Fluvial morphology of Naktong Vallis,Mars: A late activity with multiple processes

The morphology of fluvial valleys on Mars provides insight into surface and subsurface hydrology, as well as to Mars’ past climate. In this study, Naktong Vallis and its tributaries were examined from high-resolution stereoscopic camera (HRSC) images, thermal emission imaging system (THEMIS) daytime IR images, and mars orbiter laser altimeter (MOLA) data. Naktong Vallis is the southern part of a very large fluvial basin composed by Mamers, Scamander, and Naktong Vallis with a total length of 4700 km, and is one of the largest fluvial system on Mars. Naktong Vallis incised along its path a series of smooth intercrater plains. Naktong's main valley cut smooth plains during the Early Hesperian period, estimated ～3.6–3.7 Gyr, implying a young age for the valley when compared to usual Noachian-aged valley networks. Branching valleys located in degraded terrains south of the main Naktong valley have sources inside a large plateau located at more than 2000 m elevation. Connections between these valleys and Naktong Vallis have been erased by the superimposition of late intercrater plains of Early to Late Hesperian age, but it is likely that this plateau represents the main source of water. Small re-incisions of these late plains show that there was at least one local reactivation. In addition, valley heads are often amphitheatre-shaped. Despite the possibility of subsurface flows, the occurrence of many branching valleys upstream of Naktong's main valley indicate that runoff may have played an important role in Naktong Vallis network formation. The importance of erosional landforms in the Naktong Vallis network indicates that fluvial activity was important and not necessarily lower in the Early Hesperian epoch than during the Noachian period. The relationships between overland flows and sapping features suggest a strong link between the two processes, rather than a progressive shift from surface to subsurface flow.     

Stratigraphy, mineralogy, and origin of layered deposits inside Terby crater, Mars

The 174 km diameter Terby impact crater (28.0°S-74.1°E) located on the northern rim of the Hellas basin displays anomalous inner morphology, including a flat floor and light-toned layered deposits. An analysis of these deposits was performed using multiple datasets from Mars Global Surveyor, Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter missions, with visible images for interpretation, near-infrared data for mineralogical mapping, and topography for geometry. The geometry of layered deposits was consistent with that of sediments that settled mainly in a sub-aqueous environment, during the Noachian period as determined by crater counts. To the north, the thickest sediments displayed sequences for fan deltas, as identified by 100 m to 1 km long clinoforms, as defined by horizontal beds passing to foreset beds dipping by 6-10° toward the center of the Terby crater. The identification of distinct sub-aqueous fan sequences, separated by unconformities and local wedges, showed the accumulation of sediments from prograding/onlapping depositional sequences, due to lake level and sediment supply variations. The mineralogy of several layers with hydrated minerals, including Fe/Mg phyllosilicates, supports this type of sedimentary environment. The volume of fan sediments was estimated as >5000 km³ (a large amount considering classical martian fan deltas such as Eberswalde (6 km³)) and requires sustained liquid water activity. Such a large sedimentary deposition in Terby crater is characteristic of the Noachian/Phyllosian period during which the environment favored the formation of phyllosilicates. The latter were detected by spectral data in the layered deposits of Terby crater in three distinct layer sequences. During the Hesperian period, the sediments experienced strong erosion, possibly enhanced by more acidic conditions, forming the current morphology with three mesas and closed depressions. Small fluvial valleys and alluvial fans formed subsequently, attesting to late fluvial processes dated as late Early to early Late Hesperian. After this late fluvial episode, the Terby impact crater was submitted to aeolian processes and permanent cold conditions with viscous flow features. Therefore, the Terby crater displays, in a single location, geologic features that characterize the three main periods of time on Mars, with the presence of one of the thickest sub-aqueous fan deposits reported on Mars. The filling of Terby impact crater is thus one potential “reference geologic cross-section” for Mars stratigraphy.     

