The origin and timing of fluvial activity at Eberswalde crater, Mars
N. MangoldEdwin S. KiteMaarten G. KleinhansH. E. NewsomV. AnsanErnst HauberE. R. KraalC. QuantinK. L. Tanaka
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Noachian
Hesperian
Landform
Tharsis
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Noachian
Hesperian
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On Earth, the characteristics of fluvial erosion depends on two main parameters: climate (rain fall) and tectonic history. Mars is a planet that experienced erosion driven by liquid water but its geodynamics are vastly different from Earth’s. Mars therefore represents a unique opportunity to understand how landscape evolution differs on a planet with a “stagnant lid” tectonic regime. The formation of Tharsis dome, a vast volcanic province, during the early history of Mars represented a major magmato-tectonic upheaval for the planet. Over several hundreds of million years, the Tharsis region experienced large scale magmatic intrusions, crustal deformation and effusive volcanism resulting in crustal growth, dynamic uplift and true polar wander (TPW) that accounts for the present location of the Tharsis dome at the equator. This event occurred during a time when Mars had an active water cycle, although the total mass and relative proportion of ice, liquid water and vapor is not well constrained. The uplift and subsequent true polar wander of Mars have affected drainage systems across the planet with many being abandoned or modified due to the variable uplift or subsidence as a lithospheric response to the regional upheaval in the Tharsis region (load on the elastic lithosphere) and TPW. Here we present results from numerical simulations performed using a stream power law algorithm on Mars during the Noachian/Hesperian growth of Tharsis to assess how the patterns of erosion rate are affected by the distribution of atmospheric moisture and flow routing in an attempt to reproduce the observed distribution of valley networks and their geometry. For this, we adapted and used the fully-implicit and O(n)-complexity FastScape algorithm to perform the simulation at the planetary scale. The aims of this work are to quantify the effect of Tharsis dome formation on fluvial systems during the Noachian and early Hesperian, and to establish a first-order erosion rate for this period. This study could help to constrain how much water was cycling on Mars at this time.
Hesperian
Tharsis
Noachian
geodynamics
Lithospheric flexure
Hydrosphere
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Utilizing the Termoskan data set of the Phobos '88 mission we have recognized a new feature on Mars: ejecta blanket distinct in the thermal infrared (EDITH). Virtually all of the more than 100 features discovered in the Termoskan data are located on the plains near Valles Marineris. EDITHs have a startlingly clear dependence upon terrains of Hesperian age, implying a spatial or temporal dependence on Hesperian terrains. Almost no thermally distinct ejecta blankets are associated with any of the thousands of craters within the data set that occur on the older Noachian units. EDITHs also do not appear on the portions of the younger Tharsis Amazonian units seen in the data. The Hesperian terrain dependence cannot be explained by either atmospheric or impactor variations; Noachian and Hesperian terrains must have experienced identical atmospheric and impactor conditions during Hesperian times. Thermally distinct ejecta blankets therefore reflect target material differences and/or secondary modification processes. Not all lobate ejecta blankets are thermally distinct, but all EDITHs correlated with visibly discernible ejecta blankets are associated with lobate ejecta blankets. The boundaries of the thermally distinct areas usually follow closely the termini of the fluidized lobate ejecta blankets, even when the ejecta blankets show a high degree of sinuosity. Thus, the thermally distinct nature of EDITHs must be due to the primary ejecta formation process. The coupling of these thermal anomalies to morphology is unlike most sharp Martian inertia variations which are decoupled from observed surface morphology. Some thermally distinct ejecta blankets occur near otherwise similar craters that do not have thermally distinct ejecta blankets. Thus, wind patterns or locally available aeolian material cannot provide a single overall explanation for the observed variations. We compiled a data base of 110 EDITH and non‐EDITH craters ranging in diameter from 4.2 km to 90.6 km. There are almost no correlations within the data base other than occurrence on Hesperian terrains. We postulate that most of the observed EDITHs are due to excavation of thermally distinctive Noachian age material from beneath a relatively thin layer of younger, more consolidated Hesperian volcanic material. The plausibility of this theory is supported by much geological evidence for relatively thin near‐surface Hesperian deposits overlying massive Noachian megabreccias on the EDITH‐rich plains units. We suggest that absence of thermally distinct ejecta blankets on Noachian and Amazonian terrains is due to absences of distinctive near‐surface layering. Thermally distinct ejecta blankets are excellent locations for future landers and remote sensing because of relatively dust free surface exposures of material excavated from depth.
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Fluvial erosion on early Mars was dominated by valley networks created through a combination of groundwater processes and surface runoff. A reduced greenhouse effect due to CO 2 loss, together with a declining geothermal heat flux, promoted the growth of a cryosphere and a Hesperian hydrologic regime dominated by outflow channel formation. We test the hypothesis that the transition from valley network to outflow channel formation was preceded by a more subtle evolution characterized by a weakening of surface runoff, leaving groundwater processes as the dominant, final source of valley network erosion. Our hypothesis, supported by a terrestrial analog in the Atacama desert of Chile, is related to the groundwater sapping reactivation hypothesis for densely dissecting highland valley networks on Mars suggested by Baker and Partridge in 1986 and focuses on the age analysis of large, sparsely dissecting valley networks such as Nanedi Valles, Nirgal Vallis, valleys in fretted terrain, and tributaries of outflow channels and Valles Marineris chasmata. We find that these features are consistently late Noachian to Hesperian in age, younger than Noachian densely dissecting dendritic valley networks in the southern highlands. In the Tharsis region the observation of dense and sparse valley network morphologies on Hesperian terrain suggests that while surface runoff gave way to groundwater processes consistent with our hypothesis, the transition may have occurred later than elsewhere on the planet. The volcanic nature of Tharsis suggests that geothermal heat and volatile production led to episodically higher volumes of surface runoff in this region during the Hesperian.
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