Martian impact craters: Continuing analysis of lobate ejecta sinuosity

The lobate ejecta morphology surrounding most fresh Martian impact craters can be quantitatively analyzed to determine variations in ejecta sinuosity with diameter, latitude, longitude, and terrain. The results of such studies provide another clue to the question of how these morphologies formed: are they the results of vaporization of subsurface volatiles or caused by ejecta entrainment in atmospheric gases. Kargel provided a simple expression to determine the degree of non-circularity of an ejecta blanket. This measure of sinuosity, called 'lobateness', is given by the ratio of the ejecta perimeter to the perimeter of a circle with the same area as that of the ejecta. The Kargel study of 538 rampart craters in selected areas of Mars led to the suggestion that lobateness increased with increasing diameter, decreased at higher latitude, and showed no dependence on elevation or geologic unit. Major problems with the Kargel analysis are the limited size and distribution of the data set and the lack of discrimination among the different types of lobate ejecta morphologies. Bridges and Barlow undertook a new lobateness study of 1582 single lobe (SL) and 251 double lobe (DL) craters. The results are summarized. These results agree with the finding of Kargel that lobateness increases with increasing diameter, but found no indication of a latitude dependence for SL craters. The Bridges and Barlow study has now been extended to multiple lobe (ML) craters. Three hundred and eighty ML craters located across the entire Martian surface were studied. ML craters provide more complications to lobateness studies than do SL and DL craters - in particular, the ejecta lobes surrounding the crater are often incomplete. Since the lobateness formula compares the perimeter of the ejecta lobe to that of a circle, the analysis was restricted only to complete lobes. The lobes are defined sequentially starting with the outermost lobe and moving inward

The cratering record in the inner solar system: Implications for earth

Internal and external processes have reworked the Earth's surface throughout its history. In particular, the effect of meteorite impacts on the early history of the earth is lost due to fluvial, aeolian, volcanic and plate tectonic action. The cratering record on other inner solar system bodies often provides the only clue to the relative cratering rates and intensities that the earth has experienced throughout its history. Of the five major bodies within the inner solar system, Mercury, Mars, and the Moon retain scars of an early episode of high impact rates. The heavily cratered regions on Mercury, Mars, and the Moon show crater size-frequency distribution curves similar in shape and crater density, whereas the lightly cratered plains on the Moon and Mars show distribution curves which, although similar to each other, are statistically different in shape and density from the more heavily cratered units. The similarities among crater size-frequency distribution curves for the Moon, Mercury, and Mars suggest that the entire inner solar system was subjected to the two populations of impacting objects but Earth and Venus have lost their record of heavy bombardment impactors. Thus, based on the cratering record on the Moon, Mercury, and Mars, it can be inferred that the Earth experienced a period of high crater rates and basin formation prior to about 3.8 BY ago. Recent studies have linked mass extinctions to large terrestrial impacts, so life forms were unable to establish themselves until impact rates decreased substantially and terrestrial conditions became more benign. The possible periodicity of mass extinctions has led to the theory of fluctuating impact rates due to comet showers in the post heavy bombardment period. The active erosional environment on the Earth complicates attempts to verify these showers by erasing geological evidence of older impact craters. The estimated size of the impactor purportedly responsible for the Cretaceous-Tertiary mass extinctions is 10 km in diameter. Thus impactors greater than or equal to the size postulated for K-T impactor are rare within the inner solar system since the end of heavy bombardment

Revision of the Martian relative age chronology

This study has provided a more detailed chronology than currently exists in the literature and has created some changes to the currently accepted geological evolutionary sequence of Mars. The period of heavy bombardment, although dominated by impact processes, experienced many forms of volcanic activity and at least one episode of intense fracturing. Most small volcanic constructs and the ridged plains regions are found to date from this early period, contrary to common belief. The fracturing and dissection of the highlands helps to provide further constraints on the timing of events such as the formation of the hemispheric dichotomy and the formation of the Tharsis Bulge. The northern plains are found to consist of a number of differently aged regions. The difference in age between the chaotic terrain and the outflow channels together with differences in the distribution curves among craters of various erosional states found on the channels support the theory of episodic periods of flooding

