Alexander Pavlov - Academia.edu (original) (raw)
Papers by Alexander Pavlov
The Sample Analysis at Mars (SAM) instrument on the Curiosity rover is designed to determine the ... more The Sample Analysis at Mars (SAM) instrument on the Curiosity rover is designed to determine the inventory of organic and inorganic volatiles thermally evolved from solid samples using a combination of evolved gas analysis (EGA), gas chromatography mass spectrometry (GCMS), and tunable laser spectroscopy [1]. The first sample analyzed by SAM at the Rocknest (RN) aeolian deposit revealed chlorohydrocarbons derived primarily from reactions between a martian oxychlorine phase (e.g. perchlorate) and terrestrial carbon from N-methyl-N-(tert-butyl-dimethylsilyl)trifluoroacetamide (MTBSTFA) vapor present in the SAM instrument background [2]. No conclusive evidence for martian chlorohydrocarbons in the RN sand was found [2]. After RN, Curiosity trav-eled to Yellowknife Bay and drilled two holes separated by 2.75 m designated John Klein (JK) and Cumber-land (CB). Analyses of JK and CB by both SAM and the CheMin x-ray diffraction instrument revealed a mudstone (called Sheepbed) consisting of ...
Journal of Geophysical Research: Planets, 2015
Science (New York, N.Y.), Jan 23, 2015
The deuterium-to-hydrogen (D/H) ratio in strongly bound water or hydroxyl groups in ancient marti... more The deuterium-to-hydrogen (D/H) ratio in strongly bound water or hydroxyl groups in ancient martian clays retains the imprint of the water of formation of these minerals. Curiosity's Sample Analysis at Mars (SAM) experiment measured thermally evolved water and hydrogen gas released between 550° and 950°C from samples of Hesperian-era Gale crater smectite to determine this isotope ratio. The D/H value is 3.0 (±0.2) times the ratio in standard mean ocean water. The D/H ratio in this ~3-billion-year-old mudstone, which is half that of the present martian atmosphere but substantially higher than that expected in very early Mars, indicates an extended history of hydrogen escape and desiccation of the planet.
Science, 2013
No Methane to Be Found On Earth, atmospheric methane is mostly produced biologically. Atmospheric... more No Methane to Be Found On Earth, atmospheric methane is mostly produced biologically. Atmospheric methane has also been detected on Mars, but these reports have been controversial. Based on data from the Sample Analysis at Mars instrument suite on the Curiosity rover, which arrived at the surface of Mars in August 2012, Webster et al. (p. 355 , published online 19 September) report no methane, with an upper limit of only 1.3 parts per billion by volume, about 6 times lower than previous measurements.
Angewandte Chemie International Edition, 2008
Space Science Reviews, 2012
Science, 2013
The Curiosity rover discovered fine-grained sedimentary rocks, which are inferred to represent an... more The Curiosity rover discovered fine-grained sedimentary rocks, which are inferred to represent an ancient lake and preserve evidence of an environment that would have been suited to support a martian biosphere founded on chemolithoautotrophy. This aqueous environment was characterized by neutral pH, low salinity, and variable redox states of both iron and sulfur species. Carbon, hydrogen, oxygen, sulfur, nitrogen, and phosphorus were measured directly as key biogenic elements; by inference, phosphorus is assumed to have been available. The environment probably had a minimum duration of hundreds to tens of thousands of years. These results highlight the biological viability of fluvial-lacustrine environments in the post-Noachian history of Mars.
Science, 2013
Samples from the Rocknest aeolian deposit were heated to ~835°C under helium flow and evolved gas... more Samples from the Rocknest aeolian deposit were heated to ~835°C under helium flow and evolved gases analyzed by Curiosity’s Sample Analysis at Mars instrument suite. H 2 O, SO 2 , CO 2 , and O 2 were the major gases released. Water abundance (1.5 to 3 weight percent) and release temperature suggest that H 2 O is bound within an amorphous component of the sample. Decomposition of fine-grained Fe or Mg carbonate is the likely source of much of the evolved CO 2 . Evolved O 2 is coincident with the release of Cl, suggesting that oxygen is produced from thermal decomposition of an oxychloride compound. Elevated δD values are consistent with recent atmospheric exchange. Carbon isotopes indicate multiple carbon sources in the fines. Several simple organic compounds were detected, but they are not definitively martian in origin.
