Title: Cosmic Pollution on Mars

Title: Cosmic pollution on Mars

Authors: Mark Allen1,2*, IngeLoes ten Kate3, George Cody4, Edwin Kite2, Karen Willacy1, Jason Weibel5

Affiliations:

1Science Division, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA.

2Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA.

3Department of Earth Sciences, UtrechtUniversity, Budapestlaan 4,3584 CD Utrecht,The Netherlands.

4Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, DC 20015, USA.

5Chemistry Department, Shenandoah University, 1460 University Drive, Winchester, VA 22601, USA.

*Correspondence to: .

Abstract: As for Earth, Mars is regularly bombarded by particles derived from asteroids and comets that contain organic material to varying degrees. Due to the Martian thin atmosphere, much of this organic material reaches the Martian surface relatively unprocessed. We have modeled the accumulation of this exogeneous organic material—cosmic pollution in effect—on the surface and burial to depths beneath the surface, taking into account decomposition due to ultraviolet radiation and galactic cosmic rays. The computed abundance of exogeneous organic material is a few parts per billion to a few parts per million by mass at the surface and for many meters beneath the surface. This material is predominantly insoluble polymeric organics. At these abundances, the meteoritic organics should be detectable by present and future landed experiments. As such, this amount of cosmic pollution can confound current and future plans to detect endogenous organic material that might be evidence for extinct or extant Martian life.

One Sentence Summary:Exogeneous organic material regularly deposited on the Martian surface may overwhelm attempts to detect endogeneous organic material that could be evidence for extinct or extant life.

Main Text: The Martian atmosphere is regularly bombarded by organic-containing particles derived from asteroids and comets, as is the case for the Earth (1). Due to the thin Martian atmosphere, many of these particles reach the surface relatively unaltered (2). The predicted total mass of cosmic material landing on the Martian surface is between 17 and 370 gm m-2 my-1(2). If the particles are of asteroidal origin, this would imply a flux of carbon to the surface between 35 and 767 mg m-2 my-1, while if of cometary origin, between 4.2 and 92 gm m-2 my-1 is landing on the Martian surface(3). The chemical composition of the carbon-containing phases of these particles is fairly similar whether derived from asteroids or comets—80% insoluble polymeric organic and 20% soluble organic molecules, such as amino acids.

With the advent of modern searches for organic chemicals on the surface or in the near subsurface including the recent landing of the NASA Curiousity mission, a joint ESA-Roscosmos mission later in this decade, and planning for sample return, we chose to reexamine the implications of the influx of this cosmic pollution. We computed the expected surface abundance and abundances at depths beneath the surface of exogeneous organic material. The abundance at the surface and below was computed as a balance between the exogeneous flux to the surface, chemical decomposition as a function of depth beneath the surface, and physical burial due to aeolian or sedimentation processes.

The model used for the calculation herein was derived from a coupled atmospheric chemistry/vertical transport model used to simulate atmospheres throughout the solar system (4). However, for the purposes of this paper, the surface is the top boundary of the model and transport to the subsurface is what counterbalances local chemical processes. In addition, the vertical transport is treated as advection, not diffusion. The numerical values for the parameters in the model were derived from the literature.

As the chemical loss rates are a function of the chemical composition of the carbon-containing components of the exogeneous material, we treated separately the insoluble polymeric fraction and the soluble fraction. Of the possible molecules comprising the soluble fraction, we only could find in the literature measured or computed decomposition rates for amino acids, so we treat the whole soluble faction as if it were simply composed of amino acids. For the purposes of the calculation herein, we assume that the organic fractions are isolated from and not protected by the inorganic matrix in which the organic fractions are delivered to the surface (if otherwise, the organic fractions will be protected, more stable, and even more abundant on the surface or in the near subsurface than estimated below).

