ALBERT

Description: Caldera-topped volcanoes are the largest volcanic edifices on a planet, and represent relatively long term development of near surface magma chambers. The types, geometries, numbers, and distributions of calderas on the Earth, Mars, Venus, and Io are compared.

Description: Discovery of geyser-like plumes on the surface of Triton was a highlight of Voyager 2's passage through the Neptune planetary system. Remarkable as these observations were, they were not entirely without precedent. Considering the confirmed predictions for the 1979 Voyager Jovian passage, it was logical to consider other solar system bodies beside Io where tidal effects could be a significant factor in surface processes. It was our intuition that the Neptune-Triton gravitational bond acting at high inclination to the Neptune equator and the fact that Neptune was a fluid body was significant oblateness would produce tidal and mechanical forces that could be transformed into thermal energy vented on Triton's surface. Prior to the Voyager flyby, others have noted that capture and evolution of Triton's orbit from extreme eccentricity to near circular state today would have resulted in significant tidal heating, but these analysts disregard current day forces. Our calculations indicate that the time varying forces between Neptune-Triton fall midway between those exerted in the Earth-Moon and Jupiter-Io systems, and considering the low level of other energy inputs, this source of internal energy should not be ignored when seeking an explanation for surface activity. In each planet-satellite case, residual or steady-state eccentricity causes time-varying stresses on internal satellite strata. In the case of Jupiter the residual eccentricity is due largely to Galilean satellite interactions, particularly Io-Europa, but in the case of Neptune-Triton, it is the effect of Triton's inclined orbit about an oblate primary.

Description: It is pointed out that volcanoes are morphological expressions of magma composition, eruption processes, and regional tectonics. Thus, the lack of unambiguous silicic volcanoes and pyroclastic flows on Mars implies that compared to earth, its crust is deficient in volatiles and/or the planet lacks a thick granitic crust. Such deductions are examples of comparative planetology, in which knowledge of landforms and processes on one planet are utilized to interpret features and histories on others. In the present study calderas are considered in a comparative planetology context to provide perspective for students who investigate only terrestrial calderas and to search for broadscale relationships between calderas and planetary characteristics. Attention is given to terrestrial calderas, lunar calderas, Martian calderas, calderas on Jupiter's satellite Io, and calderas on Venus. A planetary definition of calderas is proposed.

Description: A description is presented of 18 Martian basins which were newly discovered on Viking photographs. A study of the basins reveals that diameter dependent differences in the morphology of Martian basins are more complex than previously realized. Basins have been classified according to ring morphology, and a diameter dependent sequence apparently exists. The obtained morphology sequence does not conform to the progression observed on the moon, Mercury, and earth. The small Martian basins are 50 to 100 km smaller than any basin on Mercury and the moon, supporting the view that they are unique to Mars. On earth there are significant terrain influences on crater and basin morphology that encourage the speculation that localized unique characteristics of the Martian crust led to basin formation at diameters where craters would normally be formed.

Description: Monogenetic volcanic activity has produced cinder cones and small shield volcanoes on the earth, moon, and Mars. Extraterrestrial cinder cones have median volumes only 25% as large as average terrestrial cinder cones, implying that their magma chambers are smaller and shallower (1 km depth vs 3 km). Ejection velocities for lunar and Martian cinder cones range from 20 to 70 m/sec, only 1/3 to 1/10 as high as for equal volume terrestrial eruptions. These low velocities imply low volatile contents for both Martian and lunar magmas.

Description: A preliminary investigation was conducted regarding the variations in the morphological characteristics of Mercurian craters that appear to be associated with a degradation of fresh craters. Craters are classified according to relative states of degradation in order to provide evidence on degradation styles and rates on Mercury, and to correlate degradation with major geologic events in the history of the planet. Processes and relative rates of degradation on Mercury and the moon are also compared. Degradation trends of crater morphology are found to be parallel on Mercury and the moon. The severe destruction of interior structures in lunar craters may have resulted from more widespread distribution of ejecta due to the moon's lower gravity. The possibility is considered that the smooth plains on the surface of Mercury were formed early in the history of that planet.

Description: The results are reported of an analysis of the characteristics of fresh crater samples occurring on the two major geologic units on the moon (maria and highlands) and on Mercury (smooth plains and cratered terrain). In particular, the onset diameters and abundances of central peaks and terraces are examined and compared for both geologic units on each planet in order to detect any variations that might be due to geologic unit characteristics. The analysis of lunar crater characteristics is based on information provided in the LPL Catalog of Lunar Craters of Wood and Andersson (1977). The Mercurian data set utilized is related to a program involving the cataloguing of Mercurian craters visible in Mariner 10 photography. It is concluded that the characteristics of the substrate have exerted a measurable influence on the occurrence of central peaks, terraces, and scallops in flash crater samples. Therefore, in order to compare the morphologic characteristics of fresh crater populations between planets, an analysis of possible substrate-related differences must first be undertaken for each planet under consideration. It is suggested that large variations in gravity do not produce major variations in crater wall failure.

Description: About 1.3 billion years ago, on one or more distant planetary bodies, silicate melts formed and produced cumulate rocks which eventually made their way to earth. Nine of these rocks have been recovered. Three distinct groups are involved, including shergottites, nakhlites, and chassignites (abbreviated as SNC). The young crystallization ages and other chemical features of SNC meteorites have prompted several workers to suggest that the specimens may be samples of igneous rock, ejected from the surface of Mars during an impact event. Others have rejected the Martian origin of SNC meteorites in favor of a more traditional asteroidal parent body. The present investigation shows that the petrologic, geochemical, and isotopic evidence is inconsistent with an asteroidal origin for SNC meteorites. It is found that the characteristics of SNC meteorites argue convincingly against their origin in a planetary object as small as the largest asteroid. That these meteorites may be fragments of the Martian surface still remains the most likely possibility.

Description: The statistics of basins on the moon, Mercury, and Mars are compared. The basic similarities of basins on Mars, Mercury, and the moon argue convincingly that they result from the same processes. Variations in onset diameters and relative abundances, however, suggest that these basic processes are modulated by second-order factors.

Description: It is believed that explosive basic to ultrabasic eruptions may be responsible for small cinder conelike features on Mars and perhaps for the ancient flank-scoured paterae. Eruptions of this type may have been driven by near-surface water/ice-magma interactions or volatile rich magmas. Noting that the paterae represent a unique style of volcanism that stopped approximately 2 b.y. ago, it is suggested that the volatiles associated with these features were derived from mantle sources and that the cessation of patera formation may coincide with the termination of the period of maximum planetary degassing. Since approximately 2 b.y. ago volcanism has been dominantly effusive, except for such minor explosive ash events as those that blanketed part of the summit of Hecates Tholus. It is proposed that such eruptions were driven by volatiles generated by differentiation of a magma chamber, similar to Icelandic explosive eruptions.