Download Allen et al. Science submission 7May2013

Title: Cosmic pollution on Mars
Authors: Mark Allen1,2*, Inge Loes ten Kate3, George Cody4, Edwin Kite2, Karen
Willacy1, Jason Weibel5
Affiliations:
1
Science Division, Jet Propulsion Laboratory, California Institute of Technology, 4800
Oak Grove Drive, Pasadena, CA 91109, USA.
2
Division of Geological and Planetary Sciences, California Institute of Technology, 1200
East California Boulevard, Pasadena, CA 91125, USA.
3
Department of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD
Utrecht, The Netherlands.
4
Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road,
NW, Washington, DC 20015, USA.
5
Chemistry Department, Shenandoah University, 1460 University Drive, Winchester, VA
22601, USA.
*Correspondence to: [email protected].
Abstract: In the same way as Earth, Mars is regularly bombarded by particles derived
from asteroids and comets that contain organic material. 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 its burial 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). One recent estimate for the surface meteoritic organic abundance is ~400 ppmm
(parts per million by mass) that is based on a correlation with nickel detected on the
Martian surface and assumes distribution over the top few meters, but no chemical
degradation (3). A much lower estimate of ~4-10 ppmm, depending on latitude, was
based just on consideration of ultraviolet degradation and limited mixing with the
subsurface (4). With the advent of modern searches for organic chemicals on the surface
or in the near subsurface, including the recent landing of the NASA Curiosity 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 abundance of exogeneous organic material both on the surface and below
the surface. The abundances were 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 calculations herein was derived from a coupled
atmospheric chemistry/vertical transport model used to simulate atmospheres throughout
the solar system (5). 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 spatial resolution for the calculations was finer near the surface and
became coarser with increasing depth. The numerical values for the parameters in the
model were derived from the literature.
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 (6).
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, as found in primitive chondritic
material (6)
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 and polycyclic aromatic hydrocarbons
(PAHs), so we decided to treat the whole soluble faction as if it were one type of
compound. 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 has been discussed since the days of the Viking mission (7), most
recently inspired by the detection of perchlorate at the Phoenix lander site (8), 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 10-6 s-1. The published value was
calculated assuming overhead illumination (9), so the value we have adopted has been
adjusted downward by a factor of three to be consistent with a global diurnal average.
Alternatively PAHs could represent the soluble fraction; the published ultraviolet
decomposition rate (10) has been scaled downward as just discussed to provide an
alternative mean loss timescale of 5.7 x 10-6 s-1 for the soluble fraction. As the insoluble
fraction is not a specific molecular compound, there have been no published
measurements for ultraviolet decomposition. However, from the work of (4), 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
(11). Again there is no work published on insoluble cosmic ray decomposition, but (12)
have shown a scaling with mass that allows an insoluble cosmic ray inverse loss
timescale to be estimated from lower mass values (13), yielding a value of 5.3 x 10-15 s-1
based on a stoichiometry of C100H120O60N4 for the insoluble fraction (14).
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 (15) and is consistent with an experimental value of 200 – 500 μm (16).
The mean attenuation depth for cosmic ray penetration of 160 cm was derived from (13);
our value was scaled upwards by a factor of 1.54 to account for differences in density
(rock ~ 2 g cm-3 versus Mars soil ~ 1.3 g cm-3 (17)).
As will be seen below, the near-surface abundance of exogeneous organic
material is very sensitive to the local rate of burial by dust, silt and sand. Burial can slow
decomposition of surface organics if not previously decomposed when exposed on the
surface. Where layered sediments are forming, burial could be as fast as 30-100 μm/yr
(18). Small craters can be filled in at a rate ≤1 μm/yr (19). General resurfacing rates are
much slower, <0.01 μm/yr (20).
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 multiple burial rates. 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) are photodecomposition (solid line) and cosmic ray decomposition (dash line)
and, for the soluble organic fraction (blue lines), are photodecomposition with a rate
coefficient of 1.7 x 10-6 e(z/0.078) (solid line) and 5.7 x 10-6 e(z/0.078) (dash-dot line) and
cosmic ray decomposition (dash line). The black lines are the timescales for burial to a
depth of 1 centimeter at a rate of 100 μm/yr (solid line), 30 μm/yr (dot line), 1 μm/yr
(dash line), and 0.01 μm/yr (dash-dot line).
Table 2 shows the specific parameters used in the different model simulations. In
all cases, the computed abundances of meteoritic organics scales linearly with the
adopted value for the infall flux of meteoritic organics. Table 1 shows that this
assumption leads to a range in results of ~103 in each case, although this will not be
shown explicitly in the rest of this paper.
