ESCAPE OF EARLY MARTIAN ATMOSPHERE AND HYDROSPHERE

46th Lunar and Planetary Science Conference (2015)
1643.pdf
ESCAPE OF EARLY MARTIAN ATMOSPHERE AND HYDROSPHERE: CONSTRAINTS FROM ISOTOPIC COMPOSITIONS Hiroyuki Kurokawa1 , Kosuke Kurosawa2 , and Tomohiro Usui3 , 1 Dept. of Phys. Nagoya
University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan (kurokawa@nagoya-u.jp), 2 Planet. Explor. Res.
Ctr., Chiba Institute of Technology, 3 Dept. of Earth & Planet. Sci., Tokyo Institute of Technology.
Model We calculate the evolution of the total amounts
of the atmosphere and hydrosphere and their isotopic
compositions individually, considering the impact erosion and thermal/nonthermal escape (Fig. 1).
First, we calculate the evolution of the total atmospheric pressure due to the impact erosion using a
stochastic bombardment model [12]. We calculate the
surface age using the cumulative number of impacts and
an empirical curve obtained from the lunar craters [13].
The total pressure of 6 mbar or 100 mbar (corresponds
thermal/nonthermal escape
(with isotopic fractionation)
impact erosion
(without fractionation)
exobase
atmosphere
Introduction Mars currently has a cold and dry surface environment with a small amount of water-ice observed at the polar caps [1]. On the contrary, increasing
evidence suggests that the early Mars sustained a warm
climate with a large amount of liquid water [e.g., 2],
though it is controversial whether the warm climate was
episodic or permanent [3, 4]. Impact erosion and thermal/nonthermal escape have possibly contributed to the
loss of the early atmosphere and hydrosphere [5]. However, the timing of the escape and the relative importance
of each process are poorly constrained.
The thermal/nonthermal escape induce isotopic fractionation that leaves behind heavier isotopes in the atmosphere and hydrosphere, whereas the impact erosion removes a fraction of atmosphere without the isotopic fractionation. The early evolution of the atmosphere and hydrosphere is constrained by the isotopic data of the martian meteorite Allan Hills 84001 (ALH 84001), which
has a crystallization age of 4.1 Ga [6]. A high hydrogen
isotope ratio (D/H = 2-4 times the Martian primordial
water) at 4.1 Ga [7, 8] suggests that a larger amount of
water was lost during the first 0.4 billion years than the
later periods by the thermal/nonthermal escape [9]. On
the other hand, isotope ratios of nitrogen and noble gases
at 4.1 Ga show unfractionated values, implying that the
atmosphere was lost after 4.1 Ga [10, 11].
We study the evolution of the martian atmosphere
and hydrosphere considering their isotopic ratios. Comparing our results with isotopic data at 4.1 Ga recorded
in the martian meteorite ALH 84001, we propose a scenario that the loss of atmosphere and hydrosphere had
proceeded before 4.1 Ga. An efficient isotopic fractionation of nitrogen and noble gases due to the thermal/nonthermal escape started after the impact erosion
of the thick early atmosphere during the heavy bombardment period.
isotopic separation
homopause
homogeneous
vapor-liquid
equilibrium
hydrosphere
Figure 1: Schematic illustration of our model. Evolution
of atmosphere and hydrosphere is calculated considering
impact erosion and thermal/nonthermal escape.
to the possible amount of CO2 in the polar regions and
regoliths [14]) at present was assumed. The evolution is
calculated backward from present to 4.5 Ga assuming a
CO2 -dominated atmosphere. The erosion efficiency at
each impact is calculated using a modified sector blowoff model [15]. The momentum of an expanding silicate vapor is calculated using the entropy method [e.g.,
16, 17] and thermodynamic data for forsterite [18].
Second, we calculate the evolution of the isotope ratios of the minor volatile elements (D/H, 15 N/14 N, and
38
Ar/36 Ar) due to the thermal/nonthermal escape. The
initial surfacial water of 100 m GEL, which is almost
equivalent to the minimum estimate of the paleo-ocean
[26], is assumed. We assume the escape rates of the ion
pick-up, sputtering, and photochemical escape given by
[19, 20]. Hydrogen is lost by the Jeans escape whose escape rate is regulated by the loss of oxygen [21]. Oxygen
is assumed to be lost by the ion pick-up [22]. Nitrogen is
lost by the sputtering [19] and photochemical escape [23]
and argon by the sputtering. The fractionation factor of
hydrogen is assumed to be 0.016 [24, 25]. We adapt the
fractionation factors of other species tabulated in [19].
