This story is part of Welcome to Mars, our series exploring the red planet.

Scientists have been working hard to untangle the mysteries of water on Mars, from current-day concerns (what’s under the southern ice cap) to the planet’s ancient past. A study using data from NASA’s Mars Reconnaissance Orbiter spacecraft suggests we may need to revise the known timeline for flowing water on the red planet.

Mars today is a dry, dusty place. Scientists have said it was once much wetter, but that the water evaporated from the surface about 3 billion years ago. The new study, published in the journal AGU Advances late last year, found evidence of flowing water as recently as 2 billion to 2.5 billion years ago.

The timeline for Mars water is important for understanding the potential habitability of the planet. «The discovery raises new questions about how long microbial life could have survived on Mars, if it ever formed at all,» NASA said in a statement on Wednesday. «On Earth, at least, where there is water, there is life.» NASA’s Perseverance rover, for example, is looking for signs of ancient microbial life inside an ancient lakebed.

Evaporating water often leaves behind telltale signs it was there. In the case of Mars, those come in the form of chloride salt deposits. MRO data allowed the researchers to map the presence of these deposits in a region of Mars marked by impact craters. «These craters were one key to dating the salts: The fewer craters a terrain has, the younger it is. By counting the number of craters on an area of the surface, scientists can estimate its age,» NASA said.

MRO collects data in multiple ways. For this study, the spacecraft’s cameras and its Compact Reconnaissance Imaging Spectrometer for Mars (Crism) instrument provided key information. Crism is able to spot mineral residue that forms from water. The researchers examined former ponds and streams and dated the deposits. That’s how they arrived at the updated timeline for flowing water on the red planet.

Our understanding of Mars is constantly evolving as robotic explorers investigate the planet from the surface and from above. Said study co-author Bethany Ehlmann, a planetary scientist at Caltech, «What is amazing is that after more than a decade of providing high-resolution image, stereo, and infrared data, MRO has driven new discoveries about the nature and timing of these river-connected ancient salt ponds.»