Mapping of Io's thermal radiation by the Galileo photopolarimeter-radiometer (PPR) instrument

Between 1999 and 2002, the Galileo spacecraft made 6 close flybys of Io during which many observations of Io's thermal radiation were made with the photopolarimeter-radiometer (PPR). While the NIMS instrument could measure thermal emission from hot spots with T>200 K, PPR was the only Galileo instrument capable of mapping the lower temperatures of older, cooling lava flows, and the passive background. We tabulate all data taken by PPR of Io during these flybys and describe some scientific highlights revealed by the data. The data include almost complete coverage of Io at better than 250 km resolution, with extensive regional coverage at higher resolutions. We found a modest poleward drop in nighttime background temperatures and evidence of thermal inertia variations across the surface. Comparison of high spatial resolution temperature measurements with observed daytime SO₂ gas pressures on Io provides evidence for local cold trapping of SO₂ frost on scales smaller than the 60 km resolution of the PPR data. We also calculated the power output from several hot spots and estimated total global heat flow to be about 2.0-2.6 W m⁻². The low-latitude diurnal temperature variations for the regions between obvious hot spots are well matched by a laterally-inhomogeneous thermal model with less than 1 W m⁻² endogenic heat flow.     

An empirically derived lunar gravity field

The heat-flow experiment is one of the Apollo Lunar Surface Experiment Package (ALSEP) instruments that was emplaced on the lunar surface on Apollo 15. This experiment is designed to make temperature and thermal property measurements in the lunar subsurface so as to determine the rate of heat loss from the lunar interior through the surface. About 45 days (1 1/2 lunations) of data has been analyzed in a preliminary way. This analysis indicates that the vertical heat flow through the regolith at one probe site is 3.3 × 10^–6 W/cm² (±15%). This value is approximately one-half the Earth's average heat flow. Further analysis of data over several lunations is required to demonstrate that this value is representative of the heat flow at the Hadley Rille site. The mean subsurface temperature at a depth of 1 m is approximately 252.4K at one probe site and 250.7K at the other. These temperatures are approximately 35K above the mean surface temperature and indicate that conductivity in the surficial layer of the Moon is highly temperature dependent. Between 1 and 1.5m, the rate of temperature increase as a function of depth is 1.75K/m (±2%) at the probe 1 site. In situ measurements indicate that the thermal conductivity of the regolith increases with depth. Thermal-conductivity values between 1.4 × 10^–4 and 2.5 × 10^–4 W/cm K were determined; these values are a factor of 7 to 10 greater than the values of the surface conductivity. If the observed heat flow at Hadley Base is representative of the moonwide rate of heat loss (an assumption which is not fully justified at this time), it would imply that overall radioactive heat production in the Moon is greater than in classes of meteorites that have formed the basis of Earth and Moon bulk composition models in the past.Lamont-Doherty Geological Observatory Contribution Number 1800.     

Atlas of lunar radar maps at 70-cm wavelength

The intensity distribution of lunar radar echoes has been mapped for two-thirds of the earth-visible lunar surface at a wavelength of 70 cm. The depolarizing effects of the lunar surface were observed by simultaneously receiving the radar echoes in opposite polarizations. These echoes were mapped with areal resolutions of 25–100 km². Mappings with this resolution confirmed that the young craters have enhanced returns. A few craters were found to have enhanced echoes only from their rims. Backscattering differences were also observed between various areas within a mare, between different highland areas, and between maria and adjacent highlands. These scattering differences were interpreted with a simple model, which assumed that the surface backscattered with varying amounts of quasi-specular and diffuse power. Only an increase in the diffuse power was needed to give the numerical values of the enhancements.     

Lunar remnant magnetic field mapping from orbital observations of mirrored electrons

Areas of lunar surface magnetic field are observed to ‘mirror’ low energy electrons present in the normal lunar space environment. The ambient electrons provide, in effect, a probe along the ambient magnetic field lines down to the lunar surface for remote sensing of the presence of surface fields. This probe, unlike direct measurement by the magnetometer, does not require low altitude or a very stable (magnetotail) ambient field to provide a mapping of regions of occurrence of such fields. Use of the on-board vector magnetometer measurements of the ambient magnetic field orientation allows accurate projection of such mapping onto the lunar surface. Preliminary maps of the lunar surface magnetic areas underlying the orbit of the ‘Particles and Fields Satellite deployed from Apollo-16’ have been generated, obtaining 40% coverage from partial data to demonstrate feasibility of the technique. As well as providing independent verification of areas such as Van de Graaff already discovered in the magnetometer data, these maps reveal many previously unreported areas of surface magnetism. The method is sensitive to fields of less than 0.1γ at the surface. Application to the full body of available PFS-1 & 2 electron data is expected to provide complete mapping of the lunar surface for areas of magnetization up to latitudes of 35–40 deg. The surface field regions observed are generally due to sources smaller than 10–50 km in size, although many individual regions are often so close together as to give much larger regions of effectively continuous mirroring. Absence of consistent mirroring by any global field places an upper limit on the size of any net lunar dipole moment of less than 10¹⁰ γ km³. Much additional information regarding the magnetic regions can be obtained by correlated analysis of both the electron return and vector magnetometer measurements at orbital altitude, the two techniques providing each other with directly complimentary measurements at the satellite and along the ambient field lines to the surface.     

