Pierre Drossart

Recent hotspot volcanism on venus from VIRTIS emissivity data

Hotspots on Venus The surface of Venus shows clear signs of volcanism, but are there active volcanoes on Venus today? The answer to this question will bear on our understanding of the planets climate evolution and interior dynamics. Using surface thermal emissivity data returned by the Venus Express spacecraft, Smrekar et al. (p. 605, published online 8 April) looked at three hotspots on Venus. These places were identified by analogy with terrestrial hotspots like Hawaii, which are believed to overlie mantle plumes and to be the most likely sites for current volcanic activity. Lava flows at the three hotspots have anomalously high thermal emissions when compared with their surroundings. Low emissivity is generally interpreted as the result of surface alteration by the corrosive atmosphere of Venus. High emissivity implies that not much alteration took place and thus that the hotspots must represent recently active volcanoes younger than 2.5 million years. Satellite observations suggest that Venus is a geologically active planet. The questions of whether Venus is geologically active and how the planet has resurfaced over the past billion years have major implications for interior dynamics and climate change. Nine “hotspots”—areas analogous to Hawaii, with volcanism, broad topographic rises, and large positive gravity anomalies suggesting mantle plumes at depth—have been identified as possibly active. This study used variations in the thermal emissivity of the surface observed by the Visible and Infrared Thermal Imaging Spectrometer on the European Space Agency’s Venus Express spacecraft to identify compositional differences in lava flows at three hotspots. The anomalies are interpreted as a lack of surface weathering. We estimate the flows to be younger than 2.5 million years and probably much younger, about 250,000 years or less, indicating that Venus is actively resurfacing.

ISO-SWS Observations of Jupiter: Measurement of the Ammonia Tropospheric Profile and of the 15N/14N Isotopic Ratio

We present the results of the Infrared Space Observatory Short Wavelength Spectrometer (ISO-SWS) observations of Jupiter related to ammonia. We focus on two spectral regions; the first one (the 10-μm region), ranging from 9.5 to 11.5 μm, probes atmospheric levels between 1 and 0.2 bar, while the second one (the 5-μm window), ranging from 4.8 to 5.5 μm, sounds the atmosphere between 8 and 2 bar. The two spectral windows cannot be fitted with the same ammonia vertical distribution. From the 10-μm region we infer an ammonia distribution of about half the saturation profile above the 1-bar level, where the N/H ratio is roughly solar. A totally different picture is derived from the 5-μm window, where we determine an upper limit of 3.7×10−5 at 1 bar and find an increasing NH3 abundance at least down to 4 bar. This profile is similar to that measured by the Galileo probe. The discrepancy between the two spectral regions most likely arises from the spatial heterogeneity of Jupiter, the 5-μm window sounding dry areas unveiled by a locally thin cloud cover (the 5-μm hot spots), and the 10-μm region probing the mean jovian atmosphere above 1 bar. The 15NH3 mixing ratio is measured around 400 mbar from ν2 band absorptions in the 10-μm region. We find the atmosphere of Jupiter highly depleted in 15N at this pressure level [(15N/14N)[formula]=1.9+0.9−1.0)×10−3, while (15N/14N)⊕=3.68×10−3]. It is not clear whether this depletion reveals the global jovian 15N/14N ratio. Instead an isotopic fractionation process, taking place during the ammonia cloud condensation, is indicated as a possible mechanism. A fractionation coefficient α higher than 1.08 would explain the observed isotopic ratio, but the lack of laboratory data does not allow us to decide unambiguously on the origin of the observed low 15N/14N ratio.

Cassini Visual and Infrared Mapping Spectrometer Observations of Iapetus: Detection of CO2

The Visual and Infrared Mapping Spectrometer (VIMS) instrument aboard the Cassini spacecraft obtained its first spectral map of the satellite Iapetus in which new absorption bands are seen in the spectra of both the low-albedo hemisphere and the H2O ice-rich hemisphere. Carbon dioxide is identified in the low-albedo material, probably as a photochemically produced molecule that is trapped in H2O ice or in some mineral or complex organic solid. Other absorption bands are unidentified. The spectrum of the low-albedo hemisphere is satisfactorily modeled with a combination of organic tholin, poly-HCN, and small amounts of H2O ice and Fe2O3. The high-albedo hemisphere is modeled with H2O ice slightly darkened with tholin. The detection of CO2 in the low-albedo material on the leading hemisphere supports the contention that it is carbon-bearing material from an external source that has been swept up by the satellites orbital motion.

Atmospheric structure and dynamics as the cause of ultraviolet markings in the clouds of Venus

When seen in ultraviolet light, Venus has contrast features that arise from the non-uniform distribution of unknown absorbers within the sulphuric acid clouds and seem to trace dynamical activity in the middle atmosphere. It has long been unclear whether the global pattern arises from differences in cloud top altitude (which was earlier estimated to be 66–72 km), compositional variations or temperature contrasts. Here we report multi-wavelength imaging that reveals that the dark low latitudes are dominated by convective mixing which brings the ultraviolet absorbers up from depth. The bright and uniform mid-latitude clouds reside in the ‘cold collar’, an annulus of cold air characterized by ∼30 K lower temperatures with a positive lapse rate, which suppresses vertical mixing and cuts off the supply of ultraviolet absorbers from below. In low and middle latitudes, the visible cloud top is located at a remarkably constant altitude of 72 ± 1 km in both the ultraviolet dark and bright regions, indicating that the brightness variations result from compositional differences caused by the colder environment rather than by elevation changes. The cloud top descends to ∼64 km in the eye of the hemispheric vortex, which appears as a depression in the upper cloud deck. The ultraviolet dark circular streaks enclose the vortex eye and are dynamically connected to it.

