No abstract

Topography and gravity measured by the Mars Global Surveyor have enabled determination of the global crust and upper mantle structure of Mars. The planet displays two distinct crustal zones that do not correlate globally with the geologic dichotomy: a region of crust that thins progressively from south to north and encompasses much of the southern highlands and Tharsis province and a region of approximately uniform crustal thickness that includes the northern lowlands and Arabia Terra. The strength of the lithosphere beneath the ancient southern highlands suggests that the northern hemisphere was a locus of high heat flow early in martian history. The thickness of the elastic lithosphere increases with time of loading in the northern plains and Tharsis. The northern lowlands contain structures interpreted as large buried channels that are consistent with northward transport of water and sediment to the lowlands before the end of northern hemisphere resurfacing.

A key pacemaker of ice ages on the Earth is climatic forcing due to variations in planetary orbital parameters. Recent Mars exploration has revealed dusty, water-ice-rich mantling deposits that are layered, metres thick and latitude dependent, occurring in both hemispheres from mid-latitudes to the poles. Here we show evidence that these deposits formed during a geologically recent ice age that occurred from about 2.1 to 0.4 Myr ago. The deposits were emplaced symmetrically down to latitudes of approximately 30 degrees--equivalent to Saudi Arabia and the southern United States on the Earth--in response to the changing stability of water ice and dust during variations in obliquity (the angle between Mars' pole of rotation and the ecliptic plane) reaching 30-35 degrees. Mars is at present in an 'interglacial' period, and the ice-rich deposits are undergoing reworking, degradation and retreat in response to the current instability of near-surface ice. Unlike the Earth, martian ice ages are characterized by warmer polar climates and enhanced equatorward transport of atmospheric water and dust to produce widespread smooth deposits down to mid-latitudes.

Elevations measured by the Mars Orbiter Laser Altimeter have yielded a high-accuracy global map of the topography of Mars. Dominant features include the low northern hemisphere, the Tharsis province, and the Hellas impact basin. The northern hemisphere depression is primarily a long-wavelength effect that has been shaped by an internal mechanism. The topography of Tharsis consists of two broad rises. Material excavated from Hellas contributes to the high elevation of the southern hemisphere and to the scarp along the hemispheric boundary. The present topography has three major drainage centers, with the northern lowlands being the largest. The two polar cap volumes yield an upper limit of the present surface water inventory of 3.2 to 4.7 million cubic kilometers.

Abstract. The chronology of lunar volcanism is based on radiometric ages determined from Apollo and Luna landing site samples, regional stratigraphic relationships, and crater degradation and size-frequency distribution data for units largely defined prior to the end of the Apollo program. Here we report on new crater size-frequency distribution data for 139 spectrally and morphologically defined basalt units which are exposed in six nearside impact basins (Australe, Tranquillitatis, Humboldtianum, Humorum, Serenitatis, and Imbrium). Crater size-frequency distribution measurements are a statistically robust and accurate method to derive absolute model ages of unsampled regions of the Moon. Compared to crater degradation ages, crater size-frequency ages, performed on spectrally defined units, offer significant improvements in accuracy. Our investigation showed that (1) in the investigated basins, lunar volcanism was active for at least 1.5-2.0 b.y., starting at about 3.9-4.0 b.y. and ceasing at-2.0 b.y., (2) most basalts erupted during the late Imbrian Period at about 3.6-3.8 b.y., (3) significantly fewer basalts were emplaced during the Eratosthenian Period, (4) basalts of Copernican age were not found in any of the investigated basins, (5) lunar basin-filling volcanism probably started within -100 m.y. after the formation of the individual basins. We also assessed the relationship between impact basin age and the history of mare basalt emplacement in these basins. We found that (1) in all pre-Nectarian basins (Australe and Tranquillitatis) as well as in the Humboldtianum basin, which is of Nectarian age, the distribution of surface ages is clearly dominated by only a single peak in the number of erupted units at 3.6-3.8 b.y., (2) in the younger basins (Humorum, Serenitatis, and Imbrium) a second peak at 3.3-3.5 b.y. is observed, (3) basalt eruptions younger than 2.6 b.y. occur only intermittently, and (4) in the youngest basins, Serenitatis and Imbrium, we see an extended period of active basin-filling volcanism (1.5-1.6 b.y.) which is 500 m.y. longer than in the Australe and Humorum and even -1.0 b.y. longer than in Tranquillitatis and Humboldtianum.