Thermophysical properties of lunar material returned by Apollo missions

Data on thermophysical properties measured on lunar material returned by Apollo missions are reviewed. In particular, the effects of temperature and interstitial gaseous pressure on thermal conductivity and diffusivity have been studied. For crystalline rocks, breccias and fines, the thermal conductivity and diffusivity decrease as the interstitial gaseous pressure decreases from 1 atm to 10^–4T. Below 10^–4T, these properties become insensitive to the pressure. At a pressure of 10^–4T or below, the thermal conductivity of fines is more temperature dependent than that of crystalline rocks and breccias. The bulk density also affects the thermal conductivity of the fines. An empirical relationship between thermal conductivity, bulk density and temperature derived from the study of terrestrial material is shown to be consistent with the data on lunar samples. Measurement of specific heat shows that, regardless of the differences in mineral composition, crystalline rocks and fines have almost identical specific heat in the temperature range between 100 and 340K. The thermal parameter calculated from thermal conductivity, density and specific heat shows that the thermal properties estimated by earth-based observations are those characteristic only of lunar fines and not of crystalline rocks and breccias. The rate of radioactive heat generation calculated from the content of K, Th and U in lunar samples indicates that the surface layer of the lunar highland is more heat-producing than the lunar maria. This may suggest fundamental differences between the two regions.Now at Lamont-Doherty Geological Observatory, Columbia University, Palisades, New York, U.S.A.     

Mapping global lunar abundance of plagioclase,clinopyroxene and olivine with Interference Imaging Spectrometer hyperspectral data considering space weathering effect

Mineral mapping of the lunar surface is critical to understanding the Moon’s geological diversity and history, yet the global lunar abundance of minerals has not been mapped using hyperspectral data. The Interference Imaging Spectrometer (IIM) of Chang’E-1 mission obtained hyperspectral data of the global lunar surface within the wavelength of 480–960 nm in which major minerals can be discriminated by faint differences in 32 contiguous hyperspectral bands. The effect of space weathering produces multiple endmembers of lunar minerals by obscuring the pure spectra of minerals in different levels. In this study, the distributions of plagioclase, clinopyroxene and olivine on the global lunar surface were mapped with IIM hyperspectral data based on the modified Multiple Endmember Spectral Mixture Analysis (MESMA) method considering the space weathering effect. The distribution of lunar space weathering levels was retrieved as a byproduct of mineral mapping. The mineral mapping results were compared with recent mapping results. Although the wavelength of IIM is limited, it shows that our results are basically consistent with the recent research at both global and local scales. The distribution of space weathering levels is also consistent with the map of optical maturity parameter (OMAT) in most parts of the global lunar surface, especially in the highlands. This study demonstrates that the modified MESMA method is an effective approach to quantitative mapping of the lunar minerals and space weathering levels using hyperspectral data. In the future, more minerals can be mapped with higher accuracy if hyperspectral data with a wider spectral range are used based on the method proposed in this study.     

The evolution of the moon

The thermal evolution of the Moon as it can be defined by the available data and theoretical calculations is discussed. A wide assortment of geological, geochemical and geophysical data constrain both the present-day temperatures and the thermal history of the lunar interior. On the basis of these data, the Moon is characterized as a differentiated body with a crust, a 1000-km-thick solid mantle (lithosphere) and an interior region (core) which may be partially molten. The presence of a crust indicates extensive melting and differentiation early in the lunar history. The ages of lunar samples define the chronology of igneous activity on the lunar surface. This covers a time span of about 1.5 billion yr, from the origin to about 3.16 billion yr ago. Most theoretical models require extensive melting early in the lunar history, and the outward differentiation of radioactive heat sources.Thermal history calculations, whether based on conductive or convective computation codes define relatively narrow bounds for the present day temperatures in the lunar mantle. In the inner region of the 700 km radius, the temperature limits are wider and are between about 100 and 1600°C at the center of the Moon. This central region could have a partially or totally molten core.The lunar heat flow values (about 30 ergs/cm²s) restrict the present day average uranium abundance to 60 ± 15 ppb (averaged for the whole Moon) with typical ratios of K/U = 2000 and Th/U = 3.5. This is consistent with an achondritic bulk composition for the Moon.The Moon, because of its smaller size, evolved rapidly as compared to the Earth and Mars. The lunar interior is cooling everywhere at the present and the Moon is tectonically inactive while Mars could be and the Earth is definitely active.     

