They estimate ages by counting impact craters in the regions containing these channels.
A depression created by the hypervelocity impact of a smaller body on the surface of a planet, moon, or other solid body in the Solar System or elsewhere is known as an impact crater.
Even when the records are hidden beneath the surface, impact craters give researchers access to a planet's geological past.
Our understanding of the past and present of the Solar System has been greatly aided by lessons learned from impact craters.
Older surfaces have had more time to be exposed to hitting bodies like meteoroids, asteroids, and comets than younger surfaces. Older surfaces therefore have more impact craters.
Therefore, astronomers estimate that the runoff channels seen on Mars must be at least 3.9 billion years old by counting impact craters in the regions containing these channels.
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