Elsevier

Icarus

Volume 211, Issue 1, January 2011, Pages 316-329
Icarus

Chaos formation by sublimation of volatile-rich substrate: Evidence from Galaxias Chaos, Mars

https://doi.org/10.1016/j.icarus.2010.09.005Get rights and content

Abstract

Galaxias Chaos deviates significantly from other chaotic regions due to the lack of associated outflow channels, lack of big elevation differences between the chaos and the surrounding terrain and due to gradual trough formation.

A sequence of troughs in different stages is observed, and examples of closed troughs within blocks suggest that the trough formation is governed by a local stress field rather than a regional stress field. Moreover, geomorphic evidence suggests that Galaxias Chaos is capped by Elysium lavas, which superpose an unstable subsurface layer that causes chaotic tilting of blocks and trough formation.

Based on regional mapping we suggest a formation model, where Vastitas Borealis Formation embedded between Elysium lavas is the unstable subsurface material, because gradual volatile loss causes shrinkage and differential substrate movement. This process undermines the lava cap, depressions form and gradually troughs develop producing a jigsaw puzzle of blocks due to trough coalescence.

Observations of chaos west of Elysium Rise indicate that this process might have been widespread along the contact between Vastitas Borealis Formation and Elysium lavas. However, the chaotic regions have probably been superposed by Elysium/Utopia flows to the NW of Elysium Rise, and partly submerged with younger lavas to the west.

Introduction

Chaotic terrains are common on the martian surface, however the origin of this landform is enigmatic and many different formation models have been proposed.

Chaotic terrain was thoroughly described by Sharp (1973) as irregular jumbles of angular blocks of various size (kilometers to tens of kilometers) many preserving remnants of the upland surface and sometimes controlled by linear features. Generally the blocks are flat topped, their sizes decrease away from the bounding head scarp and the terrain consists of alcove-like depressions that commonly have a subcircular shape and can be divided into cells (Nummedal and Prior, 1981, Chapman and Tanaka, 2002, Rodriguez et al., 2006). Usually, the chaotic terrain lies several hundreds of meters up to a couple of kilometers below the surrounding plateau and thus, the topographic characteristics are very distinctive for chaotic terrain. Meresse et al. (2008) used topographic profiles from the Xanthe and Margaritifer Terra to resolve different stages of chaos formation, showing an initial stage of shallow ground subsidence of a few tens to hundreds of meters, where the ancient plateau is heavily fractured and collapse is limited. During the collapse, the highland material is fractured into mesas making the chaos margins well defined and resulting in a vertical displacement of 1000–3000 m. Equally, Rodriguez et al. (2005) has delineated different stages of chaos formation based on morphologic and topographic observations of the Xanthe Terra region.

Chaotic terrain is found widespread and is generally geographically associated with huge equatorial troughs like Valles Marineris, with outflow channels like Chryse outflow channels and with the dichotomy boundary (e.g., Soderblom and Wenner, 1978).

Several different geologic scenarios have been proposed as chaos forming processes, often involving H2O as an important agent. Sharp (1973) suggested both degradation of ground ice and excavation of magma as plausible formation processes and Soderblom and Wenner (1978) elaborated on a ground ice model suggesting that erosion of strata originating from different zones of H2O stability would form chaotic regions. Recently, Zegers et al. (2010) proposed a new variant of this hypothesis suggesting that water ice is a part of the rock sequence. As the rock overburden reaches a thickness of 1–3 km the ice begins to melt due to the thermal insulation caused by overlying sediment, and thus, the overlying material destabilizes and water escapes creating chaos terrain.

The close spatial connection between chaotic terrain and outflow channels has resulted in suggestions that chaotic terrains are source regions for channel formation. Carr, 1978, Carr, 1979 argued that outflow channels emerging full born from chaotic regions are strongly suggestive for catastrophic flooding by rapid release of a confined aquifer under great pressure resulting in collapse in adjacent areas. Triggering mechanisms could either be impact cratering or pore pressure reaching lithostatic pressure. Magma–ground ice interactions is another possible triggering mechanism, both as a heat source for catastrophic melting of ground ice, as well as disrupting the cryosphere releasing a confined aquifer under pressure (Chapman and Tanaka, 2002, Head and Wilson, 2002, Meresse et al., 2008). This model has been supported by Glotch and Christensen (2005), who observed grey, crystalline hematite along with phyllosilicates and possible sulfate in Aram Chaos providing evidence for episodic pulses of water release as a chaos forming process. Cabrol et al. (1997) stress the importance of volcano tectonic strains as another triggering mechanism based on a survey of Shalbatana Vallis, where crossing fault systems are suggested to allow hydrothermal drainage of confined aquifers.

