The planet Venus that scientists know today is a hellish world characterized by a carbon dioxide atmosphere that is 90 times thicker than Earth's atmosphere.
The planet has almost no water vapor and with surface temperatures reaching up to 864 degrees Fahrenheit, the idea that the second planet from the sun can host life as we know it may seem far out.
Projections of a NASA climate model, however, have revealed that Venus may have once been habitable. The planet may have once hosted a shallow and liquid-water ocean and a habitable surface temperature.
Scientists working at NASA's Goddard Institute for Space Studies (GISS) developed a model similar to those used by climate scientists to explore the past history of Venus. They wanted to know if the planet may once had conditions similar to those in habitable Earth despite its hot water-less surface and carbon dioxide-choked atmosphere.
The speed of a planet's rotation on its axis is known to influence the habitability of its climate. Venus has a notably slow rotation rate. A day on Venus is equivalent to 117 days on Earth. Scientists previously thought that the planet's slow rotation rate is due to its thick atmosphere.
Newer research, however, has revealed that a thin Earth-like atmosphere may have also produced the same slow rotation rate, which means that an early Venus with an atmosphere similar to that of Earth may have had the same rotation rate as today.
"We find that such a world could have had moderate temperatures if Venus had a rotation period slower than ~16 Earth days, despite an incident solar flux 46 - 70% higher than Earth receives," the researchers wrote in their study, which was published in the journal Geophysical Research Letters on Aug. 11.
"At its current rotation period, Venus's climate could have remained habitable until at least 715 million years ago."
Scientists also learned that the ancient Venus likely had more dry regions compared to Earth but it had enough water and geographical variability that could guard against rapid evaporation and host life. Venus at the time also benefited from a younger sun. The solar system's star was 30 percent dimmer during its infancy.
"In the GISS model's simulation, Venus' slow spin exposes its dayside to the sun for almost two months at a time," said Anthony Del Genio, from GISS, explaining that this causes the surface to warm up and produce rain that generated a thick layer of clouds that shields the surface from solar heating.
"The result is mean climate temperatures that are actually a few degrees cooler than Earth's today."