The antiquity indicator argon‐40/argon‐36 for lunar surface samples calibrated by uranium‐235‐xenon‐136 dating

Abstract— Several solar gas rich lunar soils and breccias have trapped ⁴⁰Ar/³⁶Ar ratios >10, although solar Ar is expected to yield a ratio of <0.01. Radiogenic ⁴⁰Ar produced in the lunar crust from ⁴⁰K decay was outgassed into the lunar atmosphere, ionized, accelerated in the electromagnetic field of the solar wind, and reimplanted into lunar surface material. The ⁴⁰Ar loss rate depends on the decreasing abundance of ⁴⁰K. In order to calibrate the time dependence of the ⁴⁰Ar/³⁶Ar ratio in lunar surface material, the period of reimplantation of lunar atmospheric ions and of solar wind Ar was determined using the ²³⁵U‐¹³⁶Xe dating method that relies on secondary cosmic‐ray neutron‐induced fission of ²³⁵U. We identified the trapped, fissiogenic, and cosmogenic noble gases in lunar breccia 14307 and lunar soils 70001‐8, 70181, 74261, and 75081. Uranium and Th concentrations were determined in the 74261 soil for which we obtain the ²³⁵U‐¹³⁶Xe time of implantation of 3.25^+0.38_‐0.60 Ga ago. On the basis of several cosmogenic noble gas signatures we calculate the duration of this near surface exposure of 393 ± 45 Ma and an average shielding depth below the lunar surface of 73 ± 7 g/cm². A second, recent exposure to solar and cosmic‐ray particles occurred after this soil was excavated from Shorty crater 17.2 ± 1.4 Ma ago. Using a compilation of all lunar data with reliable trapped Ar isotopic ratios and pre‐exposure times we infer a calibration curve of implantation times, based on the trapped⁴⁰ Ar/³⁶Ar ratio. A possible trend for the increase with time of the solar ³He/⁴He and ²⁰Ne/²²Ne ratios of about 12%/Ga and about 2%/Ga, respectively, is also discussed.     

The local inhomogeneities of the chemical composition on the surface of the strontium Ap-star HD 140160

The surface inhomogeneities of the chemical composition of HD 140160 (χ Ser) were studied from the spectral line profiles. The method of quantitative analysis of local abundance proposed by Khokhlova (1974) was used. The overabundance of Sr up to 10³ was found in three ‘spots’ on the surface of the star and normal abundance outside the spots. Iron was found to be overabundant up to 10 times in four spots and deficient by 10 times outside the spots. Some arguments in favour of a supernova hypothesis are mentioned to explain the deficiency of iron.     

Volatile emission on the moon; Possible sources and release mechanisms

Evidence for very recent emission of volatiles on the Moon is primarily of four types: (1) transient lunar optical events observed by Earth-based astronomers; (2) excursions on Apollo SIDE and mass spectrometer instruments; (3) localized Rn²²²/Po²¹⁰ enhancements on the lunar surface detected by Apollo 15 and 16 orbital alpha spectrometers; (4) presence in lunar fines of retrapped Ar⁴⁰ and other volatiles. Available evidence indicates that the release rate of volatile substances into the lunar atmosphere is not steady, but instead sporadic and episodic. Rn²²²/Po²¹⁰ anomalies are at locations that are among those from which transient events have most often been reported (edges of maria, certain specific craters), and are probably related to them. Volatiles emitted at maria rims may originate in the Moon's fluid core, reaching the surface through deep cylindrical fault systems that ring the maria borders. The sources of volatiles emitted at craters such as Aristarchus or Tsiolkovsky, which possess floors which are cracked or filled with dark lava and possess central peaks, are more likely to be local pockets of magma or trapped gas at shallower depths. The volatiles are produced directly by radioactive decay (He⁴, Ar⁴⁰, Rn) and by heating (other volatiles). The release by heating can occur either during melting or by ‘bakeout’ of unmelted materials. Release of gas into the lunar atmosphere is probably triggered by buildup of its own pressure. This may be assisted by tidal forces exerted on the Moon by the Earth. In addition to independent release, volatile emission is also expected to accompany other lunar activity, such as ash flows, if any lunar volcanism is presently active.

