S. F. Pratt

Extensive Hesperian-aged south polar ice sheet on Mars : Evidence for massive melting and retreat, and lateral flow and ponding of meltwater

Local and regional Mars Orbiter Laser Altimeter (MOLA) topographic data support the presence of an extensive Hesperian-aged volatile-rich south polar deposit (the Dorsa Argentea Formation, Hd, and related units) underlying the present Amazonian-aged polar cap (Api, residual ice, and Apl, layered terrain) and covering a surface area that could be as large as 2.94×106 km2 (about 2% of the surface of Mars), over twice the area of the present Amazonian-aged south polar deposits. The deposit characteristics indicate that it contained significant quantities of water ice in amounts comparable to present-day polar deposits. Several lines of evidence for melting indicate that the ice sheet deposits underwent melt back and liquid water drainage into surrounding lows, including a large valley near the crater Schmidt and the Argyre basin. Narrow sinuous ridges lie in a broad linear depression extending from a high near the present polar cap continuously downslope to near the distal portion of Hd. The new topographic data support the interpretation of these ridges as eskers, representing meltwater distribution networks at the base of the receding deposit. Extensive development of large pits and depressions (cavi) have previously been interpreted as eolian etching or basal melting of ice-rich deposits. Analysis of MOLA topography supports the interpretation that they represent basal melting of ice-rich deposits and shows that they have links to the esker systems. Volumetric considerations and topographic lineations suggest that some of the basal melting occurred beneath regions presently occupied by Apl, and that some of the liquid water formed ponds and lakes in the distal parts of Hd. The presence of pedestal craters is further evidence of the removal of extensive volatile-rich deposits and contributes to the quantitative measure of the former deposit thickness. Where did the melt products go? Inspection of the margins of the Dorsa Argentea Formation reveals several large channels that begin there and drain downslope for distances between 900 and 1600 km onto the floor of the Argyre basin, some 3.5–4.0 km below their origin. These channels do not exhibit tributaries. Their broad lateral distribution supports other evidence that deposit melting was areally very widespread and volumetrically significant, and that a large part of the meltwater entered the surface distribution system and was deposited on the floor of the Argyre basin over 1000 km away. Estimates of the present deposit thicknesses together with amounts of the deposit removed by meltback suggest that the original volume could have been as much as 5.9×106 km3, equivalent to a global layer of water ∼20 m deep if the deposit consisted of ∼50% volatiles. A portion of these volatiles migrated across the surface to pond in adjacent valleys and basins, and into the groundwater system. A significant portion of the volatiles remain in the deposit, representing a net removal from the atmosphere and from the active hydrologic system in early to middle Mars history, and forming an accessible record of aqueous conditions and possible biological environments dating from that time.

The Color of the Surface of Venus

Multispectral images of the basaltic surface of Venus obtained by Venera 13 were processed to remove the effects of orange-colored incident radiation resulting from interactions with the thick Venusian atmosphere. At visible wavelengths the surface of Venus appears dark and without significant color. High-temperature laboratory reflectance spectra of basaltic materials indicate that these results are consistent with mineral assemblages bearing either ferric or ferrous iron. A high reflectance in the near-infrared region observed at neighboring Venera 9 and 10 sites, however, suggests that the basaltic surface material contains ferric minerals and thus may be relatively oxidized.

Meltback of Hesperian-aged ice-rich deposits near the south pole of Mars: Evidence for drainage channels and lakes

ponding (minimum volumes � 10 12 m 3 ), overtopping, downcutting, and further drainage of material through a series of craters and into the Prometheus Basin near the edge of the current cap. Topography data show that water filled some craters to depths of at least 200 m and possibly as much as 600 m. Materials derived from melting of this lobe of the DAF drained over a lateral distance of � 600 km and a vertical height of � 800 m, partially emptying into the Prometheus Basin. These combined observations imply that meltback of the older, volatile-rich deposit was a widespread event, occurring on both sides of the deposit over a distance of at least 1800 km, and in an area possibly as much as � 2.9 � 10 6 km 2 . Candidate top-down and bottom-up melting scenarios are examined; the presence of associated Hesperian-aged volcanic deposits and possible subglacial edifices suggests that local and regional basal heating was a major factor in generating the meltwater that formed the drainage features. INDEX TERMS: 6225 Planetology: Solar System Objects: Mars; 5416 Planetology: Solid Surface Planets: Glaciation; 5470 Planetology: Solid Surface Planets: Surface materials and properties; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 5418 Planetology: Solid Surface Planets: Heat flow; KEYWORDS: Mars, polar, glaciers, Hesperian, channels, water

Spectral signature of oxidized pyroxenes - Implications to remote sensing of terrestrial planets

The Fe2+ ions in pyroxenes occurring in basalts on surfaces of terrestrial planets are vulnerable to oxidation, particularly in magma in contact with the atmosphere; this produces Fe3+-bearing assemblages that may mask spectral features of remnant Fe2+ in pyroxenes in telescopic reflectance spectra of oxidized planetary surfaces. To assess such interferences, reflectance spectra (0.3–2.6 urn) were obtained for a variety of pyroxenes that had been heated in air at elevated temperatures. Mossbauer spectral measurements enabled the relative proportions of Fe3+ and Fe2+ ions and Fe3+-bearing phases to be determined in the reaction products. Nanophase hematite having particle diameters smaller than 10 nm obscures the pyroxene Fe2+/M2 site crystal field (CF) bands of oxidized enstatite and pigeonite. Intensification of Fe2+ → Fe3+ intervalence charge transfer transitions in calcic clinopyroxenes mask pyroxene Fe2+/M1 site CF bands, as well as the Fe2+/M2 site CF bands of augites and subcalcic hedenbergites. The ferric iron derived from oxidation of Fe2+ ions may remain as structural Fe3+ ions in the oxidized calcic clinopyroxenes or exist as clusters of nanophase hematite particles having diameters of a few unit cells.

Cavi Angusti, Mars: Characterization and assessment of possible formation mechanisms

Received 26 August 2002; revised 5 February 2003; accepted 20 February 2003; published 24 May 2003. [1] Cavi Angusti represent a series of large irregular depressions localized in part of the south circumpolar area previously mapped as the Hesperian-aged Dorsa Argentea Formation. Their origin has primarily been interpreted to be due to eolian deflation or subglacial melting. We use MGS MOLA and MOC data to analyze the largest of these features (� 100 � 50 km, and up to about 1500 m deep). These data reveal terraced interiors, centrally located equidimensional and elongated edifices, and lava-flow-like structures that strongly suggest that this basin formed as a result of magmatic intrusion and extrusion, causing heating and melting of a volatile-rich substrate and drainage and loss of the liquid water. Volume estimates and heat transfer calculations are consistent with a mechanism involving a combination of intrusion and extrusion very similar to that observed to be responsible for Icelandic subglacial eruptions and meltwater generation. Mounds and ridges in the floors of other depressions suggest that this mechanism may have operated in at least several other features of the Cavi. Eolian activity, sublimation, and solution are also likely to have played a role in further modification of these features. Meltwater from basin formation appears to have drained laterally and may also have reentered the regional subsurface groundwater system. INDEX TERMS: 6225 Planetology: Solar System Objects: Mars; 5480 Planetology: Solid Surface Planets: Volcanism (8450); 5462 Planetology: Solid Surface Planets: Polar regions; 5416 Planetology: Solid Surface Planets: Glaciation; KEYWORDS: Mars, Cavi Angusti, Dorsa Argentea Formation