Post contributed by Jennifer Scully, Dept. of Earth, Planetary & Space Sciences, University of California Los Angeles
Vesta is the second most massive asteroid in the asteroid belt, with a mean diameter of 526 km (e.g. Russell et al., 2012). High resolution images from the Dawn Mission have detected curvilinear and linera gully forms and lobate deposits in craters and on steep slopes on its surface (Scully et al., 2015).
Linear and curvilinear gullies are geomorphologically dissimilar. The linear gullies (Image 1) are straight, non-intersecting, parallel and do not incise the substrate. They originate at spurs of more competent material. Linear gullies form as the intersection between lobes of homogeneous talus deposits, which are interpreted to form by dry flow of talus material.
In contrast, the curvilinear gullies (Image 2) are interconnected, non-linear, sub-dendritic and sub-parallel networks that do incise the substrate. They originate in crater walls and end in lobate deposits near the crater floors. The lobate deposits are incised by networks of curvilinear gullies and cross-cut one another to form bajada-like deposits. The lobate deposits are also commonly covered in pitted terrain, assemblages of coalescing pits interpreted to form via impact-induced degassing of volatiles (Denevi et al., 2012).
The curvilinear gullies and lobate deposits on Vesta are morphologically analogous to gullies and lobate deposits formed by debris flows on Earth (e.g. Kumar et al., 2010; Blair, 1999) and Mars (e.g. Johnsson et al., 2014; Schon & Head, 2012; Malin & Edgett, 2000): all share interconnected networks of gullies, which frequently end in cross-cutting lobate deposits that are incised by the gullies (Figure 2). As a result, the curvilinear gullies and lobate deposits are interpreted to form via a debris-flow-like process. Flow of impact melt and dry flow of material were considered as alternate formation mechanisms, but were ruled out on account of morphology, composition, temperature properties, surface slopes, and grain sizes (discussed further in Scully et al., 2015).
Curvilinear gullies and lobate deposits are proposed to form when impact-induced meltwater is released onto crater walls, and mobilizes the impact-formed debris into a debris flow, which carves the curvilinear gullies until they end in the deposition of the lobate deposits. The meltwater is proposed to be sourced in sub-surface ice-bearing deposits. Water is unstable on the surface of Vesta because of the very low surface temperatures and pressures. Thus, the water will evaporate during flow, and any water remaining in the lobate deposits will evaporate to form pitted terrain. Laboratory experiments and quantitative modeling support this formation mechanism (discussed further in Scully et al., 2015).
Curvilinear gullies and lobate deposits are only observed in relatively young (< 100s Ma old) craters, but may have formed throughout Vesta’s history because they are shallow landforms that would easily be erased by depositional processes. This study, in addition to remote sensing (De Sanctis et al., 2012; McCord et al., 2012; Prettyman et al., 2012; Reddy et al. 2012) and meteorite observations (Sarafian et al., 2013; Warren et al., 2013; Treiman et al., 2004), suggest that Vesta may not be an entirely dry body as was previously assumed. Thus, Vesta, along with other asteroids and main belt comets (e.g. Küppers et al., 2014; Hsieh and Jewitt, 2006), suggest that the solar system may not consist of entirely dry or entirely wet bodies, but may be more accurately characterized by a continuum of bodies with many intermediate states of hydration.
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