     

Amalthea: Implications of the temperature observed by Voyager

Voyager 1 IRIS observations of Amalthea, although initially indicating an unusually high temperature, now give a temperature of only 164 ± 5°K, a value consistent with the Earth-based measurement by G. H. Rieke [Icarus25, 333–334 (1975)] of 155 ± 15°K. We numerically modeled the temperature profile in the satellite's surface layer as a function of location and time of day, assuming a triaxial ellipsoid shape and thermal properties similar to those of the lunar soil. The major heat source is direct insolation, but temperatures are increased slightly by thermal radiation from Jupiter (?9°K), by sunlight reflected from the planet (?5°K), and by charged particle bombardment (?2°K). Maximum calculated temperatures reach 166°K, and we estimate that the temperature that Voyager would have measured under these circumstances is ≈160°K, in agreement with the observed temperature. Possible sources of error in the model are discussed in detail, including satellite shape effects, unusually low emissivity, uncommonly rough surface, abnormal thermal intertia, variability of the charged particle flux, and Joule heating. The IRIS observation strongly suggests that (i) the Amalthean surface has an emissivity near unity; (ii) the charged particle flux on the satellite at the time of observation was no more than 20 times larger than the flux indicated by Pioneer observations; and (iii) Joule heating of the satellite is insignificant (a conclusion also supported by rough calculations). The IRIS observation cannot, however, put any useful limits on the thermal inertia of the Amalthean surface layer.     

The atmosphere of Titan: An analysis of the Voyager 1 radio occultation measurements

Two coherently related radio signals transmitted from Voyager 1 at wavelengths of 13 cm (S-band) and 3.6 cm (X-band) were used to probe the equatorial atmosphere of Titan. The measurements were conducted during the occultation of the spacecraft by the satellite on November 12, 1980. An analysis of the differential dispersive frequency measurements did not reveal any ionization layers in the upper atmosphere of Titan. The resolution was approximately 3 × 10³ and 5 × 10³ electrons/cm³ near the evening and morning terminators, respectively. Abrupt signal changes observed at ingress and egress indicated a surface radius of 2575.0 ± 0.5 km, leading to a mean density of 1.881 ± 0.002 g cm^?3 for the satellite. The nondispersive data were used to derive profiles in height of the gas refractivity and microwave absorption in Titan's troposphere and stratosphere. No absorption was detected; the resolution was about 0.01 dB/km at the 13-cm wavelength. The gas refractivity data, which extend from the surface to about 200 km altitude, were interpreted in two different ways. In the first, it is assumed that N₂ makes up essentially all of the atmosphere, but with very small amounts of CH₄ and other hydrocarbons also present. This approach yielded a temperature and pressure at the surface of 94.0 ± 0.7°K and 1496 ± 20 mbar, respectively. The tropopause, which was detected near 42 km altitude, had a temperature of 71.4 ± 0.5°K and a pressure of about 130 mbar. Above the tropopause, the temperature increased with height, reaching 170 ± 15°K near the 200-km level. The maximum temperature lapse rate observed near the surface (1.38 ± 0.10°K/km) corresponds to the adiabatic value expected for a dry N₂ atmosphere—indicating that methane saturation did not occur in tbis region. Above the 3.5-km altitude level the lapse rate dropped abruptly to 0.9 ± 0.1°K/km and then decreased slowly with increasing altitude, crossing zero at the tropopause. For the N₂ atmospheric model, the lapse rate transition at the 3.5-km level appears to mark the boundary between a convective region near the surface having the dry adiabatic lapse rate, and a higher stable region in radiative equilibrium. In the second interpretation of the refractivity data, it is assumed, instead, that the 3.5 km altitude level corresponds to the bottom of a CH₄ cloud layer, and that N₂ and CH₄ are perfectly mixed below this level. These assumptions lead to an atmospheric model which below the clouds contains about 10% CH₄ by number density. The temperature near the surface is about 95°K. Arguments concerning the temperature lapse rates computed from the radio measurements appear to favor models in which methane forms at most a limited haze layer high in the troposphere.     

