Fresh Shallow Valleys and Exit Breach Mid-Latitude Craters: Formation Below a Thick Ice Cover

Introduction: Fresh Shallow Valleys (FSVs) are relatively undegraded dominantly mid-latitude  valleys and valley networks that are generally shallowly incised (<20 m). into mantle substrate [1].  They are primarily of Hesperian to Amazonian age and are found in the mid-latitudes (30-50° N/S).  The primary interpretation is formation through surface runoff of snowmelt, although a sub-ice hypothesis has been suggested for some crater-interior FSVs [2]. Exit-breach craters (EBCs), or pollywog craters, are relatively fresh craters primarily in the 1-7 km diameter range with no contributing drainage beyond the rim, and one or more exit valleys [1, 3].

Observations: Systematic ArcGIS mapping of FSVs and EBCs was conducted in the southern mid-latitudes using the Murray Lab CTX mosaic at a 1:100,000 scale. Of 1668 craters mapped as possible EBCs, 435 of these were designated as firm detections by having evident rim incision, visibly incised valleys, and no exterior contributing drainage.  FSVs were mapped only exterior to craters, although many interior crater rims are dissected by FSV-like valleys. A total of 7276 FSVs were mapped averaging 7.2 km in length. Fig. 1a shows the latitudinal distribution of FSVs and EBCs and Fig. 1b the size distribution of EBCs. FSVs and EBCs have similar latitudinal distribution. To assess the possible hydrology of FSVs and EBCs, topographic relationships were analyzed using CTX and HRSC DTMs.

 The most deeply incised FSVs occur across divides in settings connecting intercrater basins or connecting intercrater basins to crater interiors (Figs. 2 and 3). Fig. 2 shows FSVs on the northeast exterior rim of a 46 km diameter crater at 44.3°S, 165.2°W.  The interior of the crater is indicated by “#”.  A ~1 km wide valley leads from the exterior crater rim northward (white arrow).  Three ~150m wide FSVs (“a”, “b” & “c”) extend from the valley axis east and northeast with gradients of about 0.04. Each of these FSVs breach the eastern rim of the north-trending valley. The depression on the exterior crater rim at “x” is drained by at least five east-trending FSVs that incise a low ejecta ridge. 

Fig. 3 is centered on the southeastern exterior rim of the same crater. Part of the crater rim is at “#”. The exterior rim depression “d” was drained by two FSVs, one of which extends to the rim depression at “a”.  This basin in turn was drained by the FSV shown at the arrow, which incises through an exterior rim ridge at “b”.  The lowpoint of the southern rim of basin “a” is at “c”, which is about 30 m lower that the elevation of the FSV rim breach at “b”.

Most EBCs have a single exit breach, but some have multiple breaches. Fig. 4 shows an exit breach crater at 120.56°W, 43.88°S with a minimum of three exit breaches and possibly as many as seven. 

Analysis and Conclusions:

FSV morphology is distinct from typical terrestrial and the older martian valley networks: 1) they are mostly shallowly incised; 2) they are often isolated features; 3) depth of dissection is <20 m unless they cross divides; 4) many display sinuous planforms; 5) FSVs commonly connect basins across topographic divides, often through multiple breaches and extending to nearly the basin floor in both basins; 6) FSV divide breaches often do not occur at the topographic lowpoints between basins. The exit channels of EBCs are morphologically like FSVs and often accordantly drain into FSVs.  As such, FSVs and EBCs probably share a common hydrological environment. Because of their relative youthfulness and mid-latitude environment, FSVs have been suggested to be fed from snowmelt, indicating a post-Noachian environment that could support occasional surface temperatures exceeding freezing [1, 4]. However, melting of seasonal or meter-scale snow cover does not readily account for FSVs breaching divides unless one of the basins overflowed.

EBCs are likewise enigmatic.  Filling of craters by groundwater influx to overflow conditions has been suggested [1,3], but many EBCs occur on divides, as in Fig. 4.  Filling of craters to overflow from precipitation (presumably melting of snow and ice) could lead to catastrophic breach incision [3, 5].  We suggest a common hydrological scenario for both FSVs and EBCs as resulting from episodic melting of a >100 m deep snow/ice cover accompanied by flow through basal ice tunnels (R-channels) [6].  Estimation of FSV discharge from gradient and meander wavelength using the approach of [7] suggests values <100 cumecs. In terrestrial glacial settings, divide-crossing subglacial channels generally create eskers due to bedload aggradation.  On Mars, FSVs may be largely incised into fine-grained mid-latitude mantle deposits, so little coarse bedload would be entrained and channel beds might erode through scour or melting rather than aggrading. Divides breached by FSVs seldom exceed 50 m of relief relative to adjoining basins, suggesting 150-250 m of ice cover might be sufficient to support R-channel flows. Whether wider braided/anastomosing FSVs [1] can form beneath ice is uncertain. Perennial water might exist below the ice cover in the relatively undegraded EBCs, causing overflow either from surface ice accumulation or melt percolating through the ice cap.  If flow rates overspilling EBCs are modest, rim incision may be restricted to removing the fine-grained mid-latitude mantle materials. The lack of large EBCs probably reflects insufficient ice cover to create overflow.

 

References: [1] Wilson, S. A. et al., (2016), JGR Planets 121, 1667-94, [2]       Hobley, D. E. J. et al., (2014), JGR Planets 119, 128-53. [3] Warren, A. O. et al., (2021), EPSL 554, doi:/10.1016/j.epsl.2020.116671. [4] Howard, A. D., Moore, J. M., (2011), JGR 116, E05003. [5] Goudge, T. A. et al., (2021), Nature 597, 645-49. [6] Walder, J. S., (2010), J. Glaciol. 56, 1079-85. [7] Howard, A. D. et al., (2008), LPSC 39, Abs. 1629.