Icy Surfaces Research Articles

Mass and energy transport in sublimating cometary ice cracks

As the spacecraft encounters to comet Halley have confirmed, comets seem to be bodies of rather low bulk density; i.e., they must contain a rather large fraction of hollow space both on a microscopic scale and on a macroscopic scale. Cracks of different sizes are most probably very common topological features on a cometary surface. Furthermore, if the cometary ice contains a lot of pore space, the thermal behavior and, consequently, the net sublimation rate might be affected. In the present paper we develop a model which describes the mass and energy transport by the gas in a rectangular crack of arbitrary cross section and depth. The model is valid as long as the conditions of free molecular gas flow inside the crack are fulfilled. Hereby it is assumed that the gas sublimates from the walls according to the local ice temperature and that the molecules hitting the ice walls recondense there. The model allows us to calculate gas flux and recondensation rates inside the gap and at its exit provided the temperature profile along the walls is known. It is found that the sublimation rate from an icy surface with many small pores is almost the same as in the case of a flat surface, while the existence of a few larger holes (leading to the same total pore cross section) may drop the sublimation rate of the surface to 80%. If an inverse temperature profile (inward increasing) exists in the ice, gaps can be the major source of emission. Energetically the recondensation of particles in the deeper parts of the gaps does not significantly contribute to the heating of the ice. On the other hand, cooling by net sublimation from the ice walls is important.

Implications of the stability and radiative time constant of Triton's atmosphere

This paper examines both Triton's atmospheric escape rates and radiative timescales.We first calculate the range of plausible atmospheric escape rates and examine the implications of these escape fluxes. If even modest hydrodynamic escape is occurring, escape rates can exceed the ion pickup rates predicted by Delitsky, et al. (1989). This process is expected to develop a torus of gas in orbit around Neptune. If Triton's escape flux exceeds ∼ 1010 cm−2 sec−1, a substantial amount of ancient icy surface topography may have been lost to escape.We also calculate the radiative timescale in Triton's atmosphere as a function of surface temperature, atmospheric pressure, and composition. It is shown that if N2 obtains saturation vapor pressure equilibrium, then diurnal variations will be negligible. If N2 is far from saturation or if only CH4 is present, then both diurnal and seasonal effects may be expected. Recent observations indicate the presence of decadal‐timescale atmospheric variations. Radiative timescale constraints imply that if these observations are correct, then the present atmospheric pressure is unlikely to much exceed ≈100 mb.

The first UV spectrum of Triton - IUE observations from 2600 to 3200 A

view Abstract Citations (14) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The First UV Spectrum of Triton: IUE Observations from 2600 to 3200 Angstrom Stern, S. Alan ; Skinner, T. E. ; Brosch, N. ; van Santvoort, J. ; Trafton, L. M. Abstract The results of the first observations of Triton's spectrum below 3300 A are reported. Triton's 2700 A geometric albedo is found to be 0.28 + or - 0.04. The albedo increases monotonically from 2600 to 3200 A, with a slope of 0.13 + or - 0.03 per 1000 A. This positive slope is qualitatively similar to, but shallower than, Triton's visible albedo slope and argues against a strong Rayleigh scattering signature. Triton's integrated flux is found to be 1.5 times Pluto's, indicating Triton to be either more reflective in the UV, or bigger, or both. The redness of Triton's visible and UV colors argues against an extremely bright surface and for a larger radius than Pluto's. A lower limit on the radius of 1240 + or - 90 km is derived. The spectrum observed by IUE is consistent with either an icy surface or a suspended aerosol layer which preferentially absorbs UV light. Publication: The Astrophysical Journal Pub Date: June 1989 DOI: 10.1086/185469 Bibcode: 1989ApJ...341L.107S Keywords: Albedo; Planetary Atmospheres; Triton; Ultraviolet Spectra; Absorption Spectra; Aerosols; Iue; Optical Reflection; Pluto (Planet); Rayleigh Scattering; Lunar and Planetary Exploration; PLANETS: ATMOSPHERES; PLANETS: SATELLITES; PLANETS: SPECTRA; SOLAR SYSTEM: GENERAL; ULTRAVIOLET: SPECTRA full text sources ADS | data products MAST (1) INES (1)

Crater formation and modification on the icy satellites of Uranus and Saturn: Depth/diameter and central peak occurrence

