Impacts on early Mars can produce H2 and CH4 in the thermal plume. In a thick CO2 atmosphere, collision‐induced absorptions between CO2‐H2 and CO2‐CH4 can boost the greenhouse effect. We construct a ...simple model of the impact history of Mars and show that for a variety of impactor types and CO2 surface pressures >0.5 bars, postimpact surface temperatures due to H2 alone can exceed the melting point of water for much longer periods of time than from the dissipation of the heat derived from the impactor's kinetic energy. This longer timescale is set by hydrogen escape rather than radiation to space. Cumulatively, the Noachian surface may have been above the melting point of water for millions of years by this mechanism. These greatly extended postimpact warm environments may have played a larger role in the erosion and mineralogy of the surface than previously thought and may partly explain some of the observed fluvial features.
Plain Language Summary
We propose that meteorites could warm ancient Mars by degassing H2 into a thick CO2 atmosphere after they impact the surface. The impact creates a hot thermal plume where water oxidizes the meteorite's reduced materials producing H2 gas in the process. Collisions between CO2 and H2 molecules enhances the greenhouse effect and warms the surface. If the impactor is large enough in diameter (>100 km), surface temperatures can rise above freezing for many thousands of years—a time scale much longer than previously envisioned. This mechanism must have operated to some degree and could partly explain the observed erosion, fluvial features, and mineralogy of ancient Martian surfaces.
Key Points
Reductants in meteorites oxidized by water and CO2 can produce significant quantities of CO and H2 in the thermal plume following an impact
Degassing of H2 following a large impact could warm Mars above the melting point for tens to hundreds of thousands of years
During the Noachian epoch the total time for above‐melting surface temperatures could have been millions of years
Humanity has long been fascinated by the planet Mars. Was its climate ever conducive to life? What is the atmosphere like today and why did it change so dramatically over time? Eleven spacecraft have ...successfully flown to Mars since the Viking mission of the 1970s and early 1980s. These orbiters, landers and rovers have generated vast amounts of data that now span a Martian decade (roughly eighteen years). This new volume brings together the many new ideas about the atmosphere and climate system that have emerged, including the complex interplay of the volatile and dust cycles, the atmosphere-surface interactions that connect them over time, and the diversity of the planet's environment and its complex history. Including tutorials and explanations of complicated ideas, students, researchers and non-specialists alike are able to use this resource to gain a thorough and up-to-date understanding of this most Earth-like of planetary neighbours.
Observations by the Mars Color Imager (MARCI) onboard the Mars Reconnaissance Orbiter (MRO) in the ultraviolet (UV, Band 7; 320 nm) are used to characterize the spatial and temporal behavior of ...atmospheric water ice over a period of 6 Mars Years. Exploiting the contrast of the bright ice clouds to the low albedo surface, a radiative transfer-based retrieval algorithm is developed to derive the column-integrated optical depth of the ice (τice). Several relatively unique input products are created as part of the retrieval development process, including a zonal dust climatology based on emission phase function (EPFs) sequences from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), a spatially variable UV-reflectance model for Band 7 (as well as for Band 6, 260 nm), and a water ice scattering phase function based on a droxtal ice habit. Taking into account a radiometric precision of 7%, an error analysis estimates the uncertainty in τice to be ∼0.03 (excluding particle size effects, which are discussed separately). Zonal trends are analyzed over the full temporal extent of the observations, looking at both diurnal and interannual variability. The main (zonal) features are the aphelion cloud belt (ACB) and the polar hoods. For the ACB, there can be an appreciable diurnal change in τice between the periods of 14h30–15h00 and 15h00–15h30 Local True Solar Time (LTST). The amplitude of this effect shows relatively large interannual variability, associated mainly with changes in the earlier time block. When averaged over the interval 14h00–16h00 LTST, the interannual differences in the ACB structure are appreciably smaller. When the MARCI τice are compared to those from the Thermal Emission Spectrometer (TES), there is a good correlation of features, with the most significant difference being the seasonal (LS) evolution of the ACB. For TES, the ACB zonal profile is relative symmetric about LS = 90°. In the MARCI data, this profile is noticeably asymmetric, with the centroid shifted to later in the northern summer season (LS = 120°). The MARCI behavior is consistent with that observed by several other instruments. The correspondence of MARCI τice zonal and meridional behaviors with that predicted by two Global Circulation Models (GCM) is good. Each model captures the general behavior seen by MARCI in the ACB, the polar hoods, and the major orographic/topographic cloud features (including Valles Mariners). However, the mismatches between GCM results and MARCI reinforce the challenging nature of water ice clouds for dynamical models. The released τice are being archived at Malin Space Science Systems at https://www.msss.com/mro_marci_iceclouds/.
