N 2, CO 2. N % [e.g., Chassefiere et al., 2004, 2007]

Transcription

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2 Outline H 2 O H O 2

3 N 2, CO 2 N % [e.g., Chassefiere et al., 2004, 2007] 3

4 ~200 km ~120 km ~30 km UV H 2 O 2, O 3 H H 2 H x CO y 0 km O, C CO 2, H 2 O H + O 2 + M HO 2 + M HO 2 + O OH + O 2 CO + OH CO 2 + H HO 2 + H H 2 + O 2 CO 2, H 2 O H, O, C H 4

5 H, O, C ~200 km H 2. H 2 H, H 2 CO 2 O, C ~120 km CO 2 H 2 ~30 km 1. H 2 0 km H 2 O H 2 1. H 2 2. H 2, H H O C 1 2. CO 2 O, C 5

6 H ~200 km H 2. H 2 H, H 2 ~ ~120 km CO 2 H 2 ~30 km 0 km H 2 O 1. H 2 ~ 1. [Catling and Kasting, 2017] Φ i = b i f i / H a f i b i H a 6

7 H H 2 ~ MEX, MAVEN H 2 O H Alex Hubig ~ MAVEN/IUVS [Clarke et al., 2017] 7

8 O H O H H ~10 27 atom/s O ~10 26 atom/s C ~10 25 atom/s O O 8

9 O H O 2 OH H 2 [Zahnle et al., 2008] [cm/s] O H 2 H H O [McElroy, 1972; Zahnle et al., 2008] 9

10 H [Zahnle et al., 2008] H H H 2 O H 10

11 O O H O [Krasnopolsky, 2002] H O O 11

12 I0 I0 DENSITY (Cl~ ~') H x CO y [Liu and Donahue, 1976] [Chassefiere and Leblanc, 2011] O / 12

13 dry deposition [Zahnle et al., 2008] H 2 O 2, O 3 3~50 Chassefiere et al. [2013] 13

14 Atmospheric pressure [bar] (3) (1) (4) the web version of this article.) [Kurokawa et al., 2018] (6) (3) (2) (5) (5) Time [Ga] Atmospheric collapse at high pressure (7) Upper limit of total CO 2 in AIR system (8) 2 Present-day atmosphere 41 Time [Ga] δ 15 N [ ] Time [Ga] Fig. 7. The evolution of the nitrogen isotope ratio (the δ 15 N value) in the best-fit model ( X gas = 1%, C vol = 5, C Ne,IDP = 10, and f comet = 0.1%). (a) A hundred Monte Carlo simulations. (b) Selected three cases corresponding to Fig. 5 c. Collapsed and uncollapsed cases are shown by thick colored lines and thin black lines, respectively. The averaged value of the collapsed cases at 0 Ga is shown with 1- σ error bar (purple). Ranges of observed present-day atmospheric ratios are from Curiosity (red solidline, Wong et al., 2013 ), Viking (red dashed-line, Nier and McElroy, 1977 ), and ALH (red data point at 4.1 Ga, Mathew and Marti, 2001 ), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to cause the magnetic protection was assumed. After the quasi-steady state between 3.5 Ga to 0.5 Ga, the N isotope ratio started to increase as the volcanic degassing diminished. The N isotope ratio in the collapsed cases successfully reproduced the present-day value obtained by Curiosity ( Wong et al., 2013 ). The isotopic compositions of Ne ( 20 Ne/ 22 Ne), Ar ( 38 Ar/ 36 Ar), and Kr ( 86 Kr/ 84 Kr as a representative ratio) in the Martian atmosphere were also reproduced in our model, while Xe ( 136 Xe/ 130 Xe as a representative ratio) was not ( Figs ). As well as N, the evolutionary tracks of Ne and Ar are classified into two branches depending on the presence/absence of the atmospheric collapse ( Figs. 8 a and 9 a). In contrast to the isotope ratios moderately evolving in a (b) [Kurokawa et al., 2018]

