Distribution Of Water Ice Research Articles

Water ice and dust in the innermost coma of comet 103P/Hartley 2

On November 4th, 2010, the Deep Impact eXtended Investigation (DIXI) successfully encountered comet 103P/Hartley 2, when it was at a heliocentric distance of 1.06AU. Spatially resolved near-IR spectra of comet Hartley 2 were acquired in the 1.05–4.83μm wavelength range using the HRI-IR spectrometer. We present spectral maps of the inner ∼10km of the coma collected 7min and 23min after closest approach. The extracted reflectance spectra include well-defined absorption bands near 1.5, 2.0, and 3.0μm consistent in position, bandwidth, and shape with the presence of water ice grains. Using Hapke’s radiative transfer model, we characterize the type of mixing (areal vs. intimate), relative abundance, grain size, and spatial distribution of water ice and refractories. Our modeling suggests that the dust, which dominates the innermost coma of Hartley 2 and is at a temperature of 300K, is thermally and physically decoupled from the fine-grained water ice particles, which are on the order of 1μm in size. The strong correlation between the water ice, dust, and CO2 spatial distribution supports the concept that CO2 gas drags the water ice and dust grains from the nucleus. Once in the coma, the water ice begins subliming while the dust is in a constant outflow. The derived water ice scale-length is compatible with the lifetimes expected for 1-μm pure water ice grains at 1AU, if velocities are near 0.5m/s. Such velocities, about three order of magnitudes lower than the expansion velocities expected for isolated 1-μm water ice particles (Hanner, 1981; Whipple, 1951), suggest that the observed water ice grains are likely aggregates.

Saturn’s icy satellites and rings investigated by Cassini–VIMS: III – Radial compositional variability

In the last few years Cassini–VIMS, the Visible and Infrared Mapping Spectrometer, returned to us a comprehensive view of the Saturn’s icy satellites and rings. After having analyzed the satellites’ spectral properties (Filacchione, G., Capaccioni, F., McCord, T.B., Coradini, A., Cerroni, P., Bellucci, G., Tosi, F., D’Aversa, E., Formisano, V., Brown, R.H., Baines, K.H., Bibring, J.P., Buratti, B.J., Clark, R.N., Combes, M., Cruikshank, D.P., Drossart, P., Jaumann, R., Langevin, Y., Matson, D.L., Mennella, V., Nelson, R.M., Nicholson, P.D., Sicardy, B., Sotin, C., Hansen, G., Hibbitts, K., Showalter, M., Newman, S. [2007]. Icarus 186, 259–290, paper I) and their distribution across the satellites’ hemispheres (Filacchione, G., Capaccioni, F., Clark, R.N., Cuzzi, J.N., Cruikshank, D.P., Coradini, A., Cerroni, P., Nicholson, P.D., McCord, T.B., Brown, R.H., Buratti, B.J., Tosi, F., Nelson, R.M., Jaumann, R., Stephan, K. [2010]. Icarus 206, 507–523, paper II), we proceed in this paper to investigate the radial variability of icy satellites (principal and minor) and main rings average spectral properties. This analysis is done by using 2264 disk-integrated observations of the satellites and a 12×700 pixels-wide rings radial mosaic acquired with a spatial resolution of about 125km/pixel. Using different VIS and IR spectral indicators, e.g. spectral slopes and band depths, we perform a comparative analysis of these data aimed to measure the distribution of water ice and red contaminant materials across Saturn’s system. The average surface regolith grain sizes are estimated with different indicators through comparison with laboratory and synthetic spectra. These measurements highlight very striking differences in the population here analyzed, which vary from the almost uncontaminated and water ice-rich surfaces of Enceladus and Calypso to the metal/organic-rich and red surfaces of Iapetus’ leading hemisphere and Phoebe. Rings spectra appear more red than the icy satellites in the visible range but show more intense 1.5–2.0μm band depths. Although their orbits are close to the F-ring, Prometheus and Pandora are different in surface composition: Prometheus in fact appears very water ice-rich but at the same time very red at VIS wavelengths. These properties make it very similar to A–B ring particles while Pandora is bluer. Moving outwards, we see the effects of E ring particles, generated by Enceladus plumes, which contaminate satellites surfaces from Mimas out to Rhea. We found some differences between Tethys lagrangian moons, Calypso being much more water ice-rich and bluer than Telesto. Among outer satellites (Hyperion, Iapetus and Phoebe) we observe a linear trend in both water ice decrease and in reddening, Hyperion being the reddest object of the population. The correlations among spectral slopes, band depths, visual albedo and phase permit us to cluster the saturnian population in different spectral classes which are detected not only among the principal satellites and rings but among co-orbital minor moons as well. These bodies are effectively the “connection” elements, both in term of composition and evolution, between the principal satellites and main rings. Finally, we have applied Hapke’s theory to retrieve the best spectral fits to Saturn’s inner regular satellites (from Mimas to Dione) using the same methodology applied previously for Rhea data discussed in Ciarniello et al. (Ciarniello, M., Capaccioni, F., Filacchione, G., Clark, R.N., Cruikshank, D.P., Cerroni, P., Coradini, A., Brown, R.H., Buratti, B.J., Tosi, F., Stephan, K. [2011]. Icarus 214, 541–555).

