MESSENGER Bids Farewell to Venus

ESA’s Venus Express and NASA’s MESSENGER booked an appointment at Venus late in the evening of 5 June, to look at the oddities of this mysterious planet in tandem for a few hours. Just a few weeks on, scientists from both teams are ready to present a first set of images.

Image right: As NASA’s MESSENGER departed from 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. Initially, images were acquired at a rate of one of every 20 minutes and then, with increasing distance, the timing interval was increased to 60 minutes. Click image to view movie. Credit: NASA/APL

This unique opportunity to make multi-point observations of the Venusian atmosphere was possible thanks to the MESSENGER (MErcury Surface, Space ENvironment, Geochemistry, and Ranging) swingby of Venus – a key step during its long journey to Mercury - while Venus Express was already orbiting the planet in the course of its mission.

The two spacecraft carry sets of instruments employing different observation techniques which complement each other. The data collected at Venus are now being analysed by teams on both sides of the Atlantic and, as can be appreciated in the first images presented here, already hints at the potential of the results to come.

The particular 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.

VIRTIS Image of the Area Overflown by MESSENGER

Image left: This grey-scale image, obtained by the VIRTIS instrument on board ESA’s Venus Express, shows the atmospheric region of Venus over which NASA’s MESSENGER passed on 5 June 2007. The region of MESSENGER’s closest approach is in the night side (marked by a circle). Credits: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA

MESSENGER made its closest approach at a distance of about 338 km from the planet over the planetary coordinates 12.25° South and 165° East, on the night side of the planet. Meanwhile, Venus Express was behind the horizon, almost right above the South Pole, at about 35 000 km from Venus.

So how could they make true joint observations of the same regions and phenomena? Scientists came up with a highly creative solution.

Two Hunters for the Same Cloud

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.

VIRTIS Images of the Clouds That MESSENGER Flew Over

Image above: The images in this panel were obtained by the VIRTIS imaging spectrometer on board Venus Express on 5 and 6 June 2007, before and after MESSENGER’s closest approach to the planet. These panels from VIRTIS provide a night-side view of the same region that Messenger flew over and imaged. The images where obtained at 1.7 micrometres, revealing atmospheric details down to an altitude of 50 km from the surface. Credit: NASA/JHUAPL

The VIRTIS imaging spectrometer on board Venus Express probed this cloud (top row of this image composite) at several wavelengths. These observations provided a view of the cloud at about 45-50 km altitude (bottom row) from the planet. The clouds below the point of closest approach can be seen in the top row.

The Mercury Laser Altimeter (MLA) instrument on board MESSENGER probed the same cloud structure at 50-75 km from the surface, like VIRTIS.

Such an observation – a typical example of atmospheric structure at Venus – with cross-sections obtained at different altitudes and with different instruments, is a unique opportunity for researchers hoping to solve the puzzle of the Venusian atmosphere’s dynamics and composition.

Cloud Structures at Venus at Time of MESSENGER Flyby

Image right: This movie consists of a sequence of six images obtained by the VIRTIS imaging spectrometer on board ESA’s Venus Express on 5 and 6 June 2007, before and after NASA MESSENGER’s closest approach to the planet. The image sequence, obtained by VIRTIS, provides a night-side view of the same region that Messenger flew over and imaged. Click image to view animated GIF. Credits: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA

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.

Thermal and Radar Maps of Venus’ Surface Compared

Image above: An unprocessed 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). Correlations between topographic and thermal data similar to the ones shown in this image-composite will allow the scientists to understand if the measured temperature of the surface depends only on the altitude – where higher altitudes simply corresponds to colder, temperatures such as on Earth – or if it depends on the presence of previously undetected sources of heat such as active volcanoes. Credits: Left panel: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA, Right panel: NASA

A spectacular view obtained by VIRTIS (left), 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).

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.

Image left: ESA’s Venus Express, in orbit around Venus since 11 April 2006, was joined for a few hours by NASA’s MESSENGER spacecraft, flying by Venus while on its way to Mercury. This animation shows both the Venus Express and MESSENGER spacecraft in orbit around Venus at the time of the fly-by. Earth-based observatories and telescopes in orbit around Earth were also watching. Looking at Venus together, spacecraft and ground observatories obtained a unique set of data each, so many different ‘eyes’ observed the same regions and phenomena during the same time frame. Click image to view animation. Credit: NASA/APL

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 mature results from this joint observation campaign are expected by the end of the year.