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1.
The possible effects of trace-gas induced climatic changes on Pyramid and Yellowstone Lakes are assessed using a model of lake temperature. The model is driven by years of hourly meteorological data obtained directly from the output of double-CO2 experiments (2 × CO2) conducted with a regional climate model nested in a general circulation model. The regional atmospheric model is the climate version of the National Center for Atmospheric Research/Pennsylvania State University mesoscale model, MM4.Average annual surface temperature of Pyramid Lake for the 2 × CO2 climate is 15.5 ± 5.4°C (±1 σ), 2.8°C higher than the control. Annual overturn of the lake ceases as a result of these higher temperatures for the 2 × CO2 climate. Evaporation increases from 1400 mm yr−1 in the control to 1595 mm yr−1 in the 2 × CO2 simulation, but net water supplied to the Pyramid Lake basin increases from −6 mm yr−1 in the control to +27 mm yr−1 in the 2 × CO2 simulation due to increased precipitation.For the open water periods, the average annual surface temperature of Yellowstone Lake is 13.2 ± 5.1°C for the 2 × CO2 climate, a temperature 1.6°C higher than the control. The annual duration of ice cover on the lake is 152 days in the 2 × CO2 simulation, a reduction of 44 days relative to the control. Warming of the lake for the 2 × CO2 climate is mostly confined to the near-surface. Simulated spring overturn for the 2 × CO2 climate occurs earlier in the year and fall overturn later than in the control. Evaporation increases from 544 mm yr−1 to 600 mm yr−1 in the 2 × CO2 simulation, but net water supplied to the Yellowstone Lake basin increases from +373 mm yr−1 in the control to +619 mm yr−1 due to increased precipitation. The effects of these climatic changes suggest possible deterioration of water quality and productivity in Pyramid Lake and possible enhancement of productivity in Yellowstone Lake.  相似文献   

2.
The purpose of this study was to determine if any temporal variability in the broadband UV spectral properties of the icy Galilean satellites exists, and if so, to characterize its spatial distribution as a function of longitude in order to attempt to correlate any temporal changes with satellite surface interactions with the space environment. The temporal time period examined is between 1978-1984 (referred to as the 1980s data) and those from 1995-1996 (referred to as the 1990s data). The plausible temporal characteristics detected appear to vary from one satellite to the next. For Europa possible temporal variations are concentrated on the leading, anti-jovian quadrant. Example broadband UV spectra show Europa's spectral slope decreases (and darkens) with time on the leading and anti-jovian hemispheres, but remains essentially constant with time on the trailing hemisphere. The data quality does not support any definitive temporal changes for Ganymede. Possible temporal changes seen in the Callisto data set are concentrated on the jovian hemisphere. Example broadband UV spectra for Callisto show no definitive change in slope with time. The hypothesis is that these temporal differences in UV spectral properties are caused by variations in the surface ice chemistry due to temporal variability in the space environment. It is postulated that the UV spectral changes suggested for Europa may be linked to changes in H2O2 concentrations, whereas the changes on Callisto may be linked to variability in SO2 concentration.  相似文献   

3.
Interannual variability of regional climate was investigated on a seasonal basis. Observations and two global climate model (GCM) simulations were intercompared to identify model biases and climate change signals due to the enhanced greenhouse effect. Observed record length varies from 40 to 100 years, while the model output comes from two 100-year equilibrium climate simulations corresponding to atmospheric greenhouse gas concentrations at observed 1990 and projected 2050 levels. The GCM includes an atmosphere based on the NCAR CCM1 with the addition of the radiative effects of CH4, N2O and CFCs, a bulk layer land surface and a mixed-layer ocean with thermodynamic sea-ice and fixed meridional oceanic heat transport.Because comparisons of interannual variability are sensitive to the time period chosen, a climate ensemble technique has been developed. This technique provides comparisons between variance ratios of two time series for all possible contiguous sub-periods of a fixed length. The time autocorrelation is thus preserved within each sub-period. The optimal sub-period length was found to be 30 years, based on which robust statistics of the ensemble were obtained to identify substantial differences in interannual variability that are both physically important and statistically significant.Several aspects of observed interannual variability were reproduced by the GCM. These include: global surface air temperature; Arctic sea-ice extent; and regional variability of surface air temperature, sea level pressure and 500 mb height over about one quarter of the observed data domains. Substantial biases, however, exist over broad regions, where strong seasonality and systematic links between variables were identified. For instance, during summer substantially greater model variability was found for both surface air temperature and sea-level pressure over land areas between 20–50°N, while this tendency was confined to 20–30°N in other seasons. When greenhouse gas concentrations increase, atmospheric moisture variability is substantially larger over areas that experience the greatest surface warming. This corresponds to an intensified hydrologic cycle and, hence, regional increases in precipitation variability. Surface air temperature variability increases where hydrologic processes vary greatly or where mean soil moisture is much reduced. In contrast, temperature variability decreases substantially where sea-ice melts completely. These results indicate that regional changes in interannual variability due to the enhanced greenhouse effect are associated with mechanisms that depend on the variable and season.  相似文献   