Migrating and UV screening subsurface zone on Mars as target for the analysis of photosynthetic life and astrobiology

Analyzing the UV shielding capacity of two Mars simulant materials in laboratory (JSC-1 and basaltic sand), 1–2mm thickness of these granular materials were enough to provide the required UV shielding for some extremophiles under Martian surface radiations, while light enough for photosynthesis could penetrate down to 4–5mm depth. Between these two depth levels a PAR zone exists (photosynthetically active radiation between 400 and 700nm that can be used by photosynthetic organism on Earth), where hypothetic organisms could photosynthesize in theory if other environmental parameters are tolerable. To identify the possible ideal locations we surveyed the spatial and temporal distribution of water ice on Mars inside this PAR zone at periods and locations where no CO2 ice is present on the surface.It was found that this PAR zone is filled with ice in the northern hemisphere of Mars for 31–35 Earth days for any certain location, and these locations form a receding ring-like zone there. Because of few observations and probably the small quantity of water ice there, the characteristics of such a ring could not be identifies for the southern hemisphere. The focused analysis of this ideal zone could be interesting in the future as based on theoretical reasons ephemeral liquid water as a thin film might appear there and give insight into the characteristics of astrobiological important locations on Mars.

Research on the distribution and content of water ice in lunar pole regions using Clementine UVVIS data

Interest in the Moon started to increase at the beginning of the 21st century, and hence- forth, more and more attention has been paid to the content and distribution of water ice in the lunar polar regions. The existence of water or ice in the regolith can apparently change its dielectric features. Therefore, in this article, the Dobson model is adopted and improved according to the Moon's envi- ronmental features, to construct the relationship between the volumetric water ice content and the di- electric constant. Thereafter, a lunar regolith dielectric distribution map is generated based on the im- proved Dobson model and the Clementine UVVIS data. The map indicates that the imaginary part of the dielectric constants in the lunar mare is much higher than that in the highlands. However, the maximum dielectric constants occur at the north- and south-pole regions, whose values are apparently bigger than those in the middle and low latitudes. Then, an abnormal map of the dielectric constant is gained if the threshold is put as 0.053 7, which is the highest value in the middle and low latitudes. The statistical results indicate that the number of abnormal pixels is 110 596, and the average is about 0.057 9. Assuming that the mean dielectric constant in the lunar mare is the normal dielectric constant at the south and north poles and e1=11.58+i0.057 9 is the abnormal one, the volumetric water ice con- tent can be evaluated using the advanced Dobson model. The results show that the average volumetric water ice content is about 1.64%, and the total area is about 25 294 km 2 , where 10 956 km 2 belongs to

