Was Earth once a bit more like Venus?

The degassing of early Earth’s magma ocean may have produced an atmosphere on Earth similar to that found on Venus today – rich in carbon dioxide and relatively poor in nitrogen – new research suggests.

Modelling by a team led by Paolo Sossi from Switzerland’s ETH Zürich indicates that the atmospheric differences between the two planets have more to do with Earth’s mass and its distance from the Sun rather than the respective accretionary histories.

Scientists agree, Sossi says, that Earth was covered entirely by magma 4.5 billion years ago, but it is less clear what the atmosphere was like at the time.

To make their calculations, he and colleagues from France, Australia and the US created their own magma in the laboratory by creating a powder that matched the composition of Earth’s molten mantle and heating it.

That required temperatures of around 2000 degrees Celsius, and thus a special furnace that was heated by a laser. The researchers could levitate the magma inside by by letting streams of gas mixtures flow around it.

These gas mixtures were plausible candidates for the primeval atmosphere that influenced ancient magma, they say, and thus, with each mixture of gases that flowed around the sample, the magma turned out a little different.

“The key difference we looked for was how oxidised the iron within the magma became,” Sossi says. When iron meets oxygen, it turns into rust, so when the gas mixture blown over their magma contained a lot of oxygen, the iron within the magma became more oxidised.

This gave the researchers something they could compare to naturally occurring rocks that make up Earth’s mantle today.

“What we found was that, after cooling down from the magma state, the young Earth had an atmosphere that was slightly oxidising, with carbon dioxide as its main constituent, as well as nitrogen and some water,” Sossi says.

The surface pressure was much higher – almost 100 times that of today – as was the atmosphere, due to the hot surface. These characteristics made it more similar to the atmosphere of today’s Venus than to that of today’s Earth.

Sossi and colleagues say they draw two main conclusions. The first is that Earth and Venus began with similar atmospheres, but Venus lost its water because it is closer to the Sun and is thus hotter. Earth kept its water, primarily in the form of oceans, which absorbed much of the CO₂ from the air.

The second is that theories on the emergence of life on Earth based on the “Miller-Urey experiment”, in which lightning strikes interact with certain gases (notably ammonia and methane) to create amino acids, are much less likely, because the necessary gases were not sufficiently abundant.

Related reading: Evidence mounts for habitability of Venus-like worlds