Fresh Martian impact craters display a variety of ejecta blanket morphologies. The fluidized appearance of most fresh ejecta types is commonly ascribed to heating and vaporization of subsurface volatiles during crater formation. We have conducted a study of the distribution of the three dominant fluidized ejecta morphologies (single layer ejecta (SLE), double layer ejecta (DLE), and multiple layer ejecta (MLE)) within the ±60° latitude zone on Mars. We have subdivided this region into 5° × 5° latitude‐longitude boxes and have computed the following for each box: (1) percentage of craters showing any ejecta morphology as a function of total number of craters, (2) percentage of SLE craters as a function of craters with an ejecta morphology, (3) percentage of DLE craters as a function of craters with an ejecta morphology, and (4) percentage of MLE craters as a function of craters with an ejecta morphology. We confirm previous reports that the SLE morphology is the most common ejecta type within the study area, constituting >70% of all ejecta morphologies over most of the study area. The DLE and MLE morphologies are much less common, but these morphologies are concentrated in localized regions of the planet. Using these results, we discuss how subsurface volatile reservoirs may be distributed across the planet. The regional variations found in this study generally correlate with the proposed locations of near‐surface H 2 O reservoirs detected by Mars Odyssey.
The Mars Crater Morphology Consortium recommends the use of a standardized nomenclature system when discussing Martian impact crater ejecta morphologies. The system utilizes nongenetic descriptors to identify the various ejecta morphologies seen on Mars. This system is designed to facilitate communication and collaboration between researchers. Crater morphology databases will be archived through the U.S. Geological Survey in Flagstaff, where a comprehensive catalog of Martian crater morphologic information will be maintained.
Abstract Impacts on early Mars can produce H 2 and CH 4 in the thermal plume. In a thick CO 2 atmosphere, collision‐induced absorptions between CO 2 ‐H 2 and CO 2 ‐CH 4 can boost the greenhouse effect. We construct a simple model of the impact history of Mars and show that for a variety of impactor types and CO 2 surface pressures >0.5 bars, postimpact surface temperatures due to H 2 alone can exceed the melting point of water for much longer periods of time than from the dissipation of the heat derived from the impactor's kinetic energy. This longer timescale is set by hydrogen escape rather than radiation to space. Cumulatively, the Noachian surface may have been above the melting point of water for millions of years by this mechanism. These greatly extended postimpact warm environments may have played a larger role in the erosion and mineralogy of the surface than previously thought and may partly explain some of the observed fluvial features.
The ‘Workshop on Surface Ages and Histories: Issues in Planetary Chronology’ brought together researchers from the impact cratering, radiometric analysis, and numerical modeling communities to discuss the current status of chronologic techniques and identify future research directions. Heavily cratered terrains suggest that the inner solar system impact rate was much higher 3.8–4.4 Ga ago (1 Ga = 10 9 years). Impact rates declined after this late heavy bombardment (LHB) period to a level that has been approximately constant for the past 3.8 Ga. A major question is whether the LHB was a gradual decline or a spike near 4.0 Ga. Dynamical modeling of outer planet migrations reveals a mechanism for perturbing material into the inner solar system between 3.8 and 4.0 Ga. Geochronologic evidences from the Moon, Earth, and asteroids also support a cataclysmic LHB.
Photoclinometry is being utilized to quantify the degree of degradation suffered by Martian impact craters (1‐ to 5‐km‐diameter range) in the Arabia and Maja Valles regions. Results indicate that present crater depths can vary substantially from crater depths expected for the original fresh craters at all sizes examined. Localized regions of high, moderate, and low degradation of the order of 10 2 km 2 have been delineated using the percentage change in crater depth/diameter between current and assumed fresh crater morphology. Regions of low degradation occur randomly across these regions and probably represent recently produced impact craters which have undergone little degradation since their formation. Aeolian activity, including current mantling deposits, appears to dominate the processes degrading craters in the Arabia region, with fluvial and impact processes playing a secondary role. In the Maja Valles region, crater degradation is associated primarily with fluvial outflow channel activity, with aeolian and impact processes contributing secondary effects to the destruction of impact craters.
The sinuosities of 2213 Martian rampart ejecta craters are quantified through measurement of the ejecta flow front perimeter and ejecta area. This quantity, called lobateness, was computed for each complete lobe of the 1582 single lobe (SL), 251 double lobe (DL), and 380 multiple lobe (ML) craters included in this study. A lobateness value of 1 indicates a circular ejecta blanket, whereas more sinuous ejecta perimeters have lobateness values >1. Although resolution does have an effect on the absolute values of lobateness, the general relationships between lobateness and morphology exist regardless of resolution. Evaluation of the lobateness values reveals that the outer lobes of DL and ML craters have higher median lobateness values (i.e., are more sinuous) than the inner lobes. The outermost lobe of ML craters displays higher lobateness values than the outer lobe of DL craters or the single lobe of SL craters. Previous reports of lobateness‐diameter, lobateness‐latitude, and lobateness‐terrain relationships for rampart craters are not supported by this study. Many of the differences between the results of this study and the previous lobateness analyses can be attributed to the inclusion of resolution effects and the distinction between different ejecta morphologies in this study. The results of this study taken together with a previous analysis of the distribution and diameter dependence of different ejecta morphologies are most consistent with the theory that Martian lobate ejecta morphologies form from impact into subsurface volatiles.
