Description: Publication date: Available online 2 June 2018 Source: Planetary and Space Science Author(s): Vincenzo Orofino, Giulia Alemanno, Gaetano Di Achille, Francesca Mancarella The presence of ancient fluvial systems on Mars, known as valley networks, is considered one of the most compelling evidence that liquid water was once stable on the planet's surface. To understand the formation mechanisms of these valleys and to acquire information on the ancient climatic conditions of the planet, we determined the formation time of a sample group of 63 Martian valley networks. Our sample group was divided into two subgroups: valleys with an interior channel (13 valleys); and valleys without visible interior channels (50 valleys). For the first subgroup we estimated the formation time using a method based on the calculation of water and sediment discharges. We assumed four different possibilities: continuous flow; 5% of intermittence, typical of terrestrial humid or sub-humid conditions; 1% as for semiarid or arid environments; and finally, 0.1% for hyper-arid conditions. Once obtained the formation times for each valley we evaluated the erosion rates. Subsequently, the mean erosion rates obtained for the first subgroup of valleys were used to calculate the formation times for the remaining 50 valleys using the ratio between the eroded volume and the erosion rate. For the whole sample group of the fluvial systems, we found formation timescales ranging from 5 × 10 2  yr to 8 × 10 6  yr (with a median of 3 × 10 4  yr) for a continuous sediment flow, while the range is from 1 × 10 4 to 2 × 10 8  yr (median 5 × 10 5  yr) with an intermittence of 5%, from 5 × 10 4 to 8 × 10 8  yr (median 3 × 10 6  yr) with an intermittence of 1%, and from 5 × 10 5 and to 8 × 10 9 yr (median 3 × 10 7 yr) with a 0.1% intermittence. However, based on our results, a continuous sediment flow as well as an intermittence of 0.1% seem unlikely. Plausible values of formation timescales are instead obtained with intermittencies of 5% and 1% corresponding to humid and semiarid/arid environment, respectively. Our results do not allow to discriminate between the two scenarios; on the contrary the scenario of a permanently cold and icy Noachian Mars can be ruled out by our findings.