Scientists normally estimate the age of a rocky planet like Mars by counting the number, size and distribution pattern of craters on its surface. The idea: the older a surface, the more craters should have accumulated over time. Crater counts thus give an indication of the relative age of different Martian regions. Scientists would then compare the crater counts with similar regions on the moon, which have already been exactly dated by using the rocks from the moon, and thus arrive at an exact date for the martian regions.
However, it looks like this method of computing the age of different Martian regions is flawed.

A typical Martian crater (Courtesy: NASA)According to Nadine Barlow of Northern Arizona University in Flagstaff, USA, pictures from NASA's Mars Explorer spacecraft detected faint ray-like lines emanating from even the smallest of craters. This suggests that a lot of debris went up in the air when the meteorite hit the surface, and that falling debris must have caused a lot of secondary craters in the region. This would mean that the way crater-counting is done is flawed, since the current methods would have counted the secondary craters as craters from meteorite impacts. This would skew the result, and the region would seem to be much older than it actually is.
The findings will significantly change our estimations on when different regions of Mars were volcanically active. The vast majority of craters smaller than 2 kilometres in diameter may be secondary craters, making them virtually useless for dating surfaces. Therefore, all estimations that are based on craters, have to be revisited. This will have implications for our understanding of the evolution of Mars, when water last existed on Mars, and consequently, our understanding of why it disappeared and when life might have formed there or gone extinct.