Episodes of terrestrial geologic activity during the past 260 million years: A quantitative approach

Records of major geologic events of the past 260 Myr including: biologic extinction events, ocean-anoxic and black-shale events, major changes in sea level, major evaporite (salt) deposits, continental flood-basalt eruptions, first-order discontinuities in sea-floor spreading, and major mountain building events, have been aggregated and analyzed with moving-window and spectral techniques that facilitate recognition of clustering and possible cyclieity. Significant clustering of events suggests a model in which changes in rates and directions of sea-floor spreading (ridge jumps) are associated with episodic rifting, volcanism, mountain building, global sea level and changes in the composition of the earth's atmosphere via the carbon cycle. Variation in atmospheric CO₂ affects global climate, ocean circulation and marine productivity. The geologic data formally show a statistically significant underlying periodicity of 26.6 Myr for the Mesozoic and Cenozoic (the exact period differs with minor changes in geologic dating). Phase information suggests that the most recent maximum of the cycle occurred within the last 9 Myr, and may be close to the present time. The quasi-regular pulses of activity might be related to internal earth processes. However, a similar periodicity in impact craters and in galactic dynamics, and a one-to-one correlation among mass extinctions, large impact events, and flood-basalt volcanism, indicate an extraterrestrial pacemaker.     

The global distribution of pyroclastic deposits on Mercury: The view from MESSENGER flybys 1–3

We present a global survey of candidate pyroclastic deposits on Mercury, derived from images obtained during MESSENGER flybys 1–3 that provided near-global coverage at resolutions between 5 and 0.5 km/pixel. Thirty-five deposits were identified and characterized and are located principally on the floors of craters, along rims of craters, and along the edge of the Caloris basin. Deposits are commonly centered on rimless, often irregularly shaped pits, mostly between 5 and 45 km in diameter. The deposits identified are generally similar in morphology and absolute reflectance to lunar pyroclastic deposits. Spectrally the deposits appear brighter and redder than background Mercury terrain. On the basis of the available coverage, the candidate pyroclastic deposits appear to be essentially globally distributed. The diameters of the deposits, when mapped to lunar gravity conditions, are larger than their lunar counterparts, implying that more abundant volatiles were present during the typical eruptive process than on the Moon. These observations indicate that if these deposits resulted from hawaiian-style eruptions, the volatile contents required would be between ～1600 and 16,000 ppm CO or an equivalent value of H₂O, CO₂, SO₂, or H₂S (for a more oxidizing interior), or N₂, S₂, CS₂, S₂Cl, Cl, Cl₂, or COS (for a more reducing interior). These abundances are much greater than those predicted by existing models for Mercury's formation. An apparent lack of small deposits, compared with the Moon, may be due to resolution effects, a topic that can be further assessed during the orbital phase of the MESSENGER mission. These results provide a framework within which orbital observations by MESSENGER and the future BepiColombo mission can be analyzed.     

Cosmogenic activation of a natural tellurium target

¹³⁰Te is one of the candidates for the search for neutrinoless double beta decay. It is currently planned to be used in two experiments: CUORE and SNO+. In the CUORE experiment TeO₂ crystals cooled at cryogenic temperatures will be used. In the SNO+ experiment ^natTe will be deployed up to 0.3% loading in the liquid scintillator volume. A possible background for the signal searched for, are the high Q-value, long-lived isotopes, produced by cosmogenic neutron and proton spallation reaction on the target material. A total of 18 isotopes with Q-value larger than 2 MeV and T_1/2 > 20 days have been identified as potential backgrounds. In addition low Q-value, high rate isotopes can be problematic due to pile-up effects, specially in liquid scintillator based detectors. Production rates have been calculated using the ACTIVIA program, the TENDL library, and the cosmogenic neutron and proton flux parametrization at sea level from Armstrong and Gehrels for both long and short lived isotopes. The obtained values for the cross sections are compared with the existing experimental data and calculations. Good agreement has been generally found. The results have been applied to the SNO+ experiment for one year of exposure at sea level. Two possible cases have been considered: a two years of cooling down period deep underground, or a first purification on surface and 6 months of cooling down deep underground. Deep underground activation at the SNOLAB location has been considered.     

