Mangala Valles, Mars: A reassessment of formation processes based on a new geomorphological and stratigraphic analysis of the geological units

Abstract

Mangala Valles has always been viewed as the typical outflow channel formed by catastrophic floods of water. A new analysis has shown that the geomorphological traces of fluvial or lacustrine processes within Mangala Valles can be better explained by fluid lava flooding the channels and filling pre-existing impact craters. As for the circum-Chryse outflow channels, where no clear source of water or mechanism able to replenish water at its hydraulic head is observed, there is no geologic trace of a sudden removal of a volume of water (ice) necessary to carve Mangala Valles. Neither maars nor rootless cones, typical volcanic features indicative of interaction between lava and ground ice, were found. Past works suggested that the formation of Mangala Valles occurred in late Amazonian age when the climate of Mars was similar to that seen today, that is absolutely not liquid water friendly. The present work shows how the origin of Mangala Valles may go back to Noachian or even Pre-Noachian when other studies have concluded that the climate was not liquid water friendly. Even assuming limited periods of obliquity favourable to liquid water in the history of Mars, which is at odds with the widespread presence of unaltered olivine and jarosite, it is very difficult to find plausible mechanisms of aquifer recharge or signs of catastrophic water release at the Notch of Mangala Valles that could feed the multiple episodes, or even a single episode, of fluvial flooding suggested in the literature. This evidence and other analysis will show that the presence of water and, eventually, ground ice is not incontrovertible in the equatorial regions and should not be given for granted as commonly done so far in the literature. The geomorphological analysis of the Mars Reconnaissance Orbiter (MRO) images provided in this paper, combined with THEMIS and MOLA data, show how Mangala Fossa, from which Mangala Valles originated as a breakout, is an erosional channel formed by the flow of lava in a original tube coming from Daedalia Planum rather than a tectonic graben or the sign of a dike rupturing to the surface.

The Absence of an Ocean and the Fate of Water all Over the Martian History

“…Combined, impact and sputtering processes may even account from 95% to 99% of the primordial atmosphere (Brain & Jakosky, 1998). The strongest phase of volcanism that Mars had in its first 500 Ma, as consequence of the SPGI, then might have replenished some CO 2 but degassed any remaining water in the mantle as final process (Leone, 2017). This is consistent with the analysis of the shergottites formed by melting of an original wet mantle that degassed over time (Balta & McSween, 2013).…”

“…In any case the outflow channels were emplaced on Hesperian and Amazonian geological units, estimated from crater counts (Tanaka et al, 2014), periods when water was mainly gone since the pre‐Noachian to Noachian. A reassessment of the stratigraphic units at Mangala Valles even opened a possible case whether the outflow channels might be older than expected and maybe ascribed to the peak of volcanism occurred in the pre‐Noachian (Leone, 2017).…”

“…Vastitas Borealis and Utopia Planitia also received the lava flows coming from Tharsis (Ivanov & Head, 2006) and Elysium (Hopper & Leverington, 2014), respectively. The olivine naturally contained in lava appears globally unaltered since the Noachian (Ehlmann et al, 2010; Leone, 2017, 2019; Leverington, 2009, 2011, 2019a; McSween, Wyatt, et al, 2006) as can also be seen from the corresponding units in the geochronological map of Mars (Tanaka et al, 2014). After such an abundant evidence, it was recently suggested that the outflow channels of Tharsis would have fed an ocean in the lowlands thought to exist at the frozen state until the Hesperian (Carr & Head, 2019).…”

“…Surely not present in the lowlands, where the ocean was thought to be, and not present at all. A possible explanation is that this serpentine was emplaced on the surface by volcanic processes after underground alteration in hydrothermal environment where the lithostatic pressure allowed the survival of liquid water (Ehlmann et al, 2011; Evans, 2004, 2010; Leone, 2017).…”

“…The giant impact event that formed the Martian dichotomy must have removed much of the primordial atmosphere and part of the water that survived after the accretion (Leone et al, 2014). The remaining water was then lost to space (Gillmann et al, 2011; Krasnopolsky, 2015; Kurokawa et al, 2014; Villanueva et al, 2015) through degassing from a still wet mantle (Balta & McSween, 2013; Leone, 2017). Today, the amount of water present on the surface and in the atmosphere is nearly negligible.…”