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
Surface-based measurements of terrestrial and martian dust devils/convective vortices provided from mobile and stationary platforms are discussed. Imaging of terrestrial dust devils has quantified ...their rotational and vertical wind speeds, translation speeds, dimensions, dust load, and frequency of occurrence. Imaging of martian dust devils has provided translation speeds and constraints on dimensions, but only limited constraints on vertical motion within a vortex. The longer mission durations on Mars afforded by long operating robotic landers and rovers have provided statistical quantification of vortex occurrence (time-of-sol, and recently seasonal) that has until recently not been a primary outcome of more temporally limited terrestrial dust devil measurement campaigns. Terrestrial measurement campaigns have included a more extensive range of measured vortex parameters (pressure, wind, morphology, etc.) than have martian opportunities, with electric field and direct measure of dust abundance not yet obtained on Mars. No martian robotic mission has yet provided contemporaneous high frequency wind and pressure measurements. Comparison of measured terrestrial and martian dust devil characteristics suggests that martian dust devils are larger and possess faster maximum rotational wind speeds, that the absolute magnitude of the pressure deficit within a terrestrial dust devil is an order of magnitude greater than a martian dust devil, and that the time-of-day variation in vortex frequency is similar. Recent terrestrial investigations have demonstrated the presence of diagnostic dust devil signals within seismic and infrasound measurements; an upcoming Mars robotic mission will obtain similar measurement types.
•245 convective vortices were detected from pressure measurements obtained by MSL during the first 707 sols of operation.•The lack of UV flux signatures corresponding to identified vortices implies ...they are mainly dust free.•These dust free, low intensity vortices support the theoretical prediction that the planetary boundary layer is suppressed at Gale crater.
Atmospheric convective vortices, which become dust devils when they entrain dust from the surface, are prominent features within Mars' atmosphere which are thought to be a primary contributor to the planet's background dust opacity. Buoyantly produced in convectively unstable layers at a planet's surface, these vertically aligned vortices possess rapidly rotating and ascending near-surface warm air and are readily identified by temporal signatures of reduced atmospheric surface pressure measured within the vortex as it passes by. We investigate such signatures in surface pressure measurements acquired by the Rover Environmental Monitoring Station aboard the Mars Science Laboratory rover located within Gale crater. During the first 707 sols of the mission, 245 convective vortices are identified with pressure drops in the range of 0.30–2.86Pa with a median value of 0.67Pa. The cumulative distribution of their pressure drops follows a power law of slope −2.77 and we observe seasonal and diurnal trends in their activity. The vast majority of these pressure signatures lack corresponding reductions in REMS-measured UV flux, suggesting that these vortices rarely cast shadows upon the rover and therefore are most often dust-free. The relatively weak-magnitude, dustless vortices at Gale crater are consistent with predictions from mesoscale modeling indicating that the planetary boundary layer is suppressed within the crater and are also consistent with the almost complete absence of both dust devils within Mars Science Laboratory camera images and Gale crater surface dust devil streaks within orbiter images.
The nature of the early Martian climate has been the subject of debate for decades, with geologic evidence suggesting an environment with prolonged precipitation and flowing liquid water on the ...surface, while climate models have struggled to reproduce such conditions. In this paper, we test the impact heating hypothesis for warming early Mars as presented in Segura et al. (2008) using a new early Mars version of the NASA Ames Research Center 3-D Mars Global Climate Model. We simulate impacts of asteroids 30-, 50-, and 100- km in diameter into atmospheres possessing 150-mbar, 1-bar, and 2-bar surface pressure conditions, accounting for both radiatively active and radiatively inert water clouds. Based on the scenarios simulated here, we find that the evolution of post-impact initially hot and moist conditions can be characterized in four phases: 1) a rapid radiative cooling phase, 2) a latent heat phase in which cloud formation and the radiative effects of water vapor induce a temporary warm period with significant precipitation, 3) a transition phase in which cooling accelerates due to sublimation at the surface and the lack of available water in the atmosphere for greenhouse warming and in which water vapor begins to contribute less to surface warming than water clouds, and 4) a steady state phase with mean annual surface temperatures below freezing and minimal precipitation. In these post-impact climate scenarios, global average surface temperatures remain above freezing for only 0.043 to 6.25 Mars years, accompanied by 0.23 to 5.8 m of cumulative precipitation (global equivalent) falls during 10 simulated Mars years. Ultimately, periods of warm temperatures and significant precipitation are short-lived and even in the warmest cases, do not support sustained conditions in which valley networks are likely to form in the long run either by liquid precipitation or by seasonal melting of surface ice. Scenarios with high surface pressures and radiatively active clouds experience the longest periods of above-freezing post-impact temperatures and result in the highest mean annual temperatures during the fourth phase (272.8 K in our warmest scenario), highlighting the potential significance of water clouds in the early Martian climate and the importance of their careful physical treatment in models. Future studies addressing sustained warm and wet early Mars conditions should investigate the potential effects of obliquity, initial surface ice distribution, and possible delivery of reducing greenhouse gases on these post-impact climates.