4.
Atmospheric composition in the turbopause regime was determined by four rocket-borne mass spectrometers, which employed shock-freezing cryo-ion sources. Number densities of N2, O2, O, Ar and CO2 are presented for these experiments. The results are compared with those of other rocket experiments taken from the literature. Ar/N2 ratios are analyzed with respect to atmospheric turbulence. Magnitude and variability of atomic oxygen layer maximum density and layer content are discussed. Variations of O densities and simultaneous Ar/N2 ratio changes are compared. Six CO2 measurements are discussed in terms of CO2/N2 ratios.  相似文献   

5.
The global climate–vegetation model HadSM3_TRIFFID has been used to estimate the equilibrium states of climate and vegetation with pre-industrial and last glacial boundary conditions. The present study focuses on the evaluation of the terrestrial biosphere component (TRIFFID) and its response to changes in climate and CO2 concentration. We also show how, by means of a diagnosis of the distribution of plant functional types according to climate parameters (soil temperature, winter temperature, growing-degree days, precipitation), it is possible to get better insights into the strengths and weaknesses of the biosphere model by reference to field knowledge of ecosystems.The model exhibits profound changes between the vegetation distribution at the Last Glacial Maximum and today that are generally consistent with palaeoclimate data, such as the disappearance of the Siberian boreal forest (taiga), an increase in shrub cover in Europe and an increase of the subtropical desert area. The effective equatorial and sub-tropical tree area is reduced by 18%. There is also an increase in cover of wooded species in North-Western Africa and in Mexico. The analysis of bioclimatic relationships turns out to be an efficient method to infer the contributions of different climatic factors to vegetation changes, both at high latitudes, where the position of the boreal treeline appears in this model to be more directly constrained by the water stress than by summer temperature, and in semi-humid areas where the contributions of temperature and precipitation changes may partly compensate each other. Our study also confirms the major contribution of the decrease in CO2 to environmental changes and carbon storage through its selective impact on gross primary productivity of C3 and C4 plants and a reduction by 25% of water-use efficiency. Specifically, the reduction in CO2 concentration increases the amount of precipitation necessary to sustain at least 20% of grass fraction by 50 mm/year; the corresponding threshold for trees is increased by about 150 mm/year. As a consequence, a reduction in CO2 concentration considerably widens the climatic range where grasses and shrubs dominate.  相似文献   

6.
An enhanced “greenhouse effect” due to an increase in atmospheric CO2 is expected to produce significant climatic changes. If the combustion of fossil fuels is the only anthropogenic source of atmospheric CO2, measurements show that 54% resides in the atmosphere. The largest reservoir for the remaining 46% is the oceans. Known oceanic processes can account for 35% and the major uncertainty appears to be the role played by the intermediate waters. If, however, deforestation is as large a source of additional atmospheric CO2 as some have suggested, carbon balance cannot be obtained with presently identified removal processes. Various computer models have been used to calculate the effects of increasing atmospheric CO2. These include energy balance, radiative-convective and general circulation models (GCM's). Many feedback mechanisms must be considered including water vapour, clouds, oceans and the cryosphere. Although representing a considerable advance over other models, GCM's still require many approximations, of which the treatment of oceans and clouds are the most questionable. These models predict, for the scenario of the doubling of atmospheric CO2, an increase in global surface temperature of about 3°C with larger increases, up to 10° at higher latitudes. Significant changes in evaporation and precipitation patterns are also indicated.  相似文献   