Trapped Ar isotopes in meteorite ALH 84001 indicate Mars did not have a thick ancient atmosphere

Water is not currently stable in liquid form on the martian surface due to the present mean atmospheric pressure of ～7 mbar and mean global temperature of ～220 K. However, geomorphic features and hydrated mineral assemblages suggest that Mars’ climate was once warmer and liquid water flowed on the surface. These observations may indicate a substantially more massive atmosphere in the past, but there have been few observational constraints on paleoatmospheric pressures. Here we show how the ⁴⁰Ar/³⁶Ar ratios of trapped gases within martian meteorite ALH 84001 constrain paleoatmospheric pressure on Mars during the Noachian era [～4.56–3.8 billion years (Ga)]. Our model indicates that atmospheric pressures did not exceed ～1.5 bar during the first 400 million years (Ma) of the Noachian era, and were <400 mbar by 4.16 Ga. Such pressures of CO₂ are only sufficient to stabilize liquid water on Mars’ surface at low latitudes during seasonally warm periods. Other greenhouse gases like SO₂ and water vapor may have played an important role in intermittently stabilizing liquid water at higher latitudes following major volcanic eruptions or impact events.     

Periglacial mass-wasting landforms on Mars suggestive of transient liquid water in the recent past: Insights from solifluction lobes on Svalbard

On Earth, periglacial solifluction is a slow mass-wasting process related to freeze–thaw activity. We compare the morphology of small-scale lobate features on Mars to solifluction lobes in Svalbard to constrain their processes of formation. The analysis is based on high-resolution satellite imagery of Mars (HiRISE, ～25 cm/pxl), aerial images of Svalbard with a similar spatial resolution (HRSC–AX, ～20 cm/pxl) acquired through an air campaign in summer 2008, and ground truth obtained during two summer expeditions in 2009 and 2011 on Svalbard. We present a detailed study of two crater environments on Mars displaying two types of lobate forms, characterized as sorted (clast-banked) and non-sorted lobes. On both Svalbard and Mars such lobes typically occur as clusters of overlapping risers (lobe fronts), pointing to differential velocities in the soil. The martian small-scale lobes have well-defined arcuate risers and lobe treads (surface). Lobe widths range between 14 and 127 m and tread lengths between 13 and 105 m. Riser height is estimated to be approximately 1–5 m. The lobes on Mars share the plan view morphology of solifluction lobes on Svalbard and their morphometry is within the range of values of terrestrial solifluction lobes. The lobes are distinct from permafrost-creep landforms such as rock glaciers. We show the results of a survey of 53 HiRISE images covering latitudes between 59°N and 81°N. Similar to Svalbard, the studied lobate features on Mars occur in close spatial proximity to gullies and thermal contraction polygons. The widespread distribution of the lobate forms in the northern hemisphere and their close association to ground-ice and gullies are best explained by mass-wasting processes related to frost creep, gelifluction and/or plug-like flow. This suggests a protracted process (thousand to several thousands of years) of freeze–thaw activity at the northern high latitudes on Mars. Age constraints on lobe deposits and superposition relationships with gullies and polygons imply a process involving liquid water within the last few million years.     

Characterization of hydrated silicate-bearing outcrops in Tyrrhena Terra,Mars: Implications to the alteration history of Mars

The Tyrrhena Terra region of Mars is studied with the imaging spectrometers OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) onboard Mars Express and CRISM (Compact Reconnaissance Infrared Spectrometer for Mars) onboard Mars Reconnaissance Orbiter, through the observation of tens of craters that impacted into this part of the martian highlands. The 175 detections of hydrated silicates are reported, mainly associated with ejecta blankets, crater walls and rims, and central up-lifts. Sizes of craters where hydrated silicates are detected are highly variable, diameters range from less than 1 km to 42 km. We report the presence of zeolites and phyllosilicates like prehnite, Mg-chlorite, Mg-rich smectites and mixed-layer chlorites–smectites and chlorite–vermiculite from comparison of hyperspectral infrared observations with laboratory spectra. These minerals are associated with fresh craters post-dating any aqueous activity. They likely represent ancient hydrated terrains excavated by the crater-forming impacts, and hence reveal the composition of the altered Noachian crust, although crater-related hydrothermal activity may have played a minor role for the largest craters (>20 km in diameter). Most detected minerals formed over relatively high temperatures (100–300 °C), likely due to aqueous alteration of the Noachian crust by regional low grade metamorphism from the Noachian thermal gradient and/or by extended hydrothermal systems associated with Noachian volcanism and ancient large impact craters. This is in contrast with some other phyllosilicate-bearing regions like Mawrth Vallis where smectites, kaolinites and hydrated silica were mainly identified, pointing to a predominance of surface/shallow sub-surface alteration; and where excavation by impacts played only a minor role. Smooth plains containing hydrated silicates are observed at the boundary between the Noachian altered crust, dissected by fluvial valleys, and the Hesperian unaltered volcanic plains. These plains may correspond to alluvial deposition of eroded material. The highlands of Tyrrhena Terra are therefore particularly well suited for investigating the diversity of hydrated minerals in ancient martian terrains.     

