The rotation of Venus, the closest planet to Earth, is getting slower. According to data gathered by European Space Agency’s (ESA) Venus Express orbiter, the current rotational period of the planet is 6.5 minutes slower than the normal Venusian day of 243.0185 Earth days, the length of time required for the planet to complete one rotation, measured by NASA in 1990s. The orbiter found surface features on Venus were not quite where they should be. Scientists of ESA used the VIRTIS instrument at infrared wavelengths to penetrate the thick cloud cover to study the surface features and discovered that some were displaced by upto 20 kms from where they should be, given the accepted rotation rate as measured by NASA.

The difference seems minor, but it places some features on Venus about 20 kilometers away from where scientists were expecting – a big deal for future missions looking to set a lander or rover down at a particular site. Though the reasons for the rotational slowdown is not clear, it is attributed to friction caused by fierce weather systems, just as weather and tides cause Earth’s day to vary.

“An accurate value for Venus’ rotation rate will help in planning future missions, because precise information will be needed to select potential landing sites,” said Haken Svedhem, ESA’s Venus Express project scientist.

The detailed measurements from the orbit are helping scientists determine whether Venus has a solid or liquid core, which will ultimately help understand the evolution of the planet. If Venus has a solid core, its mass must be more concentrated towards the centre, in this case, the planet’s rotation would react less to external forces. (Courtesy ESA)