Noachian climatic conditions on Mars inferred from valley network junction angles

Abstract

Valley networks (VNs) on Mars offer convincing evidence for its past water activities. Previous research empirically and theoretically suggested that the geometry properties, such as junction angles, of streams on Earth formed under different climatic conditions are different. Thus the geometry properties of VNs can be used to infer the early Mars climatic conditions under which VNs were formed. The frequency distribution of junction angles is less influenced by the post-formational modification processes than valley density and cross-section and can be accurately extracted from low resolution data. We first analyzed the association between the junction angles and environmental factors on Earth. The results suggested that the climatic factors are stronger than or on par with the geologic factors in controlling junction angles and that climatic parameters Aridity Index (AI) and Mean Annual Precipitation (MAP) can be estimated from junction angles. We then applied the associations between terrestrial junction angle and climatic conditions to estimate the AI and MAP of Mars, considering the different solar radiation of the two planets. The spatial analysis of inferred climatic conditions showed that the Noachian Mars had an active global hydrological cycling and Mars was “warm” during the VNs' formation period. The duration of “warm” Mars is estimated by the ratio between the required water volume to form VNs and the runoff discharge derived from MAP. The range of “warm” period is from ∼4.4 to ∼77 million years. The results support the hypothesis that Mars was “episodically warm”.