Earth and Planetary Science Letters, 2003
Science (New York, N.Y.), Jun 8, 2018
Variable levels of methane in the martian atmosphere have eluded explanation partly because the m... more Variable levels of methane in the martian atmosphere have eluded explanation partly because the measurements are not repeatable in time or location. We report in situ measurements at Gale crater made over a 5-year period by the Tunable Laser Spectrometer on the Curiosity rover. The background levels of methane have a mean value 0.41 ± 0.16 parts per billion by volume (ppbv) (95% confidence interval) and exhibit a strong, repeatable seasonal variation (0.24 to 0.65 ppbv). This variation is greater than that predicted from either ultraviolet degradation of impact-delivered organics on the surface or from the annual surface pressure cycle. The large seasonal variation in the background and occurrences of higher temporary spikes (~7 ppbv) are consistent with small localized sources of methane released from martian surface or subsurface reservoirs.
Journal of Geophysical Research: Planets
ABSTRACT 13C/12C and 15 N/14 N isotopic ratios are pivotal for our understanding of the Martian c... more ABSTRACT 13C/12C and 15 N/14 N isotopic ratios are pivotal for our understanding of the Martian carbon cycle, history of the Martian atmospheric escape and origin of the organic compounds on Mars. Here we demonstrate that the carbon and nitrogen isotopic composition of the surface rocks on Mars can be significantly altered by the continuous exposure of Martian surface to cosmic rays. Cosmic rays can effectively produce 13C and 15 N isotopes via spallation nuclear reactions on oxygen atoms in various Martian rocks. We calculate that in the top meter of the Martian rocks the rates of production of both 13C and 15 N due to galactic cosmic rays (GCRs) exposure can vary within 1.5-6 atoms/cm3/s depending on rocks’ depth and chemical composition. We also find that the average solar cosmic rays (SCRs) can produce carbon and nitrogen isotopes at a rate comparable to GCRs in the top 5–10 cm of the Martian rocks. We demonstrate that if the total carbon content in a surface Martian rock is <10 ppm then the “light”, potentially “biological” 13C/12C ratio would be effectively erased by cosmic rays over 3.5 billion years of exposure. We found that for the rocks with relatively short exposure ages (e.g. 100 million years), cosmogenic changes in 15 N/14 N ratio are still very significant. We also show that a short exposure to CRs of ALH 84001 while on Mars can explain its high-temperature heavy nitrogen isotopic composition (15 N/14 N). Applications to Martian meteorites and the current MSL mission are discussed.
We determined the fraction of the liquid water and the rates of organic degradation in a simulate... more We determined the fraction of the liquid water and the rates of organic degradation in a simulated martian shallow subsurface layer as a function of ice table depth, atmospheric pressure, surface temperature, salt and oxidants' content.