•The zonal behavior is dominated by aphelion cloud belt (ACB) and polar hood structures.•The MARCI ACB can demonstrate appreciable diurnal change and annual variability.•MARCI ACB observations show differences with those of Thermal Emission Spectrometer.•Dynamical models can capture the general behavior seen by the MARCI ACB and polar hoods.
Abstract
The dust cycle is the dominant driver of meteorology and climate on present-day Mars. Despite this, few studies have investigated the impact of dust interacting with incoming stellar ...radiation on the climate, habitability, and potential spectral signature of Mars-like exo-land planets. Dust availability is positively correlated with increasing soil aridity and therefore dust has significant potential to modify dynamics on dry land planets. In this work, we use an advanced Mars general circulation model to study the coupling between radiatively active dust and land planet climate at different stellar heating rates or planetary orbits. We find that radiatively active dust can significantly modify land planet climate. At Earth orbit, dust with optical properties similar to present-day Mars warms the planetary surface above 273 K and augments both the zonal mean circulation and the thermal tide, and in particular the semidiurnal component. As dust accumulates, peak heating rises off the planetary surface and the most active regions of dust lifting shift from the summer to winter hemisphere. Simulated spectra are nearly featureless across all wavelengths. We find that in order to accurately assess the climate and habitability of land planets it is critical to carefully consider that potential atmospheric dust budget and its radiative impact.
The Context Camera (CTX) on the Mars Reconnaissance Orbiter (MRO) is a Facility Instrument (i.e., government‐furnished equipment operated by a science team not responsible for design and fabrication) ...designed, built, and operated by Malin Space Science Systems and the MRO Mars Color Imager team (MARCI). CTX will (1) provide context images for data acquired by other MRO instruments, (2) observe features of interest to NASA's Mars Exploration Program (e.g., candidate landing sites), and (3) conduct a scientific investigation, led by the MARCI team, of geologic, geomorphic, and meteorological processes on Mars. CTX consists of a digital electronics assembly; a 350 mm f/3.25 Schmidt‐type telescope of catadioptric optical design with a 5.7° field of view, providing a ∼30‐km‐wide swath from ∼290 km altitude; and a 5000‐element CCD with a band pass of 500–700 nm and 7 μm pixels, giving ∼6 m/pixel spatial resolution from MRO's nearly circular, nearly polar mapping orbit. Raw data are transferred to the MRO spacecraft flight computer for processing (e.g., data compression) before transmission to Earth. The ground data system and operations are based on 9 years of Mars Global Surveyor Mars Orbiter Camera on‐orbit experience. CTX has been allocated 12% of the total MRO data return, or about ≥3 terabits for the nominal mission. This data volume would cover ∼9% of Mars at 6 m/pixel, but overlapping images (for stereo, mosaics, and observation of changes and meteorological events) will reduce this area. CTX acquired its first (instrument checkout) images of Mars on 24 March 2006.
M stars comprise 80% of main sequence stars, so their planetary systems provide the best chance for finding habitable planets, that is, those with surface liquid water. We have modeled the broadband ...albedo or reflectivity of water ice and snow for simulated planetary surfaces orbiting two observed red dwarf stars (or M stars), using spectrally resolved data of Earth's cryosphere. The gradual reduction of the albedos of snow and ice at wavelengths greater than 1 μm, combined with M stars emitting a significant fraction of their radiation at these same longer wavelengths, means that the albedos of ice and snow on planets orbiting M stars are much lower than their values on Earth. Our results imply that the ice/snow albedo climate feedback is significantly weaker for planets orbiting M stars than for planets orbiting G-type stars such as the Sun. In addition, planets with significant ice and snow cover will have significantly higher surface temperatures for a given stellar flux if the spectral variation of cryospheric albedo is considered, which in turn implies that the outer edge of the habitable zone around M stars may be 10-30% farther away from the parent star than previously thought.
