Brian M. Hynek

Evidence for extensive denudation of the Martian highlands

High-resolution topographic data from the Mars Orbiter laser altimeter reveal evidence for widespread denudation from Margaritifer Sinus to northern Arabia Terra, an area of ∼1 × 107 km2. A major resurfacing event is indicated by: (1) a heavily degraded landscape hosting numerous inliers, (2) truncation or absence of valley networks, and (3) a break in slope at the edge of early to middle Noachian plateau materials. Geomorphic mapping was completed on the type locale of denudation (0°–30°S, 0°–30°W). Superposition relations and crater counts for geomorphic units indicate that large-scale resurfacing took place in the late Noachian, eroding the Martian highlands and resulting in transportation and deposition of ∼4.5 × 106 km3 of sediment in the northern plains. This is equivalent to a 120-m-thick uniform layer of sediment on the surface of Mars north of 30°N. Geomorphic mapping and crater counts limit the timing of denudation to the late Noachian, an interval of 350–500 m.y. Using this limit, we estimate a minimum rate of denudation of 2.0 μm/yr, comparable to denudation of typical slopes in a temperate maritime climate on Earth. The morphology, rate of denudation, and extensive nature of upland degradation suggest that precipitation-fed surface runoff is the most likely geomorphic agent capable of such a process, indicative of a warm, wet Mars during the late Noachian Epoch.

New data reveal mature, integrated drainage systems on Mars indicative of past precipitation

Earlier analyses of valley networks on Mars often concluded that they were poorly integrated, immature drainage systems. Consequently, surface runoff from precipitation was generally thought to be an unimportant geomorphic process. Combination of Mars Global Surveyor (MGS) imaging and altimetry data sets, however, provides a vast improvement in image clarity and resolution. Although we have used the same defining characteristics for valley networks as used in previous work, our mapping in the Martian highlands reveals up to an order of magnitude higher values for the number of valleys, total valley length, and drainage density. Segments can now be mapped a greater distance with the result that the heads of numerous systems reach right up to the drainage divides. Moreover, MGS data show that many previously mapped, unconnected, low-stream-order segments are part of larger, integrated, mature drainage networks. In light of these new data, it is likely that surface runoff (and, by inference, precipitation) played an important role in the sculpting of large regions of the Martian landscape early in the planet’s history.

A volcanic environment for bedrock diagenesis at Meridiani Planum on Mars

Exposed bedrocks at Meridiani Planum on Mars display chemical and mineralogical evidence suggesting interaction with liquid water. On the basis of morphological observations as well as high abundances of haematite and sulphate minerals, the rocks have been interpreted as sediments that were deposited in a shallow body of briny water with subsequent evaporation leaving behind the sulphate minerals. The iron-sulphur mineralization at Meridiani has also been inferred to be analogous to that produced during oxidative weathering of metal sulphide minerals, such as occurs at acid mine drainage sites. Neither of these interpretations, however, is consistent with the chemical composition of the rocks. Here we propose an alternative model for diagenesis of Meridiani bedrock that involves deposition of volcanic ash followed by reaction with condensed sulphur dioxide- and water-bearing vapours emitted from fumaroles. This scenario does not require prolonged interaction with a standing body of surface water and may have occurred at high temperatures. Consequently, the model invokes an environment considerably less favourable for biological activity on Mars than previously proposed interpretations.

Implications for hydrologic processes on Mars from extensive bedrock outcrops throughout Terra Meridiani.

Grey haematite was recently detected in the Terra Meridiani region of Mars by the Thermal Emission Spectrometer onboard the Mars Global Surveyor spacecraft. The formation of haematite on Earth often requires liquid water to be present for long periods of time, making this an important target for deciphering the history of water on Mars. The Mars Exploration Rover Opportunity landed in Meridiani early in 2004 and has since discovered light-toned bedrock outcrops rich in chemical and textural signatures of long-term water interaction locally at the landing site. Here I use remote sensing data to show that the light-toned outcrops at the landing site are not a local phenomenon. Instead, they are observable throughout the haematite-bearing plains in both visible and thermal infrared remote sensing data. Moreover, the light-toned material outcrops around much of the margin and is mappable for hundreds of kilometres to the north, east and west of the plains. I infer that 3 × 105 km2 of this material is exposed over 20° of longitude, indicating the extended presence of surface or near-surface water over a large region of Mars.