•We test the early Mars impact heating hypothesis with a 3-D GCM.•We simulate 30-, 50-, and 100-km diameter impactors in 150 mbar, 1 bar, and 2 bar atmospheres.•Impacts can induce warm conditions and precipitation only temporarily.•Mean annual temperatures eventually fall below freezing (warmest scenario 272.8 K).•Majority of valley networks are unlikely to form in simulated post-impact climates.
Abstract
Impacts on early Mars can produce H
2
and CH
4
in the thermal plume. In a thick CO
2
atmosphere, collision‐induced absorptions between CO
2
‐H
2
and CO
2
‐CH
4
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 CO
2
surface pressures >0.5 bars, postimpact surface temperatures due to H
2
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 H
2
into a thick CO
2
atmosphere after they impact the surface. The impact creates a hot thermal plume where water oxidizes the meteorite's reduced materials producing H
2
gas in the process. Collisions between CO
2
and H
2
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 CO
2
can produce significant quantities of CO and H
2
in the thermal plume following an impact
Degassing of H
2
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
The impact heating hypothesis has been explored as a means of warming early Mars and inducing rainfall through the potential injection of water, energy, and reducing greenhouse gases to the ...atmosphere. We simulate H2-rich post-impact scenarios with the 3D NASA Ames legacy early Mars Global Climate Model (eMGCM) for 100-km and 250-km diameter impactors to assess the ability of these environments to warm the surface above freezing and induce fluvial erosion. We find that including degassed hydrogen does not extend the short-term period of warm temperatures and heavy rainfall experienced immediately after an impact, but does provide enough warming in the final climate state to raise mean annual surface temperatures ≥273 K. These warm conditions would be long lived as hydrogen slowly escapes to space (over timescales of 105 years). Predicted precipitation rates in these long lived warm conditions suggest that a handful of large impacts (100 km in diameter or greater) could be capable of inducing a significant amount of the total Noachian erosion (~1–10 m per 100-km impact event and ~10–100 m per 250-km impact event). Simulations of a Hellas-sized impact event with H2 degassing are very sensitive to surface water reservoir assumptions. Scenarios range from wet conditions with widespread precipitation to conditions where water is trapped at the poles / over Tharsis. Overall, more work is needed to understand hydrological cycle sensitivities in H2-rich climates, but we confirm the ability of impacts to induce transient warm and wet conditions on early Mars is much better than previously thought if those impacts degassed H2 in thick CO2 atmospheres.
•Large asteroid impact events could have degassed significant quantities of hydrogen into the early Martian atmosphere.•A post-impact H2-rich climate on early Mars would have had widespread above-freezing surface temperatures.•The distribution of precipitation in post-impact H2-richs climate is sensitive to surface water reservoir availability.•A handful of large impacts could have produced a good portion of the total Noachian erosion.•Future work should explore the complexities of combining multiple warming sources such as H2, H2O clouds, and CO2 clouds.
