Using the Fourier Transform Spectrometer at the Canada–France–Hawaii Telescope, we observed a spectrum of Mars at the P-branch of the strongest CH
4 band at 3.3 μm with resolving power of 180,000 for ...the apodized spectrum. Summing up the spectral intervals at the expected positions of the 15 strongest Doppler-shifted martian lines, we detected the absorption by martian methane at a 3.7 sigma level which is exactly centered in the summed spectrum. The observed CH
4 mixing ratio is
10
±
3
ppb
. Total photochemical loss of CH
4 in the martian atmosphere is equal to
2.2
×
10
5
cm
−2
s
−1
, the CH
4 lifetime is 340 years and methane should be uniformly mixed in the atmosphere. Heterogeneous loss of atmospheric methane is probably negligible, while the sink of CH
4 during its diffusion through the regolith may be significant. There are no processes of CH
4 formation in the atmosphere, so the photochemical loss must therefore be balanced by abiogenic and biogenic sources. Outgassing from Mars is weak, the latest volcanism is at least 10 million years old, and thermal emission imaging from the Mars Odyssey orbiter does not reveal any hot spots on Mars. Hydrothermal systems can hardly be warmer than the room temperature at which production of methane is very low in terrestrial waters. Therefore a significant production of hydrothermal and magmatic methane is not very likely on Mars. The calculated average production of CH
4 by cometary impacts is 2% of the methane loss. Production of methane by meteorites and interplanetary dust does not exceed 4% of the methane loss. Methane cannot originate from an extinct biosphere, as in the case of “natural gas” on Earth, given the exceedingly low limits on organic matter set by the Viking landers and the dry recent history which has been extremely hostile to the macroscopic life needed to generate the gas. Therefore, methanogenesis by living subterranean organisms is a plausible explanation for this discovery. Our estimates of the biomass and its production using the measured CH
4 abundance show that the martian biota may be extremely scarce and Mars may be generally sterile except for some oases.
The quadrupole mass spectrometer of the Sample Analysis at Mars (SAM) instrument on Curiosity rover has made the first high‐precision measurement of the nonradiogenic argon isotope ratio in the ...atmosphere of Mars. The resulting value of 36Ar/38Ar = 4.2 ± 0.1 is highly significant for it provides excellent evidence that “Mars” meteorites are indeed of Martian origin, and it points to a significant loss of argon of at least 50% and perhaps as high as 85–95% from the atmosphere of Mars in the past 4 billion years. Taken together with the isotopic fractionations in N, C, H, and O measured by SAM, these results imply a substantial loss of atmosphere from Mars in the posthydrodynamic escape phase.
Key Points
First high‐precision measurement of primordial argon isotopes in Martian air
This measurement definitively ties the“martian” meteorites to Mars”
Requires massive loss of Martian atmosphere after the hydrodynamic escape phase
The Sample Analysis at Mars (SAM) instrument suite on the Mars Science Laboratory (MSL) measured a Mars atmospheric14N/15N ratio of 173 ± 11 on sol 341 of the mission, agreeing with Viking's ...measurement of 168 ± 17. The MSL/SAM value was based on Quadrupole Mass Spectrometer measurements of an enriched atmospheric sample, with CO2 and H2O removed. Doubly ionized nitrogen data at m/z 14 and 14.5 had the highest signal/background ratio, with results confirmed by m/z 28 and 29 data. Gases in SNC meteorite glasses have been interpreted as mixtures containing a Martian atmospheric component, based partly on distinctive14N/15N and40Ar/14N ratios. Recent MSL/SAM measurements of the40Ar/14N ratio (0.51 ± 0.01) are incompatible with the Viking ratio (0.35 ± 0.08). The meteorite mixing line is more consistent with the atmospheric composition measured by Viking than by MSL.
