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Glaciation in Mid-Latitude Terra Sirenum Craters of Mars

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Glaciation in Mid-Latitude Terra Sirenum Craters of Mars GLACIATION IN MID-LATITUDE TERRA SIRENUM CRATERS OF MARS John G. Apostolou May 06, 2020 I. INTRODUCTION II. BACKGROUND The evolution of the Martian surface, According to global circulation models, especially as pertaining to the presence of large scale transportation of glacial ice has water, is a fundamental consideration for occurred from polar regions equatorially understanding the history and current as driven by oscillatory behavior in the state of the red planet. Modern efforts obliquity of Mars (Mischna et al., 2003). have helped to develop a tentative past of This relationship is exists such that periods hydrologic activity and possibly even hab- of high obliquity correspond to extensions itability on Mars. In the present era of in- of ice-rich mantle from the poles, repre- creasing interest in the deeper exploration senting a Martian ice age, whereas peri- and potential colonization of the planet, ods of low obliquity align with a recession knowledge of the varied surface proves vi- of this ice to the poles (Head et al., 2003). tal. Based on existing observations of The orbital trends of Mars beyond ~20 Myr Mars, it is known that the presence of wa- into the past are relatively uncertain. How- ter on the surface has been influential in ever, recent estimates place the present the past as both liquid and ice through a obliquity of Mars at ~25.2°, which is con- variety of geologic processes. This has sidered low in relation to the trends of the yielded regions of considerable complexity last 20 Myr (Laskar et al., 2004). It is then displaying diverse features such as chan- reasonable to assume that the current neling and carving intermingled with ejecta state of the Martian surface is interglacial and impact cratering, such as within the with smaller scale flow features and rem- southern mid-latitudes. In deeper context, nants. evidence of glacial activity has been rec- Of these features, a considerable vari- ognized as especially prominent in the ety have been observed at the mid-latitude mid-latitude bands where it may be recog- bands as features of the craters present nized as a significant contributor to crater there. Lobate debris aprons (LDA) are de- degradation (Berman et al., 2008). In gen- scribed as protruding flows displaying pat- eral, a survey of craters within a given lati- terns of down-gradient flow, bending tude band, featuring examination of lobate around topographic obstacles, and broad- flow fronts, gullies, and carving features, ening when unrestricted by the present to- may provide insight into the latitudinal and pography (Baker et al., 2009). The for- regional distribution of particular glacial mation of these flows is not certain, lead- features as well as their extent. ing to an array of proposals regarding the methods of development, such as frost stemming from an alcoved head and end- creep, rock glacier flow, and debris-cov- ing with a fanned apron at the base of the ered glacier flow. While many of these fea- slope. A related classification of flow fea- tures are analogous to terrestrial glacial ture is the moraine-like ridge (MLR), which features, there is disagreement over clas- are accumulations of glacial debris of up to sifications for such terrestrial counterparts, 100 m in height. Systems of these ridges providing a further obstacle to identifying are sometimes seen near the fan-like out- Martian flows (Baker et al., 2009). flow of the gulley bottoms. The majority of Often associated with lobate flows is these ridges are found within craters of the evidence of sublimation, primarily textur- southern mid-latitudes. Beyond this, the ing. Sublimation, in general, is the phase most prominent, developed of these MLR transformation from solid directly to vapor features are considered to exist at the without transitioning to the liquid state. Es- base of pole-facing crater walls (Arfstrom pecially in regard to glacial ice, the subli- and Hartmann, 2004). mation is generally rate-controlled by va- There exist a variety of other features por transport from the outer surface of interest when examining Martian cra- (Douglas and Mellon, 2019). This process ters, although not necessarily tied to gla- can result in heavily textured glacial sur- cial activity. Channeling can be found in faces with patterns of varying complexity. many regions of the Martian surface but Larger scale trends in the pattern of subli- likewise can have a presence within a mation texturing may also provide insight crater or on its rim, generally indicating a into the direction of an associated lobate previous or current fluid flow. Similarly, flow. Alternatively, dissected mantle ter- craters may contain evidence of alluvial rains may be the result of subsurface sub- fanning as a result of the presence of such limation. Regardless, the combination of fluid, namely liquid water. Beyond hydro- these processes, ice