the effect of martian surface geometry on ultraviolet fluxes

1. the effect of martian surface geometry on ultraviolet fluxes abstractThe atmosphere of Mars, unlike that of the Earth, does little to attenuate incoming ultraviolet radiation. Such large amounts of ultraviolet radiation are known not only to sterilize the hardiest of terrestrial organisms within minutes [1], but to chemically alter the soil such that organic molecules in the upper few cm are rapidly destroyed [2]. Thus the survival of any putative Martian life near the surface depends to a large extent on how much ultraviolet radiation is received at its location. As such, variations in small-scale geometry of the surface such as pits, trenches, flat faces and overhangs can have a significant effect on the observed flux and create “safe havens” for organisms. Furthermore, by quantifying this reduction in radiation we can better pick targets for landed missions knowing which geometries offer us the best chance of uncovering such a safe haven. chemistry conclusions and implicationsExamining the contour plots from each geometry we can draw the following conclusions: firstly, with regards to latitude we can see that pits are more effective at reducing incident flux at high latitude (since the sun rises much lower in the sky each day), that overhangs have a very slight dependence on latitude and that north-facing faces are most effective at mid-latitudes when the characteristic ratio is small and at high latitudes when the characteristic ratio is large. Secondly, comparing between geometries, the best protection is offered by overhangs followed by pits and cracks and then northward facing surfaces. Maximum survivability on these surfaces ranges from 100 000 years/g-of-C for flat northward facing surfaces to almost 370 million years/g-of-C for undersurfaces of overhangs. In many cases, the geometry is sufficient to protect organics on the timescale of an obliquity cycle (100 000 years) if there is sufficient starting material. Thus even if some process only allows for the production of organics at only one point during an obliquity cycle, these products could be preserved until the next warm and wet episode in these protective geometries. It should be possible for a landed craft searching in these specific zones to make a positive detection. Additionally, since these zones represent areas of reduced flux they may also imply reduced production of oxidizing chemicals. This would allow any microbial life living below the soil to more closely approach the surface in these areas. Thus these sheltered geometries may indicate areas where life may be found close to the surface. In order to convert the fluxes output by the model into a useful measure of the chemical and biological impact we need to know the efficiency of UV photons in destroying organics. In particular we make use of the efficiency factor derived experimentally by Stoker and Bullock [2] of 1.46±1.0E-6 molecules/photon in the spectral range of 0.20m to 0.24m. This can be used to determine the survivability of a glycine-like organic layer with an areal carbon density of 1g/m² in terms of Martian years. We are also able to update the UVA flux from 1997 to derive a mean destruction rate of 1.399E-4g of C/year corresponding to a northern-hemisphere wide average survivability of 7100 years/g in the case of a flat (billiard ball-case) Mars. Note that this figure is a minimum since our model does not yet consider the shielding effects of ozone. Note that ultraviolet radiation is typically divided up by wavelength into three bands. These are termed UVA (0.32m to 0.40m), UVB (0.28m to 0.32m) and UVC (0.20m to 0.28m). In this poster, the term “UV” will be used to refer to the sum of these three bands, however, survivability in the geometries section is obtained by considering only the restricted wavelength range described above. north faces methodA theoretical approach was taken to determine the appropriate fluxes. A numerical 1-Dimensional model based upon the doubling and adding method coded by Martin Tomasko and Lyn Doose to study the atmosphere of Titan [3] was adapted to the atmosphere of Mars. In this code, the atmosphere is divided up into discrete layers each of which may be populated with particles and gasses as desired to achieve the correct radiative transfer properties through that layer. Thus the main focus of the initial code development was finding appropriate values for dust and ice in the atmosphere. In the end it was decided to omit the ice particles since their abundance is not well known. Furthermore, studies of the scattering properties of the Martian atmosphere working from in-situ data effectively describe a composite particle with contributions from both dust and ice. As such we use a formulation of the particles due to Johnson et al [5] consistent with observations made by Viking [6], MPF [4,6] and MER [6]. Deconvolving these two contributions will be of great interest since small particles with radii near 0.2m will much more efficiently scatter radiation at UV wavelengths then does dust near 1.6m. The results for each geometry are obtained by considering radiance tables produced by the 1D code. These tables list the radiance of the sky at 11 discrete points in elevation and 76 points in azimuth for a total of 836 meshed points. The effect of the geometry is to “black-out” specific mesh elements of the sky