We employ the NASA Ames Mars general circulation model (GCM) to investigate the dust lifting mechanisms responsible for the observed Martian dust cycle and the net surface response to the combined ...influence of dust lifting and deposition. This GCM includes lifting, transport, and deposition of radiatively active dust. Two dust lifting mechanisms are accounted for: wind stress lifting and dust devil lifting. A “baseline” simulation is presented and shown to compare well to available spatial and temporal observations of atmospheric opacity, wind stress dust lifting events, and atmospheric temperatures recorded during a nonglobal dust storm year. Multiple simulations were conducted to explore the model's sensitivity to a wide range of dust lifting parameters (the functional dependence of surface dust flux on wind stress, the wind stress threshold for lifting, etc.) Model results robustly suggest that wind stress lifting produces the peak in atmospheric dust load during southern spring and summer and that dust devils maintain the background haze of atmospheric dust during northern spring and summer. These results are consistent with previously published conclusions. Dust devil and wind stress lifting contribute equally to the simulated total amount of dust lifted annually during nonglobal dust storm years. The simulated spatial pattern of annual net deflation/deposition suggests that the low thermal inertia regions (Tharsis, Arabia, and Elysium) are not currently net dust accumulation regions. This net deflation is the result of dust devil dust lifting, suggesting that dust devils could play an important role in the present‐day pattern of surface dust reservoirs.
Extensive modeling of Mars in conjunction with in situ observations suggests that the annual average global mean surface temperature is Ts¯∼202K. Yet its effective temperature, i.e., the temperature ...at which a blackbody radiates away the energy it absorbs, is Te∼208K. How can a planet with a CO2 atmosphere have a mean annual surface temperature that is actually less than its effective temperature? We use the Ames General Circulation Model explain why this is the case and point out that the correct comparison of the effective temperature is with the effective surface temperature Tse, which is the fourth root of the annual and globally averaged value of Ts4. This may seem obvious, but the distinction is often not recognized in the literature.
We develop a 1‐D steady state photochemical model of the modern Martian atmosphere and apply it to possible Martian atmospheres present and past. A unique feature of our model is that the major ...current sink of oxygen is dry deposition (surface reactions) of highly reactive, oxidized molecules (chiefly H2O2), rather than oxygen escape to space. Another difference is that we allow hydrogen to escape to space at the diffusion limit, which gives H escape fluxes ∼70% higher than in other models. What results is a model with one free parameter: a dry deposition velocity to describe the surface sink of reactive molecules. An effective global average deposition velocity of 0.02 cm s−1 for H2O2 and O3 gives a good match to the observed abundances of O2, CO, and H2, the three abundant photochemical trace gases. We then apply our model to Martian atmospheres with different amounts of CO2, H2O, and solar forcing. We find that thick, cold, dry CO2 atmospheres are photochemically unstable with respect to conversion to CO. This may be pertinent to ancient Mars when the Sun was faint and O escape rates were likely high, for which the tipping point is computed to be ∼10 mbar of CO2. The possible photochemical instability of cold thick CO2 atmospheres, and the high likelihood that CO was abundant even if CO2 were stable, has broad implications for early Mars.
Carbon dioxide is an essential atmospheric component in martian climate models that attempt to reconcile a faint young sun with planetwide evidence of liquid water in the Noachian and Early ...Hesperian. In this study, we use mineral and contextual sedimentary environmental data measured by the Mars Science Laboratory (MSL) Rover Curiosity to estimate the atmospheric partial pressure of CO2 (PCO2) coinciding with a long-lived lake system in Gale Crater at ∼3.5 Ga. A reaction-transport model that simulates mineralogy observed within the Sheepbed member at Yellowknife Bay (YKB), by coupling mineral equilibria with carbonate precipitation kinetics and rates of sedimentation, indicates atmospheric PCO2 levels in the 10s mbar range. At such low PCO2 levels, existing climate models are unable to warm Hesperian Mars anywhere near the freezing point of water, and other gases are required to raise atmospheric pressure to prevent lake waters from being lost to the atmosphere. Thus, either lacustrine features of Gale formed in a cold environment by a mechanism yet to be determined, or the climate models still lack an essential component that would serve to elevate surface temperatures, at least locally, on Hesperian Mars. Our results also impose restrictions on the potential role of atmospheric CO2 in inferred warmer conditions and valley network formation of the late Noachian.
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