The nature of the Late Noachian climate of Mars remains one of the outstanding questions in the study of the evolution of martian geology and climate. Despite abundant evidence for flowing water (valley networks and open/closed basin lakes), climate models have had difficulties reproducing mean annual surface temperatures (MAT) > 273 K in order to generate the “warm and wet” climate conditions presumed to be necessary to explain the observed fluvial and lacustrine features. Here, we consider a “cold and icy” climate scenario, characterized by MAT ∼225 K and snow and ice distributed in the southern highlands, and ask: Does the formation of the fluvial and lacustrine features require continuous “warm and wet” conditions, or could seasonal temperature variation in a “cold and icy” climate produce sufficient summertime ice melting and surface runoff to account for the observed features? To address this question, we employ the 3D Laboratoire de Météorologie Dynamique global climate model (LMD GCM) for early Mars and (1) analyze peak annual temperature (PAT) maps to determine where on Mars temperatures exceed freezing in the summer season, (2) produce temperature time series at three valley network systems and compare the duration of the time during which temperatures exceed freezing with seasonal temperature variations in the Antarctic McMurdo Dry Valleys (MDV) where similar fluvial and lacustrine features are observed, and (3) perform a positive-degree-day analysis to determine the annual volume of meltwater produced through this mechanism, estimate the necessary duration that this process must repeat to produce sufficient meltwater for valley network formation, and estimate whether runoff rates predicted by this mechanism are comparable to those required to form the observed geomorphology of the valley networks.

When considering an ambient CO 2 atmosphere, characterized by MAT ∼225 K, we find that: (1) PAT can exceed the melting point of water (>273 K) in topographic lows, such as the northern lowlands and basin floors, and small regions near the equator during peak summer season conditions, despite the much lower MAT; (2) Correlation of PAT > 273 K with the predicted distribution of surface snow and ice shows that melting could occur near the edges of the ice sheet in near-equatorial regions where valley networks are abundant; (3) For the case of a circular orbit, the duration of temperatures >273 K at specific valley network locations suggests that yearly meltwater generation is insufficient to carve the observed fluvial and lacustrine features when compared with the percentage of the year required to sustain similar features in the MDV; (4) For the case of a more eccentric orbit (eccentricity of 0.17), the duration of temperatures >273 K at specific valley network locations suggests that annual meltwater generation may be capable of producing sufficient meltwater for valley network formation when repeated for many years; (5) When considering a slightly warmer climate scenario and a circular orbit, characterized by MAT ∼243 K, we find that this small amount of additional greenhouse warming (∼18 K MAT increase) produces time durations of temperatures >273 K that are similar to those observed in the MDV. Thus, we suggest that peak daytime and seasonal temperatures exceeding 273 K could form the valley networks and lakes with either a relatively high eccentricity condition or a small amount of additional atmospheric warming, rather than the need for a sustained MAT at or above 273 K.

The results from our positive-degree-day analysis suggest that: (1) For the conditions of 25° obliquity, 600 mbar atmosphere, and eccentricity of 0.17, this seasonal melting process would be required to continue for ∼(33–1083) × 103 years to produce a sufficient volume of meltwater to form the valley networks and lakes; (2) Similarly, for the conditions of 25° obliquity, 1000 mbar atmosphere, circular orbit, and ∼18 K additional greenhouse warming, the process would be required to continue for ∼(21–550) × 103 years. Therefore, peak seasonal melting of snow and ice could induce the generation of meltwater and fluvial and lacustrine activity in a “cold and icy” Late Noachian climate in a manner similar to that observed in the MDV. A potential shortcoming of this mechanism is that independent estimates of the required runoff rates for valley network formation are much higher than those predicted by this mechanism when considering a circular orbit, even when accounting for additional atmospheric warming. However, we consider that a relatively higher eccentricity condition (0.17) may produce the necessary runoff rates: for the perihelion scenario in which perihelion occurs during southern hemispheric summer, intense melting will occur in the near-equatorial regions and in the southern hemisphere, producing runoff rates comparable to those required for valley network formation (∼mm/day). In the opposite perihelion scenario, the southern hemisphere will experience very little summertime melting. Thus, this seasonal melting mechanism is a strong candidate for formation of the valley networks when considering a relatively high eccentricity (0.17) because this mechanism is capable of (1) producing meltwater in the equatorial region where valley networks are abundant, (2) continuously producing seasonal meltwater for the estimated time duration of valley network formation, (3) yielding the amount of meltwater necessary to incise the valley networks within this time period, and (4) by considering a perihelion scenario in which half of the duration of valley network formation is spent with peak summertime conditions during perihelion in each hemisphere, higher runoff rates are produced than in a circular orbit and the rates may be comparable to those required for valley network formation.