Martian Lakes Forming Climates Were Caused by Methane Outbursts and Short Term

Duration and rapid shutdown of Mars lake-forming climates explained by methane bursts

Authors:

Kite et al

Abstract:

Build-up of relatively young (<∼3.6 Ga) deltas and alluvial fans on Mars required lakes to persist for >3 Kyr (assuming dilute flow), but the watersheds' little-weathered soils indicate a climate history that was >99% dry. The lake-forming climates' trigger mechanism remains unknown. Here we show that these intermittency constraints, while inconsistent with many previously-proposed triggers for lake-forming climates, are consistent with a novel CH4-burst mechanism. Chaotic transitions in mean obliquity drive latitudinal shifts in temperature and ice loading that destabilize CH4 clathrate. For past clathrate hydrate stability zone occupancy fractions >∼0.2, we show that CH4(±C2H6) builds up to levels whose radiative forcing (>15 W/m2, plus feedbacks) is sufficient to modulate lake-forming climates. Such occupancy fractions are consistent with CH4+C2H6 production by >3 Ga water-rock reactions. Sub-lake CH4 destabilization provides positive feedback. UV-limited CH4 photolysis curtails individual lake-forming climates to

Evidence of a Declining Martian Hydrological Cycle Starting 3.7 Billion Years Ago at the Noachian/Hesperian Boundary

Insights into surface runoff on early Mars from paleolake basin morphology and stratigraphy

Authors:

Goudge et al

Abstract:

We present observations on the morphology and stratigraphy of more than 400 paleolake basins on Mars. We show that there are two distinct classes of Martian paleolake basins: (1) paleolakes fed by regionally integrated valley networks (N = 251), and (2) paleolakes fed by isolated inlet valleys not integrated into broader regional drainage systems (N = 174). We conclude that valley network–fed paleolakes primarily formed prior to approximately the Noachian-Hesperian boundary, ca. 3.7 Ga, while isolated inlet valley paleolakes primarily formed later in Martian history. All 174 isolated inlet valley paleolakes are closed-basin lakes; however, there are surprisingly few (31) valley network–fed closed-basin lakes compared to a large number (220) of valley network–fed open-basin lakes. This observation is consistent with declining levels of fluvial activity over time on the Martian surface. Our results imply that during the era of valley network formation, ∼90% of topographic basins breached by an inlet valley had sufficiently high ratios of water influx to losses to fill, overtop, and form an outlet valley. This conclusion provides an important constraint on the balance between surface runoff production and water losses on early Mars that must be satisfied by any model of the early Martian climate and hydrologic cycle.

The fate of ethane in Titan’s hydrocarbon lakes and seas

The fate of ethane in Titan’s hydrocarbon lakes and seas

Authors:

Mousis et al

Abstract:

Ethane is expected to be the dominant photochemical product on Titan’s surface and, in the absence of a process that sequesters it from exposed surface reservoirs, a major constituent of its lakes and seas. Absorption of Cassini’s 2.2 cm radar by Ligeia Mare however suggests that this north polar sea is dominated by methane. In order to explain this apparent ethane deficiency, we explore the possibility that Ligeia Mare is the visible part of an alkanofer that interacted with an underlying clathrate layer and investigate the influence of this interaction on an assumed initial ethane–methane mixture in the liquid phase. We find that progressive liquid entrapment in clathrate allows the surface liquid reservoir to become methane-dominated for any initial ethane mole fraction below 0.75. If interactions between alkanofers and clathrates are common on Titan, this should lead to the emergence of many methane-dominated seas or lakes.

The physical properties of Titan’s empty lake basins

Constraining the physical properties of Titan’s empty lake basins using nadir and off-nadir Cassini RADAR backscatter

Authors:

Michaelides et al

Abstract:

We use repeat synthetic aperture radar (SAR) observations and complementary altimetry passes acquired by the Cassini spacecraft to study the scattering properties of Titan’s empty lake basins. The best-fit coefficients from fitting SAR data to a quasi-specular plus diffuse backscatter model suggest that the bright basin floors have a higher dielectric constant, but similar facet-scale rms surface facet slopes, to surrounding terrain. Waveform analysis of altimetry returns reveals that nadir backscatter returns from basin floors are greater than nadir backscatter returns from basin surroundings and have narrower pulse widths. This suggests that floor deposits are structurally distinct from their surroundings, consistent with the interpretation that some of these basins may be filled with evaporitic and/or sedimentary deposits. Basin floor deposits also express a larger diffuse component to their backscatter, which is likely due to variations in subsurface structure or an increase in roughness at the wavelength scale (Hayes, A.G. et al. [2008]. Geophys. Res. Lett. 35, 9). We generate a high-resolution altimetry radargram of the T30 altimetry pass over an empty lake basin, with which we place geometric constraints on the basin’s slopes, rim heights, and depth. Finally, the importance of these backscatter observations and geometric measurements for basin formation mechanisms is briefly discussed.

