The European Space Agency’s Venus Express (VEX) is the only active spacecraft mission at the planet Venus. It carries a number of instruments: A magnetometer, a wide-angle CCD camera, a space plasma detector, a Fourier spectrometer, a thermal spectrometer, a radio science package, and a cluster of spectrometers specifically designed to study the Venusian atmosphere: SPICAV (Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus). This package contains three spectrometers, one of which operates in the ultraviolet and is the subject of this article.
The UV instrument in SPICAV (described in great detail in Bertaux et al. 2007) is a refurbished flight spare from the Mars Express spacecraft. Sensitive to wavelengths from 118 to 320 nm, it was designed to measure different aspects of the atmosphere in different modes. In nadir orientation, SPICAV-UV measures SO2 and the distribution of the mysterious UV absorber in the clouds. On the night side, it observes the γ and δ bands of NO, and it can measure vertical profiles of CO2, SO2, clouds, and aerosols in stellar occultation mode.
The UV spectrometer collects light with a 40 mm off-axis parabolic mirror that reflects light toward the spectrometer entrance. There is a configurable slit mechanism in the focal plane that, when left in place, is used for extended source viewing. The slit can be removed from the focal plane entirely for stellar occultation observations. A concave UV grating causes the spectrum to fall onto an image intensifier that is blind to wavelengths greater than 320 nm. The image created on the phosphor output screen of the intensifier is transferred to a 288x384 pixel CCD which can optionally be cooled to 270K using a Peltier cooling unit to reduce dark current. The focal length of the telescope (120mm) results in each CCD pixel having a FOV of 0.01 x 0.01°.
The slit of the spectrometer is divided into two parts with different widths to allow differing spectral resolutions during observing of an extended source. The narrow part of the slit is 50 μm wide, giving resolving power between 120 and 300 with lower flux, while the wider part (500 μm) provides more sensitivity at the expense of a lower resolving power of ≈20.
SPICAV-UV data (in fact, all ESA data more than six months old) are available to the public as part of the ESA Planetary Science Archive (PSA), and are found at http://www.rssd.esa.int/index.php?project=PSA&page=vex. NASA’s Planetary Data System (PDS) standard was adopted by the PSA as a baseline for defining the structure and format of datasets. The PSA allows members of the public to browse the entire available archive via an FTP (File Transfer Protocol) interface, and also provides a Java-based Advanced Search Interface tool that lets a user specify dozens of search parameters, select datasets from search results, and it then “delivers” the requested datasets to a special folder on their server and sends the user an email containing a hypertext link to the data.
|Figure 1: Quick look analysis of SPICAV-UV data from VEX orbit 520.|
Datasets for the SPICAV instrument cluster are stored by spacecraft orbit number. Each individual dataset comes with human-readable descriptor files that define the format, encoding, and semantic meaning of all data provided. In addition to the spectral data themselves, the dataset also include all of the contextual data about the conditions under which the observations were made, such as spacecraft altitude and orientation, instrument temperature, the time and date, exposure times, instrument configuration settings, and many more.
A SPICAV-UV dataset file includes one or more Collection objects, each containing a header descriptor and data table. A data table consists of five rows of 408 columns, which represents five rows of the CCD with each row containing 408 pixels. Each 16-bit pixel value is a digital representation of the charge collected by that pixel during integration.
The PSA also provides a data browsing area on the FTP server where summary images of each dataset are available for a “quick look” at the data (see Figure 1).
Science with SPICAV
One example of an investigation which used data from SPICAV-UV is a recent study hoping to find evidence of extant volcanic activity. Six years of ultraviolet spectrometer data were used to examine the density of sulphur dioxide above the clouds of Venus (Marcq et al. 2012). They found that SO2 column densities increased prior to 2007, and then decreased by a factor of 5 over the next five years. Read the rest of my summary of this research here
Another study a year earlier (Montmessin, et al. 2011) used SPICAV-UV data to identify, for the first time, a layer of ozone in the upper atmosphere of Venus (previously, ozone had only been identified in the atmospheres of Mars and Earth). Read more details here
Bertaux, J., Nevejans, D., Korablev, O., Villard, E., Quémerais, E., Neefs, E., Montmessin, F., Leblanc, F., Dubois, J., Dimarellis, E., Hauchecorne, A., Lefèvre, F., Rannou, P., Chaufray, J., Cabane, M., Cernogora, G., Souchon, G., Semelin, F., Reberac, A., Van Ransbeek, E., Berkenbosch, S., Clairquin, R., Muller, C., Forget, F., Hourdin, F., Talagrand, O., Rodin, A., Fedorova, A., Stepanov, A., Vinogradov, I., Kiselev, A., Kalinnikov, Y., Durry, G., Sandel, B., Stern, A., & Gérard, J. (2007). SPICAV on Venus Express: Three spectrometers to study the global structure and composition of the Venus atmosphere Planetary and Space Science, 55 (12), 1673-1700 DOI: 10.1016/j.pss.2007.01.016
Esposito, L. W. (1984). Sulfur dioxide: Episodic injection shows evidence for active venus volcanism. Science (New York, N.Y.), 223(4640), 1072-1074.
Marcq, E., Bertaux, J. L., Montmessin, F., & Belyaev, D. (2012). Variations of sulphur dioxide at the cloud top of Venus's dynamic atmosphere. Nature geoscience, 6(1), 25-28.
Montmessin, F., Bertaux, J. L., Lefèvre, F., Marcq, E., Belyaev, D., Gérard, J. C., ... & Vandaele, A. C. (2011). A layer of ozone detected in the nightside upper atmosphere of Venus. Icarus, 216(1), 82-85.