Depth/diameters (d/D) for fresh craters on the intermediate‐sized icy satellites of Uranus and Saturn have been determined using photoclinometry and shadow lengths, and are compared with similar measurements on the terrestrial planets. Simple bowl‐shaped craters on icy satellites, including those on Miranda for which the highest resolution data are available, are systematically 20–40% shallower than on the terrestrial planets. This pronounced difference between crater depths on icy and rocky surfaces indicates that differences in impact velocity or surface gravity are not as important as the differences in mechanical properties between ice and “rock” in controlling simple crater morphology. Experimental impact studies indicate that differences in material properties such as angle of internal friction can control crater depth. Alternatively, breccia lenses may be thicker in icy satellite simple craters. Complex craters on the icy satellites become significantly deeper with increasing crater diameters, unlike complex craters on terrestrial planets, which are nearly constant in depth. Central peaks, and hence floor rebound, are also volumetrically and morphologically more prominent than wall slumping (rim collapse) on the icy satellites. The magnitude of viscous relaxation of very large craters (e.g., Herschel and Odysseus) on the icy satellites can be estimated from extrapolation of the d/D fits at smaller crater diameters. The transition diameter from simple to complex morphology is inversely correlated with gravity, as it is on the terrestrial planets, but at significantly smaller diameters than would be expected from a simple extrapolation of the terrestrial trend. Thus crater modification, especially floor rebound, is easier to initiate on icy satellites. Estimated effective cohesions for shocked icy material are lower by a factor of 10–20 relative to rock, although estimated effective viscosities are similar. The distinct, systematic differences between crater morphology on icy and rocky worlds indicate that gross material property differences between rock and ices play a key role in crater formation and modification, but gravity is still the primary driving force.

Near-infrared photometry of the Galilean satellites

The present near-IR (2.2-, 3.8-, 4.8-micron) lightcurves and phase coefficients for the Galilean satellites notes the geometric albedo of Io to include volcanic emission at the two longer wavelengths, although no major outbursts were detected during the 1982-1983 period of these observations. The trend of decreasing albedo with increasing wavelength exhibited by Ganymede and Europa is consistent with their possession of icy surfaces. The results obtained for Callisto are consistent with visible-wavelength observations of other dark solar system objects.

Measurements of reflectance spectra of ion-bombarded ice and application to surfaces in the outer Solar System

A number of the icy satellites of the outer planets exhibit interesting hemispherical differences in brightness1–5 which have been attributed to enhanced bombardment by the local plasma of one hemisphere. The plasma bombardment is thought to erode the icy surfaces and implant species, thereby altering the surface reflectance spectra2,3, as well as producing fresh plasma6. Here we present the first results of laboratory measurements of the wavelength dependence of the alteration of the visible reflectance spectra of H2O ice irradiated by keV ions. When the implanted species is chemically neutral, absorption is slightly enhanced below 0.55 μm. For an incident species containing sulphur, a strong absorption feature is produced at 0.4 μm corresponding (probably) to S3. This occurs at too large a wavelength to account for the absorption feature observed at Europa by Voyager and therefore casts doubt on the recent interpretations7–9 of the reflectance data of Europa.

STUDY OF THE DE-ICING ACTION AND THE MELTING ACTION OF CHEMICALS FOR WINTER ROAD MAINTENCE

This study shows the results of indoor test at low temperature, on the de-icing action and the melting action of sodium chloride (NaCl) and other chemicals used for winter road maintenance. (1)As to the de-icing action, it was tested that the relation between the changes in the composition of phase and the skid resistance on road after applying chemicals.The solution on the road will not only be effective in lowering the freezing point, but also be helpful in improving skid resistance, as follows, (1)The skid resistance is already documented to decrease remarkably as soon as the water on the road has frozen. However, the solution on the road does not become slippenly at the freezing point.(2)Even when the temperature goes down below the freezing point of the solution, increase of the skid resistance is expected more than the case of perfectly icy road surface.(2)As to the melting action, the relation between the temperature of ice and the amount of melting ice after adding chemicals was tested to clarify the ice-melting effect by chemicals. As the result, it was concluded that ice-melting degree (ice-melting amount, ice-melting velocity) by chemical is liable to be controlled by the up-and-down variation of the tem- perature.

Proton and oxygen plasmas at Uranus

Despite the presence of several large, icy moons within the Uranian magnetosphere, the Voyager 2 spacecraft did not detect any heavy ion plasma. This paper estimates the heavy ion density that would be consistent with a heavy neutral torus formed by photosputtering, charged particle sputtering, and micrometeoroid impact vaporization of icy surfaces on the moons, taking into account the large 60° angle between the satellite orbit plane and the magnetic equator. The expected heavy ion density is unobservably small. The observed proton plasma of the inner magnetosphere can be maintained by ionization of the atomic hydrogen corona and by the ionospheric proton source driven by photoelectron escape, for a plasma residence time of ∼30 days. These two proton sources are comparable near L = 5. The same 30‐day residence time for energetic protons implies an upper limit of 10−5 on the fraction of incident solar wind protons that enter the magnetosphere and penetrate to within L < 6 while conserving their first adiabatic invariants.