Sea level forcing in the Mediterranean Sea between 1960 and 2000

Sea level trends and inter-annual variability in the Mediterranean Sea for the period 1960–2000 is explored by comparing observations from tide gauges with sea level hindcasts from a barotropic 2D circulation model, and two full primitive equation 3D ocean circulation models, a regional one and the Mediterranean component of a global one,. In the 2D model, 50% of the sea level variance was found to result from the wind and atmospheric pressure forcing. In the 3D models, 20% of the sea level variance was explained by the steric effects. The sea level residuals at the tide gauges locations, calculated by subtraction of the 2D model output from the sea level observations are significantly correlated (r = 0.4) with the steric signals from the 3D models. After the removal of the atmospheric and the steric contributions the tide-gauge sea level records indicate a period where sea level was stable (1960–1975) and a period where sea level was rising (1975–2000) with rates in the range 1.1–1.8 mm/yr. A part of the residual trend can be explained by the contribution of local land movements (0.3 mm/yr) while its major part indicates a global signal, probably mass addition, appearing after 1975.     

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

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

Hydrous Silicates and Water on Venus

We used thermochemical equilibrium calculations to predict stabilities of pure rock-forming hydrous silicates on Venus' surface as a function of elevation, atmospheric H₂O and SO₂concentrations, and oxygen fugacity (f_O2). About 50 different hydrous silicates were included in our calculations. We find that many of these are unstable on Venus's surface because of the low atmospheric H₂O content of 30–45 parts per million by volume (ppmv) and the high surface temperatures (660 K on Maxwell Montes to 740 K in the plains). Hydrous Fe²⁺-bearing silicates are unstable due to oxidation to magnetite and/or hematite at the f_O2of the near-surface atmosphere. Ca-bearing hydrous silicates are unstable because of sulfatization to anhydrite. Some Fe-free micas (e.g., eastonite, eastonite–phlogopite micas), and some alkali amphiboles might be stable on Venus' surface, especially in the lower temperature highlands. We discuss hydrous mineral formation in the interior and on the surface of Venus. We review the literature on mica and amphibole thermal decomposition and find that dehydration of phlogopitic micas and fibrous amphiboles produces (metastable) dehydroxylated anhydrides that decompose to more stable minerals at temperatures hundreds of degrees higher than the onset of dehydroxylation. These observations raise the possibility that anhydrides formed from hydrous silicates, which may have been present during a wetter period in Venus' history, may persist somewhere on Venus' present surface. We discuss experiments that could be used on future spacecraft missions to detect hydroxyl in rocks and hydrous silicates on Venus. Finally, we review estimates of the amount of water and OH (hydroxyl) in the Earth's mantle. Based on this review, we suggest that even if no hydrous silicates are stable on Venus, significant amounts of water are plausibly present in surface rocks as OH in nominally anhydrous minerals.     

HiRISE observations of gas sublimation-driven activity in Mars’ southern polar regions: II. Surficial deposits and their origins

The High Resolution Imaging Science Experiment (HiRISE) onboard Mars Reconnaissance Orbiter (MRO) has been used to monitor the seasonal evolution of several regions at high southern latitudes and, in particular, the jet-like activity which may result from the process described by Kieffer (JGR, 112, E08005, doi:10.1029/2006JE002816, 2007) involving translucent CO₂ ice. In this work, we mostly concentrate on observations of the Inca City (81°S, 296°E) and Manhattan (86°S, 99°E) regions in the southern spring of 2007. Two companion papers, [Hansen et al. this issue] and [Portyankina et al. this issue], discuss the surface features in these regions and specific models of the behaviour of CO₂ slab ice, respectively. The observations indicate rapid on-set of activity in late winter initiating before HiRISE can obtain adequately illuminated images (Ls < 174° at Inca City). Most sources become active within the subsequent 8 weeks. Activity is indicated by the production of dark deposits surrounded by brighter bluer deposits which probably arise from the freezing out of vented CO₂ [Titus et al., 2007. AGU (abstract P41A-0188)]. These deposits originate from araneiform structures (spiders), boulders on ridges, cracks on slopes, and along linear cracks in the slab ice on flatter surfaces. The type of activity observed can often be explained qualitatively by considering the local topography. Some dark fans are observed to shorten enormously in length on a timescale of 18 days. We consider this to be strong evidence that outgassing was in progress at the time of HiRISE image acquisition and estimate a total particulate emission rate of >30 g s⁻¹ from a single typical jet feature. Brighter deposits at Inca City become increasingly hard to detect after Ls = 210°. In the Inca City region, the orientations of surficial deposits are topographically controlled. The deposition of dark material also appears to be influenced by local topography suggesting that the ejection from the vents is at low velocity (<10 m s⁻¹) and that a ground-hugging flow process (a sort of “cryo-fumarole”) may be occurring. The failure up to this point to obtain a clear detection of outgassing though stereo imaging is consistent with low level transport. The downslope orientation of the deposits may result from the geometry of the vent or from catabatic winds. At many sites, more than one ejection event appears to have occurred suggesting re-charging of the sources. Around Ls = 230°, the brightness of the surface begins to drop rapidly on north-facing slopes and the contrast between the dark deposits and the surrounding surface reduces. This indicates that the CO₂ ice slab is being lost completely in some areas at around this time. By Ls = 280°, at Inca City, the ice slab has effectively gone. CRISM band ratios and THEMIS brightness temperature measurements are consistent with this interpretation.     