ABSTRACT Methane was arguably an important greenhouse gas during the Archean and early Paleoprote... more ABSTRACT Methane was arguably an important greenhouse gas during the Archean and early Paleoproterozoic Eras, prior to the rise of O2 at ~2.3 Ga (1,2). Atmospheric CH4 concentrations of 1000 ppmv or more are predicted, assuming that methanogenic bacteria evolved early and that they were widely distributed in the prevailing anaerobic biosphere (2,3). Indeed, previous models (1-3) may have underestimated both the concentration of CH4 and the greenhouse effect itself at this time. Knauth and Lowe (4) have used oxygen isotopes in cherts to argue that the mean surface temperature at 3.2-3.5 Ga was between 55oC and 85oC. The Sun is thought to have been some 23 percent dimmer at that time (5), so this would have required substantial greenhouse warming by both CH4 and CO2. Further constraints are imposed by the formation of hydrocarbon haze, and an accompanying "anti-greenhouse effect", if the CH4 concentration exceeds that of CO2 (2). Thus, only certain combinations of CH4 and CO2 are possible. The rise of O2 at 2.3 Ga destabilized the methane greenhouse, leading to widespread (possibly global) glaciation (1,6). The next 1.5 billion years were warm, however, as evidenced by the absence of glacial deposits and the continuing high temperatures implied by O isotopes in cherts (7). CH4 levels could have remained relatively high during this time, 20-100 ppmv, as a consequence of low concentrations of dissolved O2 and sulfate in the deep oceans, and increased recycling of organic matter by fermentation and methanogenesis (8,9). An increase in either O2 or dissolved sulfate at around 0.75 Ga may have once again decreased atmospheric CH4 levels and triggered the widespread Neoproterozoic glaciations. References: 1. Pavlov, A. A., Kasting, J. F., Brown, L. L., Rages, K. A. & Freedman, R. J. Geophys. Res. 105, 11,981-11,990 (2000). 2. Pavlov, A. A., Kasting, J. F. & Brown, L. L. J. Geophys. Res. 106, 23,267-23,287 (2001). 3. Kasting, J. F., Pavlov, A. A. & Siefert, J. L. Origins of Life Evol. Biosph. 31, 271-285 (2001). 4. Knauth, L. P. & Lowe, D. R. GSA Bull. 115, 566-580 (2003). 5. Gough, D. O. Solar Phys. 74, 21-34 (1981). 6. Evans, D. A., Beukes, N. J. & Kirshvink, J. L. Nature 386, 262-266 (1997). 7. Knauth, L. P. NASA Astrobiol. Inst. Spring Meeting Abstracts (Tucson, AZ, Feb., 2003). 8. Pavlov, A. A., Hurtgen, M. T., Kasting, J. F. & Arthur, M. A. Geology 31, 87-90 (2003). 9. Canfield, D. E. Nature 396, 450-453 (1998).
Agu Fall Meeting Abstracts, Nov 29, 2003
ABSTRACT Decreased solar luminosity (Gough, 1981) and multiple lines of geologic evidence in favo... more ABSTRACT Decreased solar luminosity (Gough, 1981) and multiple lines of geologic evidence in favor of a "liquid" ocean in the Archean set a puzzle known as "Faint Young Sun" paradox. For several decades, elevated atmospheric CO2 levels were considered to be the most self-consistent solution for the warm Archean climate (Walker et al., 1977; Kasting et al., 1993). However, to offset a ~25% decreased solar luminosity (at ~3.5 Gyr ago) and keep the mean global surface temperature at ~288K, CO2 should have been at a steady-state concentration of about 0.3 bars. At such high levels CO2 would condense (Mellon, 1996) in the Earth's polar regions (as it does on Mars today) and no longer could be considered as the only "stabilizer" of the Archean climate. Lack of siderite in paleosols (Rye et al., 1995) and lack of glaciations in Archean/Proterozoic also does not support large CO2 concentrations and pure CO2 greenhouse in the Precambrian. Climate simulations (Pavlov et al., 2000) show that 100-1000 ppm of methane would be sufficient to maintain warm climate under decreased solar luminosity without invoking huge CO2 levels. Therefore, the key question is how to maintain such high CH4 levels. In the anoxic Archean environment (Pavlov & Kasting, 2002), the lifetime of methane molecule would be long ~10000 years. Previous photochemical calculations show that to maintain the "steady-state" 1000 ppm of CH4, the methane flux into Archean atmosphere should have been close to the present day biogenic methane flux (Pavlov et al., 2001) which is debatable. However, previous calculations assumed a high ("diffusion-limited") rate of hydrogen loss to space. If atmosphere was anoxic, hydrogen should have been lost at much (5-100 times) slower rate (Tian et al., 2003). Here we demonstrate that 100-1000 ppm could be maintained with much smaller methane flux in the hydrogen-rich Archean atmosphere. In the oxygenated Proterozoic atmosphere the lifetime of methane becomes much shorter. However, the biogenic flux from the oxygen/sulfate-poor Proterozoic ocean could have been even higher than the present total biogenic flux. The methane abundance in the oxygenated atmosphere is a non-linear function of methane source because methane molecules destroy their major sink - OH radicals (Prather, 1996). We showed (Pavlov et al., 2003) that ~100 ppm of methane in Proterozoic could be maintained with only 7-10 times increased present biogenic flux. We conclude that methane was abundant throughout Archean and Proterozoic and most likely was responsible for lack of glaciations in the Precambrian.