Icy Galilean satellite reflectance spectra: Less ice on Ganymede and Callisto?

Recent interpretations of the reflectance spectra of the icy Galilean satellites (Europa, Ganymede, and Callisto) have implied very ice-rich surfaces, as high as 90 wt% ice even on the dark surface of Callisto. A reevaluation of the spectra, taking into account the depth of the 3-μm fundamental water ice absorption feature as well as the shorter wavelength bands, suggests that the spectra of at least Ganymede and Callisto may also be consistent with much lower ice abundances if the ice is segregated from the nonicy material. Reasonable fits to all band depths (including the shallow 1.04- and 1.25-μm bands) are obtained with around 50% areal coverage of ice on Ganymede and 10% on Callisto, the rest of the surface being occupied by carbonaceous chondrite-like material which has a strong 3-μm absorption due to bound water. Europa's spectrum probably indicates a homogeneous icy surface. The darkness beyond 3 μm, and lack of a 3.6-μm peak, for all three objects may be consistent with the presence of small quantities of sulfuric acid on the satellite surfaces.

The radar-glory theory for icy moons with implications for radar mapping

The anomalous radar echoing properties of three ice-clad moons of Jupiter appear to be due to glory-like backscattering from buried craters. The enormous glare from these sources would impair geologic studies based on standard methods of radar mapping. It is not known whether similar or different problems will arise in the radar study of other icy surfaces in the outer solar system, or of the unseen surface of Titan. In any event, the results from the moons of Jupiter illustrate the role of exploratory measurements and the importance of possible bistatic radar-mapping techniques based on the use of separated transmitters and receivers.

Ion bombardment of interplanetary dust

The recent discovery of ion tracks in interplanetary dust and the increasing evidence for carbon and carburized materials in these objects are strongly suggestive that chemical processing by energetic charged-particle bombardment has occured during the dust lifetimes. The track density gives a measure of the total ion fluence experienced by the grains. We use this information and laboratory data on the modification of icy surfaces by incident ions and electrons to discuss the likelihood that chondritic interplanetary dust particles could have been proceed, by plasma bombardment, from aggregates of particles which had volatile and/or ogranic mantles. Such a processing would leave carbon and carburized deposits and can affect estimates of the temperature of formation of these dust grains.

Slip-resistance on icy surfaces of shoes, crampons and chains — A new machine

Bruce, M., Jones, C. and Manning, D.P., 1986. Slip-resistance on icy surfaces of shoes, crampons and chains — a new machine. Journal of Occupational Accidents, 7: 273–283. A new machine for measuring friction of shoe solings is described. Shoes and attachments such as crampons and chains worn by a test subject are dragged across the substrate, the horizontal force being measured by a load cell. The machine was used to measure coefficient of friction (COF) on ice and was capable of discriminating between very low friction readings. There was good correlation between the horizontal force recorded by two test subjects using the same shoes. There was also a good correlation between hardness of solings and friction. The highest friction values were generated by the softest materials. It is likely that on ice the optimum hardness is in the range 20 to 30 Shore A. Friction of the worst solings was half the value of the best and it should therefore be possible to reduce accidents on ice by specifying the softest solings. Nevertheless, friction from solings cannot be expected to give complete protection from slipping accidents on smooth ice and only crampons give adequate friction values. Chains attached to shoes have been found to be dangerous on smooth ice.

Polar frost formation on Ganymede

The suggested models of polar frost formation on Ganymede are reviewed. A model in which plasma bombardment changes the reflectance characteristics of the icy surface is proposed.

The composition and origin of the Iapetus dark material

Telescopic observations, laboratory simulations, and photogeological studies have been conducted to investigate the composition of the dark material on Iapetus and the reasons for its peculiar distribution. The results of our earlier studies ( D.P. Cruikshank, J.F. Bell, M.J. Gaffey, R.H. Brown, R. Howell, C. Beermen, and M. Rognstad, 1983, Icarus 53 90–104) are confirmed and extended. Improved telescopic spectra of leading and trailing hemispheres were obtained over the range 0.3–2.6 μm. A mixing model was used to correct the leading-side spectral data for the presence of regions of bright terrain at both poles. The resulting true dark-unit spectrum is much redder in the visible and near-IR than previous uncorrected spectra, but gradually flattens near 2.0 μm. This unusual spectrum is reproduced with an intimate mixture of simulated meteoritic organic polymers (10%) and hydrated silicates (90%) which corresponds to an extension of the compositional trends in known carbonaceous meteorites toward lower formation temperatures. Structures in the dark/bright transition zone visible in the Voyager 2 images are inconsistent with an eruptive origin for the dark material; this zone is probably the region of grazing impacts of dust spiraling in from Phoebe. The dark material is probably a native component of the original icy surface concentrated in a thin, devolatilized regolith produced by the Phoebe-dust bombardment. Similar “ultracarbonaceous” material dominates the surfaces of the D-type asteroids and may be the elusive nonice component in the surfaces of other icy satellites. An Iapetus-like bombardment regime may be the cause of the large hemispheric asymmetry in regolith properties on Callisto.

