Venus Keeps Upping Its Rebel Status in the Solar System

Venus, our inner cosmic neighbor, is an enigma. Sometimes referred to as the Earth's twin since the planets are roughly the same size, Venus is more like an inside out and backwards Earth. Venus rotates in the opposite direction, not just of Earth but of all the planets in the Solar System; if you could stand on Venus, you'd see the sun rise in the west and set in the east. It's also hot with surface temperatures around 460 degrees Celsius (or 860 degrees Fahrenheit). But now there's a new piece of the Venusian mystery. Its rotation seems to be slowing down. New data suggests that the days on Venus have gotten remarkably longer in less than two decades.

A planet's rotation isn't set in stone. Case in point: Earth. The very young Earth had an 8 hour day, but the great collision that astronomers hypothesize created our Moon affected its rotation, and the Moon continues to affect the Earth to this day. The Moon pulls on the Earth as it recedes, gradually slowing the planet's rotation and lengthening its day. But the rate is minute, just tiny fractions of a second as the Moon literally inches away from the Earth each year. We can barely measure the change.

Venus has experienced a more drastic change to its day, and its unclear why. The planet doesn't have a moon to affect its rotation, and astronomers would know if something huge smashed into it.

The data comes form the European Space Agency (ESA), whose Venus Express spacecraft is currently in orbit around the planet. It's armed with a visible and infrared thermal imaging spectrometer – VIRTIS – that combines the three observing channels into one instrument. Using VIRTIS to peer through the thick cloud cover, the ESA found a disparity between its observations and the commonly accepted map of the Venusian surface NASA created from the data gathered by its Magellan spacecraft in the early 1990s.

Magellan watched surface features as the planet rotated and determined that the Venusian day was 243.0185 Earth days long. Now, 16 years later, Venus Express has measured the Venusian day as about 6.5 minutes longer. Tracking surface features, the ESA spacecraft found that landmarks were up to 12 miles from where Magellan said they should be.

At first, the Venus Express team thought their measurements were off. They double checked their calculations and considered the possibility of short-term random variations to Venus' rotation. But all evidence suggests they weren't mistaken, and that any variations would average out over a long timeframe. Their findings appeared to be correct. And, interestingly, this figure supports recent Earth-based observations.

The answer might lie in Venus' unique qualities. Part of Venus Express' purpose is to take detailed measurements of the planet to help scientists determine its nature. Specifically, if it has a solid or liquid core. In the former case, with its mass concentrated in the centre, the planet's rotation would be less affected by external forces. A solid core could also explain Venus' lack of a magnetic field.

But even if Venus is less susceptible to external forces, there are formidable forces at work there. Its dense atmosphere is 90 times thicker than Earth's, it's really hot, and the high-speed weather systems can cause enough friction with the planet's rotation to affect its speed. It's also not impossible for Venus to have weather cycles stretching over decades, which could have the observed effect on the planet's rotation.

This happens on Earth, but on a much smaller scale. Our planet experiences tides, both water and land, from the Moon and the Sun as well as occasional strong winds. These factors can affect the Earth's rotation. The Earth's day can change by a millisecond depending on wind patterns, temperature and seasons that change throughout the year. Earth and Venus approaching one another in their orbits could be another cause behind the change in the latter planet's rotation.

But without a clear understanding of the cause, Venus Express mission scientists can't really say anything conclusive beyond asserting that the data merits further study. Understanding such a significant change to a planet in a short timeframe will help scientists understand the formation of the planet, the inner solar system, and help plan future missions to our backwards twin.