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The latter are derived from the results of the GCM described by Forget et al. It is noteworthy that despite a retrieval approach totally independent from model expectations, we achieve a very close quantitative correspondence. However, the TES climatology is derived mostly from early afternoon observations 14 local time and cannot account for the likely daily basis variations of the ice content [ Formisano et al. Consequently, the occurrence of substantial ice load during nighttime cannot be excluded. The presence of water ice has a twofold effect on our retrievals:.

Water ice in the atmosphere may represent an additional source of opacity and may induce a depression of radiance level. If this opacity is not consistently taken into account during retrieval, resulting temperatures may be underestimated.

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In particular, the ice content map discussed in section 4. Despite the theoretical capability to provide a numerical modeling of this phenomenon, its inclusion in a complete retrieval scheme has not yet been implemented, making it difficult to distinguish between this fluorescence, the thermal emission of atmosphere in the band wings, and scattering of solar radiation. The effect of dust scattering is also attenuated in a deeply saturated spectral region, since most of the dust lies in the lower atmosphere, and solar photons cannot reach these altitudes.

The absolute intensity of radiation observed at 4. Figure 9b illustrates this trend for two selected latitudes. Namely, at a given latitude, pixels observed during morning show higher signal than those observed in the afternoon at the same solar zenith angle. A peak efficiency around 11 LT is observed in both latitude ranges. This trend can be interpreted as due to an additional contribution to the signal measured at 4. Actually, direct numerical experiments demonstrated that signal measured at the bottom of the band LTE components only may increase by almost two orders of magnitude with respect to clear sky condition if an ice cloud of realistic opacity 0.

The solid curve in Figure 10a shows the intensity measured at 4. The curve is an average of the radiances measured between columns and , in the upper part of frame i.

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  • Scattering of solar radiation by aerosols and thermal emission of the atmosphere background account almost entirely for the signal measured between 0 and 50 km altitude. Conversely, at 4. This result is in fair agreement with the trends reported in Figure 20 of the work of Formisano et al. Despite the nonuniqueness of solution for this inverse problem, results can be considered as robust above 90 km, where the emission is effectively modeled by a Gaussian function peaked at km, with a standard deviation of 20 km and maximum intensity of 8.

    About Rosetta

    Eventually, this implied increased systematic errors. Further sources of errors with respect to SPICAV case are represented by 1 high Martian signal, which causes phenomena of saturation for the adopted exposure time in the central part of the disk, characterized by low Sun zenith angle and high surface albedo, and 2 residual flat field corrections.

    Combining these contributions, systematic retrieval errors up to 10 MR cannot be excluded. On the other hand, random errors due to individual pixel noise more important for the definition of relative trends remain below 5 MR.

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    The final result is presented in Figure 11b. The main feature is the sharp emission enhancement above both polar regions, more evident on the Southern Hemisphere. Once the systematic errors are kept in mind, these figures are quantitatively consistent with the SPICAM results presented by Fedorova et al.

    The emission can be easily observed above the polar caps, where the planet limb remains in sunlight and ozone photodissociation can take place. In our data, the Sun is already illuminating the limb of the planet, pointing toward ozone dissociation as the main source of emission.

    Unfortunately, during our observation sessions, the signal from the continuum due to scattering of solar radiation by aerosols was so high that the detector was saturated while observing at tangent altitudes lower than 65 km. Radiance profile is, within errors, monotonically decreasing from 65 to km, where the signal reaches the effective noise level. Even if not deconvolved for limb view geometry, this excludes the occurrence of individual emission layers in this altitude range. The depth of the former feature is by far more evident, but its usage in a retrieval scheme is made very difficult by the need to distinguish its effects from the hydration signature of surface materials.

    On the other hand, the 1. Our retrieval scheme is based on a parameterization of feature depth on the basis of 1 total ice load, 2 emission, and 3 Sun zenith angles. This parameterization was derived from a series of synthetic spectra computed for different input conditions, on the basis of the radiative transfer scheme described by Ignatiev et al. This method further includes a multiple scattering treatment. Water ice cloud retrieval was not possible on polar areas, where surface ice features become dominant.