7.
Abstract– A model is presented in which the aqueous conditions needed to generate phyllosilicate minerals in the absence of carbonates found in the ancient Noachian crust are maintained by an early CO2‐rich atmosphere, that, together with iron (II) oxidation, would prevent carbonate formation at the surface. After cessation of the internal magnetic dynamo, a CO2‐rich primordial atmosphere was stripped by interactions with the solar wind and surface conditions evolved from humid to arid, with ground waters partially dissolving subsurface carbonate and sulfide minerals to produce acid‐sulfate evaporitic deposits in areas with upwelling ground water. In a subsequent geochemical state (Late Noachian to Hesperian), surface and subsurface acidic solutions were neutralized in the subsurface through interaction with basaltic crust, allowing the precipitation of secondary carbonates. This model suggests that, in the early Noachian, the surface waters of Mars maintained acidity because of a drop in temperature. This would have favored increased dissolution of CO2 and a reduction in atmospheric pressure. In this scenario, physicochemical conditions precluded the formation of surface carbonates, but induced the precipitation of carbonates in the subsurface.  相似文献   

8.
The role of tropical ecosystems in global carbon cycling is uncertain, at least partially due to an incomplete understanding of climatic forcings of carbon fluxes. To reduce this uncertainty, we simulated and analyzed 1982–1999 Amazonian, African, and Asian carbon fluxes using the Biome-BGC prognostic carbon cycle model driven by National Centers for Environmental Prediction reanalysis daily climate data. We first characterized the individual contribution of temperature, precipitation, radiation, and vapor pressure deficit to interannual variations in carbon fluxes and then calculated trends in gross primary productivity (GPP) and net primary productivity (NPP). In tropical ecosystems, variations in solar radiation and, to a lesser extent, temperature and precipitation, explained most interannual variation in GPP. On the other hand, temperature followed by solar radiation primarily determined variation in NPP. Tropical GPP gradually increased in response to increasing atmospheric CO2. Confirming earlier studies, changes in solar radiation played a dominant role in CO2 uptake over the Amazon relative to other tropical regions. Model results showed negligible impacts from variations and trends in precipitation or vapor pressure deficits on CO2 uptake.  相似文献   

9.
Over the last few decades, General Circulation Models (GCM) have been used to simulate the current martian climate. The calibration of these GCMs with the current seasonal cycle is a crucial step in understanding the climate history of Mars. One of the main climatic signals currently used to validate GCMs is the annual atmospheric pressure cycle. It is difficult to use changes in seasonal deposits on the surface of Mars to calibrate the GCMs given the spectral ambiguities between CO2 and H2O ice in the visible range. With the OMEGA imaging spectrometer covering the near infra-red range, it is now possible to monitor both types of ice at a spatial resolution of about 1 km. At global scale, we determine the change with time of the Seasonal South Polar Cap (SSPC) crocus line, defining the edge of CO2 deposits. This crocus line is not symmetric around the geographic South Pole. At local scale, we introduce the snowdrop distance, describing the local structure of the SSPC edge. Crocus line and snowdrop distance changes can now be used to calibrate GCMs. The albedo of the seasonal deposits is usually assumed to be a uniform and constant parameter of the GCMs. In this study, albedo is found to be the main parameter controlling the SSPC recession at both global and local scale. Using a defrost mass balance model (referred to as D-frost) that incorporates the effect of shadowing induced by topography, we show that the global SSPC asymmetry in the crocus line is controlled by albedo variations. At local scale, we show that the snowdrop distance is correlated with the albedo variability. Further GCM improvements should take into account these two results. We propose several possibilities for the origin of the asymmetric albedo control. The next step will be to identify and model the physical processes that create the albedo differences.  相似文献   

10.
The dynamic climate in the Northern Hemisphere during the early Holocene could be expected to have impacted on the global carbon cycle. Ice core studies however, show little variability in atmospheric CO2. Resolving any possible centennial to decadal CO2 changes is limited by gas diffusion through the firn layer during bubble enclosure. Here we apply the inverse relationship between stomatal index (measured on sub-fossil leaves) and atmospheric CO2 to complement ice core records between 11,230 and 10,330 cal. yr BP. High-resolution sampling and radiocarbon dating of lake sediments from the Faroe Islands reconstruct a distinct CO2 decrease centred on ca. 11,050 cal. yr BP, a consistent and steady decline between ca. 10,900 and 10,600 cal. yr BP and an increased instability after ca. 10,550 cal. yr BP. The earliest decline lasting ca. 150 yr is probably associated with the Preboreal Oscillation, an abrupt climatic cooling affecting much of the Northern Hemisphere a few hundred years after the end of the Younger Dryas. In the absence of known global climatic instability, the decline to ca. 10,600 cal. yr BP is possibly due to expanding vegetation in the Northern Hemisphere. The increasing instability in CO2 after 10,600 cal. yr BP occurs during a period of increasing cooling of surface waters in the North Atlantic and some increased variability in proxy climate indicators in the region.The reconstructed CO2 changes also show a distinct similarity to indicators of changing solar activity. This may suggest that at least the Northern Hemisphere was particularly sensitive to changes in solar activity during this time and that atmospheric CO2 concentrations fluctuated via rapid responses in climate.  相似文献   