The Sample Analysis at Mars (SAM) instrument on the Curiosity rover is designed to determine the ... more The Sample Analysis at Mars (SAM) instrument on the Curiosity rover is designed to determine the inventory of organic and inorganic volatiles thermally evolved from solid samples using a combination of evolved gas analysis (EGA), gas chromatography mass spectrometry (GCMS), and tunable laser spectroscopy [1]. The first sample analyzed by SAM at the Rocknest (RN) aeolian deposit revealed chlorohydrocarbons derived primarily from reactions between a martian oxychlorine phase (e.g. perchlorate) and terrestrial carbon from N-methyl-N-(tert-butyl-dimethylsilyl)trifluoroacetamide (MTBSTFA) vapor present in the SAM instrument background [2]. No conclusive evidence for martian chlorohydrocarbons in the RN sand was found [2]. After RN, Curiosity trav-eled to Yellowknife Bay and drilled two holes separated by 2.75 m designated John Klein (JK) and Cumber-land (CB). Analyses of JK and CB by both SAM and the CheMin x-ray diffraction instrument revealed a mudstone (called Sheepbed) consisting of ...
Journal of Geophysical Research: Planets, 2015
Science (New York, N.Y.), Jan 23, 2015
The deuterium-to-hydrogen (D/H) ratio in strongly bound water or hydroxyl groups in ancient marti... more The deuterium-to-hydrogen (D/H) ratio in strongly bound water or hydroxyl groups in ancient martian clays retains the imprint of the water of formation of these minerals. Curiosity's Sample Analysis at Mars (SAM) experiment measured thermally evolved water and hydrogen gas released between 550° and 950°C from samples of Hesperian-era Gale crater smectite to determine this isotope ratio. The D/H value is 3.0 (±0.2) times the ratio in standard mean ocean water. The D/H ratio in this ~3-billion-year-old mudstone, which is half that of the present martian atmosphere but substantially higher than that expected in very early Mars, indicates an extended history of hydrogen escape and desiccation of the planet.
Science, 2013
No Methane to Be Found On Earth, atmospheric methane is mostly produced biologically. Atmospheric... more No Methane to Be Found On Earth, atmospheric methane is mostly produced biologically. Atmospheric methane has also been detected on Mars, but these reports have been controversial. Based on data from the Sample Analysis at Mars instrument suite on the Curiosity rover, which arrived at the surface of Mars in August 2012, Webster et al. (p. 355 , published online 19 September) report no methane, with an upper limit of only 1.3 parts per billion by volume, about 6 times lower than previous measurements.
Angewandte Chemie International Edition, 2008
Space Science Reviews, 2012
Science, 2013
The Curiosity rover discovered fine-grained sedimentary rocks, which are inferred to represent an... more The Curiosity rover discovered fine-grained sedimentary rocks, which are inferred to represent an ancient lake and preserve evidence of an environment that would have been suited to support a martian biosphere founded on chemolithoautotrophy. This aqueous environment was characterized by neutral pH, low salinity, and variable redox states of both iron and sulfur species. Carbon, hydrogen, oxygen, sulfur, nitrogen, and phosphorus were measured directly as key biogenic elements; by inference, phosphorus is assumed to have been available. The environment probably had a minimum duration of hundreds to tens of thousands of years. These results highlight the biological viability of fluvial-lacustrine environments in the post-Noachian history of Mars.
Science, 2013
Samples from the Rocknest aeolian deposit were heated to ~835°C under helium flow and evolved gas... more Samples from the Rocknest aeolian deposit were heated to ~835°C under helium flow and evolved gases analyzed by Curiosity’s Sample Analysis at Mars instrument suite. H 2 O, SO 2 , CO 2 , and O 2 were the major gases released. Water abundance (1.5 to 3 weight percent) and release temperature suggest that H 2 O is bound within an amorphous component of the sample. Decomposition of fine-grained Fe or Mg carbonate is the likely source of much of the evolved CO 2 . Evolved O 2 is coincident with the release of Cl, suggesting that oxygen is produced from thermal decomposition of an oxychloride compound. Elevated δD values are consistent with recent atmospheric exchange. Carbon isotopes indicate multiple carbon sources in the fines. Several simple organic compounds were detected, but they are not definitively martian in origin.
