At the Astrobiology department within the LISA, an Atmospheric Systems Laboratory. (LISA – Laboratoire Inter-universitaire des Systèmes Atmosphériques) Créteil, France During my internship, I have to improve a computing program which simulates the abundance of organics at the surface and in the subsurface of Mars. I explained previously how the “ModelMars” program works to calculate the abundance of organics at the Martian surface under UV radiation. My next step is obviously to explain how the “ModelMars” program works to calculate the abundance of organics in depth under UV radiation. I - “DepthCalculation.f90” file functioning Aim of the file : Determine the organic abundance of Glycine or Polycyclic aromatic hydrocarbon (PAHs) in the subsurface of Mars. Hypothesis : All organic matter is destroyed on the Martian surface because the destruction rate of organics is higher than the supply of organics thanks to micrometeorites. However, there is a possibility that a small quantity of organic matter can be preserved in the subsurface of Mars. - Reason 1. Micrometeorites, containing organic matter, arrive at the surface of Mars with a speed which allows them to bury themselves under the Martian surface. - Reason 2. The Martian surface is composed of regolith*. Due to storms, regolith dust can be found in suspension in the atmosphere and then accumulates on the surface. These events may allow to bury and protect organic matter contained in micrometeorites. * Regolith : “Regolith is a layer of loose, heterogeneous superficial material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestrial planets and moons.” (En.wikipedia.org, 2017) To determine the photons flux at different depth in the regolith soil, we used the Beer-Lambert law : Instead of Intensity, we replace it by Flux. (KHALFALLAH, B., 2016) UV(Z) = I0*exp(- α *Z) (1) UV(Z) : UV Flux according to the depth Z (protons/km2/yrm) Z : Depth under the surface in micrometers (µm) I0 : UV Flux arriving at the Martian surface which depends on the latitude (protons/km2/yrm) α : The absorption constant of UV by the regolith [equal to 0,027 µm-1 (Caro, Mateo-Martí and Martínez-Frías, 2006) ] Thus, to model the evolution of the organic matter concentration, we used the program to simulate the degradation of organic matter during few years on Mars. Thanks to these simulations, we obtained pictures that show us if the organic matter resists or if it is destroyed by UV radiation. It gives us also indications about the destruction rate of organics according to their species and at which depth it is possible to detect them. II – My contribution to this part of the program I will not resume all modifications that I have made during the first part of my internship. For every problem met since the beginning of my internship, I was able to find solutions and improve this part of the program. If you want more details, just ask ! Now, let’s see the first results ! III – Destruction of organics by UV radiation after 1 year This is our hypothesis before any simulation : Hypothesis : PAHs will be more difficult to destroy than Glycine. Explanation : A molecule containing many atomic bonds requires more energy to break all of these bonds and thus destroy the molecule. As PAHs contain more atomic bonds than Glycine, It seems logical that PAHs take more energy and thus more time to be destroyed. After some simulations, this hypothesis came true ! However, I am not allowed to give you more details. I just will show you which kind of results we can obtain. On the Figure 1, you can observed the initialization of the computer program. At the beginning of the simulation, the abundance of organic matter is equal to 2e+18 molecules/km^2/yrm at any depth (where yrm is a Martian year). This represents hypothetic sources of organic matter in the past, exogenous or endogenous. On the Figure 2, you can observe how organics may be destroyed in depth, based on the equation (1) above. IV – Destruction of organics by UV radiation during 10 years The simulation is divided in several steps : 1. Add Sources (Add the flux of organics brought by micrometeorites) 2. Subtract Losses (Subtract the flux of organics destroyed by UV radiation) 3. Add the burying factor (Add the burying of the organic matter under 400 microns of regolith dust, every year, to simulate the uprising of dust created by storms.) On the Figure 3, you can observe that the organic matter is buried under 400 microns each year and thus, the organic matter is pushed deeper in the soil each year. V – What you have to remember !
Bibliography Caro, G., Mateo-Martí, E. and Martínez-Frías, J. (2006). Near-UV Transmittance of Basalt Dust as an Analog of the Martian Regolith: Implications for Sensor Calibration and Astrobiology. Sensors, 6(6), pp.688-696. En.wikipedia.org. (2017). Regolith. [online] Available at: https://en.wikipedia.org/wiki/Regolith [Accessed 27 Mar. 2017]. KHALFALLAH, B. (2016). Modélisation de l’abondance des molécules organiques sur Mars. Stage effectué au Laboratoire Inter-universitaire des Systèmes Atmosphériques (LISA). pp.9-10. Posted by Alexandra Perron on April 03, 2017
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