    Other possible sources of systematic errors are represented by scattering approximations, assumption on particles properties size distribution and refractive index , and residual flat field corrections. Effects of these sources are difficult to estimate, but systematic errors on retrieved opacity in the order of 0. It is evident that the ice content depends on the solar energy input i. Morning west side of the image shows higher loads and deeper bands than its evening counterpart Figure 15 , suggesting that a substantial fraction of ice load outside polar regions is involved in the condensation and sublimation processes on a daily basis.

    Values observed toward the evening limb are probably overestimated because of the high viewing angle, but the increase of ice load with respect to disk center midday has to be considered as a genuine feature. In general terms, the ice daily cycle is more evident in our data than in the expectations of EMCD 4. This fact and the longitudinal confinement of radiance suggest that aerosols, instead of gases, are the responsible scattering centers.

    List of minor planets and comets visited by spacecraft

    In this case, a minimum altitude of km for clouds top can be inferred from this image. Moreover, it demonstrated the flexibility of the instrument that was operated according very complex modes not foreseen in the original design e. These findings provide incentive for planning similar observations using other spacecrafts. In addition, the VIRTIS data will be essential in addressing several other investigations, mostly by comparison with the existing simultaneous observations by Mars Express instruments during Rosetta flyby.

    Our colleague L. Brower is acknowledged for her support in language and style revision of most parts of the text. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries other than missing content should be directed to the corresponding author for the article. Volume , Issue E4. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account. If the address matches an existing account you will receive an email with instructions to retrieve your username.

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    Introduction [2] Flyby observations of targets of opportunity have successfully contributed to the study of many important phenomena in our solar system.

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    Calibration [10] A detailed description of the instrument and its calibration on ground has been given in several papers [ Coradini et al. The observations can be placed into three categories: [21] 1. However, no one knows precisely how long the lander will survive on the comet. Could activity on the comet's surface damage or destroy the lander? Survival of the lander depends on a number of factors, such as power supply, temperature, or surface activity on the comet.

    For example, dust may cover the solar panels, preventing the battery from recharging. In any case, by March , when the comet is closer to the Sun, it is likely that the lander will become too hot to operate. What scientific instruments are on board the spacecraft and what will they do?

    Rosetta's goal is to examine the comet in great detail. The instruments on the Rosetta orbiter include several cameras, spectrometers, a number of sensors, and experiments that work at different wavelengths — infrared, ultraviolet, microwave, and radio. They will provide, among other things, very high-resolution images and information about the shape, density, temperature, and chemical composition of the comet. What scientific instruments are on board the lander and what function will they perform?

    A drilling system will obtain samples down to 23 cm below the surface and will feed these to the spectrometers for analysis, such as to determine the chemical composition. Other instruments will measure properties such as near-surface strength, density, texture, porosity, ice phases and thermal properties. Microscopic studies of individual grains will tell us about the texture. In addition, instruments on the lander will study how the comet changes during the day-night cycle, and while it approaches the Sun.

    How were the instruments selected? The most important factors in the selection of each instrument were their expected scientific performance and their technical feasibility. How the instruments fitted together was another consideration, as well as the experience of the team proposing the instrument. This selection was done on the basis of the so-called 'Announcement of Opportunity' AO issued by ESA to the scientific community, which is basically an open competition.

    This AO defines the mission scientific objectives and requirements, and the scientific community had to be compliant with these when submitting their proposals. How does Rosetta fit into the overall scheme of cometary exploration? Europe has been a pioneer in exploring comets and asteroids. Giotto continued its successful journey and in flew within km of the comet Grigg-Skjellerup, detecting its nucleus. The mission was the first to observe a comet nucleus and confirm theories suggesting that comets were not mere rubble piles or conglomerates of small fragments.

    Like Giotto, these probes also visited Halley in Among other comet missions were a trio of NASA probes: Deep Space 1, which flew by the comet Borelly in ; Stardust, which returned samples from the coma of Wild 2 in and later flew by Tempel 1; and Deep Impact, which in shot a massive block of copper into the nucleus of Tempel 1 before going on to fly by Hartley 2 and image the comet ISON.