Structure and dynamics of the Martian lower and middle atmosphere as observed by the Mars Climate Sounder: Seasonal variations in zonal mean temperature, dust, and water ice aerosols

The first Martian year and a half of observations by the Mars Climate Sounder aboard the Mars Reconnaissance Orbiter has revealed new details of the thermal structure and distributions of dust and water ice in the atmosphere. The Martian atmosphere is shown in the observations by the Mars Climate Sounder to vary seasonally between two modes: a symmetrical equinoctial structure with middle atmosphere polar warming and a solstitial structure with an intense middle atmosphere polar warming overlying a deep winter polar vortex. The dust distribution, in particular, is more complex than appreciated before the advent of these high (∼5 km) vertical resolution observations, which extend from near the surface to above 80 km and yield 13 dayside and 13 nightside pole‐to‐pole cross sections each day. Among the new features noted is a persistent maximum in dust mass mixing ratio at 15–25 km above the surface (at least on the nightside) during northern spring and summer. The water ice distribution is very sensitive to the diurnal and seasonal variation of temperature and is a good tracer of the vertically propagating tide.

The distribution of water ice in the interior of Comet Tempel 1

The Deep Impact flyby spacecraft includes a 1.05 to 4.8 μm infrared (IR) spectrometer. Although ice was not observed on the surface in the impact region, strong absorptions near 3 μm due to water ice are detected in IR measurements of the ejecta from the impact event. Absorptions from water ice occur throughout the IR dataset beginning three seconds after impact through the end of observations, ∼45 min after impact. Spatially and temporally resolved IR spectra of the ejecta are analyzed in conjunction with laboratory impact experiments. The results imply an internal stratigraphy for Tempel 1 consisting of devolatilized materials transitioning to unaltered components at a depth of approximately one meter. At greater depths, which are thermally isolated from the surface, water ice is present. Up to depths of 10 to 20 m, the maximum depths excavated by the impact, these pristine materials consist of very fine grained ( ∼ 1 ± 1 μm ) water ice particles, which are free from refractory impurities.

The distribution of water ice in the interior of Comet Tempel 1

The Deep Impact flyby spacecraft includes a 1.05 to 4.8 μm infrared (IR) spectrometer. Although ice was not observed on the surface in the impact region, strong absorptions near 3 μm due to water ice are detected in IR measurements of the ejecta from the impact event. Absorptions from water ice occur throughout the IR dataset beginning three seconds after impact through the end of observations, ∼45 min after impact. Spatially and temporally resolved IR spectra of the ejecta are analyzed in conjunction with laboratory impact experiments. The results imply an internal stratigraphy for Tempel 1 consisting of devolatilized materials transitioning to unaltered components at a depth of approximately one meter. At greater depths, which are thermally isolated from the surface, water ice is present. Up to depths of 10 to 20 m, the maximum depths excavated by the impact, these pristine materials consist of very fine grained ( ∼ 1 ± 1 μm ) water ice particles, which are free from refractory impurities.

