ESA’s Venus Express and NASA’s MESSENGER looked at Venus in tandem for a few hours in June. Here is the first set of images.

The orbital geometry of Venus Express when MESSENGER skimmed past Venus on 5 June meant that the two spacecraft were not at the same location (with respect to the surface of the planet) at the exact same time. So how could they make true joint observations of the same regions and phenomena? Scientists came up with a highly creative solution.

Bye-bye Venus. As NASA’s MESSENGER departed from planet Venus on 5 June 2007 to continue its journey towards Mercury, its Wide Angle Camera captured a sequence of 50 images (480-nm wavelength filter) showing the planet disappearing in the distance. At the start of the sequence MESSENGER was 60 688 kilometres from the planet, by the end it was at 89 310 kilometres. Initially, images were acquired at a rate of one of every 20 minutes and then, as Venus shrank, the timing interval was increased to 60 minutes. Credits: NASA/APL

The scientists used a computer simulation based on real atmospheric data about Venus obtained from previous ground and space observations. Knowing the speed of the local winds, which depend both on the altitude and the latitude, they were able to predict where a particular set of clouds would be at a given point in time.

For their observation, the Venus Express scientists selected a cloud that – moving west by about 90° longitude every day - was visible to Venus Express and would be in view of MESSENGER 12 hours later, at the time of its closest approach. The same cloud became visible again for Venus Express 12 hours after MESSENGER’s closest approach, this time on the night-side.

Over about 24 hours, not only did the two spacecraft observe the same clouds, but MESSENGER also flew closely over the atmospheric region. Again, these dual-spacecraft, multi-instrument observations may provide additional atmospheric details.

The panels provide a night-side view of the same region over flown and imaged by MESSENGER (first three images) and the same cloud observed by MESSENGER (last three images). The images where obtained at 1.7 micron, revealing atmospheric details down to 50 kilometres altitude from the surface. Credits: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA

A spectacular view obtained by VIRTIS (below), in the region of MESSENGER's closest approach to Venus provides, even if still unprocessed, a ‘thermal view’ of the Venusian surface. The image is compared here with an image of the same feature synthesized by data from NASA’s Magellan spacecraft in the 1990s (right).

A ‘thermal’ map of the Venusian surface obtained by VIRTIS on 5 June 2007 (left) is compared here with a radar image of the same area obtained by NASA’s Magellan spacecraft in the 1990s (right).Credits: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA

Magellan provided radar imaging and altimetry maps, providing information on the topography (elevation) and the radar reflectivity of the surface. Venus Express’ VIRTIS is providing ‘thermal maps’ of the surface containing information on the emissivity in the infrared. Correlations between topographic and thermal data similar to the ones shown here, will allow scientists to understand if the measured temperature of the surface depends on the altitude – where ‘higher’ simply corresponds to ‘colder’ – or if it depends on the presence of previously undetected sources of heat, such as active volcanoes or other geological activities.

The Venus Express and MESSENGER scientists are now continuing the analysis of this rich and complex set of data collected at Venus. The data also involve several other instruments studying not only Venus’ cloud deck and surface, but also the plasma environment, magnetic fields, and the atmospheric oxygen airglow.

More results are expected by the end of the year.

Source: ESA