There are three forms of chemical degradation discussed in the literature—oxidation processes, photolytic decomposition, and decomposition by ionizing radiation, particularly galactic cosmic rays. While oxidative decomposition of organic molecules on the Martian surface as been discussed since the days of the Viking mission (5), there are no published oxidative decomposition timescales. In previous work, the consequences of ultraviolet and cosmic ray decomposition were treated separately; in this work we consider the consequences of these processes acting simultaneously. Ultraviolet decomposition of amino acids has been quantified for Mars conditions and we adopt a value for the inverse mean loss timescale of 1.7 x 10-6s-1. The published value was calculated assuming overhead illumination (6), so the value we have adopted has been adjusted downward by a factor of three to be consistent with a global diurnal average. As the insoluble fraction is not a specific molecular compound, there have been no published measurements for ultraviolet decomposition. However, from the work of (7), we derived an inverse mean decomposition lifetime of 1.3 x 10-10 s-1. Cosmic ray decomposition has been studied for amino acids; we adopt of value of 1.3 x 10-16 s-1 based on the work of (8). Again there is no work published on insoluble cosmic ray decomposition, but (9) have shown a scaling with mass that allows a insoluble cosmic ray inverse loss timescale to be estimated from lower mass values (10), yielding a value of 5.3 x 10-15 s-1 based on a stoichiometry of C100H120O60N4 for the insoluble fraction (11).

The calculation includes these processes varying with depth beneath the surface. The mean attenuation depth for ultraviolet radiation of 0.078 cm (780 μm) was derived from the work of (12) and is consistent with an experimental value of 200 – 500 μm(13). The mean attenuation depth for cosmic ray penetration of 160 cm was derived from (10); our value was scaled upwards by a factor of two to account for differences in density (rock ~ 2 gm cm-3 versus Mars soil ~ 1 gm cm-3(14)).

Low areas on Mars may be filled in with sand from wind erosion. Sediments have been laid down over time. Either process can bury exogeneous organic material deposited onto the surface if not otherwise decomposed when exposed on the surface. For the calculation herein we have estimated a burial rate between 30 and 100 μm/yr from various published estimates for dust deposition and sediment layer formation (15-17).

To test the model formulation, we used the values in (7) for surface accretion of meteoritic carbon and ultraviolet decomposition. We have used a slow burial velocity since (7) modeled some mixing in the near-surface volume. The computed surface abundance for insoluble organics of 1.7 parts per million by mass (ppmm) is comparable to the estimate of ~ 4 ppmm carbon load at low latitudes.

Table 1 summarizes the model parameters adopted for the model simulations reported herein. Figure 1 shows the chemical loss mean timescales corresponding to the photodecomposition and cosmic ray decomposition processes in Table 1. Also in Figure 1 is the timescale for burial to 1 cm depth at a burial rate of 30 μm/yr. These timescales will be useful in understanding the model simulation results that follow. If oxidative loss timescales are ever computed for the Martian surface, these values can be compared to the timescales shown in Figure 1 from which the effect of oxidative loss on exogeneous organic abundances can be estimated.

Fig. 1. Mean timescales for loss of meteoritic organic fractions deposited on the Martian surface as a function of depth below the surface. Shown for the insoluble organic fraction (red lines) and soluble organic fraction (blue lines) are the photodecomposition (solid lines) and cosmic ray decomposition (dashed lines) timescales. In addition the black line is the timescale for burial to a depth of 1 centimeter at a rate of 30 μm/year.

Table 2 shows the specific parameters used in the different model simulations. Within the range of parameter values in Table 1, the maximum surface meteoritic organic abundance is calculated by using the maximum surface deposition fluxes for insoluble and soluble organics and the slowest burial rate (model simulation 1) (Figure 2). Note that the chemical loss processes for insoluble organics are comparable to or much slower than burial so the insoluble organic abundance is large all the way to several meter depths. On the other hand, chemical loss for the soluble organic fraction exceeds burial with the result that the soluble organic fraction fully decomposes within ~40 μm of the surface. Scaling up the burial rate by a factor of ~3 (model simulation 2) leads to opposite effects for the two different meteoritic organic fractions. For the chemically long-lived insoluble fraction, faster burial just redistributes material from the surface reservoir to depths below the surface and thus the reduction in the near-surface abundance. On the other hand, the faster burial redistributes some of the short-lived soluble organic fraction to depth below the surface where the material is more protected from ultraviolet decomposition, thus leading to a small enhancement in abundance in the near-surface layer. When the surface deposition fluxes are reduced to the low end of the values in Table 1, the model simulation 3 results show the abundances being reduced accordingly.

