Team:Brown-Stanford/PowerCell/Background

From 2011.igem.org

Photosynthesis on Mars

Photosynthesis on Earth represents an efficient way of converting solar to chemical energy on a large scale. However, photosynthetic output depends on variables such as atmospheric composition and amount of accessible sunlight. In this section we offer an estimate of the usefulness of photosynthesis on Mars by evaluating several environmental conditions.

Solar irradiance

The first factor to consider is the amount of solar energy available on Mars. Irradiance is a measure of how much solar energy reaches a planet from the Sun, and is calculated from the length of the day-night cycle, distance from the Sun, position of orbit, etc. (http://ccar.colorado.edu/asen5050/projects/projects_2001/benoit/solar_irradiance_on_mars.htm).

Figure: http://www-mars.lmd.jussieu.fr/mars/time/solar_longitude.html

Based on this method, solar irradiance at the mean distance between Mars and the Sun reaches a theoretical maximum of 590W/m^2 (assuming no distorting effects from the Martian atmosphere).

As a point of comparison, solar irradiance for Earth (measured from satellite instrumentation above the atmosphere) is approximately 1360W/m^2 (Li 2010). This means that Mars, which is 1.52 times as far from the Sun as Earth, receives 43% as much solar energy per m^2.

To achieve a more meaningful measure of irradiance for photosynthesis, however, we have to factor absorption in the atmosphere before solar rays reach the Martian surface.

Atmosphere

H2O Content

Water vapor content on Mars is measured from orbiting satellites (Smith 2001, Melchiorri 2006, Fouchet 2007). Cameras circling Mars can determine the presence of water by using spectrometers to image the planet and noting the intensity of peaks at certain wavelengths (http://tes.asu.edu/about/technique/index.html). From this information, they can quantify the amount of water vapor existing in a hypothetical column of atmosphere (this is measured in pr-µm, or micrometers of precipitable H2O)

FIGURE Melchiorri 2006: H2O column density at High Northern latitudes (think the “Arctic circle”) for Ls = 101-105

Planetary scientists have systematically recorded the density of atmospheric H2O column for the entire surface of Mars, compiling a global map of water vapor.

FIGURE Smith 2001: over entire surface, seasonally averaged water vapor column abundance (in um precipitation)

From this map it becomes clear that, as on Earth, there are variations in the distribution of atmospheric water vapor based on longitude and latitude. In particular, the Northern hemisphere contains significantly more atmospheric H2O than the South. The figures represent a seasonal average because there are also fluctuations in water vapor content corresponding to the phases of the Martian solar cycle.

FIGURE Smith 2001: Fluctuation of total atmospheric water vapor levels over time, northern and southern hemispheres

Near-surface water vapor, most relevant to uses in a settlement, is affected by these atmospheric readings. To determine water content in the lower atmosphere, researchers have looked at the role of surface-atmosphere flux in the Martian water cycle (Fouchet 2007, de Vera 2010). In recent models, up to 10% of the total vapor column exchanges with yet-to-be-determined subsurface H2O sources in a single day-night cycle (Fouchet 2007), suggests that a significant portion of atmospheric H2O can be accessible from the surface.

Particulates in the Martian atmosphere

Estimating the extent of energy conversion