrate, ≈176 K, triggers differentiation of rock and ice. The resulting structure comprises a rocky core surrounded by liquids and ice, enclosed within a >100-km thick undifferentiated crust of rock and ice. This structure is especially conducive to the retention of subsurface liquid, and bodies the size of Charon or larger (radii >600 km) are predicted to retain some subsurface liquid to the present day. We discuss the possibility that this liquid can be brought to the surface rapidly via self-propagating cracks. We conclude that cryovolcanism is a viable process expected to affect the surfaces of large KBOs including Charon.     

New estimates for Io eruption temperatures: Implications for the interior

The initial interpretation of Galileo data from Jupiter's moon, Io, suggested eruption temperatures . Tidal heating models have difficulties explaining Io's prodigious heat flow if the mantle is , although we suggest that temperatures up to may be possible. In general, Io eruption temperatures have been overestimated because the incorrect thermal model has been applied. Much of the thermal emission from high-temperature hot spots comes from lava fountains but lava flow models were utilized. We apply a new lava fountain model to the highest reported eruption temperature, the SSI observation of the 1997 eruption at Pillan. This resets the lower temperature limit for the eruption from 1600 to . Additionally, viscous heating of the magma may have increased eruption temperature by as a result of the strong compressive stresses in the ionian lithosphere. While further work is needed, it appears that the discrepancy between observations and interior models is largely resolved.     

Coring experiments with cryogenic water and carbon dioxide ices—toward planetary surface operations

As a prelude to the design of sampling devices able to extract materials from the icy surfaces of comets, outer-planet satellites, and the martian poles, it is necessary to understand some of the physical properties of these ices. To this end we have investigated the mechanical resistance displayed by two ices subjected to coring operations at low temperatures and under vacuum. The ices used in this study were water ice, frozen from liquid water, and carbon dioxide ice grown from its vapour. The coring tool employed had dimensions and required power levels that were comparable to a sample extraction system designed for a present-day spacecraft lander. The specific cutting strength, a parameter that measures the toughness of the material, has been measured while coring these two ices. For water ice this property rose from at an ice temperature of , to at . At the lower temperature of , pore-free carbon dioxide ice has also been measured to have a specific cutting strength approximately half that of water ice at the same temperature. These laboratory-based measurements may be used as guides for the power levels needed to core solid water and CO₂ ices at certain rates.     

Importance of aeolian processes in the origin of the north polar chasmata, Mars

A detailed examination of the location and orientation of sand dunes and other aeolian features within the north polar chasmata indicates that steep scarps strongly influence the direction and intensity of prevailing winds. These steep scarps are present at the heads and along the margins of the north polar chasmata. Topographic profiles of the arcuate head scarps and equator-facing wall of Chasma Boreale reveal unusually steep polar slopes ranging from ∼6°-30°. The relatively steep-sloped (∼8°), sinuous scarp at the head of two smaller chasmata, located west of Chasma Boreale, exhibits an obvious resistant cap-forming unit. Scarp retreat is occurring in places where the cap unit is actively being undercut by descending slope winds. Low-albedo surfaces lacking sand dunes or dust mantles are present at the base of the polar scarps. A ∼100-300 m deep moat, located at the base of the scarps, corresponds with these surfaces and indicates an area of active aeolian scour from descending katabatic winds. Small local dust storms observed along the equator-facing wall of Chasma Boreale imply that slope wind velocities in Chasma Boreale are sufficient to mobilize dust and sand-sized particles in the Polar Layered Deposits (PLD). Two amphitheater forms, located above the cap-forming unit of the sinuous scarp and west of Chasma Boreale, may represent an early stage of polar scarp and chasma formation. These two forms are developing within a younger section of polar layered materials. The unusually steep scarps associated with the polar chasmata have developed where resistant layers are present in the PLD, offering resistance during the headward erosion and poleward retreat of the scarps. Steep slopes that formed under these circumstances enhance the flow of down-scarp katabatic winds. On the basis of these observations, we reject the fluvial outflood hypothesis for the origin of the north polar chasmata and embrace a wind erosion model for their long-term development. In the aeolian model, off-pole katabatic winds progressively remove materials from the steep slopes below chasmata scarps, undermining resistant layers at the tops of scarps and causing retreat by headward erosion. Assuming a minimum age for the onset of formation of Chasma Boreale (10⁵ yr) results in a maximum volumetric erosion rate of . Removal of this volume of material from the equator-facing wall and head scarps of chasma would require a rate for scarp retreat of .     