Experimental constraints on the volatility of germanium,zinc, and lithium in Martian basalts and the role of degassing in alteration of surface minerals

The surface of Mars is enriched in Cl and S which is linked to volcanic activity and degassing. Similarly, elevated Ge and Zn levels in Gale crater sedimentary bedrock indicate a magmatic source for these elements. To constrain the relative effects of Cl and S on the outgassing of these trace metals and chemical characteristics of primary magmatic vapor deposits incorporated to Martian surface, we conducted a set of degassing and fumarolic alteration experiments. Ge is found to be more volatile than Zn in all experiments. In S-bearing runs, the loss of Ge and Zn was less than any other experiments. In Cl-only runs, degassing of Zn was more than twice that of Ge within the first 10 min and percent loss increased for both elements with increasing time. In Cl + S runs, S-induced reduction of GeO₂ and ZnO to metallic Ge and Zn switches the preference of chloride formation from Zn to Ge. Up to 90% of Ge and Zn loss in the 1-h no volatile-added (NVA) experiments might be due to the small amounts of Cl contamination in NVA mixes via other oxides used for synthesis. Alteration experiments show different phases between 1-h and 24-/72-h runs. In 1-h runs, anhydrite and langbeinite dominate while in 24-/72-h runs halite and sylvite dominate the condensate assemblages. S-bearing phases form as the intermediate products of fumarolic deposition, while chlorides are common when the system is allowed to cool gradually. One-hour exposure was sufficient to form alteration phases and vapor deposits such as NaCl, KCl, CaSO₄, and langbeinites on the Martian analog minerals. These salts were identified in Martian meteorites and in situ measurements. Our results provide evidence that volcanic degassing along with fumarolic alteration could be a potential source for the enrichment and varying abundances of Cl, S, Fe, Zn, Ge in Martian surface, as well as a cause for Ge depletion in shergottites.     

South polar residual cap of Mars: Features, stratigraphy, and changes

The south residual polar cap of Mars, rich in CO₂ ice, is compositionally distinct from the north residual cap which is dominantly H₂O ice. The south cap is also morphologically distinct, displaying a bewildering variety of depressions formed in thin layered deposits, which have been observed to change by scarp retreat over an interval of one Mars year (Malin et al., 2001, Science 294, 2146-2148). The climatically sensitive locale of the residual caps suggests that their behavior may help in the interpretation of recent fluctuations or repeatability of the Mars climate. We have used Mars Global Surveyor Mars Orbiter Camera (MOC) images obtained in three southern summers to map the variety of features in the south residual cap and to evaluate changes over two Mars years (Mars y). The images show that there are two distinct layered units which were deposited at different times separated by a period of degradation. The older unit, ∼10 m thick, has layers approximately 2 m thick. The younger unit has variable numbers of layers, each ∼1 m thick. The older unit is eroding by scarp retreat averaging 3.6 m/Mars y, a rate greater than the retreat of 2.2 m/Mars y observed for the younger unit. The rates of scarp retreat and sizes of the different types of depressions indicate that the history of the residual cap has been short periods of deposition interspersed with longer erosional periods. Erosion of the older unit probably occupied ∼100-150 Mars y. One layer may have been deposited after the Mariner 9 observations in 1972. Residual cap layers appear to differ from normal annual winter deposits by having a higher albedo and perhaps by having higher porosities. These properties might be produced by differences in the depositional meteorology that affect the fraction of high porosity snow included in the winter deposition.     

