From: Mars Society
Posted: Monday, July 23, 2001
Today, July 23, upon recommendation from Pascal Lee and Charles Cockell at HMP base camp, we performed an EVA to survey a large erratic block located on the western margin of the crater (which turned out to be at (4)18.92 km east ; (83)68.84 km north coordinates in the UTM reference map, i.e. 4.9 km south of the habitat). We nicknamed this boulder "Cabin Rock". Lee and Cockell supplied us with a proposed itinerary which took us downslope of the crater rim and east from the habitat across patterned ground, then southwest along a river and gravel bar to Trinity Lake. South of the lake, coming out of a narrow pass, we encountered a large block, about 7 meters tall and 4 meters across (this is in hindsight, we did not measure it), sticking out of the slope like the monolith in 2001 A Space Odyssey. We heard no hum when the sun lined up with it, however. But there were definite signs of superior intelligence grafted all over it : someone was there before us ! The "Monolith" appeared to be shattered limestone or dolomite. From this vantage point we could see our objective in the distance, the larger erratic boulder to the south.
Description and origin of "Cabin Rock"
Upon reaching this "Cabin Rock" (see photo below), the crew figured out its dimensions by measuring base lines and similar triangles (thanks, Euclid !). See photo # 256. The estimate was : 25 feet in length, 14 feet in width, and 20 feet in height, which to correspond to 7.5 meters in length, 4.2 meters in width, and 6.0 meters in height in the metric system. Further calculations will be in metric, as advised by the Climate Observer Failure Report Investigatory Panel. The block appeared to be impact breccia. On the downhill side, it showed fragments of limestone or dolomite, caught up in a light gray, frothy matrix that we suspect was molten at the time of impact and fallback (see photo # 263). Because there appeared to be no basement, granitic/gneissic clasts in the breccia, its source was probably less than a mile deep (the depth to bedrock in the crater region, at present). Cabin Rock did not show pervasive fracturing on a large scale, unlike the Monolith. It had very few fractures on a metric scale but showed fine fracturing in many places on a centimeter scale (one to two centimeters apart), along subhorizontal bedding planes that were still visible in some places (perhaps in very large clasts of dolomite ?), crisscrossed by subvertical "hair fractures" as well. We were puzzled that the apparent bedding planes were subhorizontal, rather than in a random direction in this ejecta block, and attributed this to chance or to the fact that the block was broken off by the impact along a bedding plane which defined its long dimension and thus its equilibrium resting position. We observed a centimeter-thick dykelet running subvertically over a foot or so, jutting out in relief over a few millimeters. The dykelet was itself sliced by horizontal fractures, offsetting segments by a centimeter or so. We suspect this to be an injection of carbonate melt into a fracture at the time of impact. With respect to the mode of emplacement of Cabin Rock, we observed it to be resting on moraine-looking gravel. Although the boulder was created and initially transported by impact processes, it thus reached its final position either by a glacier, or by rolling down the hill (perhaps from higher relief that has now been eroded back).
Rock alteration and biomass
Where it was not altered (where the breccia texture was apparent), the rock was extremely hard and virtually impossible to chip off with a hammer. In other places, such as the southern and western sides, the rock was attacked by lichen, including a vermilion-red species and a black one (see photo # 262), and probably microbial lifeforms. On the under belly (at eye level), upslope side of the boulder, we observed circular, centimeter-scale whitish spots that looked biologic but had a surprising hardness. They were superficial and flaked off when hammered. In its upper reaches, the surface of the rock was flaking off in onion peel fashion, over a depth of approximately .3 meter. If we assume, for matter of simplification, that this biomass affects the entire surface of the rock (approximately 200 square meters), the altered volume is approximately 60 cubic meters. We do not include the effects of fracturing in the penetration of biomass in this volume, and assume it to be uniformly penetrated. We defer to further investigation, when properly funded, the task of guessing the ratio of biomass to altered rock.
Trinity Lake breccia
During our return to the Hab, we sampled water and took a foot-long soil core at Trinity Lake. The soil is a dark brown, very fined-grained mud,. with few sand-sized particles. During the few minutes on this stop, we sampled small pieces of beautiful impact breccia, with millimeter-scale clasts in a gray matrix, with the appearance of chocolate-chip gray vanilla ice cream. There were also fragments of salt-and-pepper-colored shattered basement rock, with preserved gneissic black and white banding. These fragments were of an unusually low density: We interpret this to mean that the basement rock was partially molten and that some of its minerals were vaporized at the time of impact. Cathrine and Charles are eager to return to this location to take more samples.
We are intrigued about the distance such a large ejecta block, which as an estimated mass of 473 metric tons, (if we assume its density to be about 2.5 g/cm3), could have flown out of the transient crater at the moment of impact. Perhaps mission control could dig up in Jay Melosh's Impact Cratering book an estimate of the maximal travel distance of a block of such a size, for a 23 km final-diameter crater.
If you need further photos of different aspects of our survey, or further clarifications, please advise us. We have photos of the boulder from all angles. We have sampled the bio-alteration of the rock at three different locations, and also have three small samples of breccia from Trinity Lake. Lab photos of the sample will be available later today.
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