Gravity field and interior of Rhea from Cassini data analysis

The Cassini spacecraft encountered Rhea on November 26, 2005. Analysis of the Doppler data acquired at and around closest approach yields the mass of Rhea and the quadrupole moments of its gravity field with unprecedented accuracy. We obtained which corresponds to a density of . Our results for J₂ and C₂₂ are (7.947±0.892)×¹⁰⁻⁴ and (2.3526±0.0476)×¹⁰⁻⁴, respectively. These values are consistent with hydrostatic equilibrium. From the value of C₂₂, we infer the non-dimensional moment of inertia C/MR²=0.3721±0.0036. Our models of Rhea's interior based on the gravity data favor an almost undifferentiated satellite. A discontinuity between a core and a mantle is possible but not required by the data. Models with a constant silicate mass fraction throughout the body cannot account for the determined quadrupole coefficients. The data exclude fully differentiated models in which the core would be composed of unhydrated silicates and the mantle would be composed of pure ice. If the mantle contains 10% in mass of silicates, the core extends to 630 km in radius and has a silicate mass fraction of 40%. A continuous model in which the silicates are more concentrated toward the center of the body than in the outer layers is allowed by the gravity data but excluded by thermal evolution considerations. The one model that fits the gravity data and is self-consistent when energy transport and ice melting are qualitatively considered is an “almost undifferentiated” Rhea, in which a very large uniform core is surrounded by a relatively thin ice shell containing no rock at all.     

Comparison between the findings of Deep Impact and our experimental results on large samples of gas-laden amorphous ice

The findings of Deep Impact on the structure and composition of Tempel-1 are compared with our experimental results on large (20 cm diameter and up to 10 cm high) samples of gas-laden amorphous ice. The mechanical ∼tensile strength inferred for Tempel-1: ∼65 Pa is 30 to 60 times smaller than our experimental findings of 2-4 kPa. This means that Tempel-1 is even fluffier than our very fluffy, talcum like, ice sample. The thermal inertia: is very close to our value of 80. The density of , is close to our value of 250-300 kg m⁻³, taking into account an ice/silicate ratio of 1 in the comet, while we study pure ice. Surface morphological features, such as non-circular depressions, chaotic terrain and smooth surfaces, were observed in our experiments. The only small increase in the gas/water vapor ratio pre- and post-impact, suggest that in the area excavated by the impactor, the 135 K front did not penetrate deeper than a few meters. Altogether, the agreement between the findings of Deep Impact and our experimental results point to a loose agglomerate of ice grains (with a silicate-organic core), which was formed by a very gentle aggregation of the ice grains, without compaction.     

The global shape of Europa: Constraints on lateral shell thickness variations

The global shape of Europa is controlled by tidal and rotational potentials and possibly by lateral variations in ice shell thickness. We use limb profiles from four Galileo images to determine the best-fit hydrostatic shape, yielding a mean radius of 1560.8±0.3 km and a radius difference a−c of 3.0±0.9 km, consistent with previous determinations and inferences from gravity observations. Adding long-wavelength topography due to proposed lateral variations in shell thickness results in poorer fits to the limb profiles. We conclude that lateral shell thickness variations and long-wavelength isostatically supported topographic variations do not exceed 7 and 0.7 km, respectively. For the range of rheologies investigated (basal viscosities from 10¹⁴ to ) the maximum permissible (conductive) shell thickness is 35 km. The relative uniformity of Europa's shell thickness is due to either a heat flux from the silicate interior, lateral ice flow at the base of the shell, or convection within the shell.     