The Circum-Hellas Volcanic Province,Mars: Overview

Building on previous studies of volcanoes around the Hellas basin with new studies of imaging (High-Resolution Stereo Camera (HRSC), Thermal Emission Imaging System (THEMIS), Mars Orbiter Camera (MOC), High-Resolution Imaging Science Experiment (HiRISE), Context Imager (CTX)), multispectral (HRSC, Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA)), topographic (Mars Orbiter Laser Altimeter (MOLA)) and gravity data, we define a new Martian volcanic province as the Circum-Hellas Volcanic Province (CHVP). With an area of >2.1 million km², it contains the six oldest central vent volcanoes on Mars, which formed after the Hellas impact basin, between 4.0 and 3.6 Ga. These volcanoes mark a transition from the flood volcanism that formed Malea Planum ～3.8 Ga, to localized edifice-building eruptions. The CHVP volcanoes have two general morphologies: (1) shield-like edifices (Tyrrhena, Hadriaca, and Amphitrites Paterae), and (2) caldera-like depressions surrounded by ridged plains (Peneus, Malea, and Pityusa Paterae). Positive gravity anomalies are found at Tyrrhena, Hadriaca, and Amphitrites, perhaps indicative of dense magma bodies below the surface. The lack of positive-relief edifices and weak gravity anomalies at Peneus, Malea, and Pityusa suggest a fundamental difference in their formation, styles of eruption, and/or compositions. The northernmost volcanoes, the ～3.7–3.9 Ga Tyrrhena and Hadriaca Paterae, have low slopes, well-channeled flanks, and smooth caldera floors (at tens of meters/pixel scale), indicative of volcanoes formed from poorly consolidated pyroclastic deposits that have been modified by fluvial and aeolian erosion and deposition. The ～3.6 Ga Amphitrites Patera also has a well-channeled flank, but it and the ～3.8 Ga Peneus Patera are dominated by scalloped and pitted terrain, pedestal and ejecta flow craters, and a general ‘softened’ appearance. This morphology is indicative not only of surface materials subjected to periglacial processes involving water ice, but also of a surface composed of easily eroded materials such as ash and dust. The southernmost volcanoes, the ～3.8 Ga Malea and Pityusa Paterae, have no channeled flanks, no scalloped and pitted terrain, and lack the ‘softened’ appearance of their surfaces, but they do contain pedestal and ejecta flow craters and large, smooth, bright plateaus in their central depressions. This morphology is indicative of a surface with not only a high water ice content, but also a more consolidated material that is less susceptible to degradation (relative to the other four volcanoes). We suggest that Malea and Pityusa (and possibly Peneus) Paterae are Martian equivalents to Earth's giant calderas (e.g., Yellowstone, Long Valley) that erupted large volumes of volcanic materials, and that Malea and Pityusa are probably composed of either lava flows or ignimbrites. HRSC and OMEGA spectral data indicate that dark gray to slightly red materials (often represented as blue or black pixels in HRSC color images), found in the patera floors and topographic lows throughout the CHVP, have a basaltic composition. A key issue is whether this dark material represents concentrations of underlying basaltic material eroded by various processes and exposed by aeolian winnowing, or if the material was transported from elsewhere on Mars by regional winds. Understanding the provenance of these dark materials may be the key to understanding the volcanic diversity of the Circum-Hellas Volcanic Province.     

Is the primordial crust of Mars magnetized?

A meteorite impact capable of creating a 200 km diameter crater can demagnetize the entire crust beneath, and produce an appreciable magnetic anomaly at satellite altitudes of ～400 km in case the pre-existing crust is magnetized. In this study we examine the magnetic field over all of the craters and impact-related Quasi-Circular Depressions (QCDs) with diameters larger than 200 km that are located on the highlands of Mars, excluding the Tharsis bulge, in order to estimate the mean magnetization of the highland crust. Using the surface topography and the gravity of Mars we first identify those QCDs that are likely produced by impacts. The magnetic map of a given crater or impact-related QCD is derived using the Mars Global Surveyor high-altitude nighttime radial magnetic data. Two extended ancient areas are identified on the highlands, the South Province and the Tempe Terra, which have large number of craters and impact-related QCDs but none of them has an appreciable magnetic signature. The primordial crust of these areas is not magnetized, or is very weakly magnetized at most. We examine some plausible scenarios to explain the weak magnetization of these areas, and conclude that no strong dynamo existed in the first ～100 Myr of Mars’ history when the newly formed primordial crust was cooling below the magnetic blocking temperatures of its minerals.     

