Although the origin and composition of many units are obscure or controversial, their interpretations are based on objective descriptions of morphologic characteristics visible on Viking photomosaics and images. LOWLAND TERRAIN MATERIALS Consist of all plains-forming units between the highland-lowland boundary scarp and the north edge of the map area, exclusive of materials of the western volcanic assemblage on the Tharsis swell. Northern plains assemblage Materials deposited in widespread sheets on northern plains. Within each formation, members mapped at places on basis of crater density; these contacts are approximately located. Assemblage postdates highland-lowland boundary scarp (Scott, 1979). ARCADIA FORMATION—Forms low-lying plains in Arcadia, Amazonis, and Acidalia Planitiae. Embays highland margins and partly buries outflow channels of Kasei, Shalbatana, Simud, Tiu, and Ares Valles. Members distinguished on basis of morphology, albedo, and crater density; common boundaries of older members mapped arbitrarily at places. Flows with lobate margins and small hills with summit craters visible in many places. High-resolution pictures show that sources of some flows are small cratered cones. Interpretation: Mostly lava flows and small volcanoes Aa5 Member 5—Relatively small areal extent. Dark, fresh-appearing flows; few superposed impact craters. Type area: lat 47° N., long 30° Aa4 Member 4—In Arcadia Planitia underlies member 5 and has similar appearance; one other occurrence in channel system of Chryse Planitia. Type area: lat 45° N., long 175° Aa3 Member 3—Forms smooth plains west of Olympus Mons aureoles; embays both the aureoles and fractured terra of Acheron Fossae. Flow fronts visible in places. Type area: lat 15° N., long 155° Aa2 Member 2—Underlies members 3, 4, and 5 in Arcadia Planitia. Includes many small (<10-km-diameter) structures resembling volcanoes and cinder cones. Curved concentric ridges visible on surfaces of flows. Type area: lat 45° N., long 155° Aa1 Member l—Widespread in Chryse and Amazonis Planitiae. Mare-type (wrinkle) ridges common. Type area: lat 30° N., long 40° MEDUSAE FOSSAE FORMATION—Consists of extensive, relatively flat sheets, generally smooth to grooved and gently undulating; deposits appear to vary from soft to indurated; albedo moderate. Occurs near equator in western part of map area. Total thickness may exceed 3 km Amu Upper member—Discontinuous but widespread deposits extend from south of Olympus Mons westward across Amazonis Sulci to map boundary. Smooth, flat to rolling, light-colored surfaces; sculptured into ridges and grooves in places (as in Medusae Fossae); broadly curved margins, locally serrated. Type area: lat 0° N., long 160°. Interpretation: Nonwelded ash-flow …

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Geologic map of the western equatorial region of mars

A global stratigraphy of Mars was developed from a global geologic map series derived from Viking images; the stratigraphy is composed of three maps. A new chronostratigraphic classification system which consists of lower, middle, and upper Noachian, Hesperian, and Amazonian systems is described. The crater-density boundaries of the chronostratigraphic units and the absolute ages of the Martian epochs aer estimated. The relative ages of major geologic units and featues are calculated and analyzed. The geologic history of Mars is summarized on the maps in terms of epochs.

The stratigraphy of Mars

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Geologic map of the eastern equatorial region of Mars

This work reviews factors which are important for the evolution of habitable Earth-like planets such as the effects of the host star dependent radiation and particle fluxes on the evolution of atmospheres and initial water inventories. We discuss the geodynamical and geophysical environments which are necessary for planets where plate tectonics remain active over geological time scales and for planets which evolve to one-plate planets. The discoveries of methane–ethane surface lakes on Saturn’s large moon Titan, subsurface water oceans or reservoirs inside the moons of Solar System gas giants such as Europa, Ganymede, Titan and Enceladus and more than 335 exoplanets, indicate that the classical definition of the habitable zone concept neglects more exotic habitats and may fail to be adequate for stars which are different from our Sun. A classification of four habitat types is proposed. Class I habitats represent bodies on which stellar and geophysical conditions allow Earth-analog planets to evolve so that complex multi-cellular life forms may originate. Class II habitats includes bodies on which life may evolve but due to stellar and geophysical conditions that are different from the class I habitats, the planets rather evolve toward Venus- or Mars-type worlds where complex life-forms may not develop. Class III habitats are planetary bodies where subsurface water oceans exist which interact directly with a silicate-rich core, while class IV habitats have liquid water layers between two ice layers, or liquids above ice. Furthermore, we discuss from the present viewpoint how life may have originated on early Earth, the possibilities that life may evolve on such Earth-like bodies and how future space missions may discover manifestations of extraterrestrial life.

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What makes a planet habitable

From gravity and topography data collected by the Mars Global Surveyor spacecraft we calculate gravity/topography admittances and correlations in the spectral domain and compare them to those predicted from models of lithospheric flexure. On the basis of these comparisons we estimate the thickness of the Martian elastic lithosphere (T_e) required to support the observed topographic load since the time of loading. We convert T_e to estimates of heat flux and thermal gradient in the lithosphere through a consideration of the response of an elastic/plastic shell. In regions of high topography on Mars (e.g., the Tharsis rise and associated shield volcanoes), the mass-sheet (small-amplitude) approximation for the calculation of gravity from topography is inadequate. A correction that accounts for finite-amplitude topography tends to increase the amplitude of the predicted gravity signal at spacecraft altitudes. Proper implementation of this correction requires the use of radii from the center of mass (collectively known as the planetary “shape”) in lieu of “topography” referenced to a gravitational equipotential. Anomalously dense surface layers or buried excess masses are not required to explain the observed admittances for the Tharsis Montes or Olympus Mons volcanoes when this correction is applied. Derived T_e values generally decrease with increasing age of the lithospheric load, in a manner consistent with a rapid decline of mantle heat flux during the Noachian and more modest rates of decline during subsequent epochs.