Results The evolution of the total atmospheric pressure due to the impact erosion is shown in Fig 2a. The
total pressure decreases in several orders of magnitude
during the first several hundred million years which corresponds to the heavy bombardment period, whereas the
change is relatively insignificant after this period. For
both cases (6 mbar and 100 mbar), the total pressure ex-
46th Lunar and Planetary Science Conference (2015)
1643.pdf
Atmospheric pressure [bar]
2
102
4.1 Ga, whereas the D/H ratio is lower than the data.
(a)
P(t=0) = 6 mbar
P(t=0) = 100 mbar
101
100
10-1
10-2
10-3
4
3
2
Time [Ga]
1
0
Degree of fractionation
1
References
solid lines:P(t=0) = 6 mbar
0.8 dashed-lines:P(t=0) = 100 mbar
D/H
0.6
15
38
(b)
N/14N
Ar/36Ar
0.4
0.2
0
Discussion The discrepancy in the hydrogen isotope
ratios can be explained by some additional mechanisms
of the oxygen loss because the escape rate of hydrogen
is determined by the escape rate of oxygen in our model.
Cold ion flow can be a dominant mechanism of oxygen
loss in the early Mars [30]. Also, the oxidation of surface
material would act as another oxygen sink [9].
Estimates of atmospheric nitrogen isotope composition of ALH 84001 vary significantly (∼ 7 per mil [10]
to > 200 per mil [31, 32]). Identification of the actual
nitrogen isotope ratio at 4.1 Ga would help to constrain
the early evolution of the martian surface environment.
4.5
4.4
4.3
4.2
4.1
4.0
3.9
3.8
Time [Ga]
Figure 2: (a) Evolution of total atmospheric pressure due
to impact erosion. The results of 10 Monte Carlo simulations are plotted for each case. (b) Evolution of isotopic
compositions due to thermal/nonthermal escape. Degree of fractionation, defined as (It −I4.5Ga )/(Ipresent −
I4.5Ga ) where It is the isotopic ratio at the time t, is plotted. Isotopic compositions at 4.1 Ga recorded in martian
meteorite ALH 84001 is shown [7, 8, 10, 20, 27, 28, 29].
ceeds one bar before 4.1 Ga. Because the early atmosphere is much thicker than that of the current Mars, the
difference in the current total pressure does not affect the
total pressure during the early period (> 4 Ga).
The evolution of the isotope ratios due to the thermal/nonthermal escape is shown in Fig. 2b. The nitrogen
and argon isotope ratios start to increase as the total pressure decreases. On the contrary, the D/H ratio increases
independently because the major reservoir of hydrogen
is the hydrosphere. The obtained nitrogen and argon isotope ratios agree with the isotopic data of ALH 84001 at
[1] Christensen, P. R. (2006) Elements, 2,
151-155. [2] Di Achille, G. & Hynek, B. M. (2010)
Nature Geosci., 3, 459-463.
[3] Halevy, I.
& Head, J. W., III. (2014) Nature Geosci., 7, 865-868.
[4] Ramirez, R. M. et al. (2014) Nature Geosci., 7, 5963. [5] Lammer, H. et al. (2008) Space Sci. Rev., 139,
399-436. [6] Lapen, T. J. et al. (2010) Science, 328,
347-351.
[7] Boctor, N. Z. et al. (2003) GCA, 67,
3971-3989. [8] Greenwood, J. P. et al. (2008) GRL,
35, L05203.
[9] Kurokawa, H. et al. (2014) EPSL,
394, 179-185. [10] Mathew, K. J. & Marti, K. (2001)
JGR, 106, 1401-1422. [11] Jakosky, B. M. & Phillips,
R. J. (2001) Nature, 412, 237-244. [12] Kurosawa, K.
et al. (2013) LPS XXXXIV, 2547. [13] Chyba, F. C.
(1991) Icarus, 92, 217-233.
[14] Lammer, H. et al.
(2013) Space Sci. Rev., 174,113-154. [15] Vickery,
A. M. & Melosh, H. J. (1990) Geol. Soc. Am. Spec.
Pap. 247, 289-300. [16] Ahrens, T. J. & O’Keefe, J.
D. (1972) Moon, 4, 214-249. [17] Kurosawa, K. et al.
(2012) EPSL, 337-338, 68-76. [18] Sekine, T. et al.
(2012) AGU Fall Meeting, MR32A-04. [19] Jakosky,
B. M. et al. (1994) Icarus, 111, 271-288. [20] Pepin,
R. O. (1994) Icarus, 3, 289-304.
[21] Liu,
S. C. & Donahue, T. M. (1976) Icarus, 28, 231-246.
[22] Luhmann, J. G. et al. (1992) GRL, 19, 21512154. [23] Fox, J. L & Dalgarno, A. (1983) JGR, 88,
9027-9033.
[24] Krasnopolsky, V. A. et al. (1998)
Science, 280, 1576-1580.
[25] Krasnopolsky, V.
(2000) Icarus 148, 597-602. [26] Head, J. W., III. et
al. (1999) Science, 286, 2134-2137. [27] Usui, T. et
al. (2012) EPSL, 357, 119-129. [28] Webster, C. R. et
al. (2013) Science, 341, 260-263. [29] Pepin, R. O. et
al. (2006) EPSL, 252, 1-14.
[30] Terada, N. et al.
(2009) Astrobiol., 9, 55-70. [31] Grady, M. M. et al.
(1998) Meteor. Planet. Sci., 33, 795-802. [32] Miura,
Y. N. & Sugiura, N. (2000) GCA, 64, 559-572.