Earth and Planetary Science Department
University of Tennessee, The
Mars Geologic Mapping part of Cutting Edge:GIS and Remote Sensing:Activities2
Devon Burr, Earth and Planetary Sciences Department, University of Tennessee Knoxville Summary This final laboratory exercise in an introductory planetary geology course requires the students to use Google Mars to ...
Landscape inversion on Earth and Mars part of Vignettes:Vignette Collection
PROCESS: Landscape inversion is the process whereby relative topographic elevations become inverted such that previously low-standing features become high-standing. This relief inversion occurs through induration (increased hardness or resistance to erosion) of a specific feature or features within the landscape, and the subsequent erosion of the less-indurated surrounding material. Thus, an inverted landscape immediately implies a multi-stage history of development. The induration may occur through a variety of mechanisms, including geochemical cementation, armoring by a coarse-grained deposit, or capping or infill by lava. On Earth the regional erosion that follows is usually a result of relative sea level drop, due either to glaciation or to tectonic uplift. EXAMPLES ON EARTH: Relief inversion may occur in a variety of landscapes. For example, mesas and buttes exist on the central Colorado Piedmont. These landforms are capped by Eocene age conglomerates and tuffs, which were originally deposited into topographic lows. These resistant lithologies have preserved the more erodible underlying formation, so that these previous topographic lows now form the high-standing mesas or buttes dotting the region [Figure 1]. In some cases, erosion of the capping lithologies produces mantling of the lower slopes of the buttes, resulting in erosion in the butte centers. The products of this secondary inversion are annular features with a topographic low in the middle, termed 'talus flatiron rings' (Morgan et al. 2008) [Figure 1]. Landscape inversion has also been documented on hillslopes as a result of armoring, a process also termed 'gully gravure. In western Virginia, USA., former valleys or 'dells' now form higher-elevation 'noses' (Mills 1981). In eastern Tennessee, armoring of rilles in roadcuts has been shown to produce surfaces elevated by differential erosion above the surroundings (Osterkamp and Toy 1994). Former stream channels provide other salient examples of inversion. On Earth, inverted channels resulting from lava infill are found in northern California [Figure 2] and in southern Utah, USA, and in Australia (Pain and Ollier 1995). Inverted channels resulting from geochemical cements have been documented in Oman (Maizels 1987) and in Utah (Williams et al. 2007). EXAMPLES ON MARS: On Mars, a variety of inverted landforms have been suggested (Pain et al. 2007). For example, small, flat-topped buttes are hypothesized to be inverted impact craters. Although some common indurating mechanisms such as lava infill may be ruled out by the context, the precise indurating mechanism(s) for these features are not yet established. Former channels are perhaps the most distinctive inverted feature on Mars. In some cases, the largest and oldest of these proposed inverted channels have been attributed to lava capping [Figure 3] (Rhodes 1981). More recent and higher resolution data of Mars has revealed a plethora of sinuous ridges, inferred to be inverted paleochannels. Some of these features are found on high-standing terrain, such as near the rim of Valles Marineris, the 5000-km-long, 10-km-deep rift system on Mars. The location and morphology of these features indicate former rainfall at this elevated location. Other inverted fluvial features are being exhumed from within surrounding units. Indeed, the most populous cluster of inverted fluvial features on Mars is located at low elevation between the Aeolis and Zephyria Plana, where the features are being exhumed by aeolian abrasion from within the friable Medusae Fossae Formation [Figure 4] (Burr et al. 2009). The exhumation process has revealed a range of feature morphologies, interpreted as inverted channels and floodplain meander belts, showing scroll bars. These features are often closely spaced and may even be stacked stratigraphically [Figure 5], indicating a substantial history of repeated fluvial flow. In orbital imagery, inverted stream channels can be hard to distinguish from eskers, linear sedimentary deposits associated with glacial meltwater stream. The longitudinal profile of the feature can provide diagnostic information – rivers only flow downhill, whereas esker may trend uphill due to the pressure of the overlying ice. However, uneven inversion or post-formation tectonism can also produce uphill-trending longitudinal profiles. Thus, additional evidence of glaciation, in the form of other glacially produced landforms, is useful to substantiate a glaciogenic hypothesis. SUMMARY: Inverted morphologies comprise an unusual suite of landforms, including inverted basins, hillslopes, impact craters, river channels, and floodplains. Their correct interpretation, though challenging, provides evidence of a multi-stage sequence of events, including the original creation of the landform, then its induration, and finally its inversion. On Mars, inverted landforms are providing us with important clues to the geologic history of that planet.
