Earth observation goes planetary – SCIAMACHY observes Venus

Average NO2 measured by SCIAMACHY
17 July 2009

The SCIAMACHY sensor on ESA’s Envisat satellite has provided scientists with invaluable data on our planet, allowing them to map global air pollution and the distribution of the most important greenhouse gases. Earlier this year, its sights were set on our nearest planetary neighbour, Venus.

In March and June, SCIAMACHY (Scanning Imaging Spectrometer for Atmospheric Chartography) captured visible and near-infrared spectra of sunlight reflected by Venus.

These observations, acquired from a distance of between 43 and 127 million km, supplement close-up measurements by the SPICAV and VIRTIS instruments on ESA’s Venus Express (VEX) mission. VEX has been orbiting this perpetually cloudy, steamy, hot terrestrial planet since 2006 and is expected to continue for several years, which coincides with the extension of Envisat through to 2013.

Since SCIAMACHY observes our neighbour under different illumination and viewing geometries than Venus Express, it can also provide new information about the Venusian atmosphere. However, the main benefit of observing Venus is to provide an example of how the spectral signatures of a terrestrial planet might appear when viewed from far away.

Venus spectra from SCIAMACHY
Venus spectra from SCIAMACHY

Since the first planet around a solar-type star was discovered in 1995, one of the biggest challenges in astronomy has become finding an Earth-like planet around another star – a Second Earth or Exo-Earth. Most of the currently known exoplanets are giant planets, like Jupiter.

In the coming years, improved and dedicated instruments will bring smaller, terrestrial planets into reach. Because of the exoplanets’ enormous distance from Earth, they will always appear point-like. In order to find out whether they might be suitable for harbouring life, scientists will have to measure and analyse their spectra.

Acquiring Venus with SCIAMACHY is not an easy task, as it is only within the instrument’s reach when it is rising above the Earth’s limb. With a diameter of 12 103 km, Venus is almost as large as our home planet. When viewed from Earth, Venus’ diameter appears smaller and it fits entirely in SCIAMACHY’s field-of-view. Keeping Venus exactly within SCIAMACHY’s view long enough for the spectra to be measured requires timing the observations within a tenth of a second.

Sun-Venus-Earth constellation
Sun-Venus-Earth constellation

During the March observations, Venus was already close to inferior conjunction, when the planet passes between the Earth and the Sun. SCIAMACHY, therefore, mainly saw the night side of Venus and only a very small part of the planet (3.5%) that was lit by sunlight. This narrow crescent was sufficient, however, to yield spectra with detailed absorption features of carbon dioxide (CO2) – the gas that forms Venus’ atmosphere and sustains the extreme greenhouse effect that has turned its surface into a furnace making it unsuitable for life as we know it.

In June, Venus was much farther away, with Sun-Venus-Earth forming almost a right-angled triangle. Thus, almost half of Venus’ sunlit side was in view. The obtained spectra show similar absorption features as in March but with rather different depths.

In addition to more planned Venus observations, investigations are ongoing about how SCIAMACHY can observe the other bright solar system bodies – Mars, Jupiter and Saturn.

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