Volatile transport on inhomogeneous surfaces: I – Analytic expressions,with application to Pluto’s day

The two orders of magnitude drop between the measured atmospheric abundances of non-radiogenic argon, krypton and xenon in Earth versus Mars is striking. Here, in order to account for this difference, we explore the hypothesis that clathrate deposits incorporated into the current martian cryosphere have sequestered significant amounts of these noble gases assuming they were initially present in the paleoatmosphere in quantities similar to those measured on Earth (in mass of noble gas per unit mass of the planet). To do so, we use a statistical-thermodynamic model that predicts the clathrate composition formed from a carbon dioxide-dominated paleoatmosphere whose surface pressure ranges up to 3 bars. The influence of the presence of atmospheric sulfur dioxide on clathrate composition is investigated and we find that it does not alter the trapping efficiencies of other minor species. Assuming nominal structural parameters for the clathrate cages, we find that a carbon dioxide equivalent pressure of 0.03 and 0.9 bar is sufficient to trap masses of xenon and krypton, respectively, equivalent to those found on Earth in the clathrate deposits of the cryosphere. In this case, the amount of trapped argon is not sufficient to explain the measured Earth/Mars argon abundance ratio in the considered pressure range. In contrast, with a 2% contraction of the clathrate cages, masses of xenon, krypton and argon at least equivalent to those found on Earth can be incorporated into clathrates if one assumes the trapping of carbon dioxide at equivalent atmospheric pressures of ～2.3 bar. The proposed clathrate trapping mechanism could have then played an important role in the shaping of the current martian atmosphere.     

Valleys,paleolakes and possible shorelines at the Libya Montes/Isidis boundary: Implications for the hydrologic evolution of Mars

We describe the results of our morphologic, stratigraphic and mineralogic investigations of fluvial landforms, paleolakes and possible shoreline morphologies at the Libya Montes/Isidis Planitia boundary. The landforms are indicative of aqueous activity and standing bodies of water, including lakes, seas and oceans, that are attributed to a complex hydrologic cycle that may have once existed on Mars in the Noachian (>3.7 Ga) and perhaps also in the Hesperian (>3.1 Ga). Our observations of the Libya Montes/Isidis Planitia boundary between 85°/86.5°E and 1.8°/5°N suggest, that (1) the termination of valley networks between roughly ?2500 and ?2800 m coincide with lake-size ponding in basins within the Libya Montes, (2) an alluvial fan and a possible delta, layered morphologies and associated Al-phyllosilicates identified within bright, polygonally fractured material at the front of the delta deposits are interpreted to be the results of fluvial activity and discharge into a paleolake, (3) the Arabia “shoreline” appears as a series of possible coastal cliffs at about ?3600 and ?3700 m indicating two distinct still stands and wave-cut action of a paleosea that temporarily filled the Isidis basin the Early Hesperian, and (4) the Deuteronilus “shoreline” appears at ?3800 m and is interpreted to be a result of the proposed sublimation residue of a frozen sea that might have filled the Isidis basin, similar to the Vastitas Borealis Formation (VBF) identified in the northern lowlands. We interpret the morphologic–geologic setting and associated mineral assemblages of the Libya Montes/Isidis Planitia boundary as results of fluvial activity, lake-size standing bodies of water and an environmental change over time toward decreasing water availability and a cold and dry climate.     