Glacial-interglacial mean sea level pressure change due to sea level, ice sheet and atmospheric mass changes

The change in the global mean atmospheric pressure between glacial and interglacial periods is evaluated at sea level. This change originates in a modification of topography and in a possible variation in the atmospheric mass. In this calculation the atmosphere is at hydrostatic equilibrium, and the parameters describing the glacial period are varied in a plausible range. The result, with constant atmospheric mass, is a mean sea level pressure decrease of 9–15 hPa linked with the deglaciation. The corresponding pressure change at the reference level corresponding to the present day sea level does not exceed one hPa. When considering only the change in the atmospheric mass, an increase which does not exceed 2 hPa is found, linked with the deglaciation.     

Light dement geochemistry of the Tagish Lake CI2 chondrite: Comparison with CI1 and CM2 meteorites

Abstract— We have studied the carbon and nitrogen stable isotope geochemistry of a small pristine sample of the Tagish Lake carbonaceous chondrite by high‐resolution stepped‐combustion mass spectrometry, and compared the results with data from the Orgueil (CI1), Elephant Moraine (EET) 83334 (CM1) and Murchison (CM2) chondrites. The small chip of Tagish Lake analysed herein had a higher carbon abundance (5.81 wt%) than any other chondrite, and a nitrogen content (?1220 ppm) between that of CI1 and CM2 chondrites. Owing to the heterogeneous nature of the meteorite, the measured carbon abundance might be artificially high: the carbon inventory and whole‐rock carbon isotopic composition (δ¹³C ? +24.4%_o) of the chip was dominated by ¹³C‐enriched carbon from the decomposition of carbonates (between 1.29 and 2.69 wt%; δ¹³C ? +67%_o and δ¹⁸O ? +35%_o, in the proportions ?4:1 dolomite to calcite). In addition to carbonates, Tagish Lake contains organic carbon (?2.6 wt%, δ¹³C ? ?9%_o; 1033 ppm N, δ¹⁵N ? +77%_o), a level intermediate between CI and CM chondrites. Around 2% of the organic material is thermally labile and solvent soluble. A further ?18% of the organic species are liberated by acid hydrolysis. Tagish Lake also contains a complement of presolar grains. It has a higher nanodiamond abundance (approximately 3650–4330 ppm) than other carbonaceous chondrites, along with ?8 ppm silicon carbide. Whilst carbon and nitrogen isotope geochemistry is not diagnostic, the data are consistent with classification of Tagish Lake as a CI2 chondrite.     

Interannual sea level variability in the South China Sea and its response to ENSO

Sea level observed by altimeter during the 1993–2004 period, thermosteric sea level from 1945 through 2004, and tide gauge records are analyzed to investigate the interannual variability of sea level in the South China Sea (SCS) and its relationship with ENSO (El Niño and Southern Oscillation). Both the interannual variations of the observed sea level and the thermosteric sea level are closely related to ENSO. An ‘enigma’ that the SST and sea level in the SCS have inverse response to ENSO is revealed. It is found that the thermosteric sea level has an excellent correspondence to seawater temperature at 100 m depth, and their variations are unsynchronized to SST. Detailed analysis denotes that the warming of seawater occurs only in the upper 75 m during and after the mature phase of El Niño, while the cooling appears in the layers deeper than 75 m during El Niño years. The volume transports between the SCS and the adjacent oceans and the anomalous Ekman pumping contribute a lot for the sea level fall in the developing stage of El Niño, while the mass exchange, which is dominated by precipitation, plays a more significant role in the following continuous negative sea level anomalies.     

Remnant Massifs of Layered Deposits at High Northern Latitudes of Mars

Ice deposits of the permanent northern polar cap of Mars exhibit a clearly expressed layered structure connected with consecutive accumulation of a solid phase of H₂O (ice) with different impurity fractions of dust. The observed structure of the northern polar cap deposits is direct evidence of the uneven accumulation of ice associated with climate variations on Mars over a long period of time. Outside the boundaries of the present northern polar cap, there are isolated remnant massifs of bright deposits with a clearly distinguishable layered structure. In this paper, we analyze the morphology, spatial distribution, and material composition of remnant massifs within the latitude belt of 60°–85°. The similarity in the structure and material composition found in the remnant massifs and the layered deposits of the northern polar cap of Mars apparently suggests that these formations were genetically connected in the past climatic epochs.     