Rodriguez et al. (2005) suggest that the different stages of chaos formation result from multiple episodes of reactivation of the hydraulic head and propose a suite of processes such as: renewed magmatic activity, topographic lowering with respect to the groundwater table, subsiding cavern roofs or thickening of a permafrost seal, as mechanisms for increased hydrostatic pressure. Analysis by Wang et al. (2006) show that freezing-induced pressurization is a possible mechanism for releasing groundwater, however not on the order of big outflow channel systems.

Debris flow mechanisms, triggered by large scale failures of subsurface material, have also been proposed to account for the development of chaos (Tanaka, 1999, Nummedal and Prior, 1981). Based on observations of the Simud/Tiu deposits, Tanaka (1999) proposed that seismic activity could liquefy water-rich sediments causing a collapse and lowering of the chaos surface versus the adjacent plateau, resulting from subsurface removal rather than rotational slumping or lobate landslide masses. Wang et al. (2005) support this idea by suggesting impacts as a quaking agent and find evidence for liquefaction in chaotic regions by checkerboard patterns of gaps between blocks of chaotic terrain, which is similar to liquefaction patterns on Earth.

Dissociation of clathrates as a mechanism of subsurface removal has also been put forward in several papers (e.g., Milton, 1974, Hoffmann, 2000, Komatsu et al., 2000, Rodriguez et al., 2006) starting with Milton (1974) who proposed carbon dioxide hydrate as a possible phase, under which explosive dissociation would result in a catastrophic dewatering. Komatsu et al. (2000) argue that the stress from decomposed clathrates releases a large quantity of CO2 and CH4, which would subject water-saturated sediments to so much stress that the sediment would liquefy. A similar model, including a series of runaway degassing events for Ganges Chaos, has been proposed by Rodriguez et al. (2006), who mention deep fracture propagation, magmatic intrusions and climatic thawing and thinning of the cryosphere as plausible trigger mechanisms.

Section snippets

Geologic setting

Galaxias Chaos is situated in a highly complex region in the transition zone between Elysium Rise to the south, Utopia Basin to the north, and is bounded by Hecates Tholus to the east and by Elysium/Utopia flows to the west (Fig. 1). Galaxias Chaos was first recognized by Schaber and Carr (1977) and later mapped by Scott and Carr (1978) (1:25,000,000) as a knobby material in a zone between the lowland surface of Vastitas Borealis and Noachian Terrain. Based on Viking imagery, Mouginis-Mark, 1985

Galaxias Chaos

Since the last investigations of Galaxias Chaos, a lot of new satellite data has been provided by Mars Global Surveyor (MGS), Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter (MRO) allowing more thorough and detailed studies of chaos forming processes.

Model for formation of Galaxias Chaos

Galaxias Chaos is significantly different from other chaotic regions. First of all the topography is very different and in Galaxias Chaos the elevation of mesas is in the same range as the surroundings, whereas other chaotic terrains have a significant elevation difference up to several thousand meters between blocks and plateau. Moreover, Galaxias Chaos is not associated with huge outflow channels, and Hrad Vallis is modifying rather than fed by the western part of Galaxias Chaos. The

Discussion

This proposed formation model raises several important questions considering the nature of VBF; what mechanisms controlled the sublimation of volatiles? And do other examples of similar stratigraphic relationships exist both in the region of Elysium and elsewhere?

The role of the VBF in the history of the martian hydrologic cycle is crucial, especially with respect to resolving mechanisms of deposition and later modification, linking the present day morphology to its origin. This work stresses

Conclusion

Galaxias Chaos deviates significantly from other chaotic regions due to the lack of associated outflow channels, lack of large elevation differences between the chaos and the surrounding terrain, and due to gradual trough formation. A sequence of troughs in different stages is detected, and examples of closed troughs within blocks suggest that the trough formation is governed by a local stress field rather than a regional stress field. The geomorphic evidence suggests that Galaxias Chaos is

Acknowledgments

Thanks are extended to James Dickson and Caleb I. Fassett from Brown Planetary Geoscience Group for their very valuable help in GIS mapping and data processing. Moreover, we thankfully acknowledge Per Noernberg and the rest of the Mars Simulation Laboratory staff at Aarhus University as well as Sandra Schumacher and an anonymous reviewer for their improvement of the manuscript. Furthermore, we appreciate the efforts of the HiRISE, CTX, HRSC, MOLA, MOC and THEMIS teams to make their data

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