Magmatic intrusions and hydrothermal systems: Implications for the formation of Martian fluvial valleys.

Abstract

This dissertation investigates the possible role of hydrothermally driven groundwater outflow in the formation of fluvial valleys on Mars. Although these landforms have often been cited as evidence for a past wanner climate and denser atmosphere, recent theoretical modeling precludes such climatic conditions on early Mars when most fluvial valleys formed. Because fluvial valleys continued to form throughout Mars' geological history and the most earth-like stream valleys on Mars formed well after the decline of the early putative earth-like climate, it may be unnecessary to invoke drastically different climatic conditions for the formation of the earliest stream valleys. The morphology of most Martian fluvial valleys indicates formation by ground-water sapping which is consistent with a subsurface origin. Additionally, many Martian fluvial valleys formed on volcanoes, impact craters, near fractures, or adjacent to terrains interpreted as igneous intrusions; all are possible locales of vigorous, geologically long-lived hydrothermal circulation. Comparison of Martian valley morphology to similar features on Earth constrains valley genesis scenarios. Volumes of measured Martian fluvial valleys range from 10¹⁰ to 10¹³ m³. Based on terrestrial analogs, total water volumes required to erode these valleys range from -10¹⁰ to 10¹⁵ m³. The clustered distribution of Martian valleys within a given terrain type, the sapping dominated morphology, and the general lack of associated runoff valleys all indicate the importance of localized ground-water outflow in the formation of these fluvial systems. An analytic model of a conductively cooling cylindrical intrusion is coupled with the U.S. Geological Survey's numerical ground-water computer code SUTRA to evaluate the magnitude of ground-water outflow expected from magmatically-driven hydrothermal systems on Mars. Results indicate that magmatic intrusions of several 10² km³ or larger can provide sufficient ground-water outflow over periods (several 10⁵ years) required to form Martian fluvial Valleys. Therefore, a vastly different climate on early Mars may not be necessary to explain the formation of the observed Valleys. Martian hydrothermal systems would have also produced long-lived sources of near-surface water; these localized regions may have provided oases for any microbial life that may have evolved on the planet.