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Localized gravity/topography admittance and correlation spectra on Mars: Implications for regional and global evolution

Abundant grabens transect the volcano Alba Patera. Their complex geometry and formation mechanisms are still poorly understood. Tectonic processes and magmatic intrusions are responsible for these long surface features. Cross-cutting relationships of the grabens show radial fractures that were formed during early stages and were progressively overprinted by concentric fractures on the mid and upper flanks of the volcano. Two modeling methods are used to understand the formation of the observed structures and to evaluate their implications for hidden subvolcanic processes. Surface deformation and fault arrangements predicted in finite element models are compared to the graben systems observed in Viking images. The orientation and position of the concentric grabens are found to be best reproduced by local crustal subsidence, superimposed on a regional NW-SE oriented extension with decreasing magnitude from south to north. In analogue sandbox models we also simulate surface structures of arrangements that almost perfectly mimic the observed lineaments on Alba Patera. Formation of the grabens spans a period on the order of a billion years, suggesting long-term geodynamic processes to be responsible for the subsidence of the central Alba Patera area. The progressive change toward higher concentricity is likely resultant from an increase in density in the crust by accumulation of intrusive material and cooling, thus causing subsidence of the region above this volcanic root.

/pdf/modeling-volcanic-deformation-in-a-regional-stress-field-28kz2xac8p.pdf

Modeling volcanic deformation in a regional stress field: Implications for the formation of graben structures on Alba Patera, Mars

Unveiling the origin of radial grabens on Alba Patera volcano by finite element modelling

The relative ages of the major geologic units on and around Olympus Mons are considered, together with an interpretation of the gravity anomaly found for this area. The crater data for this investigation have been acquired and interpreted according to the method outlined by Neukum and Hiller (1981). After careful geological mapping, craters clearly identified as impacts are measured and counted. Crater frequency values per sq km for craters greater than or equal to 1 km ('crater retention ages') are read from the individual counts by fitting the Martian cumulative crater production size-frequency distribution to the individual counts. In addition to age dating, the problem of the origin and nature of the aureole materials using gravity data is addressed. This is done by determining whether the line-of-sight gravity extending from Olympus Mons to the northwestern part of the aureole can be explained by the aureole masses alone or whether additional high-density intrusive masses must be assumed in the aureola area.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/JB087iB12p09905%4010.1002/%28ISSN%292169-9356.MARS3

Mars: Stratigraphy and gravimetry of Olympus Mons and its aureole

Bell Regio is a highland fragment south of Ishtar Terra, extending 1300 km in N-S direction and 900 km in E-W direction. South of this region Eisila Regio is located with an E-W extension of 8000 km and a width of 2000 km. Bell Regio consists of two large massifs: a northern massif with maximum altitudes of 2.5 to 3.0 km above the 6051 km datum and with a semi-corona (other coronae on Venus are associated with volcanic-tectonic processes) and a southern massif with a maximum of 4 to 4.5 km above the datum. The possible shield volcano Tepev Mons of 250 km in diameter is superimposed on the southern massif. It shows a radar dark crater of 40 km diameter on its eastern flank, a crater-like feature of 15 km diameter on the top and a radar bright area extending from the dark crater across the summit. South of Tepev Mons are several volcanic structures with summit depressions. The crest of Bell Regio exhibits a N-S extending fossa system. The whole fresh appearing plain-like area has been classified as rather young compared to other units. Gravity data show a maximum of 33 mGal at Bell Regio and 35 mGal at eastern Eisila Regio. The basins north and south of the highland fragments are associated with gravity lows. Density models have been calculated along the gravity profile Rev. 163 of Pioneer Venus Orbiter across Bell and Eisila Regiones assuming Airy isostatic compensation of the topography and considering several boundary conditions (e.g. mean crustal thickness T 100 km. The highland of Beta Regio has, like Bell Regio, a N-S rifting system, volcanic structures, a fresh appearing plain-like surface and either deep-seating compensating masses or near surface surplus masses. Bell can be considered as little sister of Beta. The geological and geophysical results imply a volcanic-tectonic uplift over a hot spot. The conditions of Atla Regio in eastern Aphrodite Terra are similar. Thus the existence of volcanic-tectonic uplifts support the important role of hot spot volcanism on Venus.

Peter Janle

Papers

Modeling volcanic deformation in a regional stress field: Implications for the formation of graben structures on Alba Patera, Mars

Unveiling the origin of radial grabens on Alba Patera volcano by finite element modelling

Mars: Stratigraphy and gravimetry of Olympus Mons and its aureole

Morphologic and gravimetric investigations of Bell and Eisila regiones on Venus

Investigation of the isostatic state of the Elysium dome on Mars by gravity models