Major episodes of geologic history of Isidis Planitia on Mars

We have mapped the area of Isidis Planitia (1–27°N, 75–103°E) in order to assess the geologic history of this region using modern data sets such as MOLA topography and the high-resolution images provided by the HRSC, CTX, and HiRISE cameras. Results of our mapping show that the geologic history of Isidis Planitia consists of three principal episodes. (1) Impact dominated episode (Noachian, until ～3.8 Ga): During this time, the oldest materials in the study area were formed mostly by impact reworking and mass-wasting. Other processes (e.g., volcanism and fluvial/glacial activity) likely operated at this time but played a subordinate role. (2) An episode related to volcanic and fluvial/glacial activities (late Noachian–early Amazonian, ～3.8–2.8 Ga): Volcanism appears as the most important process at the beginning of this episode (～3.8–3.5 Ga) and was responsible for the formation of a large circum-Isidis volcanic province by the early Hesperian epoch. Volcanic materials covered large portions of the Isidis rim, almost completely buried the previous crater record on the floor of the Isidis basin, and probably were the major contributors to the filling of the basin. Fluvial/glacial processes prevailed closer to the end of the episode (early Hesperian–early Amazonian, ～3.5–2.8 Ga) and were responsible for widespread resurfacing in the Isidis Planitia region, mostly at ～3.1–3.4 Ga. Glaciers and/or ice sheets probably resulted in a massive glaciation of the rim and the floor of the Isidis basin. The total volume of material eroded from the Isidis rim by glacial and fluvial activity is estimated to be about 35,000–50,000 km³, which is equivalent to a composite layer about 40–60 m thick on entire floor of the basin. More important, however, is that the eroded materials were likely saturated with ice/water and could form wet deposits on the floor. (3) Wind-dominated episode (since early Amazonian, ～2.8 Ga): Wind activity dominated the later geologic history of Isidis Planitia but resulted only in minor modification of the surface.     

Search for methane and upper limits to ethane and SO2 on Mars

The IRTF/CSHELL observations in February 2006 at L_S = 10° and 63–93°W show ～10 ppb of methane at 45°S to 7°N and ～3 ppb outside this region that covers the deepest canyon Valles Marineris. Observations in December 2009 at L_S = 20° and 0–30°W included spectra of the Moon at a similar airmass as a telluric calibrator. A technique for extraction of the martian methane line from a combination of the Mars and Moon spectra has been developed. The observations reveal no methane with an upper limit of 8 ppb. The results of both sessions agree with the observations by Mumma et al. (Mumma, M.J. et al. [2009]. Science 323, 1041–1045) at the same season in February 2006 and are smaller than those in the PFS and TES maps. Production and removal of the biological methane on Mars do not significantly change the redox state of the atmosphere and the balance of hydrogen. A search for ethane at 2977 cm^?1 results in an upper limit of 0.2 ppb. However, this limit does not help to establish the origin of methane on Mars. Reanalysis of our search for SO₂ using TEXES confirms the recently established upper limit of 0.3 ppb. Along with the lack of hot spots and gas vents with endogenic heat sources in the THEMIS observations, the very low upper limit to SO₂ on Mars does not favor geological methane that is less abundant than SO₂ in the outgassing from the terrestrial planets.     

Experimental simulation of martian gully forms

Gullies are widespread on slopes on the surface of Mars and have been investigated by numerous authors, yet their formation processes remain elusive. In an attempt to understand the possibility of a water-based origin for these forms, we undertook a series of flume experiments at Earth surface temperatures and pressures. Our objectives were to produce forms that resemble those most commonly observed on Mars, documenting their morphometric characteristics and identifying any statistically significant relationships between form and controlling factors of slope and flow rate. Experiments were conducted in a 1×1.5 m² flume filled with medium grain size sand. The experiments were run over a slope angle range of 10–30°, corresponding to the range for gullies on Mars. Water from a constant-head tank fed through 5 mm silicone hose to a rotameter and then released just below the surface at the top of the slope. Gullies were produced at slope angle values of 10°, 20°, and 30° and flow rate values of 445, 705, 965, and 1260 mL min^?1 at each angle. Eighteen parameters were identified and subsequently measured on each gully produced in the flume. Gully forms were successfully reproduced and displayed development of the fundamental morphological components observed on Mars: alcove, channel, and apron. Slope–gully form relationships for each component revealed the following results: higher slope angles formed shorter gullies with thicker apron deposits. Moreover, longer gullies were seen at higher flow rates. We concluded that forms visually similar to those observed on Mars can be created by water in the laboratory flume under terrestrial conditions. Morphometric parameters can be measured and permit identification of controlling factors. Experimental simulation of gullies appears possible with proper scaling of experimental parameters. Although not directly scalable to Mars, flume gully parameters may be used to develop numerical models in the future.     