11.
Methane hydrate dissociation due to obliquity-driven temperature change has been suggested as a potential source of atmospheric methane plumes recently observed on Mars. This work uses both equilibrium and time-dependent models to determine how geothermal gradients change on Mars as a result of obliquity and predict how these changes affect gas hydrate stability zones (HSZs). The models predict that the depth to the HSZ decreases with increasing latitude for both CO2 and CH4 hydrate, with CO2 hydrate occurring at shallower depths than CH4 hydrate over all latitudes. The depth of the HSZ increases as surface temperatures warm and decreases as surface temperatures cool with changing obliquity, with the largest change in HSZ volume predicted near the equator and the poles. Therefore, changes in the depth to the HSZ may cause hydrate dissociation near the equator and poles as the geothermal gradient moves in and out of the hydrate stability field over hundreds of thousands of years. Sublimation of overlying ice containing diffused methane could account for recent observations of seasonal methane plumes on Mars. In addition, near-surface gas hydrate reservoirs may be preserved at mid-latitudes due to minimal changes in surface temperature with obliquity over geologic time scales. Comparisons of the predicted changes in the HSZ with hydrate dissociation and diffusion rates reveal that metastable hydrate may also remain in the near subsurface, especially at high latitudes, for millions to billions of years. The presence of methane hydrate in the near subsurface at midlatitudes could be an important analytical target for future Mars missions, as well as serving as a source of fuel for future spacecraft.  相似文献   

12.
A global to regional modeling system has been developed to evaluate precipitation under doubled CO2. The National Centers for Environmental Prediction (NCEP) regional spectral model (RSM) is initialized and forced by current and doubled CO2 simulations from the NCAR community climate model (CCM3). Three RSM simulations, RSM0, RSM1, and RSM2, with resolution of 280, 50 and 15 km, are examined. The RSM0 setup resolution matches the T42 CCM3 simulations. The RSM2 simulation is centered over Taiwan. Due to incompatibility of the model physics, noticeable differences between RSM0 and CCM3 are found, especially in wintertime, which suggests that simulation from RSM0, rather than CCM3, should be used to contrast high-resolution regional variations produced by RSM1 or RSM2 simulations.While the spatial distributions of RSM1 and RSM2 simulations over Taiwan are greatly improved over the CCM3 simulation, the intensity of the unique wintertime drizzle is overestimated, especially in RSM2. There is also a spurious northward extension of the precipitation pattern from the subtropical warm-pool region. Thus the regional response to doubled CO2, which consists of more summerlike wintertime precipitation characteristics over the northeastern and eastern sides of Taiwan, with increased intensity mostly in the extreme events, is still in doubt and must be examined with improved global and regional models.  相似文献   

13.
One response of vegetation to future increases in atmospheric CO2 may be a widespread increase in stomatal resistance. Such a response would increase plant water usage efficiency while still allowing CO2 assimilation at current rates. The associated reduction in transpiration rates has the potential of causing significant modifications in climate on regional and global scales.This paper describes the effects of a uniform doubling of the stomatal resistance parameterization in a global climate model (GENESIS). The model includes a land-surface transfer scheme (LSX) that accounts for the physical effects of vegetation, including stomatal resistance and transpiration, which is described in detail in an appendix. The atmospheric general circulation model is a heavily modified version of the NCAR Community Climate Model version 1 with new treatments of clouds, penetrative convection, planetary boundary layer mixing, solar radiation, the diurnal cycle, and semi-Lagrangian transport of water vapor. The other surface models include multi-layer models of soil, snow and sea ice, and a 50-m slab ocean mixed layer.The effects of doubling the stomatal resistance parameterization are largest in heavily forested regions: tropical South America, and parts of the Northern Hemispheric boreal forests in Canada, Russia and Siberia in summer. The primary surface changes are a decrease in evapotranspiration, an increase in upward sensible heat flux, and a surface-air warming. Secondary effects include shifts in the ITCZ which cause large increases in precipitation, soil moisture and runoff in western tropical South America, and decreases in these quantities in northern subtropical Africa. Noticeable changes in relative humidity, cloudiness and meridional circulation occur throughout the troposphere. The global effects on atmospheric temperature and specific humidity are small fractions of those found in other doubled CO2 experiments. However, unlike doubled CO2 the signs of those changes combine to give relatively large reductions in relative humidity and cloudiness. It is suggested that the stomatal-resistance effect and other plant responses to large-scale environmental perturbations should be included in models of future climate.  相似文献   