A review of lunar chronology revealing a preponderance of 4.34–4.37 Ga ages

Data obtained from Sm‐Nd and Rb‐Sr isotopic measurements of lunar highlands’ samples are renormalized to common standard values and then used to define ages with a common isochron regression algorithm. The reliability of these ages is evaluated using five criteria that include whether: (1) the ages are defined by multiple isotopic systems, (2) the data demonstrate limited scatter outside uncertainty, (3) initial isotopic compositions are consistent with the petrogenesis of the samples, (4) the ages are defined by an isotopic system that is resistant to disturbance by impact metamorphism, and (5) the rare‐earth element abundances determined by isotope dilution of bulk of mineral fractions match those measured by in situ analyses. From this analysis, it is apparent that the oldest highlands’ rock ages are some of the least reliable, and that there is little support for crustal ages older than approximately 4.40 Ga. A model age for ur‐KREEP formation calculated using the most reliable Mg‐suite Sm‐Nd isotopic systematics, in conjunction with Sm‐Nd analyses of KREEP basalts, is 4389 ± 45 Ma. This age is a good match to the Lu‐Hf model age of 4353 ± 37 Ma determined using a subset of this sample suite, the average model age of 4353 ± 25 Ma determined on mare basalts with the ¹⁴⁶Sm‐¹⁴²Nd isotopic system, with a peak in Pb‐Pb ages observed in lunar zircons of approximately 4340 ± 20 Ma, and the oldest terrestrial zircon age of 4374 ± 6 Ma. The preponderance of ages between 4.34 and 4.37 Ga reflect either primordial solidification of a lunar magma ocean or a widespread secondary magmatic event on the lunar nearside. The first scenario is not consistent with the oldest ages reported for lunar zircons, whereas the second scenario does not account for concordance between ages of crustal rocks and mantle reservoirs.     

Minerals mapping of the lunar surface with Clementine UVVIS/NIR data based on spectra unmixing method and Hapke model

The distribution of minerals on the lunar surface is information which could contribute to studying lunar origin and evolution. In this paper, the distribution of clinopyroxene, orthopyroxene, olivine, ilmenite, and plagioclase on the lunar surface has been mapped based on Hapke radiative transfer model and linear unmixing of spectra with Clementine UVVIS/NIR data. The results have been validated on the basis of minerals modal abundance data of the Apollo samples, and problems in the minerals abundance mapping have been analyzed. The validation based on analysis data of Apollo samples indicates that plagioclase mapped in this paper represents the total abundance of plagioclase and agglutinitic glass. The minerals mapping results show that the lunar surface is mainly composed of pyroxene, plagioclase, agglutinitic glass, and ilmenite. Basalt in the lunar mare is mainly composed of clinopyroxene and ilmenite, and lunar highland is mainly composed of plagioclase and agglutinitic glass. Orthopyroxene is mainly distributed on the north of Mare Imbrium, on the south of Maria and Aitken Basin. According to our results, there is probably no large area of olivine distribution on the lunar surface which is different from earlier published results. Therefore, emphasis should be put on the olivine distribution in the minerals mapping using hyperspectral data such as M³ of Chandrayaan-1 and IIM of ChangE-1.     

Chondrule cooling rates inferred from diffusive profiles in metal lumps from the Acfer 097 CR2 chondrite

Abstract— CR chondrites contain metal lumps (>300 μm) either attached to chondrule silicates or apparently isolated in the matrix. Here, laser ablation microanalysis of six metal lumps from a polished thin section of the Acfer 097 CR2 chondrite at 15 μm spatial resolution revealed zoning profiles for the volatile elements Cu and Ga. The mutual diffusivities of Cu and Ga were used to infer T ～ 1473 ± 100 K from the correlation of Cu versus Ga. The cooling rates of the metal lumps were calculated to be 0.5–50 K h^?1 for T_p ～ 1473 ± 100 K, with a maximum possible range of 0.1–400 K h^?1 for T_p ～ 1200–1800 K, overlapping the range of cooling rates inferred from petrological studies of type I chondrules (10–1000 K h^?1). Chondrule textures were established near the peak heating temperatures of chondrules (approximately 1900–2000 K), while the Cu and Ga diffusive profiles were established after solidification (T ～ 1500 K), consistent with nonlinear cooling. Furthermore, one chondrule (N2) has a more complex zoning profile that is modeled as a three‐stage cooling history involving initial cooling at approximately 1 K h^?1, followed by mild re‐heating (T ～ 1700 K) that re‐evaporated Cu and Ga from the outer approximately 100 μm of the metal lump and then cooled again at approximately 5 K h^?1. The thermal effects of parent body and other preaccretionary heating events on the Cu and Ga zoning profiles are examined. Although CR parent bodies have experienced aqueous alteration, the thermal effects of this process can neither produce nor erase the Cu and Ga diffusive profiles. Thus, metal lumps in CR chondrites record the solid‐state thermal history of chondrules as they travelled away from the chondrule‐forming region.     