Fig. 2. Mixing ratios for meteoritic insoluble organic fraction (red lines) and soluble organic fraction (blue lines) as a function of depth below the surface. Model simulations 1-3 (Table 2) are indicated by solid, dashed, and dotted lines, respectively.

Figure 2 shows that meteoritic organic infall flux can result in ppbm to ppmm mixing ratios that are well within Mars lander instrument detection capabilities. The Viking Organic Analysis Experiment had a nominal detection limit of ppbm for simple organic compounds (18). However, since this experiment only heated samples up to 500 °C, it was recognized at the time that complex polymeric material would not have been detected. As the meteoritic insoluble organic fraction is the dominant exogeneous contribution to surface materials and decomposes in laboratory experiments only above 600 °C (19), the presence of organics on the surface was undetectable. In these laboratory experiments, thermal decomposition of the meteoritic insoluble organic phase yields a range of lighter organic compounds. Since the Curiosity Sample Analysis at Mars instrument suite heats samples well above 600 °C and has ppbm sensitivity (20), the predicted meteoritic organic surface abundance should be detectable and might even be the dominant signal.

The high level of cosmic pollution possibly on the surface of Mars and to large depths below the surface can likely interfere with attempts to detect endogenous organic compounds that could serve as evidence for extinct or extant life. It may not be possible to drill to depths that are not dominated by the exogeneous organic material. Even in samples brought back to Earth, the organic component likely may be primarily exogeneous and overwhelm any remnant from a endogeneous source. Finally, the bombardment of Mars by organic-containing cosmic material extends back to the origin of Mars with the consequence that the organic content in rocks and sediments, and Martian meteoritic material found on the Earth surface (21), may be dominated by cosmic pollution.

References and Notes:

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14.K. E. Herkenhoff et al., in The Martian Surface: Composition, Mineralogy, and Physical Properties, J. F. Bell, Ed. (Cambridge University Press, New York, 2008), pp. 451-467.

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19.G. D. Cody et al., Organic thermometry for chondritic parent bodies. Earth and Planetary Science Letters 272, 446 (2008).

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Acknowledgments: We thank Y. L. Yung and R.-L. Shia for their assistance with the computer model used in this work. This research was carried out at the Jet Propulsion Laboratory, CaliforniaInstitute of Technology, under acontract with the National Aeronautics and SpaceAdministration.

Table 1. Model parameters.

Insoluble organic fraction / Soluble organic fraction
Surface deposition flux (gm cm-2 s-1) / 8.9x 10-20 - 2.4 x 10-16 / 2.2 x 10-20 – 5.9 x 10-17
Ultraviolet decomposition inverse mean timescale (s-1) / 1.3 x 10-10e(z/0.078) / 1.7 x 10-6e(z/0.078)
Cosmic ray decomposition inverse mean timescale (s-1) / 5.3 x 10-15e(z/160) / 1.3 x 10-16e(z/160)
Burial velocity (cm s-1) / 9.5 x 10-11 – 3.2 x 10-10

See text for references. z is distance below surface in cm.

Table 2.Model simulation parameters.

Surface deposition flux (gm cm-2 s-1) / Burial velocity (cm s-1)
Insoluble organics / Soluble organics
Model simulation 1 / 2.4 x 10-16 / 5.9 x 10-17 / 9.5 x 10-11
Model simulation 2 / 2.4 x 10-16 / 5.9 x 10-17 / 3.2 x 10-10
Model simulation 3 / 8.9x 10-20 / 2.2 x 10-20 / 3.2 x 10-10