Detection of iron substitution in natroalunite-natrojarosite solid solutions and potential implications for Mars

Abstract Natroalunite containing substantial amounts of Fe occurs as a prominent secondary phase during acid-sulfate alteration of pyroclastic basalts in volcanic fumaroles in Nicaragua and elsewhere, and has been observed in laboratory simulations of acid-sulfate alteration as well. Reaction path models constrained by field and experimental observations predict that Fe-rich natroalunite should also form as a major secondary phase during alteration of martian basalt under similar circumstances. Here, we evaluate the potential to use spectroscopic methods to identify minerals from the alunite group with chemical compositions intermediate between natroalunite and natrojarosite on the surface of Mars, and to remotely infer their Fe contents. X-ray diffraction and spectroscopic measurements (Raman, visible/ near infrared, mid-infrared, Mössbauer) were obtained for a suite of synthetic solid solutions with a range of Fe contents ranging from natroalunite to natrojarosite. In the visible/near infrared, minerals with intermediate compositions display several spectral features not evident in end-member spectra that could be used to remotely identify these minerals and infer their composition. In addition, Raman spectra, mid-infrared spectra, and X-ray diffraction peaks all show systematic variation with changing Fe content, indicating that these methods could potentially be used to infer mineral compositions as well. The results suggest that alunite group minerals with intermediate Fe compositions may be able to account for some visible/near-infrared and Mössbauer spectral features from Mars that had previously been unidentified or attributed to other phases. Overall, our findings indicate that consideration of solid solutions may lead to new identifications of alunite group minerals on the surface of Mars, and raise the possibility that minerals with compositions intermediate between natroalunite and natrojarosite may be widely distributed on the planet.

Late-stage formation of Martian chloride salts through ponding and evaporation

Small deposits of chloride salts have been documented at hundreds of locations on Mars through the use of multispectral orbital data. Given the small spatial extent of these deposits, their formation mechanisms, timing, and relation to other aqueous processes in Mars’ history are presently poorly constrained. Here we detail one of the chloride deposits near Meridiani Planum, the location of the Opportunity rover. This chloride deposit likely formed from fluviolacustrine processes, implying an active hydrologic cycle. Late-stage activity led to valley incision in the surrounding highlands and ponding of water to form a lake in a local basin. The lake level eventually rose and breached the drainage divide, leading to significant outflow. The remaining water evaporated and eventually precipitated the chlorides at the lowest levels. Through digital terrain models, the lake and salt flat volumes were used to calculate the salinity of the lake, which at full lake extent was ∼8% of the salinity of Earth’s oceans. Crater density measurements on the terrain cross-cut by the outlet valley constrain the maximum age to 3.60 Ga. This hydrological episode occurred after regional widespread fluvial incision and also post-dates formation of the sulfate bedrock being investigated by Opportunity. We conclude that this and other similar chloride deposits represent some of the last vestiges of habitable surface water on Mars as recorded in the mineralogical record.

Deltas and valley networks on Mars: Implications for a global hydrosphere

Deltaic depositional systems are the most prominent evidence suggesting the existence of long-lasting standing bodies of water on Mars. Due to this genetic link, Martian deltas are fundamental for the reconstruction of the paleohydrological cycles and paleoclimates of Mars, testifying to the occurrence of past climatic conditions that created hydrological settings quite different from those of modern Mars. Ancient terrains of Mars are covered with additional signatures of past water. Branching channel systems on Mars, known as “valley networks,” have long been viewed as some of the best evidence that water flowed across the surface. Recent data, including topography and high-resolution imagery, have shed new light on valley network formation. It is now believed that most valley networks were formed by precipitation and surface runoff instead of groundwater processes. Two main planet-wide equipotential surfaces that encompass complete topographic enclosures within and along the margins of the northern lowlands. The inferred levels could represent two paleoshorelines of a past ocean covering the northern hemisphere of Mars during its early history. These boundaries are generally consistent with the “Arabia shoreline” previously suggested from geomorphologic and topographic observations and also with the global distribution and age of valley networks. Reconstructed scenario implies that climatic conditions allowed the occurrence of a hydrosphere presenting similarities with that of Earth, and integrating valleys, deltas, lakes, an ocean encircling the planet, and possibly a number of suitable niches for life to exist.