Eight-color maps of Titan’s surface from spectroscopy with Huygens’ DISR

Eight-color maps of Titan’s surface from spectroscopy with Huygens’ DISR

Authors:

Karkoschka et al

Abstract:

During the descent of the Huygens probe in Titan’s atmosphere, the Descent Imager/Spectral Radiometer (DISR) acquired spectra of 3660 locations within 250 km of the landing site. Each spectrum consisted of 200 resolution elements between 480 and 960 nm wavelength. With the help of radiative transfer models, contributions from the atmosphere and surface were separated. In eight methane windows, the data were combined into a map of Titan’s surface reflectivity with 250 km diameter near the landing site. Principal component analysis revealed three significant components, a brightness component that is consistent with a mosaic based on DISR imaging of much higher spatial resolution, a spectral slope component, and a spectral curvature component. The brightness component has stronger contrasts at longer wavelengths, or brighter areas have a larger spectral slope, consistent with previous results (Keller et al. [2008]. Planet. Space Sci. 56, 728–752). The second component corresponds to small differences in spectral slopes that are not correlated with features seen before except for an area with unusual high spectral slope found by the same authors and confirmed here. Our map of the second component gives another important parameter in characterizing and understanding Titan’s surface. The third principal component is somewhat noisy and describes variation in the spectral curvature that have never seen before at similar wavelengths. These variations require processes to differentiate surface spectra. To extend this work to longer wavelengths, 62 spectra from 850 to 1600 nm wavelength were investigated too, although the much lower number of spatial resolution points revealed only two significant components in the principal component analysis. They correlate with the first two components found in the shorter wavelength data. We also compare our results with an observation by Cassini’s Visible Imager/Mapping Spectrometer (VIMS) that imaged part of our investigated area with 4096 spatial resolution elements. Both data sets are complementary. DISR data extend to about 1500 nm wavelength while most surface features are seen in the VIMS data beyond 1500 nm.

Alluvial Fan Morphology, distribution and formation on Titan

Alluvial Fan Morphology, distribution and formation on Titan

Authors:

Birch et al

Abstract:

Titan is a hydrologically active world, with dozens of alluvial fans that are evidence of sediment transport from high to low elevations. However, the distribution and requirements for the formation of fans on Titan are not well understood. We performed the first global survey of alluvial fans on Titan using Cassini Synthetic Aperture Radar (SAR) data, which cover 61% of Titan’s surface. We identified 82 fans with areas ranging from 28 km2 to 27,000 km2. A significant fraction (∼60%) of the fans are restricted to latitudes of ±50–80°, suggesting that fluvial sediment transport may have been concentrated in the near-polar terrains in the geologically recent past. The density of fans is also found to be correlated with the latitudes predicted to have the highest precipitation rates by Titan Global Circulation Models. In equatorial regions, observable fans are not generally found in proximity to dune fields. Such observations suggest that sediment transport in these areas is dominated by aeolian transport mechanisms, though with some degree of recent equatorial fluvial activity. The fan area-drainage area relationship on Titan is more similar to that on Earth than on Mars, suggesting that the fans on Titan are smaller than what may be expected, and that the transport of bedload sediment is limited. We hypothesize that this has led to the development of a coarse gravel-lag deposit over much of Titan’s surface. Such a model explains both the morphology of the fans and their latitudinal concentration, yielding insight into the sediment transport regimes that operate across Titan today.

Structure of Titan’s evaporites

Structure of Titan’s evaporites

Authors:

Cordier et al

Abstract:

Numerous geological features that could be evaporitic in origin have been identified on the surface of Titan. Although they seem to be water–ice poor, their main properties – chemical composition, thickness, stratification – are essentially unknown. In this paper, which follows on a previous one focusing on the surface composition (Cordier, D., Barnes, J.W., Ferreira, A.G. [2013b]. Icarus 226(2),1431–1437), we provide some answers to these questions derived from a new model. This model, based on the up-to-date thermodynamic theory known as “PC-SAFT”, has been validated with available laboratory measurements and specifically developed for our purpose. 1-D models confirm the possibility of an acetylene and/or butane enriched central layer of evaporitic deposit. The estimated thickness of this acetylene–butane layer could explain the strong RADAR brightness of the evaporites. The 2-D computations indicate an accumulation of poorly soluble species at the deposit’s margin. Among these species, HCN or aerosols similar to tholins could play a dominant role. Our model predicts the existence of chemically trimodal “bathtub rings” which is consistent with what it is observed at the south polar lake Ontario Lacus. This work also provides plausible explanations to the lack of evaporites in the south polar region and to the high radar reflectivity of dry lakebeds.