Searching for evidence of hydrothermal activity at Apollinaris Mons,Mars

A multidisciplinary approach involving various remote sensing instruments is used to investigate Apollinaris Mons, a prominent volcano on Mars, as well as the surrounding plains for signs of prolonged hydrologic and volcanic, and possibly hydrothermal activity. The main findings include (1) evidence from laser altimetry indicating the large thickness (1.5–2 km at some locations) of the fan deposits draping the southern flank contrary to previous estimates, coupled with possible layering which point to a significant emplacement phase at Apollinaris Mons, (2) corroboration of Robinson et al. (Robinson, M.S., Mouginis-Mark, P.J., Zimbelman, J.R., Wu, S.S.C., Ablin, K.K., Howington-Kraus, A.E. [1993]. Icarus 104, 301–323) hypothesis regarding the formation of incised valleys on the western flanks by density current erosion which would indicate magma–water interaction or, alternatively, volatile-rich magmas early in the volcano’s history, (3) mounds of diverse geometric shapes, many of which display summit depressions and occur among faults and fractures, possibly marking venting, (4) strong indicators on the flanks of the volcano for lahar events, and possibly, a caldera lake, (5) ubiquitous presence of impact craters displaying fluidized ejecta in both shield-forming (flank and caldera) materials and materials that surround the volcano that are indicative of water-rich target materials at the time of impact, (6) long-term complex association in time among shield-forming materials and Medusae Fossae Formation.The findings point to a site of extensive volcanic and hydrologic activity with possibly a period of magma–water interaction and hydrothermal activity. Finally, we propose that the mound structures around Apollinaris should be prime targets for further in situ exploration and search for possible exobiological signatures.     

Regional differences in gully occurrence on Mars: A comparison between the Hale and Bond craters

The observation of gullies on Mars raised questions about the presence of liquid water in the recent past. In some regions like Hale and Bond crater, gullies occur in one crater (Hale) but do not in another crater nearby (Bond). These regional differences have been interpreted as an argument for a formation of the gullies related to groundwater. The formation of gullies on Earth depends on rainfall and/or melting of snow as well as on several parameters such as the presence of steep slopes and sufficient amounts of fines and debris. We investigated the Hale/Bond region for differences in crater wall morphology and texture, slopes, and thermal properties to determine whether the gully formation is dependent on factors such as steep slope angles and availability of fine-grained material. Morphologically there exist two kinds of gullies in the Hale crater: Gullies on the south- and east-facing crater slopes have a pristine appearance with deep channels eroded into the talus material and well-preserved aprons. Gully-like features on the north- and west-facing slopes are degraded and superposed by craters, indicating that they are old in comparison to the pristine ones. However, their formation process is unclear and might be due to debris flows, surface runoff or dry mass wasting processes or a combination of these processes. The crater walls of Bond do not show gullies. Their morphology is most likely consistent with a degraded mantle deposit. Slope measurements reveal that the gullies in Hale crater occur on slopes between ～20° and ～30° in contrast to the slopes without gullies in Bond that are between ～10° and ～20° steep. Mean thermal inertia values on slopes with younger gullies are ～175 J m^?2 K^?1 s^?1/2 corresponding to higher amounts of fine-grained material. At slopes with older gully-like features mean thermal inertia values are ～315 J m^?2 K^?1 s^?1/2 corresponding to higher amounts of bedrock or possibly indurated grain sizes. Mean thermal inertia values of the Bond crater walls are ～230 J m^?2 K^?1 s^?1/2 indicating more consolidated terrain possibly due to the cementation of the dissected mantle material. From our investigation we conclude that the occurrence of gullies in the Hale/Bond region most likely depends on the distribution of unconsolidated material and steep slopes. The regional and local gully distribution on Mars likely varies due to differences in topography and surface material properties. Their proposed clustered distribution on Mars is not an argument for a groundwater formation mechanism of the gullies.     