15 styles represent different atmospheric pressures. The black curves in Figure 3 (right) are the number density profile of CO 2 (dashed), atomic carbon (solid), and atomic oxygen (dotted) in Case Ga (dashed curve in Figure 4) and could have reached cm 2 s 1 at 4.5 Ga. The corresponding timescales required to lose 1 bar of CO 2 from Mars ( EUV carbon atoms) are 10 and 1 Myrs respectively. Hence, these dense atmospheres could not have been maintained during the early Noachian unless the loss of CO 2 was balanced by rapid rates of volcanic outgassing. O [10] We estimate CO 2 outgassing rates on MarsCduring the early Noachian in the following manner: At present, both Earth and Venus have about 100 bar of CO 2 at their surfaces. On Venus, it is in the atmosphere; on Earth, it is mostly in carbonate rocks. 100 bar of CO 2 on the Earth is molecules. Mars has about 1/10th of Earth s mass, 1/5th of Earth s surface area, and 40% of Earth s gravity. Hence, if Mars was formed from the same material as Earth, its total CO 2 reservoir could have been as much as molecules, or 20 bars. This is consistent with the estimate based on geomorphology [Carr, 1986].Considering that Mars formed farther from the Sun than the Earth and thus could contain materials with volatile content 2 richer than Earth, a maximum 40-bar ( molecules) total CO 2 inventory is possible. If most of Mars CO 2 were emplaced in its atmosphere at 4.5 Ga, immediately following the planet s formation, the entire inventory could have been lost within 40 Myrs. [11] Alternatively, Mars could have released CO 2 gradually and thus avoided the initial fast volatile loss episode. It is estimated that 1.5 bar of CO 2 was released volcanically through the formation of the Tharsis bulge during the late Noachian [Phillips et al., 2001]. This estimate is based on the assumption that the CO 2 content in the Tharsis magmas Tian et al. [2009], Amerstrofer et al. [2017] reduced the total CO 2 inventory. This only strengthens our conclusion that Mars could not have sustained a dense CO 2 atmosphere for time scale of 10 Myrs, and thus was initially cold. (With few greenhouse gases present, the SHE? Tian et al. [2009] Figure 4. Comparison between atmospheric escape and outgassing on early Mars. The dashed curve represents the thermal escape flux of atomic carbon from a CO 2 - dominated atmosphere. The triangles represent the thermal escape fluxes of carbon from an atmosphere with an H 2 mixing ratio of (3 5) 10 4 and with associated H escape fluxes of the order of cm 2 s 1.Thetwosolidlines represent possible CO 2 volcanic outgassing fluxes on early Mars. The dash-dotted horizontal line marks CO 2 outgassing flux ( moles per year, or cm 2 s 1 ) on the present Earth. The dotted curve represents 10% of the UV CO 2 O, C ~1 41 energy-limited escape flux of carbon, F lim = ef EUV 15 4E grav,where F EUV is11.04 the solar EUV energy flux (ergs cm 2 s 1 ), E grav = GMm/r (ergs) is the potential energy of one single

16 styles represent different atmospheric pressures. The black curves in Figure 3 (right) are the number density profile of CO 2 (dashed), atomic carbon (solid), and atomic oxygen (dotted) in Case Ga (dashed curve in Figure 4) and could have reached cm 2 s 1 at 4.5 Ga. The corresponding timescales required to lose 1 bar of CO 2 from Mars ( EUV carbon atoms) are 10 and 1 Myrs respectively. Hence, these dense atmospheres could not have been maintained during the early Noachian unless the loss of CO 2 was balanced by rapid rates of volcanic outgassing. O [10] We estimate CO 2 outgassing rates on MarsCduring the early Noachian in the following manner: At present, both Earth and Venus have about 100 bar of CO 2 at their surfaces. On Venus, it is in the atmosphere; on Earth, it is mostly in carbonate rocks. 100 bar of CO 2 on the Earth is molecules. Mars has about 1/10th of Earth s mass, 1/5th of Earth s surface area, and 40% of Earth s gravity. Hence, if Mars was formed from the same material as Earth, its total CO 2 reservoir could have been as much as molecules, or 20 bars. This is consistent with the estimate based on geomorphology [Carr, 1986].Considering that Mars formed farther from the Sun than the Earth and thus could contain materials with volatile content 2 richer than Earth, a maximum 40-bar ( molecules) total CO 2 inventory is possible. If most of Mars CO 2 were emplaced in its atmosphere at 4.5 Ga, immediately following the planet s formation, the entire inventory could have been lost within 40 Myrs. [11] Alternatively, Mars could have released CO 2 gradually and thus avoided the initial fast volatile loss episode. It is estimated that 1.5 bar of CO 2 was released volcanically through the formation of the Tharsis bulge during the late Noachian [Phillips et al., 2001]. This estimate is based on the assumption that the CO 2 content in the Tharsis magmas reduced the total CO 2 inventory. This only strengthens our conclusion that Mars could not have sustained a dense CO 2 atmosphere for time scale of 10 Myrs, and thus was initially cold. (With few greenhouse gases present, the Figure 4. Comparison between atmospheric escape and outgassing on early Mars. The dashed curve represents the thermal escape flux of atomic carbon from a CO 2 - dominated atmosphere. The triangles represent the thermal escape fluxes of carbon from an atmosphere with an H 2 mixing ratio of (3 5) 10 4 and with associated H escape fluxes of the order of cm 2 s 1.Thetwosolidlines represent possible CO 2 volcanic outgassing fluxes on early Mars. The dash-dotted horizontal line marks CO 2 outgassing flux ( moles per year, or cm 2 s 1 ) on the present Earth. The dotted curve represents 10% of the energy-limited escape flux of carbon, F lim = ef EUV 4E grav,where 16 F EUV is11.04 the solar EUV energy flux (ergs cm 2 s 1 ), E grav = GMm/r (ergs) is the potential energy of one single O H 2 O CO 2 C SHE? Tian et al. [2009]

17 XUV 2 XUV 6 XUV Solar period (present) ( 2 Ga) (3.5 Ga) Process [s 1 ] Ion pickup [O ] Dissociative recombination [O*] Sputtering [O] Sputtering [CO] Sputtering [CO 2 ] Plasma clouds [O ] Cold ion outflow [O ] Total O loss [Terada et al., 2009] 1 17

18 H 2 O H O H 18