Assuming the continuous deposition of meteoritic organics with no chemical
decomposition and no burial, the Martian surface would be solidly covered with cosmic
pollution (model simulation 1). The inclusion of burial transports the surface organics to
large depths below the surface (model simulation 2, Figure 2 left). Model simulations 1
and 2, the latter with a meteoritic organic surface abundance of ~1 ppm, bracket the result
reported in (3). In model simulation 3, chemical decomposition was added to burial as
loss processes. As seen in Figure 1, the chemical decomposition timescales are much
shorter than the timescales for burial, with the result that substantial abundances of the
soluble meteoritic organics do not exist more than 1 μm beneath the surface (Figure 2
left). Figure 1 also shows that the chemical degradation timescales for the insoluble
organic fraction are comparable to burial timescales and, consequently, the insoluble
organic fraction abundance on and beneath the surface is not much different from model
simulation 1. Increasing the rate of soluble organic photodegradation by a factor of ~3
(model simulation 4) decreases the soluble organic distribution by an order of magnitude.
Decreasing the rate of burial from 100 μm/yr (model simulation 3) to 30 μm/yr
(model simulation 5) to 1 μm/yr (model simulation 6) and finally to 0.01 μm/yr (model
simulation 7) has opposite effects on the accumulation of the soluble and insoluble
organic fractions (Figure 2 right). As expected from the relative timescales in Figure 1,
expanded exposure time on the surface reduces the abundances of the meteoritic soluble
organics and more rapidly decreases the abundances with depth beneath the surface.
Since the lifetime of the meteoritic insoluble organic fraction is comparable to burial
timescales, slower burial timescales lead to increased surface accumulation. However, as
the burial rate decreases, subsurface cosmic ray degradation ultimately does limit the
depth at which substantial abundances exist. Over this range of burial rates, the surface
abundances of meteoritic insoluble organics exceed 1 ppmm (abundances bracket the
results of (4)).
Surface material burial rates of 30 - 100 μm/yr may be appropriate for the
environment that Curiosity will be sampling. Burial rates of ~1 μm/yr may be appropriate
for the sites in which the ExoMars lander will arrive. In both cases, Figure 2 shows that
the meteoritic organics may be in many ppmm amounts in materials collected on the
surface or retrieved from many meters depth.
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: (left) model
simulations 2 - 4 (Table 2) are indicated by solid, dot, and dash lines, respectively; (right)
model simulations 3, 5 – 7 are indicated by solid, dot, dash, and dash-dot lines,
respectively. The sharp bends in lines are numerical artifacts of the variable model layer
thickness with depth below the surface.
Figure 2 shows that the meteoritic organic infall flux can result in ppmm surface
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 (21). 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 only slightly decomposes in laboratory experiments
above 600 °C (22), the presence of these organics on the surface was undetectable. In
such 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 (23),
the predicted meteoritic organic surface abundance should be detectable.
The possible high level of cosmic pollution 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 an 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 (24), may be dominated by cosmic
pollution.
References and Notes:
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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, California Institute of
Technology, under a contract with the National Aeronautics and Space Administration.
Table 1. Model parameters.
Insoluble organic
fraction
Soluble organic fraction
8.9 x 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 10-10 e(z/0.078)
1.7 – 5.7 x 10-6 e(z/0.078)
Cosmic ray
decomposition inverse
mean timescale (s-1)
5.3 x 10-15 e(z/123)
1.3 x 10-16 e(z/123)
Burial velocity (cm s-1)
3.2 x 10-14 – 3.2 x 10-10
Surface deposition flux
(gm cm-2 s-1)
See text for references. z is distance below surface in cm.
Table 2. Model simulation parameters.
Model
simulation
1*
2*
Surface deposition flux
(gm cm-2 s-1)
Insoluble
organics
2.4 x 10-16
2.4 x 10-16
Soluble
organics
5.9 x 10-17
5.9 x 10-17
Ultraviolet
decomposition
inverse mean
timescale (s-1)
Soluble organics
Burial velocity
(cm s-1)
0
0
0
3.2 x 10-10
3
2.4 x 10-16
5.9 x 10-17
1.7 x 10-6 e(z/0.078) 3.2 x 10-10
-16
-17
4
2.4 x 10
5.9 x 10
5.7 x 10-6 e(z/0.078) 3.2 x 10-10
-16
-17
5
2.4 x 10
5.9 x 10
1.7 x 10-6 e(z/0.078) 9.5 x 10-11
-16
-17
6
2.4 x 10
5.9 x 10
1.7 x 10-6 e(z/0.078) 3.2 X 10-12
-16
-17
7
2.4 x 10
5.9 x 10
1.7 x 10-6 e(z/0.078) 3.2 X 10-14
Note: Values for other parameters in the model simulations are given in Table 1, unless otherwise noted.
*All decomposition rates are zero.