Some radiation-chemical aspects of chemistry in cometary nuclei

The radiation-chemical approach to chemistry in the cometary nucleus is examined. The inventory of absorbed doses in the core and icy surface layers is made after considering the contribution of cosmic rays and some radionuclides imbedded in the cometary material during accretion. The amount of radiation-induced alterations of pristine cometary material is estimated in the light of achievments in radiation chemistry and appears to be more important than generally admitted. Radiation-chemical consequences of the possible existence of a liquid core produced by the radio-genic heat of 26Al are also considered. Experimental approaches at low temperatures are examined, as well as the contribution of room temperature experiments on systems relevant to cometary nuclei. It has been pointed out that more systematic experimental work is need which should concern not only the icy surface layers but the entire cometary nucleus, including a possible liquid core. It is concluded that the radiation-chemical approach merits more attention than it has been given until now in multidisciplinary studies of cometary nuclei.

Does crater “saturation equilibrium” occur in the solar system?

Similarity is found in crater densities on the most heavily cratered surfaces throughout the solar system. This is hypothesized to result from a steady-state “saturation equilibrium” being approached or achieved by cratering processes. This hypothesis conflicts with some recent interpretations. However, it accounts for (1) a similarity in maximum relative crater density, below certain theoretically predicted values, on all heavily cratered surfaces; (2) a leveling off at this same relative density among 100-m scale (secondary?) craters in populations on lunar maria and other sparsely cratered lunar surfaces; (3) the approximate uniformity of maximum relative densities on Saturn satellites (in spite of dramatic variations predicted from nonsaturation models assuming heliocentric impactors). The lunar frontside upland crater population, sometimes described as a well-preserved production function useful for interpreting other planetary surfaces, is found not to be a production function. It was modified by intercrater plains formed (at least partly) by early upland basaltic lava flooding, recently confirmed spectrophotometrically. Consistent with this, counts in “pure uplands” (those lacking intercrater plains) match the proposed saturation equilibrium density. Variations among large ( D > 64 km) crater populations are found, but these may involve several hypothesized mechanisms that rapidly obliterate large craters, especially on icy surfaces. Recent models, in which different populations of interplanetary bodies hit different planets, need further appraisal.

Solar System Ice: Amorphous or Crystalline?

The role of meteoritic bombardment on icy surfaces in the solar system is investigated. Using recent theoretical results concerning the nature of ejecta from impact craters in ice, the author concludes that the ratio of amorphous to crystalline ice surfaces should be lower then 1.0.

Energetic charged particle erosion of ices in the solar system

Ice is known to be a pervasive constituent of the solar system and, probably, the interstellar medium. In most regions of space icy objects and ice-covered surfaces are exposed to energetic charged particle (electron and ion) bombardment. These charged particles change the surface layers via implantation, bond rearrangement, and erosion. Experiments and observations related to the erosion and modification of icy surfaces in space are briefly reviewed here.

Plasma ion‐induced molecular ejection on the Galilean satellites: Energies of ejected molecules

First measurements of the energy of ejection of molecules from icy surfaces by fast incident ions are presented. Such results are needed in discussions of the Jovian and Saturnian plasma interactions with the icy satellites. In this letter parameters describing the ion‐induced ejection and redistribution of molecules on the Galilean satellites are recalculated in light of the new laboratory data.

Methane ice on Pluto and Triton

Methane (CH4), the simplest of organic compounds, is found in the form of a gas associated with many solar system objects. Methane as a gas in the atmospheres of the Jovian planets and satellites and as a liquid on some of their surfaces probably formed in a primordial chemical reaction between hydrogen and carbon, not as a result of biological processes. Of interest to planetary scientists is the predicted occurrence of solid, crystalline methane as an important mineral phase. So far, the solid phases on the icy surfaces of the cold planets and their satellites have been identified as water (H20) ice. Recently, astronomers from the University of Hawaii and from the Jet Propulsion Laboratory have discovered crystalline methane on the surfaces of the distant planet Triton, which is a large satellite of Neptune, and on Pluto. These are the only bodies within the solar system so far that may have observable methane ice, although others may have methane mixtures.