The effects of the martian regolith on GCM water cycle simulations

This paper describes General Circulation Model (GCM) simulations of the martian water cycle focusing on the effects of an adsorbing regolith. We describe the 10-layer regolith model used in this study which has been adapted from the 1-D model developed by Zent, A.P., Haberle, R.M., Houben, H.C., Jakosky, B.M. [1993. A coupled subsurface–boundary layer model of water on Mars. J. Geophys. Res. 98 (E2), 3319–3337, February]. Even with a 30-min timestep and taking into account the effect of surface water ice, our fully implicit scheme compares well with the results obtained by Zent, A.P., Haberle, R.M., Houben, H.C., Jakosky, B.M. [1993. A coupled subsurface–boundary layer model of water on Mars. J. Geophys. Res. 98 (E2), 3319–3337, February]. This means, however, that the regolith is not able to reproduce the diurnal variations in column water vapour abundance of up to a factor of 2-3 as seen in some observations, with only about 10% of the atmospheric water vapour column exchanging with the subsurface on a daily basis. In 3-D simulations we find that the regolith adsorbs water preferentially in high latitudes. This is especially true in the northern hemisphere, where perennial subsurface water ice builds up poleward of 60° N at depths which are comparable to the Odyssey observations. Much less ice forms in the southern high latitudes, which suggests that the water ice currently present in the martian subsurface is not stable under present conditions and is slowly subliming and being deposited in the northern hemisphere. When initialising the model with an Odyssey-like subsurface water ice distribution the model is capable of forcing the simulated water cycle from an arbitrary state close to the Mars Global Surveyor Thermal Emission Spectrometer observations. Without the actions of the adsorbing regolith the equilibrated water cycle is found to be a factor of 2-4 too wet. The process by which this occurs is by adsorption of water during northern hemisphere summer in northern mid and high latitudes where it remains locked in until northern spring when the seasonal CO 2 ice cap retreats. At this time the water diffuses out of the regolith in response to increased temperature and is returned to the residual water ice cap by eddie transport.

Martian Seasonal CO 2 Ice in Polygonal Troughs in Southern Polar Region: Role of the Distribution of Subsurface H 2O Ice

The Mars Orbiter Camera onboard the Mars Global Surveyor has obtained several images of polygonal features in the southern polar region. In images taken during the end of the southern spring, when the surrounding surface is free of the seasonal frost, CO 2 ice still appears to be present within the polygonal troughs. In Earth's polar regions, polygons such as these are indicative of water ice in the ground below. We analyzed the seasonal evolution of the thermal state and the CO 2 content of these features. Our 2-D model includes condensation and sublimation of the CO 2 ice, a self consistent treatment of the variations of the thermal properties of the regolith, and the seasonal variations of the local atmospheric pressure which we take from the results of a general circulation model. We find that the residence time of seasonal CO 2 ice in troughs depends not only on atmospheric opacity and albedo of the CO 2 ice, but also and most significantly on the distribution of water ice in the regolith. Optical properties of the atmosphere and surface CO 2 ice can be independently obtained from observations. To date this is not true about the distribution of water ice below the surface. Our analysis quantifies the dependence of the seasonal cycle of the CO 2 ice within the troughs on the assumed distribution of the water ice below the surface. We show that presence of water ice in the ground at a depth smaller than the depth of the troughs reduces winter condensation rate of CO 2 ice. This is due to higher heat flux conducted from the water ice rich regolith toward the facets of the troughs.

Water ice clouds in the Martian atmosphere: General circulation model experiments with a simple cloud scheme