Methane emissions from Earth’s degassing: Implications for Mars

The presence of methane on Mars is of great interest, since one possibility for its origin is that it derives from living microbes. However, CH₄ in the martian atmosphere also could be attributable to geologic emissions released through pathways similar to those occurring on Earth. Using recent data on methane degassing of the Earth, we have estimated the relative terrestrial contributions of fossil geologic methane vs. modern methane from living methanogens, and have examined the significance that various geologic sources might have for Mars.Geologic degassing includes microbial methane (produced by ancient methanogens), thermogenic methane (from maturation of sedimentary organic matter), and subordinately geothermal and volcanic methane (mainly produced abiogenically). Our analysis suggests that ～80% of the “natural” emission to the terrestrial atmosphere originates from modern microbial activity and ～20% originates from geologic degassing, for a total CH₄ emission of ～28.0×10⁷ tonnes year^?1.Estimates of methane emission on Mars range from 12.6×10¹ to 57.0×10⁴ tonnes year^?1 and are 3–6 orders of magnitude lower than that estimated for Earth. Nevertheless, the recently detected martian, Northern-Summer-2003 CH₄ plume could be compared with methane expulsion from large mud volcanoes or from the integrated emission of a few hundred gas seeps, such as many of those located in Europe, USA, Mid-East or Asia. Methane could also be released by diffuse microseepage from martian soil, even if macro-seeps or mud volcanoes were lacking or inactive. We calculated that a weak microseepage spread over a few tens of km², as frequently occurs on Earth, may be sufficient to generate the lower estimate of methane emission in the martian atmosphere.At least 65% of Earth’s degassing is provided by kerogen thermogenesis. A similar process may exist on Mars, where kerogen might include abiogenic organics (delivered by meteorites and comets) and remnants of possible, past martian life. The remainder of terrestrial degassed methane is attributed to fossil microbial gas (～25%) and geothermal-volcanic emissions (～10%). Global abiogenic emissions from serpentinization are negligible on Earth, but, on Mars, individual seeps from serpentinization could be significant. Gas discharge from clathrate-permafrost destabilization should also be considered.Finally, we have shown examples of potential degassing pathways on Mars, including mud volcano-like structures, fault and fracture systems, and major volcanic edifices. All these types of structures could provide avenues for extensive gas expulsion, as on Earth. Future investigations of martian methane should be focused on such potential pathways.     

Rheologies and ages of lava flows on Elysium Mons,Mars

We present results of our study of the rheologies and ages of lava flows in the Elysium Mons region of Mars. Previous studies have shown that the geometric dimensions of lava flows reflect rheological properties such as yield strength, effusion rate and viscosity. In this study the rheological properties of lava flows in the Elysium Mons region were determined and compared to the rheologies of the Ascraeus Mons lava flows. We also derived new crater size-frequency distribution measurements (CSFDs) for the Elysium lava flows to identify possible changes in the rheological properties with time. In addition, possible changes in the rheological properties with the distance from the caldera of Elysium Mons were analyzed.In total, 35 lava flows on and around Elysium Mons were mapped, and divided into three groups, lava flows on the flanks of Elysium Mons, in the plains between the three volcanoes Elysium Mons, Hecates and Albor Tholus and lava flows south of Albor Tholus. The rheological properties of 32 of these flows could be determined. Based on our morphometric measurements of each individual lava flow, estimates for the yield strengths, effusion rates, viscosities, and eruption duration of the studied lava flows were made. The yield strengths of the investigated lava flows range from ～3.8 × 10² Pa to ～1.5 × 10⁴ Pa, with an average of ～3.0 × 10³ Pa. These yield strengths are in good agreement with estimates for terrestrial basaltic lava flows. The effusion rates are on average ～747 m³ s^?1, ranging from ～99 to 4450 m³ s^?1. The viscosities are on average ～4.1 × 10⁶ Pa s, with a range of 1.2 × 10⁵ Pa s to 3.1 × 10⁷ Pa s. The eruption durations of the flows were calculated to be between 6 and 183 days, with an average of ～51 days. The determined rheological properties are generally very similar to those of other volcanic regions on Mars, such as on Ascraeus Mons in the Tharsis region. Calculated yield strengths and viscosities point to a basaltic/andesitic composition of the lava flows, similar to basaltic or andesitic a’a lava flows on Earth.Absolute model ages of all 35 lava flows on Elysium Mons were derived from crater size-frequency distribution measurements (CSFD). The derived model ages show a wide variation from about 632 Ma to 3460 Ma. Crater size-frequency distribution measurements of the Elysium Mons caldera show an age of ～1640 Ma, which is consistent with the resurfacing age of Werner (2009). Significant changes of the rheologies with time could not be observed. Similarly, we did not observe systematic changes in ages with increasing distances of lava flows from the Elysium Mons caldera.     

