Venus is sometimes referred to as Earth's sister planet. It has an atmosphere and a weather system, and it is of similar size and density to our own planet. However it is the Venusian atmosphere that is the most striking contrast between the two planets. The thick, choking clouds trap heat on the planet, causing the surface temperature to reach an average 465 degrees Celsius. The atmosphere also makes studies of Venus difficult, as it shields the surface from view.
The first scientific studies of Venus from orbit were performed during a flyby in 1962 by Mariner 2. Other craft have also attempted to peer through the haze of Venus, such as the Russian Venera orbiters and landers. Unlike Mars, where rovers can roam the surface for years, the Venera landers lasted no longer than 2 hours due to the crushing surface pressure of 90 bar.
The latest era of Venusian exploration is being performed courtesy of the European Space Agency's Venus Express (VEX).
Venus Express was launched on 9 November 2005 aboard a Soyuz-Fregat rocket from the Baikonur Cosmodrome in Kazakhstan.
VEX entered orbit around Venus on 11 April 2006.
The mission was designed to last four days – Venusian days that is. Venus rotates very slowly, so one rotation of Venus is equivalent to 243 days on Earth. The mission has been extended until 2014, pending a review later this year to prove that the craft is still in a good working condition. The mission extension will see VEX continue to study the atmosphere of Venus in order to seek out any long term trends present, as well as monitor the planet during the peak of the solar cycle.
Venus Express reuses parts of the Mars Express design, such as the structural design, propulsion system and the use of dual solar arrays. However VEX receives four times more radiation than Mars Express, so it has been modified to account for this, such as being gold instead of black to reflect the Sun's radiation.
VEX set out with a lengthy list of questions to answer about our mysterious “sister planet”, such as seeking out seismic activity, finding what drives the atmospheric escape, learning about the speedy winds and atmospheric rotation, and discovering how the cloud system works.
“Venus Express has successfully achieved almost all science goals stated in the mission proposal 10 years ago,” says ESA's Dmitrij Titov. “I think the major result of VEX is that the mission has provided a comprehensive survey of the Venus atmosphere and plasma environment over the period of 6 years. Is there anything left to do? Yes. Observations of such a complex system as the Venus atmosphere require as long as possible extension, that would help to study its long term behaviour.”
Six years after entering orbit around Venus, around 250 scientific papers using results from VEX have been published detailing many of the discoveries by the orbiter. VEX uses seven scientific instruments to survey the planet and learn about why it became such a hostile environment.
The Analyser of Space Plasma and Energetic Atoms (ASPERA) instrument keeps a watchful eye on particles escaping Venus' atmosphere, and this data has allowed astronomers to calculate the escape rates of hydrogen, helium, and oxygen. Studies have revealed that Earth loses more of its atmosphere to space than Venus does, yet it was thought that the Earth’s magnetic field would offer more protection compared to a planet that only has a weak induced magnetic field. The magnetic field on Venus is caused by the solar wind interacting with the planet’s atmosphere, and the field is measured by the Venus Express Magnetometer (MAG).
The Venus Radio Science Experiment (VeRa) utilises a radio connection with Earth to probe the temperature, density, and pressure between the heights of 35 and 100 kilometres in the atmosphere Venus. VEX is the first craft to measure the global surface temperature distribution.
The Ultraviolet and Infrared Atmospheric Spectrometer (SPICAV/SOIR) also works to calculate the temperature between 80 and 180 kilometres, as well as the density. SPICAV/SOIR also searches for water in Venus’ atmosphere, and has found sulphur compounds in the middle atmosphere of Venus.
Venus Express instruments. Credit ESA
The Ultraviolet/Visible/Near-Infrared mapping spectrometer (VIRTIS) studies the composition of the atmosphere from the surface up to 70 kilometres in height, temperature structure of the mesosphere from 60 to 90 kilometres, and investigates non-thermal emissions at 100-120 kilometres altitude. It monitors clouds from the ultraviolet to the infrared, allowing atmospheric dynamics and cloud morphology to be studied, and maps themal emission emanating from the surface of Venus.
The Venus Monitoring Camera (VMC) takes global images of the planet ranging from ultraviolet wavelengths, which produces the highest contrast at the tops of the clouds making them easier to study, to the infrared, which has shown “hot spots” on the surface that indicate recent geologic activity. Imaging the clouds has allowed the wind velocities to be calculated at different altitudes, as well as monitoring the super-rotating atmosphere. The morphology of the cloud tops has also been studied using the VMC, and new cloud features have been discovered such as columns of clouds at low latitudes. Cloud morphologies at the tops of the clouds were also found to be related to cloud deeper in the atmosphere, which was unexpected due to the thickness of the atmosphere.
In monitoring the complex atmosphere of Venus, VEX has uncovered many unexpected features of the shrouded planet. “The major surprise was to see how variable Venus is,” Titov told Sen. “Global morphology of the cloud layers, wind speeds, appearance of the eye of the polar vortex can significantly change on a time scale of a few days.”
The density of the atmosphere has been probed by using the craft itself via the Venus Express Atmospheric Drag Experiment (VExADE). Reducing the pericentre altitude of the craft enabled it to experience atmospheric drag. This drag force is measurable, and it depends on the thickness of the atmosphere, enabling the craft to probe the atmospheric density between 165 to 200 kilometres above the surface.
Venus Express has answered many questions about our strange neighbouring planet and this knowledge can be used to discover why our sister planet evolved to have such a hellish environment compared to Earth.