Astronomers puzzling over why Venus, formed about the same time as Earth, has so little water may have found a culprit: solar wind, Space.com reports. Scientists think both originally had about the same amount; Venus’ warmer temperatures would have caused any oceans to evaporate. Australian scientists think solar wind carried hydrogen away on the planet’s day side, but there’s no evidence of oxygen leaving there.
On the night side, by contrast, oxygen can be seen escaping the atmosphere along with hydrogen—in a tell-tale two-to-one ratio. “I keep looking at the magnetometer data, but so far I can’t see the signature of oxygen escaping on the day side,” said one scientist. The day side’s hydrogen atoms are also higher in the atmosphere and more plentiful, meaning Venus may have more hydrogen than previously believed.