Temporal variations of Titan’s surface with Cassini/VIMS

Temporal variations of Titan’s surface with Cassini/VIMS

Authors:

Solomonidou et al

Abstract:

We analyze Cassini VIMS data of several areas on Titan’s surface looking for variations with time. Three of these locations are near the equator (10–30°S), namely Hotei Regio, Tui Regio and Sotra Patera; in some cases changes in brightness and/or in appearance were reported therein. We also investigate a portion of the undifferentiated plains, areas relatively homogeneous and dark in radar observations, located near 20–25°N. This is a follow-up on a previous paper in which we had inferred surface albedos for some distinct regions of interest (RoIs) identified within the Hotei, Tui and Sotra areas through a Principal Component Analysis (PCA) and radiative transfer (RT) modeling (Solomonidou [2014]. J. Geophys. Res. 119, 1729–1747). We apply the same methods here to a larger dataset looking for variations of the surface albedo with time and using the Huygens landing site as the ‘ground truth’ for calibration purposes. As expected, the undifferentiated plains remain unchanged from January 2010 to June 2012. Our analysis of Hotei Regio data from March 2005 to March 2009 also does not show any significant surface albedo variations within uncertainties. We note however that our RT retrievals are not optimal in this case because of the use of a plane-parallel code and the unfavorable geometry of the associated datasets. Conversely, Tui Regio and Sotra Patera show surface albedo fluctuations with time with pronounced trends for darkening and for brightening respectively. The Tui Regio spectrum exhibits a surface albedo decrease from March 2005 to February 2009, at 0.94, 1.08, 2.03, and 5 μm wavelengths, while the spectrum shape remains the same over that time. On the contrary, the Sotra Patera area became at least two times brighter within a year (April 2005–February 2006), at 1.58 μm, 2.03 μm, and 5 μm. We also retrieved surface albedo spectra for three reference regions surrounding Hotei, Tui and Sotra and for three additional regions we use as ‘test cases’ that correspond to dune fields. During the time periods explored here we find that, as expected and contrary to Tui Regio and Sotra Patera, the test cases did not show any significant changes in surface albedo. We therefore suggest that temporal variations of surface albedo exist for some areas on Titan, but that their origin may differ from one region to the other. They could be due to diverse, past and/or ongoing formation processes (endogenic and/or exogenic, possibly cryovolcanic), as discussed here.

World's Lakes are Warming Rapidly With Climate Change

Climate change is rapidly warming lakes around the world, threatening freshwater supplies and ecosystems, according to a study spanning six continents.

The study is the largest of its kind and the first to use a combination of satellite temperature data and long-term ground measurements. A total of 235 lakes, representing more than half of the world's freshwater supply, were monitored for at least 25 years. The research, published in Geophysical Research Letters, was announced today at the American Geophysical Union meeting.

The study, which was funded by NASA and the National Science Foundation, found lakes are warming an average of 0.61 degrees Fahrenheit (0.34 degrees Celsius) each decade. That's greater than the warming rate of either the ocean or the atmosphere, and it can have profound effects, the scientists say.

link.

Greenhouse gas Emissions From Freshwater HIGHER Than Thought

Do not underestimate the babbling brook. When it comes to greenhouse gases, these bucolic water bodies have the potential to create a lot of hot air.

According to a new analysis in the journal Ecological Monographs, by researchers at the University of Wisconsin-Madison and colleagues, the world's rivers and streams pump about 10 times more methane into our atmosphere than scientists estimated in previous studies. The new study also found that human activity seems to drive which streams are the biggest contributors.

"Scientists know that inland waters, like lakes and reservoirs, are big sources of methane," says Emily Stanley, a professor at the UW-Madison Center for Limnology and lead author of the paper. Yet accurately measuring emissions of methane from these sources has remained a challenge.

Like carbon dioxide, methane is a greenhouse gas that traps heat at the Earth's surface. It is less prevalent than carbon dioxide in the atmosphere but also more potent: A molecule of methane results in more warming than a molecule of carbon dioxide. Understanding how much methane is emitted into the atmosphere from all sources helps scientists account for the full global greenhouse gas budget, and take measures to mitigate its impact.

Rivers and streams haven't received much attention in accounting for that budget, Stanley says, because they don't take up much surface area on a global scale and, with respect to methane, didn't seem to be all that gassy. But over the years, measurements taken by Stanley and her lab members seemed to indicate these sources may produce more methane than scientists had previously known.

link.