and the sun at particular angles depending upon the time of day. Pits are a special case of a crack in which the width is equal to the length: since the exposure of the bottom of a crack will decrease as its length decreases pits present a good limiting case. As with north-facing faces, the geometry is illustrated in [P-B] along with the corresponding characteristic ratio. Also as in north-facing faces, a plot of typical variation over the day [P-C] and a contour plot of survivability against latitude and characteristic ratio [P-D] is also provided. While less common in currently explored regions of Mars, small pits (including craters) and cracks are identifiable in orbital imagery. An example of this blocking geometry found on the northern cap is provided in [P-A]. pits [P-A] [F-A] Characteristic ratio = Depth(d)/Height(h) [F-B] [P-C] [P-D] Characteristic ratio = Width(w)/Depth(d) [O-A] [F-C] J. E. Moores , P.H. Smith, and R. Tanner Lunar and Planetary Lab, University of Arizona 1629 E University Blvd, Tucson, Arizona, 85721. E : jmoores@lpl.arizona.edu billiard-ball mars results “Billiard-Ball” describes mars as being completely flat and without any surface geometry at all. This provides a good environment to examine the effect of increasing dust in the atmosphere as described by increasing the optical depth. Why is this important to a study on the shielding effects of geometry? While many sheltered environments on the surface will be able to provide a refuge from the sun’s direct beam, none will be able to entirely eliminate a view of the sky and hence diffuse illumination. As can be seen by the graphs below this diffuse contribution rapidly becomes important as dustiness increases, surpassing the direct beam as a radiation source along the contour shown. This effect is particularly pronounced at high latitude. In the surrounding panels we break out our results by geometry type, examining in turn flat north-facing surfaces, pits (a special case of a crack) and overhangs. In all cases, the optical depth used is 0.5, similar to the conditions observed by the Mars Pathfinder Spacecraft. Using the figures from the chemistry panel, we plot survival for each geometry in turn. = + [P-B] Idealized North Faces consist of vertical plates which have an east-west axis length equal to that of their height. This results in the geometry shown above in the cartoon [F-A] and gives a corresponding characteristic ratio. This ratio describes in relative terms how far our flux point (the black square) is located from the blocking plate. As this ratio increases we approach the billiard-ball mars case with no shelter being offered by the flat face (this property holds for all geometries). By varying the characteristic ratio, we can examine the amount of flux received on the black square [F-B] at different times of the day for a specific sol and latitude. Using the molecular destruction efficiency for organic compounds described in the chemistry panel this can then be converted into the time required to eliminate each gram of carbon from the surface. This survival time can be plotted against latitude and characteristic ratio [F-C] to demonstrate the effectiveness of the blocking geometry. Examples of plate-type blocking geometries are rocks on a plain or cliff faces. An example from the Mars Pathfinder Mission is presented below in [F-D]. While overhangs are expected to be rare they are an appealing target for this study since we expect them to be the best shelters. As with north-facing faces and pits, the geometry is illustrated in [O-D] along with the corresponding characteristic ratio. Also a plot of typical variation over the day [O-B] is shown. The contour plot of survivability against latitude and characteristic ratio is divided into two portions: the first, [O-C1], shows the lower surface being overhung, the second is located on the underside of the upper surface of an overhang, [O-C2]. Overhangs are rare in nature compared to the other geometries, however, as can be seen in [O-A] small scale features of this type do occur. Each landed spacecraft on mars also provides a shelter of this type. [O-C1] [O-C2] DIFFUSE TOTAL DIRECT PHOENIX DirectDominated Diffuse Dominated references [1] Schuerger, A. (2003) Icarus 165 pp.253-276 [2] Stoker, C. (1997) JGR 102 n°E5 pp.10881-10888 [3] Tomasko, M.G. (2005) Nature 438 n°7069 pp.765-778[4] Tomasko, M.G. (1999) JGR 104 n°E4 pp. 8987-9007[5] Johnson, J.R. (2003) Icarus 163 pp. 330-346 [6] Lemmon, M.T. (2004) Science 306 n°5702 pp. 1753-1756 Images: Background: PIA07997 Image Credit: NASA/JPL/Texas A&M/Cornell [F-D] portion of PIA02652 Image Credit: JPL[P-A] portion of MGS MOC Release n°MOC2-991 Image Credit: NASA/JPL/Malin Space Science Systems [O-A] portion of PIA03270 Image Credit: NASA/JPL-Caltech/Cornell [F-D] VL2 VL1 MPF (Sojourner) 0.2 5 0.2 5 0.2 5 MER-A MER-B Characteristic ratio =Height(h)/Width(w) [O-D] overhangs [O-B] A Sunset on Mars This true-colour image demonstrates both the dustiness of the Martian atmosphere and the role that geometry can play in the flux perceived by an observer. Were the sun just a few degrees to the left behind the Columbia Hills, the observed irradiation would be reduced. Note also the large fan-shaped cone of scattered light above the sun which shows that the proportion of energy transmitted diffusely, like the colour of the sunset itself, can be opposite our earthly experience.