Earth and Planetary Science Letters, 2003
Science (New York, N.Y.), Jun 8, 2018
Variable levels of methane in the martian atmosphere have eluded explanation partly because the m... more Variable levels of methane in the martian atmosphere have eluded explanation partly because the measurements are not repeatable in time or location. We report in situ measurements at Gale crater made over a 5-year period by the Tunable Laser Spectrometer on the Curiosity rover. The background levels of methane have a mean value 0.41 ± 0.16 parts per billion by volume (ppbv) (95% confidence interval) and exhibit a strong, repeatable seasonal variation (0.24 to 0.65 ppbv). This variation is greater than that predicted from either ultraviolet degradation of impact-delivered organics on the surface or from the annual surface pressure cycle. The large seasonal variation in the background and occurrences of higher temporary spikes (~7 ppbv) are consistent with small localized sources of methane released from martian surface or subsurface reservoirs.
Journal of Geophysical Research: Planets
ABSTRACT 13C/12C and 15 N/14 N isotopic ratios are pivotal for our understanding of the Martian c... more ABSTRACT 13C/12C and 15 N/14 N isotopic ratios are pivotal for our understanding of the Martian carbon cycle, history of the Martian atmospheric escape and origin of the organic compounds on Mars. Here we demonstrate that the carbon and nitrogen isotopic composition of the surface rocks on Mars can be significantly altered by the continuous exposure of Martian surface to cosmic rays. Cosmic rays can effectively produce 13C and 15 N isotopes via spallation nuclear reactions on oxygen atoms in various Martian rocks. We calculate that in the top meter of the Martian rocks the rates of production of both 13C and 15 N due to galactic cosmic rays (GCRs) exposure can vary within 1.5-6 atoms/cm3/s depending on rocks’ depth and chemical composition. We also find that the average solar cosmic rays (SCRs) can produce carbon and nitrogen isotopes at a rate comparable to GCRs in the top 5–10 cm of the Martian rocks. We demonstrate that if the total carbon content in a surface Martian rock is <10 ppm then the “light”, potentially “biological” 13C/12C ratio would be effectively erased by cosmic rays over 3.5 billion years of exposure. We found that for the rocks with relatively short exposure ages (e.g. 100 million years), cosmogenic changes in 15 N/14 N ratio are still very significant. We also show that a short exposure to CRs of ALH 84001 while on Mars can explain its high-temperature heavy nitrogen isotopic composition (15 N/14 N). Applications to Martian meteorites and the current MSL mission are discussed.
We determined the fraction of the liquid water and the rates of organic degradation in a simulate... more We determined the fraction of the liquid water and the rates of organic degradation in a simulated martian shallow subsurface layer as a function of ice table depth, atmospheric pressure, surface temperature, salt and oxidants' content.
ABSTRACT Methane was arguably an important greenhouse gas during the Archean and early Paleoprote... more ABSTRACT Methane was arguably an important greenhouse gas during the Archean and early Paleoproterozoic Eras, prior to the rise of O2 at ~2.3 Ga (1,2). Atmospheric CH4 concentrations of 1000 ppmv or more are predicted, assuming that methanogenic bacteria evolved early and that they were widely distributed in the prevailing anaerobic biosphere (2,3). Indeed, previous models (1-3) may have underestimated both the concentration of CH4 and the greenhouse effect itself at this time. Knauth and Lowe (4) have used oxygen isotopes in cherts to argue that the mean surface temperature at 3.2-3.5 Ga was between 55oC and 85oC. The Sun is thought to have been some 23 percent dimmer at that time (5), so this would have required substantial greenhouse warming by both CH4 and CO2. Further constraints are imposed by the formation of hydrocarbon haze, and an accompanying "anti-greenhouse effect", if the CH4 concentration exceeds that of CO2 (2). Thus, only certain combinations of CH4 and CO2 are possible. The rise of O2 at 