How hard did it rain on Mars?

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. Although there is a growing body of evidence that there was once water on Mars, it does not rain there today.

But in their new study, geologists Dr. Robert Craddock and Dr. Ralph Lorenz show that there was rainfall in the past -- and that it was heavy enough to change the planet's surface. To work this out, they used methods tried and tested here on Earth, where the erosive effect of the rain on the Earth's surface has important impacts on agriculture and the economy.

Valley networks on Mars show evidence for surface runoff driven by rainfall. "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 Dr. Craddock of the Smithsonian Institution.

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 water could then be channeled and 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," commented Dr. Lorenz of John Hopkins University, who has also studied liquid methane rainfall on Saturn's moon Titan, the only other world in the solar system apart from Earth where rain falls onto the surface at the present day.

They showed that very early on, the atmospheric pressure on Mars would have been about 4 bars (the Earth's surface today is 1 bar) and the raindrops at this pressure could not have been bigger than 3mm across, which would not have penetrated the soil. But as the atmospheric pressure fell to 1.5 bars, the droplets could grow and fall harder, cutting into the soil. In Martian conditions at that time, had the pressure been the same as we have on Earth, raindrops would have been about 7.3mm -- a millimeter bigger than on Earth.

"There will always be some unknowns, of course, such as how high a storm cloud may have risen into the Martian atmosphere, but we made efforts to apply the range of published variables for rainfall on Earth," added Dr. Craddock. "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."