Kuhn Experiments in Reduced Gravity

Sediment Settling on Mars
1. Auflage 2014
ISBN: 978-0-12-800462-3
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 50 Seiten

Experiments in Reduced Gravity: Sediment Settling on Mars is the first book to be published that reflects experiments conducted on Martian geomorphology in reduced gravity. This brief yet important book on sediment experiments assesses the theoretical and empirical foundation of the models used to analyze the increasing information we have on the past geography on Mars. The book also evaluates the need to develop new methods for analyzing new information by providing a conceptual outline and a case study on how experiments can be used to test current theoretical considerations. The conceptual approach to identifying the need for and role of experiments will be of interest to planetary scientists and geoscientists not necessarily involved with Mars, but those using experiments in their research who can apply the book's concepts. - Includes figures, diagrams, illustrations, and photographs to vividly explore experiments and outcomes in reduced gravity - Provides an outline of planned experiments and questions related to Martian geomorphology - Features results from the MarsSedEx 1 Experiment in 2012

Nikolaus J. Kuhn, Ph.D., Professor, Physical Geography and Environmental Change, University of Basel. Prof. Kuhn is currently Assistant Editor for Catena, an interdisciplinary journal of soil science, hydrology and geomorphology. As an expert in geographical sciences, Prof. Kuhn has lectured in institutions across the world. In this title he uses his research to focus on surface processes such as soil erosion, geochemical cycles and eco-hydrology. He investigates the wider impact of these processes on the landscape and the planet as a whole, linking Earth Systems Science and Geography.

Autoren/Hrsg.

Kuhn, Nikolaus

Leseproben

Chapter 1

Sediment, Life, and Models on Mars

Abstract

The search for life, or traces thereof, on Mars is one of the main drivers of current space exploration. Linked to this aim is the need for understanding the current and past Martian environment, both to reveal its past and present habitability in general, as well as to identify the sites where the preservation of traces of life is most likely. Sedimentary rocks are the prime targets for the ground exploration because they are proxies for the environmental history, i.e., habitability, of Mars and the most likely strata to preserve traces of life. The exploration of the sedimentary record relies on the ability to model surface processes, such as runoff, erosion, and deposition, on the Martian surface. These models support the selection of landing sites, ground targets, and the interpretation of the analysis conducted by rovers and landers. In this chapter, some initial results of the Mars Science Lab Curiosity’s first year on Mars are summarized and put into the context of the search for life. This is followed by a critical review of models used to describe the relevant surface processes on Earth and their applicability to Mars.

Keywords

curiosity

search for life on Mars

surface processes

models in geomorphology

1.1  SEDIMENTS AND LIFE ON MARS AT GALE CRATER 1

1.2  SEDIMENTATION AND THE TRACES OF LIFE ON MARS 7

1.3  PROCESS DESCRIPTION IN GEOMORPHIC MODELS 9

1.4  SEDIMENT TRANSPORT MODELS ON MARS 12

        BIBLIOGRAPHY 14

        INTERNET RESOURCES 15

1.1. Sediments and life on Mars at Gale crater

The Mars Science Lab Curiosity landed at Gale crater on August 6, 2012. The Gale crater landing site (Figure 1.1) was chosen because it appeared to offer a wide range of past aqueous and thus, potentially habitable environments, indicated by features such as outflow channels, an alluvial fan, as well as sequences of finely bedded deposits exposed in its center containing strata with phyllosilicates and sulfates (Figure 1.2).

Fig. 1.1 Gale crater from orbit with landing ellipse for the Mars Science Lab Curiosity. Credit: NASA/JPL-Caltech/MSSS PIA 14290.

Fig. 1.2 Map of the relevant features for studying habitability in Gale crater. Credit: NASA/JPL-Caltech/MSSS PIA 14305. Sources: Reproduced with permission from Milliken et al. (2010) and Anderson and Bell (2010).

The landing site did not disappoint those who had selected it: more or less right from the point of touchdown, many features associated with deposition by water were encountered and sparked a flurry of data revealing the history and life-bearing potential of the analyzed sites (Figure 1.3). Right near the site where Curiosity touched down, called the Bradbury Landing, layers of sedimentary rocks were discovered on the first Martian day (sol) after landing. In the outcrop named Goulburn, a well-sorted gravel conglomerate was exposed by the blast of the descent engine (Figure 1.4). Conglomerates are rocks consisting of a matrix of sand and embedded well-rounded pebbles. The pebbles at Goulburn were several centimeters in diameter and showed signs of orientation along the longitudinal axis. This type of deposition is indicative of sediment deposition from a flowing stream of water slowing down rapidly, possibly when leaving the confines of a channel. Further, similar outcrops were identified and analyzed by Curiosity during the first few weeks of the mission, which confirmed that the rover had landed on the remnants of an alluvial fan that had formed by erosion from the walls and deposition at their foot slopes in Gale crater. Based on the size of the pebbles embedded in the conglomerates and the slope of the alluvial fan, the flow velocity and depth of runoff from the crater wall could be estimated at 3–90 cm deep and flowing at a velocity of 2–75 cm/s.