Seemingly Deep Water Seep Carbonate Mounds are Really From a Stenian MesoProterozoic Alkali Lake

Deep-water seep-related carbonate mounds in a Mesoproterozoic alkaline lake, Borden Basin (Nunavut, Canada)

Authors:

Hahn et al

Abstract:

The Mesoproterozoic (1.1 Ga) Borden Basin contains extremely large, deep-water dolostone seep mounds (Ikpiarjuk Formation) whose distribution is controlled by faults. Four mounds were investigated along measured stratigraphic sections. Petrographic study revealed several depositional components, and a mixture of at least two distinct carbonate sources. Stable isotope data showed no significant methane contribution to the carbonate phases. Detritus-corrected REE + Y patterns, obtained using solution ICP-MS, depict binary mixtures between basin-water-derived precipitates and seep-fluid-derived carbonate. The purest pelagic REE + Y signal is from mound tops, suggesting that mound accumulation ceased when the seep fluid waned. The REE + Y pattern of the pelagic precipitates resembles that of modern alkaline lake water. The shale-normalised pattern of the basin water is LREE-depleted, has a positive Ce anomaly and pronounced Y-excess, but lacks the La and Gd anomalies typical of seawater. The seep-fluid-related dolomite has flat shale-normalised REE + Y patterns, no Ce anomaly, and a negative Eu anomaly. This combination of characteristics points to circum-neutral (Ca and Mg-bearing?) fluids that interacted with the underlying basement before seeping into the lake bottom through faults. The chemostratigraphic patterns of the mounds result from the relative contribution of elements from the basin water vs. seep fluids. When combined with published geochemical data for coeval black shale surrounding the mounds, the new data suggest a lacustrine setting, surrounded by catchments with a preponderance of moderately to strongly weathered alkali basalt whose runoff drove the lake to alkalinity. Seep fluid was probably evaporatively concentrated basin water that acquired new geochemical characteristics both during evaporation and through water-rock interaction in the subsurface. The new understanding of this depositional stage of the Borden Basin highlights the importance of lacustrine deposits in the Mesoproterozoic, and presents an obvious impediment to using carbonate stable isotope or trace element geochemistry to reconstruct global atmosphere-hydrosphere conditions for this time for any units that cannot be demonstrated conclusively to be of marine origin.

Fresh Water Lakes Played an Important Role in the Proterozoic Great Oxygenation Events

Enhanced organic carbon burial in large Proterozoic lakes: Implications for atmospheric oxygenation

Authors:

Spinks et al

Abstract:

The burial of organic carbon in sedimentary systems has been a fundamental part of the carbon cycle throughout the geological record, and was instrumental in major oxygenations of the atmosphere in the early Palaeoproterozoic and Neoproterozoic. While much focus has been placed on the burial of carbon in Precambrian marine carbonate and organic carbon-rich rocks deposited around the time of these major oxygenations, such deposits yield little information on the evolution of the atmosphere in the significant time between. There is, however, growing evidence from terrestrially deposited sediments to suggest the surface environment may have been at least intermittently well-oxygenated from the late Mesoproterozoic. Hence Proterozoic sediments deposited in terrestrial near-surface environments are useful targets for the study of atmospheric evolution during a time which is hitherto poorly understood.

Thus far, little attention has been paid to the contribution of large lakes and intercontinental basins to the global burial of organic carbon, and thus the progressive oxygenation of the atmosphere, especially given that the highest rates of organic carbon burial in modern aquatic environments occur in lacustrine settings, in stark contrast to the low rates observed in the contemporary marine realm. Here, we report high burial rates of organic carbon in large lacustrine systems of late Mesoproterozoic to early Neoproterozoic age, which are comparable with modern lacustrine systems, and significantly higher than modern and ancient marine deposits. These data emphasise the significance of lakes as a global repository for organic carbon, and imply Proterozoic lakes were at least as efficient, and perhaps as important, as modern lakes in the global burial of organic carbon. Such findings suggest large Proterozoic lakes and epicontinental basins played a crucial role in the progressive oxygenation of the atmosphere before the major Neoproterozoic oxygenation.

Mars' Gusev Crater had an Ephemeral Lake During the Noachian Age

Evidence for a Noachian-aged ephemeral lake in Gusev crater, Mars

Authors:

Ruff et al

Abstract:

Gusev crater has long been considered the site of a lake early in Martian history, but the Mars Exploration Rover Spirit found no apparent evidence of lake deposits along its 7 km traverse from 2004 to 2010. Although outcrops rich in Mg-Fe carbonate, dubbed Comanche, were discovered in the Noachian-aged Columbia Hills, they were inferred to result from volcanic hydrothermal activity. We now find evidence that the alteration of the Comanche outcrops is consistent with evaporative precipitation of low-temperature, near-surface solutions derived from limited water-rock interaction with rocks equivalent to nearby outcrops called Algonquin. Additional observations show that the Algonquin outcrops are remnants of volcanic tephra that covered the Columbia Hills and adjacent plains well before emplacement of basalt flows onto the floor of Gusev crater. Water-limited leaching of formerly widespread Algonquin-like tephra deposits by ephemeral waters, followed by transport and evaporative precipitation of the fluids into the Comanche outcrops, can explain their chemical, mineralogical, and textural characteristics.