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At first glance, if Earth had a twin, it would be Venus. The two planets are similar in size, mass, composition, and distance from the Sun. But the twins' similarities end there. Venus has no oceans, and its scorching surface temperature of about 484 íC (900 íF) could melt lead. Venus hides behind a persistent global shroud of sulfuric acid clouds in an atmosphere composed mostly of carbon dioxide. The atmosphere is so dense that it crushes down on the planet's surface with a pressure equal to that found at 3,000-foot depths in Earth's oceans. Oddly, Venus rotates in a direction opposite that of Earth, which means that if you were standing on Venus, you would see the Sun rising in the west and setting in the east. Venus' sluggish rotation makes one Venus "day" last as long as 243 Earth days.
Because of its convenient orbit and scientific interest, Venus has been visited by more spacecraft, both U.S. and Russian, than any other planet, with flyby missions, orbiters, surface landers, and even atmosphere-floating balloons. In 1962, the U.S. launched Mariner 2, the first successful probe to fly by another planet. Mariner 2's flyby verified Venus' high temperatures. Since then, there has been a series of successful space-flight missions to Venus revealing more and more about the cloud-veiled planet.
Despite the wealth of valuable data given to us by these missions, we still had only a rough sketch of the face of Venus. The Pioneer Venus and Venera spacecraft were able to image the surface with radar, thus answering many of our questions about Venus' large scale surface features, but many more questions remained unanswered about the extent to which Venus' surface has been shaped by volcanoes, plate tectonics, impact craters, and water and wind erosion. To address these questions, NASA, in 1989, launched a new radar imaging spacecraft named Magellan. It was named after the early Portuguese explorer Ferdinand Magellan, whose fleet was the first to circumnavigate Earth.
Magellan began its radar mapping on September 15, 1990. Within 243 Earth days, the spacecraft had achieved and even exceeded its primary objective: to map 70 percent of the planet's surface. After three complete 243-day cycles, Magellan had mapped 98 percent of Venus. Magellan began a fourth 243-day cycle a global gravity survey on September 15,1992. This survey will help scientists map the internal structure of Venus.
Magellan is unveiling on Venus a tortured surface shaped by a history of geological violence, tectonic deformation, volcanism, and impact cratering. At least 85 percent of Venus is covered by volcanic rock mostly lava flows that form the planet's vast plains. Mountains deformed by repeated geologic activity cover much of the remaining surface areas. Because no rainfall, oceans, or strong winds exist on Venus, little erosion occurs.
From data returned by Magellan scientists will create and study maps of Venus for years to come. With Venus' face unveiled, we now have a better understanding of Earth's fraternal twin, and a store of information that will help us understand the evolution of our own planet.
Venus Express is ESA's first mission to Earth's nearest planetary neighbour, Venus. The mission was born after ESA asked for proposals, in March 2001, suggesting how to reuse the design of the Mars Express spacecraft.
Venus Express was launched from the Baikonur Cosmodrome in Kazakhstan on 9 November 2005. A Soyuz-Fregat rocket carried it into space and placed the spacecraft in its transfer orbit to Venus. After an interplanetary cruise that lasted 5 months, Venus Express arrived at Venus on 11 April 2006. A 50 minute engine burn slowed down the spacecraft and allowed it to enter orbit around the planet. The first capture orbit was an eccentric polar orbit and lasted 9 days. Several manoeuvres over the period 15 April - 6 May 2006 then lowered the spacecraft into its operational orbit: a 24-hour elliptical, quasi-polar orbit. At its closest, Venus Express reached an altitude of 250 kilometres and at its furthest, it was 66 000 kilometres away from the planet. Over three years later, between 13 July and 4 August 2009 a series of manoeuvres further lowered the pericentre of the orbit into the range 185-300 km, with the apocentre orbit still at about 66 000 km.
Acid clouds and lightning
At around 60 kilometres altitude is a very thick cloud layer – a 20 kilometre-deep blanket surrounding the planet. It marks the limit between Venus’s lower and middle atmospheric layers. It is this yellowish layer that prevented for a long time Earth-based observatories and previous orbiter missions to see through.
It is known today that the upper part of this layer is mostly composed of tiny droplets of sulphuric acid, but what is happening chemically in the lower clouds is still unknown. For instance, what is the origin of the large solid particles floating in the lower clouds observed by Pioneer-Venus?
The Venus Express remote-sensing instruments will be able to see how the clouds are shaped and structured, how they form and evolve in time, how their opacity varies and what molecules they are made of. Local and global weather will be no secret for Venus Express.
During previous ground and satellite observations, visible flashes in the atmosphere have been observed, and localised emissions of radio waves have also been reported. Are they due to lightning?
The sulphuric acid droplets can be highly electrically charged, and so they offer the potential for lightning. This is very important, not only to learn whether it is possible that lightning has an influence on Venus atmospheric chemistry, but also to globally understand how the atmosphere of all terrestrial planets become electrified and then discharge.
Volcano on Venus
Date: 08 Apr 2010
Venus is known to be covered with volcanoes, but are any of these active? Results from Venus Express, using VIRTIS data (See the article on New evidence for recent volcanism on Venus) indicates that a number of lava flows on Venus are very young, and that most likely the planet is currently volcanically active.
Venus holds warning for Earth
30 November 2010
A mysterious high-altitude layer of sulphur dioxide discovered by ESA’s Venus Express has been explained. As well as telling us more about Venus, it could be sending a warning to those on Earth seeking to inject our atmosphere with sulphur droplets in an attempt to mitigate climate change.
Venus is blanketed in sulphuric acid clouds that block our view of the surface. The clouds form at altitudes of 50–70 km when sulphur dioxide from volcanoes combines with water vapour to make sulphuric acid droplets. Any remaining sulphur dioxide should be destroyed rapidly by the intense solar radiation above 70 km.
So the detection of a sulphur dioxide layer at 90–110 km by ESA’s Venus Express orbiter in 2008 posed a complete mystery. Where did that sulphur dioxide come from?
Now, computer simulations by Xi Zhang, California Institute of Technology, USA, and colleagues from America, France and Taiwan show that some sulphuric acid droplets may evaporate at high altitude, freeing gaseous sulphuric acid that is then broken apart by sunlight, releasing sulphur dioxide gas.
“However, the new findings also mean that the atmospheric sulphur cycle is more complicated than we thought.”
As well as adding to our knowledge of Venus, this new understanding may be warning us that proposed ways of mitigating climate change on Earth may not be as effective as originally thought.
The proposal stems from observations of powerful volcanic eruptions, in particular the 1991 eruption of Mount Pinatubo in the Philippines that shot sulphur dioxide up into Earth’s atmosphere. Reaching 20 km in altitude, the gas formed small droplets of concentrated sulphuric acid, like those found in Venus’ clouds, which then spread around Earth. The droplets created a haze layer that reflected some of the Sun’s rays back into space, cooling the whole planet by about 0.5°C.
However, the new work on the evaporation of sulphuric acid on Venus suggests that such attempts at cooling our planet may not be as successful as first thought, because we do not know how quickly the initially protective haze will be converted back into gaseous sulphuric acid: this is transparent and so allows all the Sun’s rays through.
ESA finds that Venus has an ozone layer too
The two top Images:
Images Credit: NASA/SDO, AIA
Image in the right:
Image credit: JAXA/NASA/Lockheed Martin
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