Philip Stooke is a cartographer at the University of Western Ontario and The International Atlas of Mars Exploration: The First Five Decades utilizes maps to help tell the story of Mars exploration from Wernher von Braun's first detailed plan for Mars exploration in 1953 through the Mars Express mission in 2003. The book includes information not only about the missions that actually flew but also the large number of programs for which planning was conducted but which never progressed beyond the planning stage. As a result, The International Atlas of Mars Exploration is an interesting historical perspective of how the plans for exploring Mars have evolved since the 1950s. Stooke has combed through archives of both published and personal documents to give the reader an insider's view of the planning involved with various programs. For example, the nine-page discussion of the selection and certification of the Viking landing sites makes the reader feel that they are sitting in the room listening to the original debates. The accompanying maps and images clearly show how perspectives of safe landing locations change as more data are acquired. I remember sitting next to Mike Carr (leader of the Viking Orbiter Imaging Team) during one of the early Mars Exploration Rover landing site workshops, where each presentation provided detailed information about the topography, roughness, thermal inertia, composition, rock abundance, geologic setting, and wind predictions of the various landing sites being proposed. Mike commented to me about the difference between the selection of the Viking landing sites and the process today and that, by comparison, the Viking landers succeeded by sheer luck! Stooke's book clearly demonstrates how the multitude of missions to Mars has helped us to refine our understanding of the planet and better understand the conditions of landing sites before arrival. The book is alternately uplifting with regard to the successes and depressing when we see where we could be today if the early plans had not been derailed. Rovers, sample return missions, geophysical stations, and human exploration have been suggested since the early fly-by missions to Mars, with many proposed to have come to fruition well before 2000. Ah, the heady days of the 1960s when we felt there was nothing we could not accomplish with the space program! In hindsight, it is good that we slowed down and became more familiar with the planet before attempting the rover missions because some of the slopes and terrains included in early traverse plans are downright terrifying with knowledge of the actual surface characteristics and our engineering capabilities. Stooke's expertise as a cartographer is highlighted throughout the book with the large number of maps used to tell the story of Mars exploration by the United States, USSR/Russia, Europe, and Japan. Maps are used to show the locations of the early Mariner imagery, proposed landing sites (including landing ellipses) for various missions, locations of surface activities by Viking and Mars Pathfinder, and proposed traverses for rovers, balloons, and airplane missions. Tables provide additional details of proposed landing sites or actual mission milestones. Prior to the arrival of the Mars Reconnaissance Orbiter with its High Resolution Imaging Science Experiment (HiRISE) camera providing resolutions up to 0.3 m, the exact location of landed missions was often uncertain by up to a few kilometers. This was particularly problematic for missions landing in flat terrain with few topographic features that would be visible from orbit, or in trying to identify the locations of missions that did not survive landing. The sections describing the attempts to find these landing sites read like a detective story, probably because Stooke himself was involved in trying to locate the position of the Viking 2 lander. Maps and images from both the surface and orbit are used to pinpoint the landing locations, with final images usually consisting of HiRISE frames showing the actual spacecraft and associated debris (parachutes, heat shields, etc.). In spite of HiRISE's capabilities, the landing sites of a few missions such as Mars Polar Lander and Beagle 2 are still unknown. The Mars-specific missions are described in the first chapter and constitute the majority of the book. The second, much shorter, chapter is dedicated to missions to Phobos and Deimos. Maps are again used to document the evolution of our knowledge about these two bodies and to locate landing sites for the Phobos 2 hopping lander and the proposed Aladdin sample return mission. Fly-by and orbital missions are mentioned in the book, but the focus is definitely on the landed missions—this is demonstrated by the approximately one page of general discussion about the 9-year-long Mars Global Surveyor mission versus the eight pages of detailed description for the 83-sol Mars Pathfinder mission. If you are looking for a thorough description of the scientific results from each mission, you will be sorely disappointed: the book provides considerable detail of the logistics of each mission, but little information about the resulting science. For example, Viking lander and Mars Pathfinder surface operations are described in an almost sol-by-sol level of detail in the text, tables, and pictures, but the scientific results are usually summarized in a few sentences if at all. There is no description of the intriguing results from the Viking biology experiments and how a biologic origin was ruled out by most scientists, nor is there any follow-up discussion of what the degradation of the conical piles of debris deposited by the landers after surface excavations revealed about the rate of wind erosion at the landing sites. The sections describing the planning of missions which never flew typically provide no information about why those programs were terminated—was it due to lack of funding, a change in direction of the country's Space Program, a transition into a different program, or some other factor? The book is well illustrated with 205 figures, but all of the figures are in black and white—some figures, such as the geologic maps, neutron spectrometer water maps, and remnant magnetization maps, would have benefited from color versions. Colorized topography could have replaced some of the shaded relief maps and provided better insights into the regional slopes. In summary, The International Atlas of Mars Exploration: The First Five Decades provides a nice compilation of the program planning and missions related to the exploration of Mars between 1953 and 2003. This book deserves a place on your bookshelf if you are a fan of maps and the history of Mars exploration. However, if you are more interested in the scientific results from the various missions, there are other books that you will find more satisfying.
The amount of obliteration suffered by Martian impact craters is quantified by comparing measurable attributes of the current crater shape to those values expected for a fresh crater of identical size. Crater diameters are measured from profiles obtained using photoclinometry across the structure. The relationship between the diameter of a fresh crater and a crater depth, floor width, rim height, central peak height, etc. was determined by empirical studies performed on fresh Martian impact craters. We utilized the changes in crater depth and rim height to judge the degree of obliteration suffered by Martian impact craters.
