Caloris Research Articles

Source processes for the alkali metals in the atmosphere of Mercury

A large (fivefold) increase in Mercury's potassium(K) column abundance on 14 October 1987, has been reported by Sprague et al.[199O](SKH), who attributed the enhancement to diffusion through the surface in the Caloris Basin, from depths of order 10 km. The postulated source rate is much larger than any previously estimated diffusion rate, and if true certainly affects consideration of the origin of other atmospheric species. However, Killen et al.[1991] have pointed out that the claim is not supported by the published observations of K or sodium(Na) as a whole. Sprague et al.[1991] have responded by further hypothesizing the existence of several other sources of gas diffusing out of the regolith, all of which are time variable. In any case the SKH data indicate large variations in abundance, and it is important to understand the cause. With this issue in mind we have examined the available abundance estimates for correlation with possible controlling physical parameters. We have found a significant correlation between the average zenith K column abundance and indices of solar activity, although we are left with the intrinsic uncertainty of a small data set.

Detecting Potassium on Mercury

A critical comment on the work of A.L. Sprague et al. (1990) is presented. It is argued that, in attributing an enhanced emission in the potassium D lines on Oct. 14, 1987 in the equatorial region of Mercury to a diffusion source centered on Caloris Basin, Sprague et al. misinterpreted the data. Sprague et al. present a reply, taking issue with the commenters.

Caloris Basin: An Enhanced Source for Potassium in Mercury's Atmosphere

Enhanced abundances of neutral potassium (K) in the atmosphere of Mercury have been found above the longitude range containing Caloris Basin. Results of a large data set including six elongations of the planet between June 1986 and January 1988 show typical K column abundances of approximately 5.4 x 10(8) K atoms/cm(2). During the observing period in October 1987, when Caloris Basin was in view, the typical K column was approximately 2.7 x 10(9) K atoms/cm(2). Another large value (2.1 x 10(9) K atoms/cm(2)) was seen over the Caloris antipode in January 1988. This enhancement is consistent with an increased source of K from the well-fractured crust and regolith associated with this large impact basin. The phenomenon is localized because at most solar angles, thermal alkali atoms cannot move more than a few hundred kilometers from their source before being lost to ionization by solar ultraviolet radiation.

Radar altimetry of Mercury: A Preliminary analysis

Measurements of Mercurian topography based on Arecibo radar observations are presented. The data, which were obtained from 1978 to 1984, cover much of the equatorial zone of Mercury between 12°N and 5°S latitude. Over thirty continuous altitude profiles were obtained, each spanning from 20 to 90 degrees of longitude at a resolution of 0.15° (longitude) by 2.5° (latitude). Radar depths for large craters support previous indications from imagery that Mercurian craters are shallower than lunar craters of the same size. One very large (800 km) impact basin shows some distinct topographic structure, although its relative shallowness suggests postimpact modification by isostatic relaxation or volcanic filling. The plains of Tir Planitia appear topographically smooth to the radar. These plains extend well into the hemisphere not imaged by Mariner 10, possibly forming part of a large annulus of smooth plains around Caloris Basin. The circum‐Caloris smooth plains are strongly down‐bowed, indicating subsidence under a load. This and other similarities to lunar maria suggest a volcanic origin for these plains. Additional areas of topographically smooth terrain have been found in both the imaged and unimaged hemispheres. Several ridges, scarps, and fault zones have been identified in the altimetry. Three mapped arcuate scarps show heights of about 700 m and cross‐sectional widths of about 70 km. One of these features is clearly a ridge, while the other two scarps have a more ridge‐like appearance than is suggested by images. One large‐scale topographic drop of 3 km correlates well with a mapped system of faults and intracrater scarps. The equatorial zone of Mercury shows 7 km of maximum relief, although the typical elevation difference between highlands and lowlands is closer to 3 km. Three major highland areas are found, the largest two of which are roughly antipodal and aligned within about 10° of the “hot poles” of Mercury. The unimaged hemisphere, possessing both large craters and topographically smooth areas, does not appear to be markedly different in its topography from the imaged hemisphere. No evidence has been found for another Caloris‐type impact structure in the unimaged hemisphere.