The bombardment history of the Moon as recorded by 40Ar‐39Ar chronology

New petrography and ⁴⁰Ar‐³⁹Ar ages have been obtained for 1–3 mm sized rock fragments from Apollo 16 Station 13 soil 63503 (North Ray crater ejecta) and chips from three rocks collected by Apollo 16 and Apollo 17 missions. Selection of these samples was aimed at the old ⁴⁰Ar‐³⁹Ar ages to understand the early history of the lunar magnetic field and impact flux. Fifteen samples were studied including crustal material, polymict feldspathic fragmental breccias, and impact melts. The impact ages obtained range between approximately 3.3 and 4.3 billion years (Ga). Polymict fragmental breccia 63503,1 exhibits the lowest signs of recrystallization observed and a probable old relic age of 4.547 ± 0.027. The plateau age of 4.293 ± 0.044 Ga obtained for impact melt rock 63503,13 represents the oldest known age for such a lithology. Possibly, this age represents the minimum age for the South Pole‐Aitken (SPA) Basin. In agreement with literature data, these results show that impact ages >3.9 Ga are found in lunar rocks, especially within soil 63503. Impact exhumation of deep‐seated warm crustal material onto the lunar surface is considered to explain the common 4.2 Ga ages obtained for weakly shocked samples from soil 63503 and Apollo 17. This would directly imply that one or more basin‐forming events occurred at that time. Some rock fragments showing none to limited petrologic features indicate thermal annealing. These rocks may have lost Ar while resident within the hot‐ejecta of a large basin. Concurrent with previous studies, these results lead us to advocate for a complex impact flux in the inner solar system during the initial approximately 1.3 Ga.     

Io’s atmosphere: Constraints on sublimation support from density variations on seasonal timescales using NASA IRTF/TEXES observations from 2001 to 2010

We present an analysis of 19 μm spectra of Io’s SO₂ atmosphere from the TEXES mid-infrared high spectral resolution spectrograph on NASA’s Infrared Telescope Facility, incorporating new data taken between January 2005 and June 2010 and a re-analysis of earlier data taken from November 2001 to January 2004. This is the longest set of contiguous observations of Io’s atmosphere using the same instrument and technique thus far. We have fitted all 16 detected blended absorption lines of the ν₂ SO₂ vibrational band to retrieve the subsolar values of SO₂ column abundance and the gas kinetic temperature. By incorporating an existing model of Io’s surface temperatures and atmosphere, we retrieve sub-solar column densities from the disk-integrated data. Spectra from all years are best fit by atmospheric temperatures <150 K. Best-fit gas kinetic temperatures on the anti-Jupiter hemisphere, where SO₂ gas abundance is highest, are low and stable, with a mean of 108 (±18) K. The sub-solar SO₂ column density between longitudes of 90–220° varies from a low of 0.61 (±0.145) × 10^?17 cm^?2, near aphelion in 2004, to a high of 1.51 (±0.215) × 10¹⁷ cm^?2 in 2010 when Jupiter was approaching its early 2011 perihelion. No correlation in the gas temperature was seen with the increasing SO₂ column densities outside the errors.Assuming that any volcanic component of the atmosphere is constant with time, the correlation of increasing SO₂ abundance with decreasing heliocentric distance provides good evidence that the atmosphere is at least partially supported by frost sublimation. The SO₂ frost thermal inertias and albedos that fit the variation in atmospheric density best are between 150–1250 W m^?2 s^?1/2 K^?1 and 0.613–0.425 respectively. Photometric evidence favors albedos near the upper end of this range, corresponding to thermal inertias near the lower end. This relatively low frost thermal inertia produces larger amplitude seasonal variations than are observed, which in turn implies a substantial additional volcanic atmospheric component to moderate the amplitude of the seasonal variations of the total atmosphere on the anti-Jupiter hemisphere. The seasonal thermal inertia we measure is unique both because it refers exclusively to the SO₂ frost surface component, and also because it refers to relatively deep subsurface layers (few meters) due to the timescales of many years, while previous studies have determined thermal inertias at shallower levels (few centimeters), relevant for timescales of ～2 h (eclipse) or ～2 days (diurnal curves).     