Comparative planetology: the geomorphology of volatile cycling and catastrophic flooding part of Vignettes:Vignette Collection
INTRODUCTION: Three planetary bodies in our Solar System show landforms that provide evidence of liquid flowing over the surface, either at present or in the past. The surface flow of liquid is evidence for a volatile cycle – the cyclic movement on a planetary surface of materials that transition easily among vapor, liquid, and solid states. Fluvial landforms result from the ordinary or average flow of liquid, whereas flood-formed landforms result from extra-ordinary liquid discharge. Earth's fluvial and flood-formed landforms and processes provide the template for understanding similar landforms on other planetary bodies. However, the flood formation mechanisms on other bodies may not all have terrestrial analogs. EARTH: Earth, of course, has an active hydrological (water-based) volatile cycle. This cycle encompasses a suite of features that includes rivers, lakes, oceans, groundwater, and glaciers. As one element of this suite, catastrophic flood channels convey extremely large flows – on the order of a million cubic meters per second – , which are similar in magnitude to massive ocean currents like the Gulf Stream. These catastrophic flood channels formed through a variety of mechanisms. The largest paleofloods on Earth are associated with glaciation, resulting from outbreaks of voluminous meltwater ponded beneath glaciers or behind glacial lobes. The Altai Mountains in Siberia (Carling et al. 2009), the Channeled Scabland in Washington state, USA (Baker 2009), and subglacial Laurentide Ice Sheet floods (Kehew et al. 2009) provide examples of this association. This glacial flooding may be triggered by volcanism or enhanced geothermal heat flow, such as may result from subsurface magma emplacement. The relationship between glacial flooding and volcanism is illustrated by contemporary jökulhlaups (glacial outburst floods) in Iceland (Figure 1) (Bjornsson 2009), in which volcanic heating produces enough glacial meltwater eventually to float or break through the glacier ice barrier as a flood. Volcanic calderas in particular can generate extreme break-out floods (Manville et al. 2007), in part because of the space available to collect large volumes of meltwater before release. Volcanism and glaciation are also indirect causes for debris-dam floods O'Connor and Beebee 2010), which are produced by failure of dams of volcanic deposits and glacial moraines, respectively. The smallest floods on Earth originate from extreme precipitation events, e.g., monsoons. MARS: On Mars, valley networks, glacial features, and polar ice caps all provide evidence for a more energetic hydrological cycle in the ancient past, although today much or all near-surface water on Mars is in the form of ice. Relict catastrophic flood channels are one of the most striking landforms on the surface of Mars. Based on age-dating of the surfaces of these flood channels, catastrophic flooding on Mars may be divided into three categories which roughly correspond with Mars' three time periods or epochs. Mars' smallest flood channels date to the Noachian epoch (3.8-3.5 Ga) and originate at large impact craters or intercrater basins (Irwin and Grant 2009). Because of their origination at impact craters and in view of other evidence for precipitation on early Mars, these flood channels are inferred to have resulted from overflow of impact crater paleolakes originally fed by precipitation. Mars' largest flood channels (Figure 2a) date to the Hesperian epoch (3.5-1.8 Ga) and generally originate at 'chaos terrain' (Figure 2b) (Coleman and Baker 2009). These scarp-bounded, collapse areas may have released pressurized groundwater due to magmatic and/or cyrospheric processes. Flood channels dated to the Amazonian epoch (1.8 Ga to present) originate at volcanotectonic fissures (Figure 3) (Burr et al. 2009a). These fissures may be the surface expression of dikes and/or extensional tectonics, either of which could have tapped groundwater aquifers. TITAN: Titan, the largest moon of Saturn, has an average surface temperature of 94K (-180C or -290F). Thus, all the water on the satellite is frozen into an icy crust. Nonetheless, Titan has a volatile cycle, apparently active, based on hydrocarbons combined with liquid nitrogen. Some results of this volatile cycle include branching networks interpreted from near-surface images as alluvial valleys with inset fluvial channels (Figure 4) (Tomasko et al. 2005, Perron et al. 2006). Radar images acquired from orbit also show fluvial networks (Lorenz et al. 2008) with a variety of network morphologies (Burr et al. 2009b). These features observed from orbit are commonly termed channels, a term which connotes periodic bankfull flow, although the large size and the morphology of these features suggests that they may instead of alluvial valleys (Burr et al. 2009b). Flooding on Titan occurs with very different materials and conditions than on Earth and Mars. Flooding on these inner planets entails water flow and the sediment is silicate material derived from the crust. In contrast, flooding on Titan likely entails liquid hydrocarbon (mostly methane) flow and the sediment is either water ice derived from the crust or organic material deposited out of Titan's atmosphere. Despite the differences in materials and in gravity, however, theoretical modeling indicates that the liquid viscosity, sediment density, and gravity on Titan combine to produce similar sediment transport parameters as for Earth and Mars. This similarity may explain in part why images from the Cassini-Huygens mission show 'surprisingly Earth-like' morphologies. In addition to fluvial networks, these morphologies include anastomosing channels similar to terrestrial desert flood channels. On Earth, these desert flood channels are produced by monsoonal rains. Titan is hypothesized to have a 'methane monsoon,' on the basis of theory and cloud observations. Unlike on Earth and Mars, glaciation likely does not occur on Titan, given the low triple point of hydrocarbons. Cryovolcanism and impact cratering may have generated debris dam floods analogous to those on Earth and Mars, although data supporting such processes is not yet evident. Thus, whereas monsoonal precipitation produces the smallest floods on the terrestrial planets, it likely produces the largest floods on Titan (Burr 2010). In summary, catastrophic flood channels are one striking landform that gives evidence of volatile cycling. A variety of mechanisms can produce flooding, and the dominant mechanism(s) change(s) according to context. Despite these differences, however, increased understanding of flood- and fluvial landforms on Earth contributes to our understanding the history and current occurrence of volatile cycling on Mars and on Titan.