14.
We present a detailed study of an Iapetus mosaic of VIMS data with high spatial resolution (0.5 × 0.5° or ∼6.4 km/pixel). The spectra were taken in August 2007 and provide the highest VIMS spatial resolution data for this object during Cassini’s primary mission. We analyze this set of data using a statistical clustering approach to reduce the analysis of a large number of data (∼104 spectra from 0.35 to 5.10 μm) to the study of seven representative groups accounting for 99.6% of the surface covered by the original sample. We analyze the spectral absorption bands in the spectra of the different clusters indicative of different composition over the observed surface. We find coherence between the distribution of the clusters and the geographical features on the surface. We give special attention to the study of the water ice and CO2 bands. We find that CO2 is widespread over the entire surface being studied, including the bright and dark areas on Iapetus’ surface, and is probably trapped at the molecular level with other materials. The strength of the CO2 band in the areas where both, H2O- and carbon-bearing materials exist, gives support to the hypothesis that this volatile is formed on the surface of Iapetus as a product of irradiation of these two components. Finally, we also compare the Iapetus CO2 with that on other satellites confirming, that there are evident differences on the center, depth and width of the band on Iapetus and Phoebe, where CO2 has been suggested to be endogenous.  相似文献   

15.
The residual south polar cap of Mars (RSPC) is distinct from the residual north polar cap both in composition and in morphology. CO2 frost in the RSPC is stabilized by its high albedo during southern spring and summer despite the relatively large insolation during that period. The morphology of the RSPC in summer displays a bewildering variety of depressions that are formed in relatively thin layers of CO2. The increase of the size of these depressions between each of the first three years of Mars Global Surveyor (MGS) observations may possibly signal some sort of climate change on the planet. For example, the erosion of the bright plateaus might reduce the RSPC albedo and affect the energy balance. The Mars Orbiter Cameras (MOC) on MGS observed Mars for four consecutive martian years before contact with the spacecraft was lost in late 2006. During this period coverage of the polar regions was particularly dense because MGS flew over them on every orbit. In this paper we report on the four-year behavior of the morphological features in the RSPC and on the large-scale variability in RSPC albedo over the period. The changes in the size of the surface features in the RSPC due to backwasting that were first observed between Mars years (MY) 24 and 25 and subsequently between MY25 and M26 was observed to continue at the same rate through MY 27. The results indicate that on average thicker layers in the RSPC retreat faster than thinner ones, roughly in proportion to their thickness. We argue that a simple difference in porosity between the A and B layers can explain this difference although other factors could be involved. The large-scale albedo of the RSPC decreases as the depressions are uncovered by sublimation of seasonal CO2. However, any interannual differences in albedo due to the backwasting process are masked by interannual differences in the summer dust opacity in the RSPC region.  相似文献   

16.
The interaction of carbon dioxide and amorphous water ice at 95 K is studied using transmission infrared spectroscopy. Samples are prepared in two ways: co-deposition of the gases admitted simultaneously or sequential deposition, in which amorphous water ice (ASW) is grown first and CO2 vapor is added subsequently. In either case, a fraction of the CO2 molecules is found to interact with water in a way that gives rise to shifts and splittings in the infrared bands with respect to those of a pure CO2 solid. In co-deposition experiments, a larger amount of carbon dioxide is trapped within the amorphous water than in sequential deposition samples, where a substantial proportion of molecules appears to be trapped in macropores of the ASW. The specific surface area of sequential samples is evaluated and compared to previous literature results. When the sequential samples are heated to 140 K, beyond the onset temperature at which water ice undergoes a phase transition, the CO2 molecules at the pores relocate inside the bulk in a structure similar to that found in co-deposited samples, as deduced by changes in the shape of the CO2 infrared bands.  相似文献   