Irradiation et pré-irradiations de la brèche 14307

Sample 14307,30, a gas-rich breccia (Group 1 of Warner, 1972) has been studied by coupling track method and light noble gas isotopic analysis. The breccia is made of a glassy dark matrix with embedded millimeter to sub-millimeter sized angular ligth xenoliths. These ones are breccia fragments of higher grade metamorphic facies (Group ? 2). A lighter strata (～ 0.5 cm thick) intersects the dark matrix. Noble gas analysis have shown the dark matrix (³⁶Ar = 5.4 × 10^?4 cc STP/g) to be enriched in solar type gases with respect to the light fractions (³⁶Ar ? 2.2 × 10^?4 cc STP/g). Themean value of the bulk ‘exposure age’ for different samplings is 180 ± 20 × 10⁶ yr, as calculated from spallogenic³He,²¹Ne and¹²⁶Xe contents, using our data and those of Bogard and Nyquist (1972). After appropriate correction for radiogenic⁴⁰Ar, the ratio⁴⁰Ar_exc/³⁶Ar_tr is about 5. A total of 390 crystals coming from 11 locations either in the dark matrix, the lighter strata or a light xenolith (0.25 cm diam), have been studied by track analysis using optical and scanning electron microscopy. 181 crystals were thoroughly investigated by means of the latter technique. The following results were obtained:

1. 72 crystals (70-300µm diam) from one location (No. 12) in the matrix show aminimum track density distribution spreading over 3 orders of magnitude (from 2 × 10⁶ up to 2 × 10⁹ cm^?2). The spectrum has at its lower edge a well defined peak (～ 50% of total crystal number) around 3 × 10⁶ cm^?2). Grains with track density variations over a factor of 3 have a low abundance as compared to average lunar soils. Moreover the mineralogy of this location is peculiar due to its large abundance in orthopyroxenes. Considering the lower edge of the track density distribution amaximum surface residence time of 5 × 10⁶ yr can be set for rock 14307 in itspresent shape;
2. 11 feldspars (1-15µm diam) and 22 clinopyroxenes (70-130µm) have been studied in the light xenolith. All crystals have minimum track densities larger than 10⁸ cm^?2. No spatial variation of track-densities (2.5 ± 0.5 × 10⁹ cm^?2) were found in feldspars inside a millimeter-sized polished section. Clearly these tracks were not acquired by an irradiation of the xenolith as an individual entity, but survived its own formation as a breccia of Group 2. Therefore, solar energetic iron particle tracks have not been erased despite a complex mechanical and thermal history involved by two subsequent brecciation processes;
3. in the 10 other locations, crystals (70-200µm diam) either from the dark matrix or the lighter strata show a significant departure from the pattern observed in lunar soils; namely:

1. the minimum track density distribution is strongly peaked at high values (～ 1-4 × 10⁹ cm^?2) for ～ 95% of the crystals, the remaining ～ 5% having low-values (0.2-1 × 10⁷ cm^?2);
2. the abundance (2%) of crystals with track density variation over a factor of 3 is about one order of magnitude less than in average lunar soils;
3. the magnitude of track density gradients within single crystals is small. In fact, thelargest track density variation versus depth found can be described by the relation? α D^?0.5, in contrast with soil grains which generally exhibit a variation of the form? α D^?1.1±0.4.

The above observations imply that the peculiar irradiation characteristics of these fossilized soils are more likely to be attributed to some wide scale process rather than to some accidental or local phenomena. Attempts to account for these findings by present solar VH particle flux and energy distribution (as determined by Crozaz and Walker, 1971; Fleischeret al., 1971b; Priceet al., 1971), current estimates of lunar fine scale erosion, accumulation and turn-over rates, have proven essentially negative. The bulk ‘exposure age’ of the breccia, rather low by lunar soil standards, makes things even worse. For lack of any better explanation, the above observations could be more easily understood by postulating a higher flux (by factors from ～ 10 up to 200) and a harder energy spectrum (at least for particles with rigidity less than 0.3 GV) for the solar cosmic rays at the time the constituents of the breccia were part as loose grains of the lunar regolith.     