Key Points
MSL's more precise results on Mars nitrogen isotopes agree with Viking
Atmospheric gas in Mars meteorites differs from MSL atmospheric composition
Geologically rapid change in atmospheric composition is a possible explanation
The in situ measurements of the Galileo Probe Mass Spectrometer (GPMS) were expected to constrain the abundances of the cloud-forming condensible volatile gases: H
2O, H
2S, and NH
3. However, since ...the probe entry site (PES) was an unusually dry meteorological system—a 5-μm hotspot—the measured condensible volatile abundances did not follow the canonical condensation-limited vertical profiles of equilibrium cloud condensation models (ECCMs) such as Weidenschilling and Lewis (1973, Icarus 20, 465–476). Instead, the mixing ratios of H
2S and NH
3 increased with depth, finally reaching well-mixed equilibration levels at pressures far greater than the lifting condensation levels, whereas the mixing ratio of H
2O in the deep well-mixed atmosphere could not be measured. The deep NH
3 mixing ratio (with respect to H
2) of (6.64±2.54)×10
−4 from 8.9–11.7 bar GPMS data is consistent with the NH
3 profile from probe-to-orbiter signal attenuation (Folkner et al., 1998, J. Geophys. Res. 103, 22847–22856), which had an equilibration level of about 8 bar. The GPMS deep atmosphere H
2S mixing ratio of (8.9±2.1)×10
−5 is the only measurement of Jupiter's sulfur abundance, with a PES equilibration level somewhere between 12 and 15.5 bar. The deepest water mixing ratio measurement is (4.9±1.6)×10
−4 (corresponding to only about 30% of the solar abundance) at 17.6–20.9 bar, a value that is probably much smaller than Jupiter's bulk water abundance. The
15N/
14N ratio in jovian NH
3 was measured at (2.3±0.3)×10
−3 and may provide the best estimate of the protosolar nitrogen isotopic ratio. The GPMS methane mixing ratio is (2.37±0.57)×10
−3; although methane does not condense on Jupiter, we include its updated analysis in this report because like the condensible volatiles, it was presumably brought to Jupiter in icy planetesimals. Our detailed discussion of calibration and error analysis supplements previously reported GPMS measurements of condensible volatile mixing ratios (Niemann et al., 1998, J. Geophys. Res. 103, 22831–22846; Atreya et al., 1999, Planet. Space Sci. 47, 1243–1262; Atreya et al., 2003, Planet. Space Sci. 51, 105–112) and the nitrogen isotopic ratio (Owen et al., 2001b, Astrophys. J. Lett. 553, L77–L79). The approximately three times solar abundance of NH
3 (along with CH
4 and H
2S) is consistent with enrichment of Jupiter's atmosphere by icy planetesimals formed at temperatures <40 K (Owen et al., 1999, Nature 402 (6759), 269–270), but would imply that H
2O should be at least 3×solar as well. An alternate model, using clathrate hydrates to deliver the nitrogen component to Jupiter, predicts O/H⩾9×solar (Gautier et al., 2001, Astrophys. J. 550 (2), L227–L230). Finally we show that the measured condensible volatile vertical profiles in the PES are consistent with column-stretching or entraining downdraft scenarios
only if the basic state (the pre-stretched column or the entrainment source region) is described by condensible volatile vertical profiles that are drier than those in the equilibrium cloud condensation models. This dryness is supported by numerous remote sensing results but seems to disagree with observations of widespread clouds on Jupiter at pressure levels predicted by equilibrium cloud condensation models for ammonia and H
2S.
Using Ockham's razor as a guide, we have tried to find the simplest model for the formation of giant planets that can explain current observations of atmospheric composition. While this “top-down” ...approach is far from sufficient to define such models, it establishes a set of boundary conditions whose satisfaction is necessary. Using Jupiter as the prototype, we find that a simple model for giant planet formation that begins with a solar nebula of uniform composition and relies on accretion of low temperature icy planetesimals plus collapse of surrounding solar nebula gas supplies that satisfaction. We compare the resulting predictions of elemental abundances and isotope ratios in the atmospheres of the other giants with those from contrasting models and suggest some key measurements to make further progress.
Titan's methane cycle Atreya, Sushil K.; Adams, Elena Y.; Niemann, Hasso B. ...