SDSS J160043.6+074802.9: a very rapid sdO pulsator

We report the serendipitous discovery of the Sloan Digital Sky Survey (SDSS) star SDSS J160043.6+074802.9 to be a very rapid pulsator. The variation is dominated by a frequency near 8380 μHz (period = 119.33 s) with a large amplitude (0.04 mag) and its first harmonic at 16760 μHz (59.66 s; 0.005 mag). In between these frequencies, we find at least another eight variations with periods between 62 and 118 s and amplitudes between about 0.007 and 0.003 mag; weaker oscillations might also be present. Preliminary spectrograms from the performance verification phase of the Southern African Large Telescope indicate that SDSS J160043.6+074802.9 is a spectroscopic binary consisting of an sdO star and a late-type main-sequence companion. This makes it the first unambiguous detection of such an sdO star to pulsate, and certainly the first found to exhibit multifrequency variations.     

Possible connection between period change and magnetic activity of the very short‐period binary VZ Piscium

New times of light minimum of the short‐period (P = 0^d.26) close binary system, VZ Psc, are presented. A period investigation of the binary star, by combining the three new eclipse times with the others collected from the literatures, shows that the variation of the period might be in an alternate way. Under the hypothesis that the variation of the orbital period is cyclic, a period of 25 years and an amplitude of 0.^d0030 for the cyclic change are determined. If this periodic variation is caused by the presence of a third body, the mass of the third body (m₃) should be no less than 0.081M_⊙. Since both components of VZ Psc are strong chromospherically active and the level of activity of the secondary component is higher than that of the primary one, the period may be more plausibly explained by cyclic magnetic activity of the less massive component. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An Investigation of the Physical Mechanisms of Orbital Period Variations of the Semidetached Binary UW Vir

The orbital period variations of the Algol-type semidetached binary UW Vir are analyzed. It is shown that in addition to a long-term rapid increase (dP/dt = + 1.37 × 10⁻⁶ day/year), its orbit period has a variation with the period of 62.3 years. Based on the basic physical parameters given by Brancewicz and Dworak in 1980, the physical mechanisms causing the orbital period variations are investigated. The analysis indicates that the periodical variation of orbital period can be interpreted by the light-travel time effect due to the presence of a third body with the mass of M₃ ≥ 0.94 M. As no observational information has been reported for this tertiary component, it might be a compact object (e.g., a white dwarf). The long-term increase of orbital period can be explained in terms of the mass transfer from the secondary to the primary component (dM₂/dt = 1.43 × 10⁻⁷ M/year). This is in agreement with the semidetached configuration of the system with a lobe-filling secondary component. But according to the evolution theory of binaries, the Algol-type semidetached binary UW Vir should be at the evolutionary stage of slow mass transfer on the nuclear-reaction timescale of the secondary component. However, the analysis shows that the timescale for the periodical variation of orbital period is much shorter than the nuclear-reaction timescale of the secondary component, but close to the thermodynamic timescale of the secondary. This reveals that: (1) This binary system is at the evolutionary stage of rapid mass transfer on the thermodynamic timescale of the secondary component; or (2) The circumstellar matter of the system makes a contribution to the rapid increase of orbital period via the angular momentum transfer.     

Magnetic cycle of LQ Hydrae: observational indications and dynamo model

We present a model for the differential rotation and dynamo activity of the young rapidly rotating K0 dwarf LQ Hya ( P _rot=1.6 d). As might be expected from observations of the similar rapid rotator AB Dor, the predicted differential rotation is small (≃0.8 per cent) but extremely efficient in generating magnetic fields. The dynamo, which is of a distributed type, produces a globally axisymmetric field with radial and azimuthal components that are of the same magnitude and display a phase-lag in their evolution of about π/2. This is consistent with the long-term Zeeman–Doppler imaging study by Donati. The latitudinal distribution of flux is, however, a little different from that observed and the cycle period of 3.2 yr is somewhat shorter than suggested by the observations.     

