Heavy rain on Mars reshaped the planet's impact craters and carved out river-like channels on its surface billions of years ago, says a study.
Changes in the atmosphere on Mars made it rain harder and harder, which had a similar effect on the planet's surface as we see on Earth, said the study published in the journal Icarus.
The fourth planet from the sun, Mars has geological features like the Earth and moon, such as craters and valleys, many of which were formed through rainfall. Valley networks on Mars show evidence for surface runoff driven by rainfall.
Although there is a growing body of evidence that there was once water on Mars, it does not rain there today.
But the new study showed that there was rainfall in the past and that it was heavy enough to change the planet's surface.
"Many people have analysed the nature of rainfall on Earth, but no one had thought to apply physics to understanding the early Martian atmosphere," said one of the researchers Robert Craddock of the Smithsonian Institution in Washington, DC.
To understand how rainfall on Mars has changed over time, the researchers had to consider how the Martian atmosphere has changed.
When Mars first formed 4.5 billion years ago, it had a much more substantial atmosphere with a higher pressure than it does now. This pressure influences the size of the raindrops and how hard they fall.
Early on in the planet's existence, water droplets would have been very small, producing something like fog rather than rain. This would not have been capable of carving out the planet we know today.
As the atmospheric pressure decreased over millions of years, raindrops got bigger and rainfall became heavy enough to cut into the soil and start to alter the craters, the study said.
The water could then be channelled and be able to cut through the planet's surface, creating valleys.
"By using basic physical principles to understand the relationship between the atmosphere, raindrop size and rainfall intensity, we have shown that Mars would have seen some pretty big raindrops that would have been able to make more drastic changes to the surface than the earlier fog-like droplets," said Ralph Lorenz of Johns Hopkins University Applied Physics Laboratory in Maryland, US.
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