Reorientation of planets with elastic lithospheres

The orientation of a planet is controlled by the positions of the principal axes of the inertia tensor relative to the planetary surface. Using the theory for the deflection of thin elastic shells the principal axes are computed after emplacement of an arbitrary axisymmetrical load. The partial compensation of the load and the partial relaxation of rotational flattening are included in the computation. It is found that the amount of reorientation is independent of lithosphere thickness. The parameters controlling the amount of reorientation are the location of the load and the size of the load compared to the rotational flattening. The results indicate that the Tharsis rise has probably reoriented Mars by only 3 to 9° and certainly less that 18°. The position of the Caloris Basin on Mercury indicates that if the surrounding lava sheet controls the planetary orientation then the lava sheet is probably less than 2000 m thick.

The dependence of reflectance spectra of Mercury on surface terrain

Reflectance spectra of Mercury, covering the spectral range of ∼0.3–1.1 μm obtained during 1963–1976, were examined for any correlations with surface terrain. Mercury's 6.1385°/day rotational rate, the phases of the planet around maximum elongations, and bidirectional reflectance spectroscopy theory were used to identify the surface area associated with each spectrum. Data from 1974–1975, re-reduced with improved standard star flux ratios, show a weak absorption band in the near infrared not see in earlier analyses. Older spectra suggest that the western longitudes of the unimaged side of Mercury are similar to the rest of the planet. Spectra of the intercrater plains in the 0–90° quadrant suggest a possible absorption band. Spectra of areas dominated by Caloris Basin with the encompassing smooth plains may show Fe 2+ abundances in the soil comparable to lunar highlands soil. No striking differences between spectra of intercrater plains and spectra of smooth plains are found. The absorption features seen in spectra of Mercury are generally weaker than features seen in lunar spectra.

Tectonics of the Caloris area on Mercury: An alternative view

Both morphologic and geometric studies of the “lineated terrains” around Caloris provide evidence of several types of tectonic motions inside the ejecta blanket of the basin. These motions preferentially occurred along a preexisting pattern. In spite of several similarities to mare-filled multiring basins on the Moon, many geometric and chronologic differences suggest that the ridge pattern inside the Caloris basin may not be produced, as observed on the Moon, exclusively by subsidence of the inner basin under volcanic loading. A model of membrane stresses which yield a decrease of the radius of Caloris and the observed tectonics is proposed.

Global volcanism and tectonism on Mercury: comparison with the Moon

Both morphologic and tectonic studies indicate that Mercury and the Moon have quite different internal histories, despite their apparently similar morphologies. The evaluation of the volcanic surfaces indicates a decreasing volcanism on Mercury at the largest impacting time, despite short and local reactivations. On the Moon, the basaltic volcanism was increasing at the same time and continued for 1 billion years. That indicates a strongly different thermal evolution for these two planetary bodies. A widespread graben pattern is present on the Moon, with a statistical dominance of radial or tangential orientation with respect to the Imbrium basin, thus suggesting a relation between this major basin and the expansion of the Moon. Azimuthal studies show that the compressive structures, observed on the stereographic covered surface of Mercury are not randomly oriented, but seem radial towards the Caloris basin, thus indicating a possible influence of this largest basin on Mercurian contraction. The qualitative and quantitative formulations of these tectonic perturbations induced by large basins will be developed in a companion paper [1].

Stratigraphy of the Caloris basin, Mercury

The 1300-km-diameter Caloris impact basin is surrounded by well-defined ejecta units that can be recognized from more than 1000 km, radially outward from the basin edge. A formal rock stratigraphic nomenclature is proposed for the Caloris ejecta units, which are collectively called the Caloris Group. Each of the individual formations within the Group are described and compared to similar rock units associated with the lunar Imbrium and Orientale basins. A crater degradation chronology, linked the the Caloris event, is also proposed to assist in stratigraphic correlation on a Mercury-wide basis.

