The ancient climate of Mars remains an enigma despite the abundance of in situ and remote-sensing data revealing hydrological activity in the past. The crux of this debate—informed by geomorphic studies, chemical alteration observations, and numerical climate models—is the amount, distribution, and nature of surface water and precipitation (i.e., rain vs. snow). Here, we combined measurements of 96 open- and closed-basin lakes with simple hydrological balances to constrain catchment-averaged precipitation, P_T, integrated over a given runoff episode of indeterminate length during early Mars history. We included 13 systems containing both open- and closed-basin lakes, providing fully bounded precipitation estimates (coupled systems). We show that, on average, P_T was ≳4 m and ≲159 m; however, local precipitation was spatially variable and consistent with a complex, planetwide climate. Aridity indexes of open-basin lakes indicate some regions may have been at least as humid as semiarid terrestrial environments. Our results offer widely distributed, quantitative hydro-climate constraints that can be used to test paleoclimate model scenarios, working toward bridging the gap between geological observations and climate theory for early Mars.

中文翻译：

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火星早期古湖的降水和干旱约束

尽管有大量的现场和遥感数据揭示了过去的水文活动，但火星的古老气候仍然是一个谜。地貌研究，化学蚀变观测和数值气候模型告知，这场辩论的重点是地表水和降水量（即雨与雪）的数量，分布和性质。在这里，我们结合对96个开放流域湖和封闭流域湖的测量以及简单的水文平衡，以限制在火星早期历史中给定径流不确定长度的流域平均降水量P _T的积分。我们包括了13个包含开放流域湖和封闭流域湖的系统，它们提供了完全有限的降水估计（耦合系统）。我们显示平均而言，P _T分别为≳4m和≲159m；然而，当地的降水在空间上是可变的，并且与复杂的全球气候一致。流域湖泊的干旱指数表明某些地区的湿度至少与半干旱陆地环境一样。我们的研究结果提供了广泛分布的定量水文气候约束，可用于测试古气候模式，以缩小火星早期地质观测与气候理论之间的差距。