Ground-based Near-infrared Emission Spectroscopy of HD 189733b

We investigate the K- and L-band dayside emission of the hot-Jupiter HD 189733b with three nights of secondary eclipse data obtained with the SpeX instrument on the NASA Infrared Telescope Facility. The observations for each of these three nights use equivalent instrument settings and the data from one of the nights have previously been reported by Swain et al. We describe an improved data analysis method that, in conjunction with the multi-night data set, allows increased spectral resolution (R ~ 175) leading to high-confidence identification of spectral features. We confirm the previously reported strong emission at ~3.3 μm and, by assuming a 5% vibrational temperature excess for methane, we show that non-LTE emission from the methane ν3 branch is a physically plausible source of this emission. We consider two possible energy sources that could power non-LTE emission and additional modeling is needed to obtain a detailed understanding of the physics of the emission mechanism. The validity of the data analysis method and the presence of strong 3.3 μm emission are independently confirmed by simultaneous, long-slit, L-band spectroscopy of HD 189733b and a comparison star.

Exposed water ice on the nucleus of comet 67P/Churyumov–Gerasimenko

Although water vapour is the main species observed in the coma of comet 67P/Churyumov–Gerasimenko and water is the major constituent of cometary nuclei, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far. The nucleus of 67P/Churyumov–Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material. Here we report the identification at infrared wavelengths of water ice on two debris falls in the Imhotep region of the nucleus. The ice has been exposed on the walls of elevated structures and at the base of the walls. A quantitative derivation of the abundance of ice in these regions indicates the presence of millimetre-sized pure water-ice grains, considerably larger than in all previous observations. Although micrometre-sized water-ice grains are the usual result of vapour recondensation in ice-free layers, the occurrence of millimetre-sized grains of pure ice as observed in the Imhotep debris falls is best explained by grain growth by vapour diffusion in ice-rich layers, or by sintering. As a consequence of these processes, the nucleus can develop an extended and complex coating in which the outer dehydrated crust is superimposed on layers enriched in water ice. The stratigraphy observed on 67P/Churyumov–Gerasimenko is therefore the result of evolutionary processes affecting the uppermost metres of the nucleus and does not necessarily require a global layering to have occurred at the time of the comet’s formation.

VLT/NACO adaptive optics imaging of Titan ?

The advent of the NAOS/CONICA adaptive optics system at the ESO Very Large Telescope recently gave us the opportunity to map the surface of Titan and to search for atmospheric variations at high spatial resolution and contrast. We report here the first results from a series of observations of Titan performed with this instrument in a number of near-infrared narrow-band filters, covering various altitude regions and three dierent longitudes (out of the 16 days of Titans orbit). We have achieved unequaled contrast on images showing complex topography on Titans trailing hemisphere and have found robust evidence for the north-south asymmetry inversion. The presence of other interesting atmospheric features at Titans South Pole is described.

A far wing lineshape for H2 broadened CH4 infrared transitions

A far wing corrective factor χ for CH4 lines broadened by H2 is proposed. It has been deduced from laboratory spectra measured in the ν3 band region at various temperatures between 100 and 300K. Its quality is demonstrated by comparisons of synthetic spectra with laboratory experiments as well as with the spectrum of the jovian atmosphere observed by ISO.

The Venus nighttime atmosphere as observed by the VIRTIS‐M instrument. Average fields from the complete infrared data set

We present and discuss here the average fields of the Venus atmosphere derived from the nighttime observations in the 1960–2350 cm−1 spectral range by the VIRTIS-M instrument on board the Venus Express satellite. These fields include: (a) the air temperatures in the 1–100 mbar pressure range (~85–65 km above the surface), (b) the altitude of the clouds top, and (c) the average CO mixing ratio. A new retrieval code based on the Bayesian formalism has been developed and validated on simulated observations, to statistically assess the retrieval capabilities of the scheme once applied to the VIRTIS data. The same code has then been used to process the entire VIRTIS-M data set. Resulting individual retrievals have been binned on the basis of local time and latitude, to create average fields. Air temperature fields confirm the general trends previously reported in Grassi et al. (2010), using a simplified retrieval scheme and a more limited data set. At the lowest altitudes probed by VIRTIS (~65 km), air temperatures are strongly asymmetric around midnight, with a pronounced minima at 3LT, 70°S. Moving to higher levels, the air temperatures first become more uniform in local time (~75 km), then display a colder region on the evening side at the upper boundary of VIRTIS sensitivity range (~80 km). As already shown by Ignatiev et al. (2008) for the dayside, the cloud effective altitude increases monotonically from the south pole to the equator. However, the variations observed in night data are consistent with an overall variation of just 1 km, much smaller than the 4 km reported for the dayside. The cloud altitudes appear slightly higher on the evening side. Both observations are consistent with a less vigorous meridional circulation on the nightside of the planet. Carbon monoxide is not strongly constrained by the VIRTIS-M data. However, average fields present a clear maximum of 80 ppm around 60°S, well above the retrieval uncertainty. Once the intrinsic low sensitivity of VIRTIS data in the region of cold collar is kept in mind, this datum is consistent with a [CO] enrichment toward the poles driven by meridional circulation.

Solar migrating atmospheric tides in the winds of the polar region of Venus

We discuss methods currently in use for determining the significance of peaks in the periodograms of time series. We discuss some general methods for constructing significance tests, false alarm probability functions, and the role played in these by independent random variables and by empirical and theoretical cumulative distribution functions. We also discuss the concept of “independent frequencies” in periodogram analysis. We propose a practical method for estimating the significance of periodogram peaks, applicable to all time series irrespective of the spacing of the data. This method, based on Monte Carlo simulations, produces significance tests that are tailor-made for any given astronomical time series. Subject headings: Methods: data analysis — Methods: statistical — Stars: oscillations