We present the first comprehensive general circulation model study of water ice condensation and cloud formation in the Martian atmosphere. We focus on the effects of condensation in limiting the vertical distribution and transport of water and on the importance of condensation for the generation of the observed Martian water cycle. We do not treat cloud ice radiative effects, ice sedimentation rates are prescribed, and we do not treat interactions between dust and cloud ice. The model generates cloud in a manner consistent with earlier one‐dimensional (1‐D) model results, typically evolving a uniform (constant mass mixing ratio) vertical distribution of vapor, which is capped by cloud at the level where the condensation point temperature is reached. Because of this vertical distribution of water, the Martian atmosphere is generally very far from fully saturated, in contrast to suggestions based upon interpretation of Viking data. This discrepancy results from inaccurate representation of the diurnal cycle of air temperatures in the Viking Infrared Thermal Mapper (IRTM) data. In fact, the model suggests that only the northern polar atmosphere in summer is consistently near its column‐integrated holding capacity. In this case, the column amount is determined primarily by the temperature of the northern polar ice cap. Comparison of the water cycle generated by the model with and without atmospheric ice condensation and precipitation shows two major roles for water ice cloud. First, clouds are essential to the observed rapid return of atmospheric water to the surface in late northern summer, as ice sedimentation forces the water column to shrink in response to the downward motion of the condensation level, concentrating water near surface sinks. Second, ice sedimentation limits the amount of water that is transported between the hemispheres through the Hadley circulation. This latter effect is used to greatly improve the model simulation of the annual water cycle by increasing ice sedimentation rates. The model is thus shown to be able to reasonably reproduce the annual cycles of vapor and ice cloud as compared to Viking data. In addition, the model is shown able to reproduce near‐instantaneous maps of water ice derived from Hubble Space Telescope images. The seasonal evolution of the geographic distribution of water ice compares reasonably well with Viking and Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) observations, except in the prediction of a weak tropical cloud belt in southern summer. Finally, it is shown that the tropical cloud belt is generated in the model by the cooling of water vapor entrained in the upwelling branch of the Hadley cell. Decline of the tropical cloud belt in mid northern summer is shown to be related to an increase in air temperatures, rather than to decreases in water vapor supply or the vigor of Hadley cell ascent. By equinox, the cloud belt experiences a second major decline event, this time due to a reduction in vapor supply. The ability of the model to emulate many aspects of observed cloud behavior is encouraging, as is the ability of enhanced ice sedimentation to improve the overall quality of the water cycle simulation. However, significant work remains to be done before all observational constraints can be matched simultaneously. Specifically, in order for the generally good fit to all other data to be attained, cloud ice particle sizes about an order of magnitude too large must be used.

The water ice distribution in Taurus determined by gas–grain chemistry

The detection of water ice in interstellar grain mantles by observation of absorbed radiation from field stars through the Taurus molecular cloud complex has revealed that the column density of water ice increases linearly,or nearly linearly with visual extinction except at the lowest values (below A(V) approximate to 3). Previous explanations of the small or negligible abundance of ice at low extinction have focused on the destruction and desorption of ice by radiation, and have ignored the efficiency of formation of ice. To understand the dependence of the ice column density on A(V) in more detail and to understand what it tells us about the Taurus dark cloud, we have run gas-grain chemical models under a variety of physical conditions. For higher values of extinction, our models predict that much of the elemental abundance of available oxygen is in the form of ice for evolutionary times greater than about 10(5) yr, and this leads to the observed near-linear relationship between ice column density and A(V). For lower extinction, our models show that if any significant amount of ice is still present, its existence can be attributed to great cloud age, to our poor understanding of dust-grain surfaces, or to cloud clumpiness. The first and third explanations pertain only if photodesorption of ice is inefficient.

Regional Variability of the Arctic Heat Budget in Fall and Winter*

In the Arctic atmosphere, the fall cooling cycle involves the evolution of the zonally symmetric circulation in late summer into the asymmetric flow of winter. This paper uses historical reanalysis data to document how the dominant components of the Arctic heat budget influence the summer–winter transition. The spatial variability of 20-yr climatologies of 700-mb temperature and geopotential height, the net surface flux, and the horizontal convergence of eddy sensible heat fluxes are examined for September through February. The development of the zonal asymmetries in the temperature and geopotential height fields in the Arctic is linked to the land–water–ice distribution that regulates the surface fluxes and the baroclinic zones in the hemispheric circulation, which lead to regional heating/cooling by the transient and standing eddies. These eddies serve to transport the heat energy gained via the surface fluxes over the North Atlantic and North Pacific to the continental and ice-covered regions of the central Arctic, where the net surface flux is small. The transient eddies are especially important in the Atlantic and Eurasian sectors of the Arctic, while the standing eddies play the larger role in the heat budget on the Pacific side of the Arctic in early to mid-winter. The Arctic oscillation (AO) has a small effect on the basinwide pattern of heating and cooling by the eddy circulations, but on smaller spatial scales there are isolated regions where the AO influences the Arctic heat budget.

