Title

An Educator's Guide to Impact Cratering in the Inner and Outer Solar System and Scientific Investigation at the Secondary Level

Publication Date

10-19-2002

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Education, Earth Science

Department

Geosciences

Major Advisor

Walter S. Snyder

Abstract

This thesis is intended for science educators who wish to implement the study of impact craters and the comparative planetary geology (i.e., formation and surface features) of the Moon, Mercury, and Callisto. It also serves as an example of student oriented scientific investigation at the high school level.

Between 1999 and 2001, twenty-two students from Nampa Senior High School participated in the investigation. The principal educational goals of this thesis were the participation of high school students in scientific investigation and, for both teacher and students, a greater understanding of impact cratering in the inner and outer solar system. The scientific goal of this thesis was the search for a hypothetical crater called a cryptopalimpsest, hypothesized by some researchers to occur on the surface of Callisto.

Callisto, the second largest moon of Jupiter, is composed of a mixture of rock and ice. Craters found on the surfaces of icy moons in the outer solar system exhibit characteristics different from those found on the terrestrial planetary bodies of the inner solar system. The inner solar system includes Mercury, Venus, Earth, the Moon, and Mars. One notable difference concerning Callisto is that it exhibits a relative lack of craters between 50-100 kilometers in diameter when compared to the terrestrial bodies such as the Moon and Mercury.

Two hypotheses regarding this relative lack of larger craters exist. Passey and Shoemaker (1982) believe that a process called viscous relaxation is responsible for the lack of craters in the 50-100 kilometer diameter range on Callisto. Viscous relaxation is defined as a materia1's resistance to flow. For example, molasses has a high viscosity or resistance to flow compared to water, which has a lower viscosity and flows very easily in comparison. They hypothesized that viscous relaxation is the cause for the complete topographic erasure of larger craters on Callisto over millions of years. A cryptopalimpsest is the term applied to these hypothetical "ghost" craters. In simpler terms, a cryptopalimspest is a crater that once was visible, and due to viscous relaxation no longer is visible. They further hypothesize that circular regions that exhibit low crater density mark the sites of these relaxed craters.

On the other hand, Woronow, Strom, and Gurnis (1982) suggest that the paucity of larger craters on Callisto was caused by the relative lack of larger impactors (i.e., comets and asteroids) that exist around Jupiter compared to the inner solar system. They exhibits clumping, would reveal the presence of recommend that viscous relaxation is only responsible for the degradation of craters but not crater obliteration. However, they do concede that the presence of a low-density circular region, if located, could be explained by viscous relaxation.

This thesis investigates this debate by utilizing more recent high-resolution images of the surface of Callisto taken by the Galileo spacecraft in 1995 to the present, since the two reports were published. The Galileo images exhibit resolutions at least 17 times better than those analyzed by Passey and Shoemaker (1982) and Woronow, Strom, and Gurnis (1982), which were taken by the Voyager spacecraft (I and II) in the 1970s.

The starting hypothesis of the investigation presented in this thesis states that if cryptopalimpsests exist on Callisto, they may be located by searching for circular regions of low crater density proposed by Passey and Shoemaker (1982). Impact cratering is assumed to be random process (Melosh, 1989). Therefore, unless altered by some process such as a large impact crater that obliterates the pre-existing record, the distribution of craters should exhibit a random pattern. A nonrandom pattern, specifically one that a process affecting the otherwise assumed random distribution. To test this hypothesis, small craters were measured and mapped. The resulting pattern was then examined to see if low-density circular regions were visible. Spatial statistical tests were performed in order to classify the pattern of craters as random or clumped. The criteria for identifying a cryptopalimpsest were threefold. First, a low-density circular region must be identified visually from the pattern of mapped craters. Secondly, the spatial statistical tests must agree that the pattern is significantly different from the assumed random pattern. Third, any circular area with low crater density, if located, must not exhibit any crater topography (i.e. - rims or crater floors) and exclude other surface processes that could produce a similar pattern. For example, debris, or ejecta, thrown from a nearby impact could cover and obliterate preexisting craters. Likewise, infilling of preexisting craters could occur from slope failure (i.e. -landslides).

Using the above criteria, no evidence existed for the presence of possible cryptopalimpsests in the nine images analyzed in this investigation. All circular low-density regions observed were smaller than craters in the 50-100 kilometer diameter range, and were explained either by the presence of visible craters or perhaps patterns inherent to random distributions. Disagreements between two of the spatial tests indicate that further simulations are required in order to better understand how these tests function together in concert.

For the educator, this thesis was written as a guide specifically related to the study of impact cratering on planetary bodies. Utilized as such, a secondary science teacher should be able to reproduce this study on any planetary body in our solar system, all of which reveal, to some extent, impact craters.

The rewards of such a project are limitless. Students who participate in such a study are exposed to the following: working on a team and contributing their own ideas, learning how to measure and classify craters throughout the inner and outer solar systems, and most importantly, learning how the process of scientific inquiry works. The students who participated in this study had, up until their involvement, viewed science as a set of memorized facts. Afterwards, the students realized that science is a complex process whereby one gathers information, develops a hypothesis, tests that hypothesis, and retrospectively examines the original hypothesis based on the outcomes. In this study, both the students and the teacher came closer to understanding the investigative nature of science. It is the author's hope that this thesis will provide science teachers useful sources regarding planetary geology, the process of cratering, and the participation of secondary students in research.

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