The origin of polygonal troughs on the Northern Plains of Mars

Viking photographs have revealed extraordinary systems of crudely polygonal troughs in three different areas of the northern plains of Mars. Trough widths average 200–800 m and average trough spacing is 5–10 km. The widest troughs have flat floors and steep sides which suggest that they are either grabens or tension cracks which have been partially filled. The following three arguments support the garben interpretation: (1) Two clear examples of vertical offsets along trough walls indicate that the troughs formed by downdropping of the floors. (2) Troughs similar in scale and morphology to those of the Martian northern plains form concentric and radial patterns in the Caloris Basin on Mercury. Most likely, they are extensional fractures or grabens resulting from uplift of the basin floor (Strom et al., 1975). The Caloris analogy suggests that the troughs on Mars are similar tectonic features related either to uplift or expansion of the planet. Fourteen rose diagrams of trough azimuths in southeastern Acidalia Planita show that locally they exhibit preferred orientations roughly parallel to the topographic contours, but overall there is no strong regional pattern. (3) The scale of tension crack systems is limited by the depth of fracturing because the extent of stress relief perpendicular to a crack is proportional to the crack depth. Observations of terrestrial tension cracks suggest a lower limit on crack depths of about one-tenth of the average spacing. Thus, if the Martian troughs are tension cracks, they would be expected to have depths of at least 500–1000 m. Mechanical considerations indicate that it is difficult to generate such deep cracks by surficial tension due to thermal cooling and contraction in permafrost, desiccation, or cooling of lava flows. Deep-seated tension of tectonic origin generally results in normal faults and not tension cracks.

The tectonic and volcanic history of mercury as inferred from studies of scarps, ridges, troughs, and other lineaments

This paper consists of separate descriptive and interpretive sections which address the issues of tectonic and volcanic modification of the Mercurian surface. In section 1 a nongenetic classification system based on planimetric form is outlined and applied to scarps, ridges, troughs, and other lineaments on Mercury. Nine landform classes are defined, and type examples of each are presented and discussed. The resulting set of observational constraints provides the basis for the following evolutionary chronology for Mercury developed in section 2. After formation of a solid lithosphere but before the end of heavy meteoritic bombardment, tidal spindown and planetary contraction owing to cooling combined to produce a global pattern of linear fractures and arcuate thrust faults. Throughout this period, intercrater plains formed by extrusion from linear vents or by local lithospheric melting. Near the end of heavy meteoritic bombardment the Caloris basin‐forming event drastically altered the morphology of much of the Mercurian surface. Soon after basin excavation, fluid material covered its floor to form smooth plains. Planimetrically irregular ridges formed inside Caloris in response to compressive stresses induced by regional subsidence. Gravitational readjustment to emplacement of plains material outside Caloris was responsible for scarp and ridge formation there. Uplift of plains inside Caloris produced extensional fractures which transect the previously established pattern of ridges. At roughly the same time, intense modification of preexisting landforms occurred in a region roughly antipodal to Caloris Basin to produce a distinctive terrain unit characterized by scattered hills and linear, open‐ended troughs. By this time, volcanic plains emplacement was largely restricted inside large, fresh craters. Irregular scarps and ridges developed in these plains in response to local stresses, perhaps induced by magma redistribution at depth. The period following the most recent smooth plains formation on Mercury has been characterized by relatively light cratering and only mild landform modification.

Tectonic implications for the gravity structure of Caloris Basin, Mercury

Studies of tectonic landforms associated with Caloris Basin on Mercury suggest that isostatic adjustment has occurred in response to basin excavation, and that the smooth plains inside Caloris were emplaced significantly before isostatic equilibrium was attained. Combined with dynamical considerations, this leads us to propose that the Caloris region is characterized by a circular negative or zero free air gravity anomaly centered inside Caloris, and an annular positive anomaly which coincides with extensive tracts of young smooth plains outside the basin. This proposed gravity pattern differs markedly from that associated with mare-filled basins on the Moon.