On the collisional disruption of porous icy targets simulating Kuiper belt objects

We present results from 27 impact experiments using porous (porosity ranging from 0.39 to 0.54) ice targets and solid ice projectiles at impact speeds ranging from 90 to 155 m/s. These targets were designed to simulate Kuiper Belt Objects (KBOs) in structure. We measured a specific energy for shattering, , of 2.1×¹⁰⁵ erg/g for those snowball targets hit by intact ice projectiles; this is of the same order as that measured for solid ice targets. The fragment mass distribution follows a power law, although the exponent is not simply related to the largest fragment size as assumed by fragmentation models. We provide the first measurement of the three-dimensional mass-velocity distribution for disrupted ice targets and find that fragment speeds range from ∼2 to ∼20 m/s. The fraction of collisional kinetic energy that is partitioned into ejecta speeds is between 1 and 15% (although it should be noted that the lower limit is more reliable than the upper).     

Ion irradiation of H2O ice on top of sulfurous solid residues and its relevance to the Galilean satellites

In this paper we present the results of new experiments of ion irradiation of water ice deposited on top of a solid sulfurous residue to study the potential formation of SO₂ at the interface ice/refractory material and discuss the possibility that this mechanism accounts for the sulfur dioxide ice detected on the surfaces of the Galilean satellites. In situ infrared spectroscopy was the used experimental technique. We have irradiated a thin film of H₂O frost on a sulfurous layer with 200 keV of He⁺ at 80 K. The used sulfurous residue was obtained by irradiation of frozen SO₂ at 16 K and it is used as a template of sulfur bearing solid materials. We have not found evidences of the efficient formation of SO₂ after irradiation of H₂O ice on top of the sulfurous residue. An upper limit to the production yield of SO₂, of interface area for each 100 eV of energy absorbed in 1 cm³ of ice-covered residue, has been estimated. These results have relevance in the context of the surfaces of the icy Galilean satellites in which SO₂ was detected. Our results show that radiolysis of mixtures of water ice and refractory sulfurous materials is not the primary formation mechanism responsible for the SO₂ present on the surfaces of the Galilean satellites.     

The structure and time variability of the ring atmosphere and ionosphere

The saturnian system is subject to constant bombardment by interplanetary meteoroids and irradiation by solar UV photons. Both effects release neutral molecules from the icy ring particles either in the form of impact water vapor or gas emission in the form of H₂O, O₂ and H₂. The observations of the Cassini spacecraft during its orbit insertion have shown the existence of molecular and atomic oxygen ions. Subsequent modeling efforts have led to the picture that an exospheric population of neutral oxygen molecules is probably maintained in the vicinity of the rings via photolytic-decomposition of ice and surface reactions. At the same time, ionized products O⁺ and ions move along the magnetic field lines and, depending on the optical local thickness rings, can thread through the ring plane or impact a ring particle, the ion principal sink. In addition, collisional interactions between the ions and neutrals will change the scale height of the ions and produce a scattered component of O₂ molecules and O atoms which can be injected into Saturn’s upper atmosphere or the inner magnetosphere. The ring atmosphere, therefore, serves as a source of ions throughout Saturn’s magnetosphere. If photolysis of ice is the dominant source of O₂, then the complex structure of the ring atmosphere/ionosphere and the injection rate of neutral O₂ will be subject to modulation by the seasonal variation of Saturn along its orbit. In this work, we show how the physical properties of the ring oxygen atmosphere, the scattered component, and the magnetospheric ion source rate vary as the ring system goes through the cycle of solar insolation. In particular, it is shown that the magnetopheric ions should be nearly depleted at Saturn’s equinox if O₂ is produced mainly by photolysis of the ring material.