Variability of rock erodibility in bedrock‐floored stream channels based on abrasion mill experiments

We quantify variations in rock erodibility, Kr, within channel cross sections using laboratory abrasion mill experiments on bedrock surfaces extracted from streams with sandstone bedrock in Utah and basaltic bedrock in the Hawaiian Islands. Samples were taken from the thalweg and channel margins, the latter at a height that is inundated annually. For each sample, a sequence of abrasion mill experiments was completed to quantify variations in erosion rate with erosion depth. Erosion rate data from these experiments shows two things. First, the erosion rate from channel margin samples is greater than for thalweg samples, with the greatest difference observed for the rock surface that was exposed in the stream channel. Second, erosion rate decreases with depth beneath the original rock surface, by an order of magnitude in most cases. The erosion rate becomes steady at depths of 1–3 mm for channel margin samples and 0.1–0.4 mm for thalweg samples. Because only rock properties and microtopography vary throughout the sequence of mill experiments, these results suggest that Kr of the bedrock surface exposed in stream channels is higher at the margins than near the channel center and that Kr decreases over depths of ~1 mm. The simplest explanation for these patterns is that Kr is enhanced, at the bedrock surface and along the channel margins, due to the effects of weathering on rock strength and surface roughness. We hypothesize that a balance exists between weathering-enhanced erodibility and episodic incision to allow channel margins to lower at rates similar to the thalweg.

Laboratory simulations of acid-sulfate weathering under volcanic hydrothermal conditions: Implications for early Mars.

We have completed laboratory experiments and thermochemical equilibrium models to investigate secondary mineral formation under conditions akin to volcanic, hydrothermal acid-sulfate weathering systems. Our research used the basaltic mineralogy at Cerro Negro Volcano, Nicaragua, characterized by plagioclase, pyroxene, olivine, and volcanic glass. These individual minerals and whole-rock field samples were reacted in the laboratory with 1 molal sulfuric acid at varying temperatures (65, 150, and 200°C), fluid:rock weight ratios (1:1, 4:1, and 10:1), and durations (1–60 days). Thermochemical equilibrium models were developed using Geochemists Workbench. To understand the reaction products and fluids, we employed scanning electron microscopy/energy dispersive spectroscopy, X-ray diffraction, and inductively coupled plasma-atomic emission spectroscopy. The results of our experiments and models yielded major alteration minerals that include anhydrite, natroalunite, minor iron oxide, and amorphous Al-Si gel. We found that variations in experimental parameters did not drastically change the suite of minerals produced; instead, abundance, size, and crystallographic shape changed. Our results also suggest that it is essential to separate phases formed during experiments from those formed during fluid evaporation to fully understand the reaction processes. Our laboratory reacted and model predicted products are consistent with the mineralogy observed at places on Mars. However, our results indicate that determination of the formation conditions requires microscopic imagery and regional context, as well as a thorough understanding of contributions from both experiment precipitation and fluid evaporation minerals.

Planetary science: Bedrock formation at Meridiani Planum (Reply)

Squyres et al. contend that our proposed volcanic origin for Meridiani Planum is inconsistent with more recently obtained data. But although the new data reveal some variation in chemical composition, this variation is small (Fig. 1a) and mainly due to modest variations in magnesium and sulphur, with concentrations of the other elements remaining essentially constant. In a volcanic model, this variation can be readily explained by mobilization of highly soluble magnesium sulphate salts during the later stages of alteration and diagenesis (Fig. 1a), as in the sedimentary/evaporite model in which sediments that were initially deposited with uniform composition are subsequently modified. Although morphological features in the bedrock may be consistent with aeolian and fluvial origins, this interpretation is not unique, particularly as features with similar grain size, sorting and morphology are seen in base surge deposits. Neither chemical nor morphological data therefore preclude a volcanic origin.