White Mars: A New Model for Mars' Surface and Atmosphere Based on CO 2

A new model is presented for the Amazonian outburst floods on Mars. Rather than the working fluid being water, with the associated difficulties in achieving warm and wet conditions on Mars and on collecting and removing the water before and after the floods, instead this model suggests that CO 2 is the active agent in the “floods.” The flow is not a conventional liquid flood but is instead a gas-supported density flow akin to terrestrial volcanic pyroclastic flows and surges and at cryogenic temperatures with support from degassing of CO 2-bearing ices. The flows are not sourced from volcanic vents, but from the collapse of thick layered regolith containing liquid CO 2 to form zones of chaotic terrain, as shown by R. St. J. Lambert and V. E. Chamberlain (1978, Icarus 34, 568–580; 1992, Workshop on the Evolution of the Martian Atmosphere). Submarine turbidites are also analagous in the flow mechanism, but the martian cryogenic flows were both dry and subaerial, so there is no need for a warm and wet epoch nor an ocean on Mars. Armed with this new model for the floods we review the activity of volatiles on the surface of Mars in the context of a cold ice world—“White Mars.” We find that many of the recognized paradoxes about Mars' surface and atmosphere are resolved. In particular, the lack of carbonates on Mars is due to the lack of liquid water. The CO 2 of the primordial atmosphere and the H 2O inventory remain largely sequestered in subsurface ices. The distribution of water ice on modern Mars is also reevaluated, with important potential consequences for future Mars exploration. The model for collapse of terrain due to ices that show decompression melting, and the generation of nonaqueous flows in these circumstances may also be applicable to outer Solar System bodies, where CO 2, SO 2, N 2, and other ices are stable.

Subaru Infrared Spectroscopy of the Pluto–Charon System

We present the results of the near-infrared spectroscopy of the Pluto-Charon system, conducted in the K band at two different sub-Earth longitudes. The spatially resolved images resulted in separated spectra with a spectral resolution of approximately 530. The spectrum of Charon showed a broad absorption feature at 2 μm during both orbital phases, suggesting a uniform distribution of water ice. In the separated spectrum of Pluto obtained in June, we confirmed the absorption bands of solid methane, carbon monoxide, and nitrogen ices. Additional weak dips have been found at 2.28, 2.32, and 2.40 μm in the spectrum of the opposite side observed in May. In order to examine the carrier of these dips, we performed model calculations of the reflectance of Pluto using Hapke's bidirectional reflectance model for icy intimate mixtures. By adding a small amount of solid ethane into the mixture, we successfully reproduced the dips at 2.28 and 2.315 μm, but the absorption at 2.40 μm is more distinct than that in the observed spectrum.

Water Ice in the Disk Around the Protostar AFGL 2136 IRS 1

The protostar AFGL 2136 IRS 1 illuminates the Juggler nebula and drives a massive molecular outflow. The line of sight toward AFGL 2136 also displays a rich spectrum of solid state absorption features. To investigate the distribution of ice-coated grains in the environment of IRS 1, we obtained narrowband images at six near-infrared wavelengths surrounding and including the 3.08 μm water ice feature. These images (specifically, a color map constructed from 2.14 and 3.60 μm images) provide vivid evidence for the presence of a disk around IRS 1 and appear to reveal, for the first time, the detailed distribution of water ice in the local environment of a protostar. Models of the 2-4 μm spectral energy distributions (SEDs) toward IRS 1 and the Juggler nebula suggest that the abundance of icy grains is larger along lines of sight through the disk than along lines of sight away from the equatorial plane of IRS 1. Thus, while we find evidence that icy grains exist throughout the cloud core containing AFGL 2136, it appears that the physical conditions in the circumstellar disk around IRS 1 are more conducive to ice mantle survival and/or growth than are physical conditions in its rarefied polar regions.

Evolution of the water regime of Phobos

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 10 5 to 10 6 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 H 2O 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.