flow, and differential logical and glacial activity remains the pos- sublimation can be responsible for more sibility of dunes and dune-like features complex and potentially curvilinear valley along a crater floor. It is suspected that terrains (Douglas and Mellon, 2019). It is such dunes would be more prevalent in therefore reasonable that such texturing larger craters where wind flow may be less can serve as evidence supporting a pres- obstructed. ence or history of glacial ice on the Martian surface. III. METHODOLOGY Gullies are features that tend to be common in the mid and upper latitudes of In attempting to examine and poten- Mars characterized as small, narrow inci- tially classify craters exhibiting glacial ac- sions resembling terrestrial features of the tivity or history, a region of study was first same name. These Martian features are selected for use with ArcGIS, ArcMap. typically found on steeper slopes, such as Given the significance of preserved glacial the inner gradients of sharp crater rims, evidence at the Martian mid-latitudes, the band of interest was chosen to be the by the northern -40.000° and southern range from -40.000° to -50.000°, corre- -50.000° boundaries. sponding to a portion of the southern mid- Beyond identifying information, the latitudes. Starting from the -180.000° lon- characteristics observed in the craters gitudinal line, the intent was to examine were broken into two additional primary the selected latitude band from the starting categories, titled ‘floor’ and ‘rim’ in refer- point to as far east as possible. Given cir- ence to the corresponding component of cumstantial constraints, the range of ex- each crater. The first recorded character- amination extended only to approximately istic of the ‘floor’ data was the type, classi- -126.700° longitude. This corresponds to fied as either flat or complex. Although ra- the Terra Sirenum region of Mars. ther intuitive, a flat rating corresponded to The data taken was organized into a relatively smooth inner surface of the three primary categories and further bro- crater, disregarding the presence of a cen- ken into several subcategories. Regarding tral peak. In contrast, the complex classifi- the selection of craters for examination, it cation was used to indicate significant was determined to consider craters with a complexity, such as a concentric crater or minimum diameter of 10.00 km, thus incor- substantial ridging and peak remnants. porating the larger, more prominent cra- Following this, the presence of a defined ters of the region. Under this criterion, for central peak was recorded along with an every crater examined, identifying infor- alternative distinction to indicate whether mation was first recorded. This consisted only remnants were visible. of the name, corresponding geographic The next floor subcategory, titled ‘Gla- coordinates, and diameter as provided by cial Carving / Flow Front,’ was the first di- the Robbins Crater Database, making up rectly related to glacial flow. The data un- the ‘identification’ primary category of the der this column was meant to indicate the project data. presence and relative location of a lobate The actual structural and characteristic flow (LDA), linear flow front, significant gla- examination of the craters was conducted cial carving, or a related feature. Figure 1 by first identifying a small group of craters provides examples of each of these fea- at the 1:1,250,000 scale and examining ture classifications as observed in the re- each crater within the group through pri- spective craters. Each entry provided a di- marily Context (CTX) imaging. Once all rection with respect to the crater center of craters in a group had been examined, fo- where such a feature was found, if appli- cus was shifted either north or south until cable. The following subcategory was then a limit of the latitude band was reached. used similarly to recognize the presence Once this occurred, the process was re- and locations of apparent sublimation tex- peated for the Martian surface to the east. turing. Although this often corresponded to Essentially, the immediate area of exami- the data entries for glacial carving and flow nation passed eastward using something fronts, there were many cases where tex- of a boxed serpentine course, constrained turing was visible without a defined flow a b c d Figure 1: (a) Annotated map displaying converging lobate flows (LDA) within crater 24-001016. (b) An- notated map displaying a more linear flow front to the north and northwest of the central peak in crater 24-000159. (c) and (d) Annotated maps exhibiting areas of glacial carving at different points (southwest and northeast, respectively) on the rim of crater 24-000413. front or carving was apparent without ac- subcategories regarding glacial flow and companying sublimation. Figure 2 pro- sublimation texturing, in cases where the vides a reasonable example of the former. feature was present over nearly all of the Furthermore, attempts were made to dis- crater floor, the term ‘areal’ was used.
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