Fig. 1.3 Curiosity on Mars, combined from images taken on sol 177 (February 3, 2013) and 270 of the mission. Credit: NASA/JPL-Caltech/MSSS PIA 16937.

Fig. 1.4 Goulburn Scour, a conglomerate consisting of sand and pebbles blasted free by the engines during Curiosity’s descent. The inset is magnified by a factor of two. The image was obtained by the Mast Camera (Mastcam) on August 19, 2012 (=sol 13, i.e., the 13th Martian day since Curiosity’s landing). Credit: NASA/JPL-Caltech/MSSS PIA 16187.

The onward journey of Curiosity led to an area called Yellowknife Bay (Figure 1.5) with further sediments deposited by water (Figure 1.6). Here, three clearly different layers of sedimentary rock, called Sheepbed, Gillespie Lake, and Glenelg, were discovered. All three show clear evidence of being deposited by water, but differ in texture and structure. Sheepbed is a fine-grained mudstone consisting of particles smaller than 62.5 µm while Gillespie Lake and Glenelg are sandstones. Glenelg is also visibly cross-stratified, which is indicative of deposition from shallow-flowing water. The chemical composition of Sheepbed and Gillespie is similar to common Martian upper crust basalts, while Glenelg, on the other hand, is more alkaline. The chemical composition of all sediments indicates limited chemical weathering and thus, only a short exposure to a wet environment.

Fig. 1.5 Route of Curiosity during the first 130 days of surface operations. Credit: NASA/JPL-Caltech/MSSS PIA 16554b.

Fig. 1.6 Shaler outcrop in Yellowknife Bay recorded by the Mastcam on the 120th sol, December 7, 2012 on Earth, after landing Curiosity on Mars. The outcrop’s patterned layers, called crossbedding, illustrate deposition by water. The rocks are part of the Glenelg formation. Credit: NASA/JPL-Caltech/MSSS PIA 16550.

With regards to Curiosity’s mission objectives, the Sheepbed sediments were most exciting because there is a strong evidence that they formed in a lacustrine, i.e., shoreline-type of environment that would offer habitability for microorganisms. This conclusion is based on a number of properties that can be used as proxies for the environmental conditions at the time of their formation. Sediment of a grain size similar to Sheepbed is abundant on Mars and mostly moved by wind. However, some features point toward a wet deposition of Sheepbed. The chemically relatively uniform layer of mudstone has a thickness of approximately 2 m. While the deposition of such a thick layer of dust is theoretically possible, it is unlikely to occur in a landscape that is shaped by fluvial processes at the same time. Dust deposition rates are low on Mars and it would take 10–20 million years to form such a thick layer of dust. Accumulation of a fine lake sediment layer of similar thickness, on the other hand, requires between 100s and 1000s of years. It is also very unlikely that such a thick dust layer has a chemical and mineralogical composition as uniform as the one observed in Sheepbed because its origin would most likely vary with the source, i.e., volcanic eruptions, each with a distinct chemistry. Further analysis of the Sheepbed sediments revealed that the sedimentary environment would have been habitable for chemolithotrophic microorganisms. The most important indicators for such habitability are the presence of hydrogen sulfides, which serve as an energy source and a pH in a near-neutral range. Furthermore, essential elements for life such as carbon, nitrogen, and phosphorus were detected in a form that would enable their use by microorganisms. Overall, the lake in which the Sheepbed sediments formed can therefore be considered as habitable for the type of microorganisms one would expect to have evolved on Mars. However, organic substances, such as methane, which are produced by living organisms, have only been found in traces lower than expected if life had been present. This leaves the Curiosity mission somewhere between a grand success because the potential habitability of Mars has been demonstrated, but the implicit hope of finding traces of life (although this was clearly not the scientific aim of the mission), has been disappointed.

The major lesson learned so far is that Mars is clearly more complex than expected. John P. Grotzinger, the lead scientist for the Curiosity Mission, summarized the findings by writing that “In this manner, the MSL mission has evolved from initially seeking to understand the habitability of ancient Mars to developing predictive models for the taphonomy of Martian organic matter”.1 Learning more about how to find these fossils is one goal the experiments described in this...