- 235 Downloads
KeywordsErosion Flooding Groundwater Ice Water
Outflow channels were first detected on images from the Mariner 9 mission (Baker and Milton 1974). They are the strongest morphological evidence that large amounts of water once flowed on the Martian surface. Characteristic landforms of outflow channels are anastomosing (building a network of streams that both branch out and reconnect) channels, streamlined erosional “islands,” dry cataracts, and scour marks and grooves on the channel floors (Baker 1982). The closest terrestrial analog for outflow channels is the huge complex of anastomosing rock-cut fluvial channels in east-central Washington (USA). This so-called Channeled Scabland was first interpreted by J Harlan Bretz as the result of the catastrophic megaflooding derived from ice-dammed Pleistocene glacial lakes (e.g., Bretz 1923), an interpretation which challenged the conventional geologic wisdom of Uniformitarianism and was, therefore, heavily debated (Baker 2009). Uniformitarianism (as described by Bates and Jackson 1980) is the fundamental principle or doctrine that geologic processes and natural laws now operating to modify the Earth’s crust have acted in the same regular manner and with essentially the same intensity throughout geologic time, and that past geologic events can be explained by phenomena and forces observable today; the classical concept that “the present is the key to the past.” Many models of the formation of Martian outflow channels favor a catastrophic formation by the sudden release of large amounts of water from subsurface reservoirs. Pressurized aquifers might have existed below an impermeable frozen upper crust (the cryosphere), which was then cracked by the ascent of volcanic dikes (e.g., Head et al. 2003). Alternatively, outflow channels might have been eroded, at least partly, by glacial processes (Lucchitta 1982). Most, if not all, outflow channels show evidence for a multistage formation with different phases of activity separated by geologically long periods of time. Outflow channel activity spans a long time, from ≫3.5 Ga to geologically very recent times (Neukum et al. 2010). Outflow channels terminate into the northern lowlands of Mars and might have fed an ancient northern ocean (Baker et al. 1991), although this scenario is contentious.
References and Further Reading
- Baker VR (1982) The channels of Mars. University of Texas Press, Austin, 198 ppGoogle Scholar
- Bates R, Jackson J (1980) Glossary of geology, 2nd edn. American Geological Institute, Falls Church, Virginia, 677 ppGoogle Scholar
- Neukum G, Basilevsky AT, Kneissl T, Chapman MG, Van Gasselt S, Michael G, Jaumann R, Hoffmann H, Lanz JK (2010) The geologic evolution of Mars: episodicity of resurfacing events and ages from cratering analysis of image data and correlation with radiometric ages of Martian meteorites. Earth Planet Sci Lett 294:204–222ADSCrossRefGoogle Scholar