Comparison of Mediterranean sea level fields for the period 1961–2000 as given by a data reconstruction and a 3D model

Two Mediterranean sea level distributions spanning the last decades are examined. The first one is a reconstruction of sea level obtained by a reduced-space optimal interpolation applied to tide gauge and altimetry data. The second distribution is obtained from a 3D (baroclinic) regional circulation model. None of the two representations includes the mechanical atmospheric forcing. Results are presented for two different periods: 1993–2000 (for which altimetry data are available) and 1961–2000 (the longest period common to both distributions).The first period is examined as a test period for the model, since the reconstruction is very similar to altimetry observations. The modelled sea level is in fair agreement with the reconstruction in the Western Mediterranean and in the Aegean Sea (except in the early nineties), but in the Ionian Sea the model departs from observations. For the whole period 1961–2000 the main feature is a marked positive trend in the Ionian Sea (up to 1.8 mm yr^− 1), observed both in the reconstruction and in the model. Also the distribution of positive trends in the Western Mediterranean (mean value of 1.1 mm yr^− 1) and the smaller trends in the Aegean Sea (0.5 mm yr^− 1) are similar in the reconstruction and in the model, despite the first implicitly accounts for sea level variations due to remote sources such as ice melting and the second does not. The interannual sea level variability associated with key regional events such as the Eastern Mediterranean Transient is apparently captured by the reconstruction but not by the model (at least in its present configuration). Hence, the reconstruction can be envisaged as a useful tool to validate further long-term numerical simulations in the region.     

Experimental hydrothermal alteration of basaltic glass with relevance to Mars

Phyllosilicates, carbonates, zeolites, and sulfates on Mars give clues about the planet's past environmental conditions, but little is known about the specific conditions in which these minerals formed within the crust and at the surface. The aim of the present study was to gain increased understanding on the formation of secondary phases by hydrothermal alteration of basaltic glass. The reaction processes were studied under varying conditions (temperature, pCO₂, water:rock ratio, and fluid composition) with relevance to aqueous hydrothermal alteration in fully and partly saturated Martian basalt deposits. Analyses made on reaction products using X‐ray diffraction (XRD) and scanning electron microscope (SEM) were compared with near infrared spectroscopy (NIR) to establish relative detectability and spectral signatures. This study demonstrates that comparable alteration minerals (phyllosilicates, carbonates, zeolites) form from vapor condensing on mineral surfaces in unsaturated sediments and not only in fully water‐saturated sediments. In certain environments where water vapor might be present, it can alter the basaltic bedrock to a suite of authigenic phases similar to those observed on the Martian surface. For the detection of the secondary phases, XRD and SEM‐EDS were found to be superior to NIR for detecting and characterizing zeolites. The discrepancy in detectability of zeolites between NIR and XRD/SEM‐EDS might indicate that zeolites on Mars are more abundant than previously thought.     

Surface changes on Io during the Galileo mission

A careful survey of Galileo SSI global monitoring images revealed more than 80 apparent surface changes that took place on Io during the 5 year period of observation, ranging from giant plume deposits to subtle changes in the color or albedo of patera surfaces. Explosive volcanic activity was discovered at four previously unrecognized centers: an unnamed patera to the south of Karei that produced a Pele-sized red ring, a patera to the west of Zal that produced a small circular bright deposit, a large orange ring detected near the north pole of Io, and a small bright ring near Io's south pole. Only a handful of Io's many active volcanoes produced large scale explosive eruptions, and several of these erupted repeatedly, leaving at least 83% of Io's surface unaltered throughout the Galileo mission. Most of the hot spots detected from SSI, NIMS and ground-based thermal observations caused no noticeable surface changes greater than 10 km in extent over the five year period. Surface changes were found at every location where active plumes were identified, including Acala which was never seen in sunlight and was only detected through auroral emissions during eclipse. Two types of plumes are distinguished on the basis of the size and color of their deposits, confirming post-Voyager suggestions by McEwen and Soderblom [Icarus 55 (1983) 191]. Smaller plumes produce near-circular rings typically 150-200 km in radius that are white or yellow in color unless contaminated with silicates, and frequently coat their surroundings with frosts of fine-grained SO₂. The larger plumes are much less numerous, limited to a half dozen examples, and produce oval, orange or red, sulfur-rich rings with maximum radii in the north-south direction that are typically in the range from 500 to 550 km. Both types of plumes can be either episodic or quasi-continuous over a five year period. Repeated eruptions of the smaller SO₂-rich plumes likely contribute significantly to Io's resurfacing rate, whereas dust ejection is likely dominated by the tenuous giant plumes. Both types of plume deposits fade on time-scales of months to years through burial and alteration. Episodic seepages of SO₂ at Haemus Montes, Zal Montes, Dorian Montes, and the plateau to the north of Pillan Patera may have been triggered by activity at nearby volcanic centers.