Lunar and Planetary Science XXXVIII (2007) 1233.pdf

A SEARCH FOR POTENTIAL BIOGENIC AND VOLCANIC GASES EMANATING FROM POINT SOURCES ON MARS. J. S. Levine, Science Directorate, NASA Langley Research Center, Hampton, VA 23681-2199. E-mail: [email protected]

Introduction: The detection of methane (CH4) [1,2,3] in Production and Destruction of Atmospheric Gases by the atmosphere of Mars has very important implications for “Near-Point” Atmospheric Processes: In addition to the the question of present-day life on Mars since on Earth the production and release to the atmosphere of gases from sur- bulk of atmospheric methane is biogenic in origin. The de- face point sources, there may be “near-point” sources and tection of certain trace atmospheric gases will provide impor- sinks within the atmosphere leading to the production and tant information about Mars as a present-day biologically and destruction of gases on Mars. The term “near-point” refers geologically active planet. Small, localized areas on the to a small, very localized region of atmospheric gas produc- surface of Mars may be sources of potential biogenic gases tion and/or destruction, as opposed to a source or sink that (e.g., methane [1,2,3,4] and ammonia (NH3)), potential vol- covers a large geographical area. It has been hypothesized canic gases (e.g., sulfur dioxide (SO2) and hydrogen sulfide that energetic electrons produced by large-scale electric (H2S)) and water vapor (H2O) resulting from possible pre- fields associated with Martian dust devils and larger convec- sent-day ground water seepage and surface run-off [5]. Due tive storms [10] may lead to the production of hydrogen per- to the significant atmospheric concentration dilution of the oxide (H2O2) [11] and the destruction of atmospheric meth- gas away from the source due to atmospheric transport proc- ane [12]. As a result of the production of hydrogen peroxide esses, coupled with the very short lifetime of many of these by energetic electrons, its atmospheric abundance can exceed gases due to their destruction by chemical and photochemical the concentration produced by atmospheric chemical and reactions, a new measurement strategy and a new measure- photochemical processes by a factor of 200 resulting in ment platform are proposed for the detection of these gases. enough hydrogen peroxide to first condense and then precipi- tate out of the atmosphere [11]. Due to the presence of these Measurement Difficulties Detecting Atmospheric Gases energetic electrons in the atmosphere of Mars, the atmos- Emanating from Surface Point Sources: Gaseous emis- pheric lifetime of methane is reduced from about 1010 s 3 sions from localized and transient point sources could easily (about 300 years) to only 10 s (about a third of a day) [12]. go undetected since typical atmospheric measurements from the Earth or Mars orbit represent averages over very large A Measurement Strategy to Detect Atmospheric Gases areas, typically the entire disk or a large fraction of it. In Emanating from Surface Point Sources: A powered, con- addition, the transient nature of such emissions would further trolled aerial vehicle flying about a kilometer above the sur- add to the difficulty of their detection from the Earth or Mars face of Mars and traversing hundreds of kilometers above the orbit. Measurements of gaseous emissions produced by a surface is the ideal platform to search for gaseous emissions transient point source would also be very difficult from a produced by surface point sources and near-surface near- rover on the surface due to the limited mobility and, and point atmospheric mechanisms that lead to the production hence, very limited geographical coverage of the vehicle. and destruction of atmospheric gases. The ARES (Aerial Using a localized point source outgassing photochemical Regional-scale Environmental Surveyor) Mars airplane, pro- model of the atmosphere of Mars, Wong, A.-S. et al. [6,7] posed for a Mars Scout Mission, is a rocket powered airplane calculate that in the relatively short time of a single hour, that will carry among other instruments, a lightweight, sensi- atmospheric convective processes alone will reduce the mix- tive, rapid-response mass spectrometer with high mass dis- ing ratios of outgassed species from a surface point source by crimination to permit measurements of species isotopes. factors of 104-105 approximately 50 km away from the ARES will also carry an atmospheric data system that will source, and the time for spreading the outgassed species measure the state of the atmosphere, including atmospheric more or less uniformly over the entire planet, with a corre- temperature and pressure and the three dimensional structure sponding dilution factor of about 108, would be only about of the wind in the planetary boundary layer over the entire one year. These calculations are based on the dilution of the flight path. If selected for a mission, the ARES mass spec- outgassed species from its surface point source due to atmos- trometer will conduct the first search for gaseous emissions pheric convective processes alone and do not include the emanating from point sources on the surface and in the near- effect of the atmospheric lifetime of the outgassed species, surface atmosphere of Mars over regional-scale distances. many of which are very short-lived. For example, the photo- The ARES mass spectrometer will not only measure the chemical lifetime of sulfur dioxide is about 107 s (40 days): primary point source emission, such as methane, ammonia, for hydrogen sulfide, the atmospheric lifetime is about 7.9 x sulfur dioxide, hydrogen sulfide, water vapor and hydrogen 105 s (9 days) [6,7]. The introduction of methane, ammonia, peroxide, but will also provide simultaneous measurements sulfur dioxide and hydrogen sulfide into the atmosphere of of the gaseous products of the primary point source gas, such Mars introduces a wide variety of chemical and photochemi- as sulfur monoxide, sulfur trioxide, sulfuric acid, formalde- cal gaseous products [8,9]. The presence of atmospheric hyde, methyl alcohol and ethane. In addition, by virtue of its methane leads to the production of formaldehyde (CH2O), greater mobility, compared to surface rovers, and hence methyl alcohol (CH3OH) and ethane (C2H6). The presence of greater geographical coverage, ARES may observe a new atmospheric sulfur dioxide leads to the production of sulfur and important atmospheric process for the first time: the monoxide (SO), sulfur trioxide (SO3) and sulfuric acid production and destruction of atmospheric gases by energetic (H2SO4). Atmospheric hydrogen sulfide is chemically trans- electrons produced by strong electric fields associated with formed to the mercapto radical (HS), then to HSO, and even- dust devils and larger convective storms in the atmosphere of tually to sulfur dioxide. Mars. Lunar and Planetary Science XXXVIII (2007) 1233.pdf

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