If viewed from above, Venus rotates on its axis in a direction that's the opposite of most planets'. That means on Venus, the sun would appear to rise in the west and set in the east. On Earth, the sun appears to rise in the east and set in the west.
The Venusian year — the time it takes to orbit the sun — is about Earth days long. Normally, that would mean that days on Venus would be longer than years. However, because of Venus' curious retrograde rotation, the time from one sunrise to the next is only about Earth days long. The last time we saw Venus transit in front of the sun was in , and the next time will be in The very top layer of Venus' clouds zips around the planet every four Earth days, propelled by hurricane-force winds traveling roughly mph kph.
This superrotation of the planet's atmosphere, some 60 times faster than Venus itself rotates, may be one of Venus' biggest mysteries. The clouds also carry signs of meteorological events known as gravity waves , caused when winds blow over geological features, causing rises and falls in the layers of air.
The winds at the planet's surface are much slower, estimated to be just a few miles per hour. Unusual stripes in the upper clouds of Venus are dubbed "blue absorbers" or "ultraviolet absorbers" because they strongly absorb light in the blue and ultraviolet wavelengths. These are soaking up a huge amount of energy — nearly half of the total solar energy the planet absorbs. As such, they seem to play a major role in keeping Venus as hellish as it is.
Their exact composition remains uncertain; Some scientists suggest it could even be life , although many things would need to be ruled out before that conclusion is accepted. The Venus Express spacecraft, a European Space Agency mission that operated between and , found evidence of lightning on the planet, which formed within clouds of sulfuric acid, unlike Earth's lightning, which forms in clouds of water. Venus' lightning is unique in the solar system.
The lightning is of particular interest to scientists because it's possible that electrical discharges from lightning could help form the molecules needed to jumpstart life, which is what some scientists believe happened on Earth. A long-lived cyclone on Venus, first observed in , appears to be in constant flux , with elements constantly breaking apart and reforming. NASA's Mariner 2 came within 21, miles 34, km of Venus in , making it the first planet to be observed by a passing spacecraft.
The Soviet Union's Venera 7 was the first spacecraft to land on another planet, having landed on Venus in December Venera 9 returned the first photographs of the Venusian surface. The European Space Agency's Venus Express spent eight years in orbit around Venus with a large variety of instruments and confirmed the presence of lightning there.
In August , as the satellite began wrapping up its mission, controllers engaged in a month-long maneuver that plunged the spacecraft into the outer layers of the planet's atmosphere. Venus Express survived the daring journey , then moved into a higher orbit, where it spent several months.
By December , the spacecraft ran out of propellent and eventually burned up in Venus' atmosphere. Japan's Akatsuki mission launched to Venus in , but the spacecraft's main engine died during a pivotal orbit-insertion burn, sending the craft hurling into space.
Mercury, which is even closer to Sol, has fairly long days as well: three for every two Mercury-years. But scientists are still puzzled by Venus's retrograde, or backward, rotation. Now a team of scientists from the French research institute Astronomie et Systemes Dynamiques have proposed a new explanation, published in this week's issue of Nature. Current theory holds that Venus initially spun in the same direction as most other planets and, in a way, still does: it simply flipped its axis degrees at some point.
In other words, it spins in the same direction it always has, just upside down, so that looking at it from other planets makes the spin seem backward. Scientists have argued that the sun's gravitational pull on the planet's very dense atmosphere could have caused strong atmospheric tides.
Such tides, combined with friction between Venus's mantle and core, could have caused the flip in the first place. They propose instead that its rotation slowed to a standstill and then reversed direction.
Taking into account the factors mentioned above, as well as tidal effects from other planets, the team concluded that Venus's axis could have shifted to a variety of positions throughout the planet's evolution. Regardless of whether it flipped or not, it is bound to settle into one of four stable rotation statestwo in either direction.
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