Io: Volcanic thermal sources and global heat flow

We have examined thermal emission from 240 active or recently-active volcanic features on Io and quantified the magnitude and distribution of their volcanic heat flow during the Galileo epoch. We use spacecraft data and a geological map of Io to derive an estimate of the maximum possible contribution from small dark areas not detected as thermally active but which nevertheless appear to be sites of recent volcanic activity. We utilize a trend analysis to extrapolate from the smallest detectable volcanic heat sources to these smallest mapped dark areas. Including the additional heat from estimates for “outburst” eruptions and for a multitude of very small (“myriad”) hot spots, we account for ～62 × 10¹² W (～59 ± 7% of Io’s total thermal emission). Loki Patera contributes, on average, 9.6 × 10¹² W (～9.1 ± 1%). All dark paterae contribute 45.3 × 10¹² W (～43 ± 5%). Although dark flow fields cover a much larger area than dark paterae, they contribute only 5.6 × 10¹² W (～5.3 ± 0.6%). Bright paterae contribute ～2.6 × 10¹² W (～2.5 ± 0.3%). Outburst eruption phases and very small hot spots contribute no more than ～4% of Io’s total thermal emission: this is probably a maximum value. About 50% of Io’s volcanic heat flow emanates from only 1.2% of Io’s surface. Of Io’s heat flow, 41 ± 7.0% remains unaccounted for in terms of identified sources. Globally, volcanic heat flow is not uniformly distributed. Power output per unit surface area is slightly biased towards mid-latitudes, although there is a stronger bias toward the northern hemisphere when Loki Patera is included. There is a slight favoring of the northern hemisphere for outbursts where locations were well constrained. Globally, we find peaks in thermal emission at ～315°W and ～105°W (using 30° bins). There is a minimum in thermal emission at around 200°W (almost at the anti-jovian longitude) which is a significant regional difference. These peaks and troughs suggest a shift to the east from predicted global heat flow patterns resulting from tidal heating in an asthenosphere. Global volcanic heat flow is dominated by thermal emission from paterae, especially from Loki Patera (312°W, 12°N). Thermal emission from dark flows maximises between 165°W and 225°W. Finally, it is possible that a multitude of very small hot spots, smaller than the present angular resolution detection limits, and/or cooler, secondary volcanic processes involving sulphurous compounds, may be responsible for at least part of the heat flow that is not associated with known sources. Such activity should be sought out during the next mission to Io.     

Keck AO survey of Io global volcanic activity between 2 and 5 μm

We present in this Keck AO paper the first global high angular resolution observations of Io in three broadband near-infrared filters: Kc (2.3 μm), Lp (3.8 μm), and Ms (4.7 μm). The Keck AO observations are composed of 13 data sets taken during short time intervals spanning 10 nights in December, 2001. The MISTRAL deconvolution process, which is specifically aimed for planetary images, was applied to each image. The spatial resolution achieved with those ground-based observations is 150, 240, and 300 km in the Kc, Lp, and Ms band, respectively, making them similar in quality to most of the distant observations of the Galileo/NIMS instrument. Eleven images per filter were selected and stitched together after being deprojected to build a cylindrical map of the entire surface of the satellite. In Kc-band, surface albedo features, such as paterae (R>60 km) are easily identifiable. The Babbar region is characterized by extremely low albedo at 2.2 μm, and shows an absorption band at 0.9 μm in Galileo/SSI data. These suggest that this region is covered by dark silicate deposits, possibly made of orthopyroxene. In the Lp-Ms (3-5 μm) bands, the thermal emission from active centers is easily identified. We detected 26 hot spots in both broadband filters over the entire surface of the minor planet; two have never been seen active before, nine more are seen in the Ms band. We focused our study on the hot spots detected in both broadband filters. Using the measurements of their brightness, we derived the temperature and area covered by 100 brightness measurements. Loki displayed a relatively quiescent activity. Dazhbog, a new eruption detected by Galileo/NIMS in August 2001, is a major feature in our survey. We also point out the fading of Tvashtar volcanic activity after more than two years of energetic activity, and the presence of a hot, but small, active center at the location of Surt, possibly a remnant of its exceptional eruption detected in February 2001. Two new active centers, labeled F and V on our data, are detected. Using the best temperature and the surface area derived from the L and M band intensities, we calculated the thermal output of each active center. The most energetic hot spots are Loki and Dazhbog, representing respectively 36 and 19% of the total output calculated from a temperature fit of all hot spots (20.6×¹⁰¹² W). Based on the temperature derived from hot spots (∼400 K), our measurement can unambiguously identify the contribution to the heat flux from the silicate portion of the surface. Because the entire surface was observed, no extrapolation was required to calculate that flux. It is also important to note that we measured the brightness of the individual hot spots when they were located close to the Central Meridian. This minimizes the line-of-sight effect which does not follow strictly a classical cosine law. Finally, we argue that despite the widespread volcanic activity detected, Io was relatively quiescent in December 2001, with a minimum mean total output of 0.4-1.2 W m⁻². This output is at least a factor of two lower than those inferred from observations made at longer wavelengths and at different epochs.