17.
Xianglei Huang 《Icarus》2003,165(2):301-314
We introduce two new techniques in analyzing martian spectrally resolved radiance data obtained by the Thermal Emission Spectrometer (TES): spectral empirical orthogonal function (EOF) analysis and the tri-spectral algorithm. Spectral EOF analysis allows us to obtain the variability of spectra and associated temporal and spatial patterns. The case study with TES 20° S-20° N data shows that the first principal component (PC1) dominates the total variance and is associated with surface or near-surface brightness temperature variations. The PC2 is associated with atmospheric variability, and a negative correlation between dust and ice absorptions can be clearly seen over many regions. The annual cycle is a major component of the PC1 temporal patterns. The fingerprint of the dust storm can be clearly seen in the PC2 temporal patterns in most areas except the highlands. Spectral EOF can be used for validation of the variability of martian GCMs. The tri-spectral algorithm is based on the differences between three bands (dust, ice and a weak CO2 absorption band) to distinguish spectra sampled in different situations: water ice cloud, dust, and surface anisothermality. We use a line-by-line radiative transfer model coupled with multiple scattering to investigate the sensitivity of this algorithm to dust and ice optical depth as well as surface emissivity. The comparisons between results of this algorithm and the TES team's retrieved dust and ice opacity are consistent over all studied periods except during the peak of the dust storm. Our algorithm is complementary to the more sophisticated TES retrieval and can be used to screen large amounts of data to get an overview.  相似文献   

18.
Current evidence indicates that the Martian surface is abundant with water presently in the form of ice, while the atmosphere was at one time more massive with a past surface pressure of as much as 1 atm of CO2. In an attempt to understand the Martian paleoclimate, we have modeled a past CO2H2O greenhouse and find global temperatures which are consistent with an earlier presence of liquid surface water, a finding which agrees with the extensive evidence for past fluvial erosion. An important aspect of the CO2H2O greenhouse model is the detailed inclusion of CO2 hot bands. For a surface pressure of 1 atm of CO2, the present greenhouse model predicts a global mean surface temperature of 294°K, but if the hot bands are excluded, a surface temperature of only 250°K is achieved.  相似文献   

19.
Tree ring chronologies provide long-term records of growth in natural environmental conditions and may be used to evaluate impacts of climatic change and CO2 increase on forest productivity. This study focuses on 21 Pinus halepensis forest stands in calcareous Provence (in the south-east of France). A chronology of net primary productivity (NPP) both for the 20th century and for each stand was estimated using tree ring data (width and density). The response of each stand to climate in terms of NPP was statistically modelled using response functions. Anomalies between estimated NPP and NPP reconstructed by response functions were calculated to evaluate the fertilising effect of CO2 increase on tree growth. The changes in anomalies during the 20th century were attributed to the effect of CO2 increase. A multiplying factor (β) linking CO2 concentration and stand productivity was then calculated, on the basis of the trend observed during the 20th century. In this study, the value of the β factor obtained under natural conditions (β=0.50) is consistent with those from controlled CO2 enrichment experiments. Both response functions and the β factor were used to predict NPP changes for a 2×CO2 scenario. The 2×CO2 climate was obtained using predictions from Météo France's ARPEGE atmospheric general circulation model (AGCM) downscaled to Marseilles meteorological station. NPP increased significantly for nine stands solely when the climatic effect was taken into account. The main factors responsible for this enhancement were increased winter and early spring temperatures. When the fertilising effect of the CO2 increase was added, NPP was significantly enhanced for 14 stands (i.e. NPP enhancement ranged from 8% to 55%). Although the effects of global change were slightly detectable during the 20th century, their acceleration is likely to lead to great changes in the future productivity of P. halepensis forests.  相似文献   

20.
Titan is known to have a young surface. Here we present evidence from the Cassini Visual and Infrared Mapping Spectrometer that it is currently geologically active. We report that changes in the near-infrared reflectance of a 73,000 km2 area on Titan (latitude 26° S, longitude 78° W) occurred between July 2004 and March of 2006. The reflectance of the area increased by a factor of two between July 2004 and March-April 2005; it then returned to the July 2004 level by November 2005. By late December 2005 the reflectance had surged upward again, establishing a new maximum. Thereafter, it trended downward for the next three months. Detailed spectrophotometric analyses suggest these changes happen at or very near the surface. The spectral differences between the region and its surroundings rule out changes in the distribution of the ices of reasonably expected materials such as H2O, CO2, and CH4 as possible causes. Remarkably, the change is spectrally consistent with the deposition and removal of NH3 frost over a water ice substrate. NH3 has been proposed as a constituent of Titan's interior and has never been reported on the surface. The detection of NH3 frost on the surface might possibly be explained by episodic effusive events occur which bring juvenile ammonia from the interior to the surface. If so, its decomposition would feed nitrogen to the atmosphere now and in the future. The lateral extent of the region exceeds that of active areas on the Earth (Hawaii) or Io (Loki).  相似文献   

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