Windy L. Jaeger

A Closer Look at Water-Related Geologic Activity on Mars

Water has supposedly marked the surface of Mars and produced characteristic landforms. To understand the history of water on Mars, we take a close look at key locations with the High-Resolution Imaging Science Experiment on board the Mars Reconnaissance Orbiter, reaching fine spatial scales of 25 to 32 centimeters per pixel. Boulders ranging up to ∼2 meters in diameter are ubiquitous in the middle to high latitudes, which include deposits previously interpreted as finegrained ocean sediments or dusty snow. Bright gully deposits identify six locations with very recent activity, but these lie on steep (20° to 35°) slopes where dry mass wasting could occur. Thus, we cannot confirm the reality of ancient oceans or water in active gullies but do see evidence of fluvial modification of geologically recent mid-latitude gullies and equatorial impact craters.

Athabasca Valles, Mars: A Lava-Draped Channel System

Athabasca Valles is a young outflow channel system on Mars that may have been carved by catastrophic water floods. However, images acquired by the High-Resolution Imaging Science Experiment camera onboard the Mars Reconnaissance Orbiter spacecraft reveal that Athabasca Valles is now entirely draped by a thin layer of solidified lava—the remnant of a once-swollen river of molten rock. The lava erupted from a fissure, inundated the channels, and drained downstream in geologically recent times. Purported ice features in Athabasca Valles and its distal basin, Cerberus Palus, are actually composed of this lava. Similar volcanic processes may have operated in other ostensibly fluvial channels, which could explain in part why the landers sent to investigate sites of ancient flooding on Mars have predominantly found lava at the surface instead.

Paterae on Io: A new type of volcanic caldera?

Paterae, defined by the International Astronomical Union as “irregular crater[s], or complex one[s] with scalloped edges,” are some of the most prominent topographic features on Io. Paterae on Io are unique, yet in some aspects they resemble calderas known and studied on Earth, Mars, and Venus. They have steep walls, flat floors, and arcuate margins and sometimes exhibit nesting, all typical of terrestrial and Martian basalt shield calderas. However, they are much larger, many are irregular in shape, and they typically lack shields. Their great sizes (some >200 km diameter) and lack of associated volcanic edifices beg comparison with terrestrial ash flow calderas; however, there is no convincing evidence on Io for the high-silica erupted products or dome resurgence associated with this type of caldera. Ionian paterae seem to be linked with the eruption of large amounts of mafic to ultramafic lavas and colorful sulfur-rich materials that cover the floors and sometimes flow great distances away from patera margins. They are often angular in shape or are found adjacent to mountains or plateaus, indicating tectonic influences on their formation. A database of 417 paterae on Io measured from images with <3.2 km pixel−1 resolution (80% of its surface) reveals that their mean diameter of 41.0 km is close to that for calderas of Mars (47.7 km), is smaller than that for Venus (∼68 km), but dwarfs those for terrestrial basalt shield calderas (6.6 km) and ash flow calderas (18.7 km). Thirteen percent of all paterae are found adjacent to mountains, 42% have straight or irregular margins, and 8% are found atop low shields. Abundant, smaller paterae with more continuously active lava eruptions are found between 25°S and 25°N latitude, whereas fewer and larger paterae are found poleward of these latitudes. Patera distribution shows peaks at 330°W and 150°W longitude, likely related to the direction of greatest tidal massaging by Jupiter. Ionian patera formation may be explained by portions or combinations of models considered for formation of terrestrial calderas, yet their unusual characteristics may require new models with a greater role for tectonic processes.

Metal–silicate partitioning of Co, Ga, and W: dependence on silicate melt composition

This study investigates the effect of silicate melt composition on metal/silicate partitioning for Co2+, Ga3+, and W4+ at 1300°C, 1 atm, and a log fO2 of −12. Five glasses in the system MgO–CaO–Al2O3–TiO2–SiO2 with nbo/t (nonbridging oxygens/tetrahedrally coordinated cations) values ranging from 0.25 to 1.52 were used as starting materials. For W and Co experiments the five glasses were equilibrated with W or Co wire loops, respectively, at the specified run conditions. For Ga experiments the glasses were doped with 2 wt.% Ga2O3 and equilibrated with pure Fe. All phases were analyzed by an electron microprobe. The metal/silicate partition coefficient for W depends strongly on melt basicity, whereas the effect of melt composition on Ga partitioning is less pronounced and for Co it is negligible. DW decreases rapidly with increasing nbo/t, DGa decreases moderately with increasing degree of melt basicity, and DCo remains relatively constant over our compositional range. The findings of this study indicate that the effect of melt composition on trace element solubility is a function of cation oxidation state such that high valency cations like W4+ are more readily dissolved in depolymerized melts which have a higher ratio of nonbridging oxygens, and lower valency cations like Co2+ are relatively independent of the parameter nbo/t. These results confirm that the composition of the primitive mantle is an important factor in constraining how siderophile trace elements distribute themselves between an Fe–metal core and the bulk silicate Earth during an early magma ocean differentiation event.

Discovery of columnar jointing on Mars

We report on the discovery of columnar jointing in Marte Valles, Mars. These columnar lavas were discovered in the wall of a pristine, 16-km-diameter impact crater and exhibit the features of terrestrial columnar basalts. There are discontinuous outcrops along the entire crater wall, suggesting that the columnar rocks covered a surface area of at least 200 km 2 , assuming that the rocks obliterated by the impact event were similarly jointed. We also see columns in the walls of other fresh craters in the nearby volcanic plains of Elysium Planitia–Amazonis Planitia, which include Marte Vallis, and in a well-preserved crater in northeast Hellas.

Response to Comment on "Athabasca Valles, Mars: A Lava-Draped Channel System"

The recent geologic history of Athabasca Valles, Mars, is controversial. Some studies report ice-rich sediment in its channels, whereas others find only lava. Data from the High-Resolution Imaging Science Experiment camera now confirm that, although certain features exhibit a superficial similarity to ice-related landforms, solidified lava coats the entire channel system.