The ridged plains as a possible landing site for the Mars sample return mission

Differences in the shape and density of crater size-frequency distribution curves have been interpreted as indicators of different impactor populations. Within the inner solar system two production populations are seen. The signature of the first is recorded in the heavily cratered regions of the Moon, Mercury, and Mars and displays a multi-sloped distribution curve which cannot be described by a power law function at all crater diameters. The signature of the second population is seen in the lightly cratered lunar and Martian plains, where the size-frequency distribution curve can be approximated by a power law function of -3 differential slope in the 8- to 70-km diameter range. Based on data obtained from the Apollo lunar samples and crater flux estimates, the first population is believed to have been emplaced during the period of heavy bombardment which, at least on the Moon, ended about 3.8 BY ago. The second population has dominated the cratering record since that time and is commonly assumed to be due to comets and asteroids

Barlow-Hall in vitro Evolution Protocol

The Barlow-Hall method for in vitro evolution is a simple alternative to methods such as DNA shuffling and is particularly applicable to predicting the evolution of target genes in nature. This detailed protocol includes the procedure itself and, as Appendices, describes particularly effective protocols for electroporation and for preparation of electrocompetent cells

Relative chronology of Martian volcanoes

Impact cratering is one of the major geological processes that has affected the Martian surface throughout the planet's history. The frequency of craters within particular size ranges provides information about the formation ages and obliterative episodes of Martian geologic units. The Barlow chronology was extended by measuring small craters on the volcanoes and a number of standard terrain units. Inclusions of smaller craters in units previously analyzed by Barlow allowed for a more direct comparison between the size-frequency distribution data for volcanoes and established chronology. During this study, 11,486 craters were mapped and identified in the 1.5 to 8 km diameter range in selected regions of Mars. The results are summarized in this three page report and give a more precise estimate of the relative chronology of the Martian volcanoes. Also, the results of this study lend further support to the increasing evidence that volcanism has been a dominant geologic force throughout Martian history

Martian crater counts on Elysium Mons

Without returned samples from the Martian surface, relative age chronologies and stratigraphic relationships provide the best information for determining the ages of geomorphic features and surface regions. Crater-size frequency distributions of six recently mapped geological units of Elysium Mons were measured to establish their relative ages. Most of the craters on Elysium Mons and the adjacent plains units are between 500 and 1000 meters in diameter. However, only craters 1 km in diameter or larger were used because of inadequate spatial resolution of some of the Viking images and to reduce probability of counting secondary craters. The six geologic units include all of the Elysium Mons construct and a portion of the plains units west of the volcano. The surface area of the units studied is approximately 128,000 sq km. Four of the geologic units were used to create crater distribution curves. There are no craters larger than 1 km within the Elysium Mons caldera. Craters that lacked raised rims, were irregularly shaped, or were arranged in a linear pattern were assumed to be endogenic in origin and not counted. A crater frequency distribution analysis is presented

TIDY, a complete code for renumbering and editing FORTRAN source programs. User's manual for IBM 360/67

TIDY, a computer code which edits and renumerates FORTRAN decks which have become difficult to read because of many patches and revisions, is described. The old program is reorganized so that statement numbers are added sequentially, and extraneous FORTRAN statements are deleted. General instructions for using TIDY on the IBM 360/67 Tymeshare System, and specific instructions for use on the NASA/AMES IBM 360/67 TSS system are included as well as specific instructions on how to run TIDY in conversational and in batch modes. TIDY may be adopted for use on other computers

Addressing neuroticism in psychological treatment

Neuroticism has long been associated with psychopathology and there is increasing evidence that this trait represents a shared vulnerability responsible for the development and maintenance of a range of common mental disorders. Given that neuroticism may be more malleable than previously thought, targeting this trait in treatment, rather than its specific manifestations (e.g., anxiety, mood, and personality disorders), may represent a more efficient and cost-effective approach to psychological treatment. The goals of the current manuscript are to (a) review the role of neuroticism in the development of common mental disorders, (b) describe the evidence of its malleability, and (c) review interventions that have been explicitly developed to target this trait in treatment. Implications for shifting the focus of psychological treatment to underlying vulnerabilities, such as neuroticism, rather than on the manifest symptoms of mental health conditions, are also discussed.First author draf