The Martian atmosphere today is thin, only 0.6 percent as thick as Earth's. That's so thin that your blood would boil in your body if you set foot on the Red Planet's surface without protection (you'd have to go 60,000 feet above sea level on Earth to experience the same effect).
But most scientists believe that this was not always the case: that once upon a time, Mars had a thick atmosphere, four times as thick as Earth's is today. Not only that, Mars may have once been water-rich, but the thinning atmosphere allowed the water vapor to escape into space instead of raining back down onto the planet as it does on Earth. This would mean that there were rains on Mars long ago – but what did they look like?
Robert Craddock and Ralph Lorenz, a pair of researchers from Johns Hopkins who published their findings in the journal Icarus, have analyzed the networks of valleys and dry channels on Mars in an attempt to better understand the movement of rains on early Mars. "Many people have analyzed the nature of rainfall on the Earth, but no one had thought to apply the physics to understanding the early Martian atmosphere," said Craddock.
After all, Earth scientists have been studying and modeling rainfall on our planet for a very long time, so they've gotten pretty good at it. While Mars isn't identical to Earth (for example, it's much smaller and has significantly weaker gravity), the two planets have plenty in common, and so it wasn't that difficult to adapt terrestrial rain models to the Red Planet.
From their data, Craddock and Lorenz constructed a model that showed Martian rain grew bigger and more intense over time. When the atmosphere was very thick, the rain drops would have been small and misty, but when the atmosphere started to depart, the rain drops would grow heavy enough to reshape the planet below.
"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 Lorenz.
"It's unlikely that rainfall on early Mars would have been dramatically different than what's described in our paper. Our findings provide new, more definitive, constraints about the history of water and the climate on Mars," added Craddock.