Centaurs, minor planets with a semi-major axis between the orbits of Jupiter and Neptune (5–30 AU), are thought to be among the most diverse small bodies in the solar system. These important targets ...for future missions may have recently been Kuiper Belt Objects (KBOs), which are thought to be chemically and physically primitive remnants of the early solar system. While the Kuiper Belt spans distances of 30–50 AU, making direct observations difficult, Centaurs' proximity to the Earth and Sun make them more accessible targets for robotic missions. Thus, we outline a mission concept designed to reconnoiter 10199 Chariklo, the largest Centaur and smallest ringed body yet discovered. Named for a legendary Centaur tamer, the conceptual Camilla mission is designed to fit under the cost cap of the National Aeronautics and Space Administration (NASA) New Frontiers program, leveraging a conservative payload to support a foundational scientific investigation to these primitive bodies. Specifically, the single flyby encounter utilizes a combined high-resolution camera/VIS-IR mapping spectrometer, a sub-mm point spectrometer, and a UV mapping spectrometer. In addition, the mission concept utilizes a kinetic impactor, which would provide the first opportunity to sample the composition of potentially primitive subsurface material beyond Saturn, thus providing key insights into solar system origins. Such a flyby of the Chariklo system would provide a linchpin in the understanding of small body composition, evolution, and transport of materials in the solar system.
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•Flyby concept for 10199 Chariklo, the largest Centaur and smallest ring system.•Opportunity to learn about Kuiper Belt Objects much closer to Earth.•Impactor would provide deepest yet subsurface sampling in the outer Solar System.•Mission concept fits well within NASA New Frontiers Program cost cap.•Mission concept may fit within NASA Discovery Program cost cap.
The nature of the early martian climate has been the subject of debate for decades. While geologic evidence of valley networks, degraded craters, and aqueously altered minerals implies prolonged warm ...conditions with precipitation, the climate modeling community has been unable to present a climate scenario that can produce these features. Characterizing the climate that supported aqueous activity and constraining the duration and intensity of warm and wet periods is crucial to understanding whether 1\Iars was habitable in the past. 1-D climate modeling studies suggest that asteroid impacts are capable of inducing greenhouse warming on early Mars by injecting energy and water into the atmosphere and on the surface (Segura et al. 2008). We use a 3-D global climate model (GCM) to simulate the climate scenarios following impacts. We present a new version of Ames Research Center (ARC) Mars GCM. the early Mars GCM (eMGCM), that includes treatments for bulk cloud condensation, precipitation and sedimentation, the radiative effects of those clouds for both liquid water and water ice particles, and moist convection. With the eMGCM, we recreate the post-impact climate conditions presented in Segura et al. (2008: 30-, 50-, and 100-km impactors in 150 mbar, 1 bar, and 2 bar atmospheres) and examine the resulting global distributions of surface temperature and precipitation to assess whether these post-impact climates can facilitate valley network formation in the southern highlands 3.5 - 3.75 Gya. We find that these post-impact scenarios result in above-freezing temperatures and lOs of cm of rainfall in the southern highlands, but that ultimately these warm periods are short lived (on the order of years) and do not support sustained warm and wet conditions that facilitate valley network formation. We find that scenarios with high surface pressures and with radiatively active clouds experience longer periods of above-freezing temperatures and result in higher final mean annual temperatures (up to 272.8K in our warmest scenario). In future work, we will investigate the delivery of other greenhouse gases by impacts in addition to water, including hydrogen and/or methane, to test whether this prolongs the warm and wet periods following impacts.
Reconciling the geology of Mars with models of atmospheric evolution remains a major challenge. Martian geology is characterized by past evidence for episodic surface liquid water, and geochemistry ...indicating a slow and intermittent transition from wetter to drier and more oxidizing surface conditions. Here we present a new model incorporating randomized injection of reducing greenhouse gases and oxidation due to hydrogen escape, to investigate the conditions responsible for these diverse observations. We find that Mars could have transitioned repeatedly from reducing (H2-rich) to oxidizing (O2-rich) atmospheric conditions in its early history. Our model predicts a generally cold early Mars, with mean annual temperatures below 240 K. If peak reducing gas release rates and background CO2 levels are high enough, it nonetheless exhibits episodic warm intervals sufficient to degrade crater walls, form valley networks and create other fluvial/lacustrine features. Our model also predicts transient buildup of atmospheric O2, which can help explain the occurrence of oxidized mineral species such as manganese oxides at Gale Crater. We suggest that the apparent Noachian--Hesperian transition from phyllosilicate deposition to sulfate deposition around 3.5 billion years ago can be explained as a combined outcome of increasing planetary oxidation, decreasing groundwater availability and a waning bolide impactor flux, which dramatically slowed the remobilization and thermochemical destruction of surface sulfates. Ultimately, rapid and repeated variations in Mars' early climate and surface chemistry would have presented both challenges and opportunities for any emergent microbial life.
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