Venusian rendezvous results: Chapter one
Fri Jul 13, 2007 at 20:52 UTC
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.
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.
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.
Image Credit: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA
This grey-scale image, obtained by the VIRTIS instrument on board ESA's Venus Express, shows the atmospheric region of Venus over flown by NASA's MESSENGER on 5 June 2007. The region of MESSENGER's closest approach is in the night side (marked by a circle).
VIRTIS obtained this image at 2.3 microns from about 35 000 kilometres from the planet, on the night side. This wavelength makes it possible to probe the atmosphere down to about 30 kilometres from the surface. Much of the contrast in this image is due to the structure of the lower clouds.
The bright areas correspond to radiation coming from the lower atmospheric layers, indicating that the clouds are less thick in those areas. At the 2.3 micron wavelength it is possible to study not only the morphology of the cloud layers, but also its chemical composition (carbon monoxide, water, sulphur-dioxide, etc).
Image Credit: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA
High resolution image
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. The panels provide a night-side view of the same region flown over and imaged by MESSENGER (top row) and the same cloud observed by MESSENGER (bottom row).
The images where obtained at 1.7 micron, revealing atmospheric details down to 50 kilometres altitude from the surface.
Image Credit: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA
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).
VIRTIS, the imaging spectrometer on board Venus Express, obtained this image at 1 micron, a wavelength that allows detection of radiation originating from the surface. The imaged region is the same as flown over by NASA's MESSENGER when the spacecraft made its closest approach to the planet.
Magellan's radar imaging and altimetry maps made it is possible to measure the elevation and the radio-optical properties of the surface. Venus Express' VIRTIS is providing the first complete set of 'thermal maps' of the surface of Venus. 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 only depend only on the altitude - where 'higher' simply corresponds to 'colder', like on Earth - or if it depends on the presence of previously undetected sources of heat such as active volcanoes.
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.
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 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.