Formation of a terraced fan deposit in Coprates Catena, Mars

We have studied a terraced fan deposit with unique characteristics located within a trough of Coprates Catena. The fan has an average length of 6.8 km, and is approximately 44 km² in area and 18 km³ in volume. The fan's broad contributing valley is approximately 35 km long and it noticeably increases in depth about 12.8 km before it intersects the trough, where a rounded knickpoint marks the transition between flat-floored upstream and V-shaped downstream cross-sections. A 14-km-long channel with no apparent source enters the contributing valley from the south. A much smaller sinuous channel has incised along a smaller V-shaped valley in the uppermost eastern portion of the fan deposit. We explored several possible origins for the terraced fan, including mass wasting, volcanic flow, alluvial fan, and delta. We propose that water sourced from volcanic melting of ice eroded and transported material along the contributing valley. This material was then deposited as a delta in a lake within the trough. The concentric terraces are most likely the result of shoreline or ice cover erosion during drops in lake level. A light-toned layered deposit to the east of the fan deposit along the floor of the trough may represent a sedimentary unit formed during the terminal stages of the lake. Although other terraced fans have been identified on Mars, the Coprates Catena fan is unique because it has many more terraces and its surface was incised by a channel and associated valley. The identification of several other valleys to the east suggests that volcanic melting of volatiles during the Hesperian Period created favorable conditions for water flow along the plains in this region.     

Power law distribution of pressure drops in dust devils: Observation techniques and Earth–Mars comparison

Data from the Pathfinder and Phoenix landers on Mars show transient pressure drops (～1–4 per day) attributed to nearby encounters with dust devils or dust-free vortices. The distribution of pressure drop amplitudes is consistent with a truncated power law distribution with a slope of ?2, similar to that suggested previously for the optical diameters of dust devils. Comparable data from terrestrial field observations are very sparse: the only published dataset is half a century old and lists only 19 pressure drops. That dataset is too small to permit a robust comparison with Mars and likely suffers from a low detection efficiency at small dust devil sizes. Observed pressure drops in these fixed-station Mars datasets (30–300 μbar) are 10× lower than those typically observed on Earth (0.3–3 mbar): some higher drops have been reported for large terrestrial devils sampled by pursuing vehicles. The needed terrestrial data for comparison with Mars in-situ data (soon to be augmented, we hope, by the Mars Science Laboratory mission) is noted. Prospects for obtaining such data via field campaigns using new data acquisition technology, and with microbarographs for nuclear test monitoring, are discussed.     

A multi-wavelength study of the 2009 impact on Jupiter: Comparison of high resolution images from Gemini,Keck and HST

Within several days of A. Wesley’s announcement that Jupiter was hit by an object on UT 19 July 2009, we observed the impact site with (1) the Hubble Space Telescope (HST) at UV through visible (225–924 nm) wavelengths, (2) the 10-m W.M. Keck II telescope in the near-infrared (1–5 μm), and (3) the 8-m Gemini-North telescope in the mid-infrared (7.7–18 μm). All observations reported here were obtained between 22 and 25 July 2009. Observations at visible and near-infrared wavelengths show that large (～0.75-μm radius) dark (imaginary index of refraction m_i ～ 0.01–0.1) particulates were deposited at atmospheric pressures between 10 and 200–300 mbar; analysis of HST-UV data reveals that in addition smaller-sized (～0.1 μm radius) material must have been deposited at the highest altitudes (～10 mbar). Differences in morphology between the UV and visible/near-IR images suggest three-dimensional variations in particle size and density across the impact site, which probably were induced during the explosion and associated events. At mid-infrared wavelengths the brightness temperature increased due to both an enhancement in the stratospheric NH₃ gas abundance and the physical temperature of the atmosphere. This high brightness temperature coincides with the center part of the impact site as seen with HST. This observation, combined with (published) numerical simulations of the Shoemaker-Levy 9 impacts on Jupiter and the Tunguska airburst on Earth, suggests that the downward jet from the terminal explosion probably penetrated down to the ～700-mbar level.     

Titan’s global crater population: A new assessment

We report a revised crater population for Titan using Cassini RADAR data through January 2010 (flyby T65), and make a size-dependent correction for the incomplete coverage (～33%) using a Monte-Carlo model. Qualitatively, Titan’s landscape is more heavily cratered than Earth, but much less than Mars or Ganymede: the area fraction covered by craters is in fact comparable with that of Venus. Quantitative efforts to interpret crater densities for Titan as surface age have been confounded by widely divergent crater production rates proposed in the literature. We elucidate the specific model assumptions that lead to these differences (assumed projectile density, scaling function for simple crater diameter, and complex crater size exponent) and suggest these are reasonable bounding models, with the Korycansky and Zahnle (2005) model representing a crater retention age of ～1 Ga, and the Artemieva and Lunine (2005) model representing a crater retention age of ～200 Ma. These estimates are consistent with models of Titan’s evolution that predict a thickening of its crust 0.3–1.2 Gyr ago.