Orientale and Caloris

Applications of experimental explosion-crater data to Orientale and recent geologic mapping of the basin have produced a new stratigraphy and genetic model for Orientale that are also applicable to Caloris. The inner-basin scarp of Orientale is thought to be a bench separating the upper parts of the basin from its deep bowl-shaped interior. The elongated and complexly fractured domes of the basin floor formed by inward compression in the terminal stages of the cratering sequence. The Inner Montes Rook are considered a central peak ring. The Montes Rook and the nonlineated knobby and associated smoother materials that overlie the Cordillera scarp around much of its circumference are the uppermost parts of the overturned rim flap which formed early in the cratering event. The knobs and smaller massifs are probably coherent blocks quarried from deep within the moon. They were among the last materials to leave the basin and had little radial momentum unlike the lineated Hevelius which formed earlier by disaggregation of the rim flap, secondary cratering, and the ground surge. The Cordillera scarp, best seen on the east side of the basin but poorly developed and discontinuous on the west, is a primary feature formed early in the crater excavation process by basinward motions of the walls and the fractured zone beyond the rim of the expanding cavity. The Cordillera scarp is overlain by ejecta over most of its extent, and post-basin internal slumping, previously thought to be important, must be a subordinate process in development of the scarp. The basin fill in Caloris has no counterpart in Orientale but the materials between the most prominent scarp and the weakly developed outer scarp appear to be the degraded and possibly mantled equivalents of the massifs and knobs associated with the Montes Rook. The radially lineated terrain that generally lies beyond the outer scarp of Caloris is considered the subdued counterpart of the Hevelius Formation, which generally shows the same relation to the Cordillera scarp at Orientale. Thus, the prominent innermost scarp of the Caloris basin is the equivalent of the Montes Rook. Beyond this scarp is the overturned flap covered by large blocks and massifs derived from a deep horizon in Mercury where the bedrock is more coherent than the upper impact-brecciated layers. The radially lineated deposits, as in Orientale, are earlier-arriving basin ejecta and secondary-crater materials mixed with the pre-basin surface all of which were modified by the ground surge. This comparison between Orientale and Caloris suggests that one or more buried ring structures should be present inside Caloris and that Mercury is also layered internally as is the moon. The differences in spacing and development of the ring structures or circumferential scarps of Orientale and Caloris are probably gravitational effects.

Comparative studies of lunar, Martian, and Mercurian craters and plains

The amount of smooth plains material in craters surrounding the Caloris basin on Mercury depends on the extent these craters have been eroded by Caloris ejecta. Therefore smooth plains surrounding Caloris must have been emplaced at least in part by saturated secondary cratering. Mercurian uplands outside the continuous belt of smooth plains deposits have a crater population which is deficient in craters smaller than 50 km relative to extrapolations of craters larger than 50 km and relative to size frequency distributions of craters ≤50 km on typical lunar upland regions. However, the typical lunar upland regions have been masked by the addition of numerous large basin secondary craters. Only rare areas southwest of Mare Serenitatis appear to be similar to Mercurian terrain at great distances from smooth plains because the areas are relatively free of basin secondaries. Martian uplands also exhibit a crater population which is deficient in craters less than 30–50 km, which was previously interpreted to have been caused by obliteration of some craters less than 30 km by surface processes. The observed crater deficiencies on the moon, Mars, and Mercury below 30–50 km are mostly a reflection of the primary crater production population; it is characterized by a power function having two different exponents (−2.0 for craters less than 50 km and −3.2 to 3.5 for craters ≥50 km). The hypothesis that the observed deficiency of small lunar craters southwest of Mare Serenitatis resulted from an episode of premare volcanism which obliterated some small craters was evaluated and rejected because highland volcanic rocks are rare in returned samples, and if significant premare upland basaltic volcanism is hypothesized, it would be circumstantial that it occurred only in those areas least affected by basin ejecta. Premare upland volcanic activity would be required also to explain the Mercurian crater counts. However, the Mercurian mapping results indicate that intercrater plains representative of hypothetical volcanic rocks were formed before craters now observed on their surfaces. The most persuasive reason for rejecting the theory of lunar premare volcanism is that reexamination of all available crater count data for lunar uplands shows no evidence for a primary production population characterized by a single power function. Therefore crater counts showing deficiencies relative to a single power function which previously was acceptable evidence for crater obliteration need no longer be considered as evidence for crater obliteration.

