THE EXTRAORDINARY MARTIAN CRATER "GREG": A PHOTO TOUR

Figure 1. THEMIS IR 512 pixel/degree mosaic of crater Greg (38.5°S, 113.1°E). Note difference in geologic landforms on north inner wall (rounded hills) and south inner wall (dissected by downhill valleys).

Figure 2. 200 m interval contours from gridded MOLA data over THEMIS IR 512 pixel/degree mosaic. The base of the wall and floor lie some 1900 to 2400 meters below the rim. At a drop of 1900m over 10 km, the mean slope of the inner walls is about 11°.

Figure 3. CTX mosaic of north wall of crater, showing at least 5 well-developed tongues, and additional transitional and final forms as distal arcuate ridges along the foot of the wall. Crater rim is near top, floor is along bottom. (CTX images P04_002676_1413 and P03_002320_1413).

Figure 4. Higher resolution images of two strikingly similar lobate tongues, from the MOC camera onboard Mars Global Surveyor.

Figure 4a (left). Observed lobate glacial tongue recognized on Mars was noted on this image by Berman & Hartmann (ca. 2001), first published by Hartmann et al. 2002 (Lun. Plan. Sci. Conf. XXXIII, abstract 1904). First published image. (MOC M18-00897).

Figure 4b (right). A nearby tongue. (MOC R20-00387).

Figure 5. Views of dissected south wall, with north (crater floor and lower end of wall valleys) at the top.

Figure 5a. This image shows dissection into valleys separated by hills. Box shows location of image 5b. (CTX images B11_013897_1406 and P03_002320_1413).

Figure 5b. Close-up view of two valleys in the south wall, showing chevron-textured fill. North (downhill) at the top; crater floor at the top. Lighting from the left. (CTX image B11_013897_1406).

Figure 5c. Detail of image 5b, showing transition away from chevron pattern as eastern valley (bottom) opens onto crater floor (top). Note 40 meter crater in the chevrons at top center. (HiRISE image PSP_003520_1405).

Figure 5d. Detail of image 5b, showing transition from chevron pattern in western valley, with a featureless zone between the chevron texture and the texture on the crater floor. Our paper concludes that the knobby texture is formed by sublimation of the ice component in a ice/dust mantle, and that when the ice-rich material flows down the valley floor, it distorts into a chevron pattern due to drag at the walls. (HiRISE image PSP_003520_1405).

Figure 6. Example of geologic formations related to the lobate glacial tongues in crater Greg, but in other areas of Mars.

Figure 6a. This 22 km diameter crater at latitude 43.1°S and longitude 161.8°W exemplifies typical erosion off the north (polar facing) wall, as studied by Berman et al.(2009) Icarus, 200, 77-95. At foot of the gullied wall are arcuate ridges resembling those at the distal ends of the lobate tongues. Floor deposits slope downward from this wall, and contain faint arcuate traces of former debris fronts. (CTX image B06_011916_1367).

Figure 6b. Context image of debris apron near Reull Vallis, about 500 km southwest of Greg. Box shows location of image 6c. (HRSC0440_0000_ND31, light from upper left).

Figure 6c. Lobate tongue (center) and isolated arcuate ridges (bottom) on south-facing slope of mountain associated with debris apron. This illustrates the common occurrence of lobate tongues on south-facing slopes in the region of Greg crater. (CTX image B11_014016_1380).

Figure 6d. Lobate tongue (left) and degraded(?) lobate tongue (right) on northern wall of a 50 km diameter crater about 150 km N of Greg. (CTX image P03_002386_1444).

Figure 7.  Examples of surfaces in crater Greg, relating to mantled terrain and the process of ice-rich mantle formation, and subsequent sublimation processes.

Figure 7a. Example of knobby terrain and associated, more heavily cratered hills on south floor of Greg. The hills are smoother textured, but have higher crater density, suggesting they are older surfaces protruding through younger, knobbly, mantled crater-floor fill.

Figure 7b. Enlargement of NE tip of the ridge in figure a, showing abrupt change in texture and a fissure along the margin of the ridge. (HiRISE image PSP_003243_1415).

Figure 7c. Portion of HiRISE image PSP_002320_1450, where crater densities were measured by WKH (as plotted in Fig. 9).

Figure 8a. Calculated history of obliquity, orbital eccentricity, and insolation at north pole, from Laskar et al. (2004) The last very high episodes (>45 degrees) occurred about 5.5, 7, 8, 15, 17, and 19 My ago.

Figure 8b. Map of Mars showing contouring of ice deposition rates during high obliquity. One of the two main regions for such deposition lies east of Hellas and is centered near the crater studied here. Vertical scale at right gives ice deposition rate in units of mm/year during deposition episodes. (Courtesy Francois Forget; cf. Forget et al. 2006).

Figure 9. Isochron diagram, showing crater counts from relatively densely cratered terrain at various locations inside Greg. Upper dark line marks crater saturation. The two short, thick diagonal lines (D 1 to 16 km) are the defining dividers between Amazonian, Hesperian, and Noachian eras. Shorter, thin diagonal lines subdivide the early, middle and late portions of the eras.

Figure 10.  Isochron diagrams showing crater counts from knobby-textured (sometimes called cantaloupe-textured) mantled terrain at various locations inside Greg.

Figure 10a.  Counts of all craters.  Largest craters probably penetrate through mantle and suggest an age of around 1-3 Gy for host crater Greg.  Smaller craters form in the mantle and suggest mantling episodes in the last few My.  (These data are discussed further in our paper.)

