The collapse of Io's primary atmosphere in Jupiter eclipse

Volcanic outgassing due to tidal heating is the ultimate source of a tenuous SO atmosphere around Jupiter's moon Io. The question of whether SO frost on the surface plays a part, and to what degree, in maintaining Io's atmosphere with the constant volcanic outgassing is still debated. It is believed that for a sublimation-supported atmosphere, the primary atmosphere should collapse during eclipses by Jupiter, as the SO vapor pressure is strongly coupled to the temperature of the ice on the surface. No direct observations of Io's atmosphere in eclipse have previously been possible, due to the simultaneous need for high spectral and time sensitivity, as well as a high signal-to-noise ratio. Here we present the first ever high-resolution spectra at 19 µm of Io's SO atmosphere in Jupiter eclipse from the Gemini telescope. The strongest atmospheric band depth is seen to dramatically decay from 2.5 ± (0.08)% before the eclipse to 0.18 ± (0.16)% after 40 min in eclipse. Further modeling indicates that the atmosphere has collapsed shortly after eclipse ingress, implying that the atmosphere of Io has a strong sublimation-controlled component. The atmospheric column density—from pre-eclipse to in-eclipse—drops by a factor of 5 ± 2.C.C.C.T. and J.R.S. at Southwest Research Institute were funded by NASA Outer Planets Research grant NNX14AC63G and Planetary Astronomy grant NNX11AD61G. M.A.L.V. was funded by the Spanish Ministerio de Economia y Competitividad and by FEDER funds under project ESP2015-65064-C2-1-P (MINECO/FEDER)Peer Reviewe

Significance Tests for Periodogram Peaks

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.Comment: 22 pages, 11 Encapsulated Postscript figures, AAS LaTeX v5.2 Submitted to Ap

Multispectral surface emissivity from VIRTIS on Venus Express

The surface composition of Venus is mostly inaccessible to remote observation due to the dense cloud cover. There are five spectral windows that show measurable thermal emission from the surface at night. The VIRTIS spectrometer on Venus Express observed three of these windows over much of the southern hemisphere of Venus. We use these data along with Magellan altimetry to map surface emissivity. The data are initially processed to correct for instrumental straylight from the dayside of Venus and to improve wavelength registration. These corrected data are then inverted to emissivity at 1020, 1100, and 1180 nm wavelength using lookup tables created by an atmospheric radiative transfer model. As in earlier studies we find residual trends of surface emissivity with respect to the Magellan altimetry that is used in the model to determine surface temperature and thickness of the atmosphere. A new observation is that these trends vary significantly from region to region, indicating some lateral variability of atmospheric parameters, most likely near surface atmospheric temperature. The trends are consistent over hundreds to thousands of km, thus it is possible to correct for them heuristically. In two regions studied in this paper there are significant deviations from the background emissivity which are associated with some geologic features. The high noise in 1100 and 1180 nm maps derived from VIRTIS data result in large uncertainties of spectral shape. The VIRTIS instrument was not designed for this task and future observations could provide high signal to noise ratio maps in at least 5 distinct bands diagnostic of major rock types and minerals

Venus surface thermal emission at 1 μm in VIRTIS imaging observations: Evidence for variation of crust and mantle differentiation conditions

The Venus Express spacecraft images the nightside thermal emissions using the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS). At 1.02, 1.10, and 1.18 μm, thermal emission from the surface is observed. The signal is attenuated by scattering and absorption in the dense atmosphere. The measured flux at the top of the atmosphere is positively correlated with surface temperature and surface emissivity. The surface temperature of Venus is relatively well constrained as being mainly a function of altitude with a gradient lesser or equal to the adiabatic lapse rate. This study examines the correlation of VIRTIS images showing a signal of the surface at 1.02 μm with viewing geometry, stray sunlight, cloud opacity, and topography and applies semiempirical relations to remove their influence. The remaining contrast can be either ascribed to surface emissivity or unexpected temperature variations. Temperature variations due to active volcanism are unlikely to be persistent over the time of observations; therefore, the mosaic of all processed images is here interpreted in terms of surface emissivity variation. The emissivity variation found is correlated with geomorphological features established from Magellan synthetic aperture radar images. It is generally lower at tessera terrain. Some, but not all, volcanic edifices show increased emissivity. Large lava flows in the Lada terra-Lavinia planitia region also show an increased thermal emission. This might indicate a more felsic surface composition of tessera highlands and large-scale extrusive volcanism of ultramafic composition

Scientific goals for the observation of Venus by VIRTIS on ESA/Venus Express mission

The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board the ESA/Venus Express mission has technical specifications well suited for many science objectives of Venus exploration. VIRTIS will both comprehensively explore a plethora of atmospheric properties and processes and map optical properties of the surface through its three channels, VIRTIS-M-vis (imaging spectrometer in the 0.3–1 µm range), VIRTIS-M-IR (imaging spectrometer in the 1–5 µm range) and VIRTIS-H (aperture highresolution spectrometer in the 2–5 µm range). The atmospheric composition below the clouds will be repeatedly measured in the night side infrared windows over a wide range of latitudes and longitudes, thereby providing information on Venus’s chemical cycles. In particular, CO, H2O, OCS and SO2 can be studied. The cloud structure will be repeatedly mapped from the brightness contrasts in the near-infrared night side windows, providing new insights into Venusian meteorology. The global circulation and local dynamics of Venus will be extensively studied from infrared and visible spectral images. The thermal structure above the clouds will be retrieved in the night side using the 4.3 µm fundamental band of CO2. The surface of Venus is detectable in the short-wave infrared windows on the night side at 1.01, 1.10 and 1.18 µm, providing constraints on surface properties and the extent of active volcanism. Many more tentative studies are also possible, such as lightning detection, the composition of volcanic emissions, and mesospheric wave propagation