Chronology of terrestrial planet evolution: The evidence from Mercury

It is premature to establish a chronology for Mars and Mercury, relative to the known lunar chronology, to better than an order of magnitude. Lunar evidence neither requires nor excludes a “cataclysmic” episode of bombardment about 4.0 b.y. ago. Such a cataclysm might have resulted naturally from tidal disruption by a planet or collisional fragmentation in the asteroid belt of either a Uranus/Neptune-scattered planetesimal or a large asteroid, in which case any lunar cataclysm would have occurred as well on other planets. There is no independent evidence in Mariner 10 imagery for (or against) an early episodic bombardment on Mercury. Crater densities on plains units of the Moon, Mars, and Mercury have not been shown to be “strikingly similar” and do not imply, in the absence of definitive dynamical calculations of planetary impact rates of plausible populations of planetesimals, any similarity in the geological chronologies for those planets. Photogeological studies alone cannot determine absolute chronologies for planets. In combination with dynamical analyses, they can help us date to no better than a factor of 3 to 10 the formation of the Caloris Basin or the epoch when the Martian rivers ran.

Tectonism and volcanism on Mercury

Mercury appears to have a tectonic framework and diastrophic history not found on other terrestrial planets explored to date (earth, Mars, and the moon). On the part of the planet viewed by Mariner 10, only two localized areas show evidence of tensional stresses, both of which are apparently associated with the Caloris basin. Lobate scarps occur in the remainder of the explored region and appear to be primarily reverse or thrust faults which have resulted from compressive stresses acting on a global scale. The period of compression represented by these scarps occurred during the final phase of heavy bombardment on Mercury and was probably caused by crustal shortening due to a small decrease in the planet's radius. Stratigraphic, volumetric, and albedo considerations together with distribution indicate that the majority of smooth plains on Mercury were produced by volcanism which occurred at the close of the period of late heavy bombardment similar to that on the moon and Mars. Several generations of plains are evident; the oldest may have resulted in part from an early differentiation of the planet.

Preliminary geologic terrain map of Mercury

A geologic terrain map of Mercury has been constructed by use of the photogeologic methods employed for the moon and Mars. The oldest and most widespread unit, intercrater plains, forms nearly level to rolling surfaces on which are superposed numerous secondary impact craters. This unit may represent a very old surface that predates the last heavy bombardment of the inner planets. The effects of this bombardment are recorded in a second widespread unit, heavily cratered terrain, consisting of closely spaced craters and basins from 30 km to several hundred kilometers in diameter. Units formed by excavation of the 1300-km-diameter Caloris basin are widespread on one hemisphere of the planet. The development of the Caloris basin was followed relatively quickly by emplacement of widespread plains materials, most of which are probably volcanic.

Photometric observations of Mercury from Mariner 10

The elimination of the residual image problem which plagued previous Mariner imaging systems allowed photometry of moderately high quality to be carried out on Mercury by Mariner 10. The conclusions from the photometric analysis are as follows. To a surface resolution of 20 km, Mercury is covered with a dark fine-grained soil similar to the lunar regolith. No regions having anomalous polarization or color were discovered, which might have implied large amounts of metallic Fe or exposed bare rock. Photoclinometry suggests that the center of the Caloris Basin may be about 9 km below the outside rim. The heavily cratered plains have albedos of about 0.17, somewhat brighter than the lunar highlands. The albedos of the darker smooth plains and the interiors of bright fresh craters are systematically higher than their lunar counterparts. One consistent interpretation of the color and albedo relationships is that the dark smooth plains resemble the low-Ti, low-Fe lunar maria and that the crust is generally low in Ti, metallic Fe, and Fe(3+).