Nitrates on Mars: Evidence from the 15/14N isotopic ratio

I. The 14/15N isotopic ratio of nitrogen in the bulk atmosphere of Mars is 170 +/- 15, while primitive nitrogen in SNCs is about 278, a fractionation factor of 1.62. Fractionating effects imply the 14/15N ratio of loss to space is between about 265 and 296 [1,2], consistent with primitive N. Estimates of N escape, 3--8E5 cm-2 s-1, imply the current time-scale for its removal is between 500 and 1300 Myr. These findings imply a source of primitive nitrogen, and a steady state fractionation, as suggested by Wallis [1]. However, our modeling of juvenile outgassing predicts an outgassing rate an order of magnitude lower than modeled escape. Here, we suggest that nitrate decomposition by impacts is the source. II. Judging from estimates of the water inventory [3], and comparison to Earth, Mars probably had between 250 and 600 mbars of N2 [1]. With the approximate 20:1 C:N ratio, this suggests between about 5 and 10 bars of CO2. During the period of intense bombardment, atmospheric nitrogen is subjected to shock heating which allows equilibrium reactions between the dissociated atoms [4,5]. Simulation of impact processes under the evolving impact flux [6] suggests that about a quarter of the initial, atmospheric nitrogen is fixed, forming nitrates in the soil -- about 60-150 mbars. III. During Mars' history, fractionating loss of N to space increases the bulk atmospheric fractionation. However, in analogy to carbonates [7], nitrates may be decomposed by the shock/heating of impacts, a process that moderates 14/15N. Nitrates are decomposed to a radius about twice that of the impactor, providing a slow recycling of N. IV. However, the distribution of nitrates in the soil affects the quantity needed to explain the current 62% fractionation. This is because the distribution of impactors is heavily weighted towards the low-mass end [8]. A concentrated surface deposit minimizes the amount of nitrates needed, while deep nitrates are out of reach of the numerous, small impactors. But nitrates are highly soluble, and are probably leached from the surface, as chlorides are at Endurance or nitrates at Atacama. If nitrates become entrained in the hydrosphere, the formation of the cryosphere would tend to produce regions of concentrated brines, perhaps near the surface where nitrate concentrations would be high. Much of the nitrates may have been washed into the northern lowlands. V. If 100 mbars of nitrates exist in the uper crust, a model consisting of a layer 100 m deep with 10% mass fraction of NaNO3 is plausible. The corresponding mass fraction of N would be 1.6% in the layer. References: [1] {Wallis}, M.~K., 1989. E&PSL, 93, 321--324, [2] Fox, J.~L., 1993. J Geophys Res, 98, 3297-3310, [3]Carr , M.~H., 1986. Icarus, 68, 187--216, [4] {Mancinelli}, R.~L. and {McKay}, C.~P., 1988. Origins of Life, 18, 311-325, [5] {Navarro-Gonz{á}lez}, R., et al. \nat, 412, 61-64, [6] Manning, C.~V., et al.,(2006). Icarus, 180, 38--59, [7] {Carr}, M.~H. 1989. Icarus, 79, 311--327, [8] Brown, P. et al., 2002. Nature 420, 294-296.