2.3 Ga destabilized the methane greenhouse, leading to widespread (possibly global) glaciation (1,6). The next 1.5 billion years were warm, however, as evidenced by the absence of glacial deposits and the continuing high temperatures implied by O isotopes in cherts (7). CH4 levels could have remained relatively high during this time, 20-100 ppmv, as a consequence of low concentrations of dissolved O2 and sulfate in the deep oceans, and increased recycling of organic matter by fermentation and methanogenesis (8,9). An increase in either O2 or dissolved sulfate at around 0.75 Ga may have once again decreased atmospheric CH4 levels and triggered the widespread Neoproterozoic glaciations. References: 1. Pavlov, A. A., Kasting, J. F., Brown, L. L., Rages, K. A. & Freedman, R. J. Geophys. Res. 105, 11,981-11,990 (2000). 2. Pavlov, A. A., Kasting, J. F. & Brown, L. L. J. Geophys. Res. 106, 23,267-23,287 (2001). 3. Kasting, J. F., Pavlov, A. A. & Siefert, J. L. Origins of Life Evol. Biosph. 31, 271-285 (2001). 4. Knauth, L. P. & Lowe, D. R. GSA Bull. 115, 566-580 (2003). 5. Gough, D. O. Solar Phys. 74, 21-34 (1981). 6. Evans, D. A., Beukes, N. J. & Kirshvink, J. L. Nature 386, 262-266 (1997). 7. Knauth, L. P. NASA Astrobiol. Inst. Spring Meeting Abstracts (Tucson, AZ, Feb., 2003). 8. Pavlov, A. A., Hurtgen, M. T., Kasting, J. F. & Arthur, M. A. Geology 31, 87-90 (2003). 9. Canfield, D. E. Nature 396, 450-453 (1998).
Agu Fall Meeting Abstracts, Nov 29, 2003
ABSTRACT Decreased solar luminosity (Gough, 1981) and multiple lines of geologic evidence in favo... more ABSTRACT Decreased solar luminosity (Gough, 1981) and multiple lines of geologic evidence in favor of a "liquid" ocean in the Archean set a puzzle known as "Faint Young Sun" paradox. For several decades, elevated atmospheric CO2 levels were considered to be the most self-consistent solution for the warm Archean climate (Walker et al., 1977; Kasting et al., 1993). However, to offset a ~25% decreased solar luminosity (at ~3.5 Gyr ago) and keep the mean global surface temperature at ~288K, CO2 should have been at a steady-state concentration of about 0.3 bars. At such high levels CO2 would condense (Mellon, 1996) in the Earth's polar regions (as it does on Mars today) and no longer could be considered as the only "stabilizer" of the Archean climate. Lack of siderite in paleosols (Rye et al., 1995) and lack of glaciations in Archean/Proterozoic also does not support large CO2 concentrations and pure CO2 greenhouse in the Precambrian. Climate simulations (Pavlov et al., 2000) show that 100-1000 ppm of methane would be sufficient to maintain warm climate under decreased solar luminosity without invoking huge CO2 levels. Therefore, the key question is how to maintain such high CH4 levels. In the anoxic Archean environment (Pavlov & Kasting, 2002), the lifetime of methane molecule would be long ~10000 years. Previous photochemical calculations show that to maintain the "steady-state" 1000 ppm of CH4, the methane flux into Archean atmosphere should have been close to the present day biogenic methane flux (Pavlov et al., 2001) which is debatable. However, previous calculations assumed a high ("diffusion-limited") rate of hydrogen loss to space. If atmosphere was anoxic, hydrogen should have been lost at much (5-100 times) slower rate (Tian et al., 2003). Here we demonstrate that 100-1000 ppm could be maintained with much smaller methane flux in the hydrogen-rich Archean atmosphere. In the oxygenated Proterozoic atmosphere the lifetime of methane becomes much shorter. However, the biogenic flux from the oxygen/sulfate-poor Proterozoic ocean could have been even higher than the present total biogenic flux. The methane abundance in the oxygenated atmosphere is a non-linear function of methane source because methane molecules destroy their major sink - OH radicals (Prather, 1996). We showed (Pavlov et al., 2003) that ~100 ppm of methane in Proterozoic could be maintained with only 7-10 times increased present biogenic flux. We conclude that methane was abundant throughout Archean and Proterozoic and most likely was responsible for lack of glaciations in the Precambrian.