Earth and Mars were pounded by space rocks about four billion years ago
A storm of meteorites that pounded Earth and Mars four billion years ago may have made the planets warmer and wetter.
Researchers superheated younger space rocks to measure the gases that would have been shed as meteorites entered fledgling atmospheres during the storm.
There would have been enough to create warmer and wetter planets more amenable to life, they say.
The work is published in Geochimica et Cosmochimica Acta.
The team started with 15 fragments of ancient meteorites from around the world, heating them using a technique called pyrolysis-FTIR.
This uses electricity to raise the temperature of the fragments by up to 20,000C per second. The gases into which they decompose are then analysed.
The researchers found that an average fragment gave up 12% of its mass as water vapour and 6% as carbon dioxide.
They then used mathematical models of how many meteoric impacts would have occurred during the Late Heavy Bombardment (LHB), a period about four billion years ago during which an unusually large number of meteorites rained down on bodies in the Solar System.
Mars may also have benefited from the bombardment
Based on published estimates of the number of LHB impacts, the researchers believe 10 billion tonnes of water and carbon dioxide would have been brought to the Earth and Mars each year.
That in turn would have led to global warming, liquid oceans, and a more habitable environment, they suggest.
"Because of their chemistry, ancient meteorites have been suggested as a way of furnishing the early Earth with its liquid water," said Imperial College London's Richard Court, lead author of the research.
"Now we have data that reveals just how much water and carbon dioxide was directly injected into the atmosphere by meteorites. These gases could have got to work immediately, boosting the water cycle and warming the planet."
Earth benefits from a magnetic field, which deflects the solar wind that in time would have whipped away the atmospheric gases that meteorites brought to Mars, according to the researchers.