Planetary and space science,
10/2006, Letnik:
54, Številka:
12
Journal Article
Recenzirano
Methane is key to sustaining Titan's thick nitrogen atmosphere. However, methane is destroyed and converted to heavier hydrocarbons irreversibly on a relatively short timescale of approximately ...10–100 million years. Without the warming provided by CH
4-generated hydrocarbon hazes in the stratosphere and the pressure induced opacity in the infrared, particularly by CH
4–N
2 and H
2–N
2 collisions in the troposphere, the atmosphere could be gradually reduced to as low as tens of millibar pressure. An understanding of the source–sink cycle of methane is thus crucial to the evolutionary history of Titan and its atmosphere. In this paper we propose that a complex photochemical–meteorological–hydrogeochemical cycle of methane operates on Titan. We further suggest that although photochemistry leads to the loss of methane from the atmosphere, conversion to a global ocean of ethane is unlikely. The behavior of methane in the troposphere and the surface, as measured by the Cassini–Huygens gas chromatograph mass spectrometer, together with evidence of cryovolcanism reported by the Cassini visual and infrared mapping spectrometer, represents a “methalogical” cycle on Titan, somewhat akin to the hydrological cycle on Earth. In the absence of net loss to the interior, it would represent a closed cycle. However, a source is still needed to replenish the methane lost to photolysis. A hydrogeochemical source deep in the interior of Titan holds promise. It is well known that in serpentinization, hydration of ultramafic silicates in terrestrial oceans produces H
2(aq), whose reaction with carbon grains or carbon dioxide in the crustal pores produces methane gas. Appropriate geological, thermal, and pressure conditions could have existed in and below Titan's purported water-ammonia ocean for “low-temperature” serpentinization to occur in Titan's accretionary heating phase. On the other hand, impacts could trigger the process at high temperatures. In either instance, storage of methane as a stable clathrate–hydrate in Titan's interior for later release to the atmosphere is quite plausible. There is also some likelihood that the production of methane on Titan by serpentinization is a gradual and continuous on-going process.
Comets have been considered to be representative of icy planetesimals that may have contributed a significant fraction of the volatile inventory of the terrestrial planets. For example, comets must ...have brought some water to Earth. However, the magnitude of their contribution is still debated. We report the detection of argon and its relation to the water abundance in the Jupiter family comet 67P/Churyumov-Gerasimenko by in situ measurement of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) mass spectrometer aboard the Rosetta spacecraft. Despite the very low intensity of the signal, argon is clearly identified by the exact determination of the mass of the isotope (36)Ar and by the (36)Ar/(38)Ar ratio. Because of time variability and spatial heterogeneity of the coma, only a range of the relative abundance of argon to water can be given. Nevertheless, this range confirms that comets of the type 67P/Churyumov-Gerasimenko cannot be the major source of Earth's major volatiles.
The origin of Titan's atmosphere: some recent advances Owen, Tobias; Niemann, H.B
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
02/2009, Letnik:
367, Številka:
1889
Journal Article
Recenzirano
Odprti dostop
It is possible to make a consistent story for the origin of Titan's atmosphere starting with the birth of Titan in the Saturn subnebula. If we use comet nuclei as a model, Titan's nitrogen and ...methane could have easily been delivered by the ice that makes up approximately 50 per cent of its mass. If Titan's atmospheric hydrogen is derived from that ice, it is possible that Titan and comet nuclei are in fact made of the same protosolar ice. The noble gas abundances are consistent with relative abundances found in the atmospheres of Mars and Earth, the Sun, and the meteorites.
Planetary atmospheres OWEN, Tobias C
Space science reviews,
06/2007, Letnik:
130, Številka:
1-4
Conference Proceeding, Journal Article
Recenzirano
Issue Title: The Composition of Matter The predominance of nitrogen in highly volatile forms and of carbon in solids set the abundance ratios of these elements in the inner planets, meteorites and ...comets. The absence of carbon compounds in an atmosphere then signals large deposits of carbon-bearing compounds in surface and/or subsurface deposits. In contrast, the icy planetesimals that contributed heavy elements to Jupiter must have had identical enrichments (relative to hydrogen) of both C and N, as well as other heavy elements that have been measured, compared to solar values. Capture of N and Ar suggests that the icy planetesimals that carried these elements must have formed at low temperatures, <40 K. New measurements of isotopes of nitrogen support this picture, but we must have more measurements in more atmospheres to be certain of this scenario. PUBLICATION ABSTRACT
Evidence for the exposure of water ice on Titan's surface GRIFFITH, Caitlin A; OWEN, Tobias; GEBALLE, Thomas R ...
Science (American Association for the Advancement of Science),
04/2003, Letnik:
300, Številka:
5619
Journal Article
Recenzirano
The smoggy stratosphere of Saturn's largest moon, Titan, veils its surface from view, except at narrow wavelengths centered at 0.83, 0.94, 1.07, 1.28, 1.58, 2.0, 2.9, and 5.0 micrometers. We derived ...a spectrum of Titan's surface within these "windows" and detected features characteristic of water ice. Therefore, despite the hundreds of meters of organic liquids and solids hypothesized to exist on Titan's surface, its icy bedrock lies extensively exposed.
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