The National Aeronautics and Space Administration's (NASA) Mars Exploration Program is focused on understanding the past or present habitability of Mars (MEPAG, 2010). Central to this aim is determining the history of liquid water on Mars. Some of the most salient evidence for liquid water is giant outflow channels that debouch into Chryse Planitia, hypothesized to be part of an ancient Martian ocean (Parker et al., 1993; Head et al., 1999; Perron et al., 2007; Di Achille and Hynek, 2010). An ocean should be a fertile nursery for Martian life, based on terrestrial evidence (e.g., Walter et al., 1980; Van Kranendonk et al., 2003). Thus, understanding whether the streamlined forms around Chryse Planitia were created in the outflow channels or within the hypothesized ocean contributes to identifying habitable zones on Mars.

Streamlined forms are familiar to us. Airplane wings and ship hulls are streamlined forms engineered to reduce drag in a flowing fluid. Nature also shapes minimum-drag forms, in flowing air as yardangs (Figs. 1A and 1B), or in flowing water as streamlined or teardrop-shaped islands (TSIs) (Figs. 1C and 1D). On Mars, streamlined forms are found in outflow channels ranging from Noachian to Amazonian in age (Burr et al., 2009; Coleman and Baker, 2009; Irwin and Grant, 2009). Scattered teardrop-shaped examples have commonly been interpreted as resulting from bedrock erosion, based on associated scour landforms and on the presence of obstacles at the wider upslope end, taken to imply protection of the lee zone of the obstacle from erosion (Carr, 2006). In contrast, clustered teardrop-shaped forms (Fig. 1D) have been attributed to sediment deposition during floodwater ponding, followed by streamlining of this sediment behind obstacles during ponded water efflux (Burr, 2005). This hypothesis for some clustered Martian streamlined forms—as a combination of sediment deposition followed by sediment erosion except behind flow obstacles—derived support from terrestrial streamlined forms in subaerial outflow channel basins. In this issue of Geology, Moscardelli and Wood (2011, p. 699–702, their figures 1 and 2) suggest a similar deposition+erosion origin for a more scattered population of streamlined forms but invoke another terrestrial analog: deep-marine Erosional Shadow Remnants (ESRs) (Moscardelli et al., 2006).

Streamlined forms are shaped during flow as a tendency to minimize total drag. Total drag is the sum of form (pressure) drag and skin (friction) drag. Form drag around a blunt body arises largely from flow separation in the lee of an obstacle, so that elongation of the form through in-filling of the leeward separation zone reduces the form drag. Conversely, skin drag acting tangentially to the obstacle surface is minimized by reducing the surface area by making the feature geometrically more compact. Consequently, minimization of total drag is accomplished through a combination of elongation and compaction. The result of these countervailing tendencies is the streamlined form (e.g., Komar, 1983).

Both erosional and depositional streamlined forms are observed in terrestrial floodscapes. On Earth, the largest floodscapes date from the Pleistocene glaciations, which provided a mechanism to pond and release great amounts of water (O'Connor and Costa, 2004; Burr, 2010). These terrestrial outflow complexes show that streamlined form type is determined largely by geologic context. In the Altai Mountains of Siberia, for example, floods produced extensive deposits (Baker et al., 1993; Rudoy and Baker, 1993), including streamlined bars, but large erosional streamlined forms are largely absent due to the channelization of floodwaters down previously incised river valleys (Herget, 2005). Conversely, along the paleo-margin of the Laurentide ice sheet in central and eastern North America, the floodwater pathways show largely erosional streamlined bars, produced in unconsolidated glacial drift, with deposition limited to isolated gravel bars and traction carpets (Kehew et al., 2009). Both types of streamlined forms occur in the Channeled Scabland of western North American, created by floods spilling over the Columbia Plateau, where Pleistocene loess was deposited on top of Neogene basalt flows (Baker, 2009). Residual streamlined forms are prevalent here as erosional remnants of this pre-existing loess (Baker, 1978). However, flood sedimentary forms are also common, of which the predominate type is the pendant bar (Baker, 1973), so-called because they “hang” downstream for bedrock projections (Malde, 1968). These bars are formed through net sediment deposition where inflow of gravels into the lee separation zone exceeded outflow.

The Channeled Scabland was a touchstone analog for interpreting early images of outflow channels on Mars (Baker, 2009), in part because of the similarity of streamlined forms. First observed 40 years ago in Mariner 9 images, Martian streamlined forms are commonly teardrop shaped, with the wider end usually anchored by an impact crater and the narrower end pointing down slope. Although some workers have pointed out that other flow processes, such as glaciation (Lucchitta, 2001) and lava flow (Leverington, 2004), create streamlined forms, an outflow origin for the channels continues to be the prevailing view (Carr, 2006).

Although most terrestrial analogs for the Martian outflow channels invoke subaerial flooding, submarine processes and morphology have also been argued. The lower gravity on Mars would produce bottom stresses in outflow channels similar to those produced by submarine flows on Earth, where buoyancy reduces the effective gravity (Komar, 1979). Streamlined forms shaped by submarine debris flows and turbidity currents have been invoked as analogs for the Martian features (Komar, 1979; Nummedal and Prior, 1981). Furthermore, in previous analysis of Martian northern plains landforms, submarine deposition and subsequent modification was suggested from morphological and contextual evidence (e.g., Parker et al., 1993).

Submarine ESRs on Earth provide a new analog for a limited set of Martian streamlined forms. Moscardelli and Wood present ESRs as potentially analogous to some TSIs in the circum-Chryse region, where the northern ocean may have extended far enough southward so that submarine processes may have occurred. However, the ESR analog may strengthen the appeal to submarine-like processes for all of the streamlined forms on Mars. Many streamlined forms exist outside of the circum-Chryse region, where contextually an ocean would not have been possible. If the physical sedimentology in outflow channel on Mars is indeed similar to that in submarine environments on Earth (Komar, 1979), then the submarine analogy revived by Moscardelli and Wood might extend beyond the limited number of circum-Chryse TSIs. In other words, this analogy may argue for the effect of lower gravity producing submarine-style processes in Martian outflow channels generally, regardless of any hypothesized submarine context.

Testing between these two interpretations—that certain streamlined forms on Mars formed in a submarine environment, or that all streamlined forms on Mars formed in an outflow environment in which particle physics mimics that in submarine environments on Earth—will require morphometric data from ESRs for comparison to Martian streamlined forms, as well as examination of the geologic context for the Martian examples. The question extends beyond Mars to other worlds. Titan has even lower gravity than Mars, a 10-times thicker atmosphere than Earth, and also shows streamlined forms (Fig. 1E). Could subaerial processes on Titan mimic outflow processes on Mars and submarine processes on Earth? Pinning down the true cause for the similar appearance between terrestrial ESRs and Martian TSIs would contribute to understanding the effect of reduced effective gravity on sedimentary landforms.