Figure 10b. Counts of sharp craters only. Selection of these craters from the total population can be somewhat subjective, but because they appear unmantled and uneroded, they give an estimate of the time since the most recent mantling episode which appears to be no more than a few My.

Figure 11.  Two oblique views of lobate glacial tongues, with no vertical exaggeration.  These give a sense of the slopes involved, and the "view from an airplane window" appearance of the features.  These views were made by co-author Veronique Ansan, by using elevation data from the European Mars Express orbiter to create a "digital elevation model" (or "DEM"), superimposing the photo on the DEM, and then viewing the DEM at a selected oblique angle.

Figure 11a. Lobate tongue designated as W1 in Figure 11c.

Figure 11b. TIn this oblique view, note how the ice-rich glacial material drapes around a hill just left of the main lobate tongue, as a result of downhill flow.   This main tongue is the one designated as V8 in Figure 13.

Figure 11c. Northeast wall of Greg, showing example of contours derived by co-author Veronique Ansan, and "traverses" along the various glacial tongues, where she measured downhill slopes of the glaciers.

Figure 12. Slope measurements by co-author Veronique Ansan on eastern part of the north wall.

Figure 13. Slope measurements by co-author Veronique Ansan on the central (northwest) part of the north wall.

Figure 14. Slope measurements by co-author Veronique Ansan on the western part of the north wall.

Figure 15. Two very high resolution views of the surface of one of the lobate tongue-shaped glaciers, marked "V8" in Figure 13, and also shown in Figure 4B.

Figure 15a. Portion of HiRISE image PSP_002320_1415 showing texture of central part of lobate tongue "V8." The near-absence of any few-meter-scale  impact craters indicates a geologically very young  age surface layers (to a depth of at least few meters).

Figure 15b. Cropped view of 15a, showing polygonal-like fracture and hillock pattern, with two unusual features, a projecting crag and shadow (upper right) and a possible degraded impact crater (lower left).

Figure 15c. Isochron diagram showing crater counts on lobate tongues in and near Greg. At D < 31 m, d < 9 m, counts fit the isochron at model age about 3-9 My (allowing for scatter at D < 8 m), suggesting relatively low disturbance since that time. At D ~ 31 m to 88 m, d ~ 9 m to 25 m, crater retention model ages (model-dependent survival times) are of the order 20-80 My. See text for discussion.

Figure 16. South wall of crater Greg showing "traverses" down the glaciated valleys, selected by co-author Veronique Ansan for measurements of slopes.  The slope measurements are shown below.  Typical slopes are 10-12 degrees.

Figure 17. Crater count isochron diagram by co-author Daniel Berman for chevron-textured material in the valley floors on the south wall. Berman outlined most of the valley fill surfaces in various valleys, counted craters in that total area, and plotted the results. The diagram suggests that the craters have survived for only about 1 My, and the surface layers (to a depth about 10 m) have been stable for no more than that period of time.

Figure. 18. Cross section of elevation, north to south across the center of Greg, showing evidence for dominant mass transport off the north wall, associated with the lobate flows. The slope off the south wall, from the wall base to the central peak, suggests a lesser amount of transport and floor fill from the south. (After Berman et al. 2009, Figure 14).

Figure 19.   Channel-erosion features on the north outer rim of Greg, once again showing chevron textures associated with north-facing slopes  (north is at the top). Light is from left, indicating that the chevron texture stands above surrounding surface.

Figure. 19a. Context view with broken north crater rim at bottom.  Several shallow valleys, ~200m wide, drain down the rim  toward a broad basin (top). Box shows location of image b. (HiRISE image PSP_002676_1415).

Figure. 19b. The formation seems to be a high-standing remnant of a flow structure (with downhill toward the top), with faint chevron-like texture. (HiRISE image PSP_002676_1415).

Figure 19c. Similar formation in another HiRISE frame east of b. Vestigial chevron texture in both b and c, at decameter scale, resembles chevron textures in valley fill of the south wall of Greg, also with equator-facing slopes. (HiRISE image PSP_003243_1415).

Figure. 19d. Close crop of feature shown in b, showing crudely developed chevron texture similar to that of flows on the similarly-oriented, equator-facing inner south wall of crater Greg. (HiRISE image PSP_002676_1415).

Figure. 19e. Close crop of chevron textures on formation seen in Figure 19c. (HiRISE image PSP_003243_1415).

Figure 20. Schematic map of planet Mars showing areas of most intense ice deposition during periods of high obliquity, from global climate model calculations, by co-author Francois Forget.  Color bar at right indicates red is strongest ice deposition rate.  Red patch at longitude 100-120 degrees E, on east rim of giant impact basin Hellas, has highest ice deposition rate.  Our crater Greg is the yellow dot, centered in that area!   The fact that we find an unusually strong concentration of glacial features in Greg supports the validity of the global climate models.

Figure 21.  Plot of derived crater retention ages (crater survival times) versus initial crater depth, on various surfaces within Greg, showing that small craters survive only since the last few high obliquity episodes.  (Crater diameter is shown at top.)   This type of diagram can be used to investigate age of subsurface layers as a function of depth, and thus will become very useful for investigating the stratigraphy of Martian surface features.

Figure 21a. Plot of age vs. crater depth for all observed craters, showing that all craters initially deeper than ~25 m survive from before the last four episodes of obliquity > 45 degrees, but most craters initially shallower than ~5 m survive only since the last few such episodes.

Figure 21b. Plot for the sharpest, bowl-shaped craters shows that most sharp craters shallower than about 5 m survive only since the last few episodes of obliquity > 35 degrees.