Observations of large valley networks on today’s Mars suggest formation by flowing water. However, most climate models cannot sustain temperatures above freezing. To understand this contradiction, a team of planetary researchers modeled the two leading theories for valley formation from precipitation (a warm wet climate) or temporarily melted ice from the edge of an ice cap (an icy cold climate). They found that the main difference between these scenarios was the location of the origin of the valleys that formed. In a warm wet setting, valleys start at many different elevations. In the icy cold scenario, valleys start only near the elevation where ice melted. The authors then examined a region of Mars with many large valley networks, focusing on the location and elevation of valley heads. Their findings showed that the distribution of valley heads matches predictions for a climate that includes precipitation rather than just runoff from melting ice caps. This suggests that precipitation played a significant role in forming these valleys, indicating that ancient Mars likely had a climate warm enough to support rain.
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