We present results from geomorphic mapping and visible to near-infrared spectral analyses of the Jezero crater paleolake basin and its associated watershed. The goal of this study is to understand the provenance of the sedimentary deposits within this open-basin lake using a source-to-sink approach. Two fan deposits located within the basin have distinct visible to near-infrared mineralogic signatures measured by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). The northern fan is spectrally characterized by a mixture of Mg-rich carbonate and olivine, while the western fan is characterized by Fe/Mg-smectite (e.g., saponite or nontronite) with variable amounts of Mg-rich carbonate and olivine in isolated exposures. The watersheds of these deposits contain a variety of geomorphic units that are likely to have supplied sediment to the Jezero crater paleolake, as the fluvial valleys that fed the basin incise these units. The geomorphic units include exposures of Fe/Mg-smectite-, olivine-, and Mg-rich carbonate-bearing terrain. We show that the difference in fan deposit mineralogy is a function of the areal exposure of the major geomorphic units within their watersheds. This indicates that the spectrally dominant aqueous alteration minerals in the fan deposits are primarily detrital, or transported, in nature and did not form in situ. We conclude that the aqueous alteration of the units in the watershed occurred prior to the fluvial activity that carved the valleys of the Jezero crater paleolake system, and that the two periods of aqueous activity are not genetically related.

Geological and physical observations and constraints are applied to the development of a model of the ascent and emplacement of basaltic magma on the earth and moon. Mathematical models of the nature and motion of gas/liquid mixtures are developed and show that gas exsolution from terrestrial and lunar magmas commonly only occurs at shallow depths (less than 2 km); thus the ascent of bubble‐free magma at depth can be treated separately from the complex motions caused by gas exsolution near the surface. Magma ascent is related to dike or conduit width; a lower limit to width is determined by the presence of a finite magma yield strength or by excessive magma cooling effects related to magma viscosity. For terrestrial basalts with negligible yield strengths and viscosities greater than 102 Pa s, widths in the range 0.2–0.6 m are needed to allow eruptions from between depths of 0.5–20 km. Fissure widths of about 4 m would be needed to account for the output rates estimated for the Columbia River flood basalt eruptions. As the magma nears the surface, bubble coalescence will tend to occur, leading to intermittent explosive strombolian‐style activity. For commonly occurring lunar and terrestrial basalts the magma rise speed must be greater than 0.5–1 m/s if strombolian activity is to be avoided and relatively steady fire fountaining is to take place. Terrestrial fire fountain heights are dictated by the vertical velocity of the magma/gas dispersion emerging through the vent, increasing with increasing magma gas content and mass eruption rate, and decreasing with increasing magma viscosity. Terrestrial fire fountain heights up to 500 m imply the release of up to 0.4 wt % water from the magma, corresponding to initial water contents up to 0.6 wt %. The presence of extremely long lava flows and sinuous rules on the moon has often been cited as evidence for very high extrusion rates and thus a basic difference between terrestrial and lunar magmas and crustal environments. However, the differences between terrestrial and lunar magma rheologies and crustal environments do not lead to gross differences between the effusion rates expected on the two planetary bodies, for similar‐sized conduits or fissures. Thus the presence of these features implies only that tectonic and other forces associated with the onset of some lunar eruptions were such as to allow wide fissures or conduits to form. The surface widths of elongate fissure vents need be no wider than 10 m to allow mass eruption rates up to 10 times larger than those proposed for terrestrial flood basalt eruptions; 25‐m widths would allow rates 100 times larger. It therefore appears unlikely that source vents on the moon with widths greater than a few tens of meters represent the true size of the unmodified vent. The main volatile released from lunar magmas was probably carbon monoxide, released in amounts proportionally less than terrestrial magmas by more than an order of magnitude. However, decompression to the near‐zero ambient lunar atmospheric pressure causes much g...

[1] 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.