Team:Brown-Stanford/PowerCell/Background

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Revision as of 00:30, 25 September 2011

Photosynthesis and 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 a rough estimate of the usefulness of photosynthesis on Mars by combining data obtained by planetary scientists. Additionally, we introduce additional environmental conditions that need to be factored into a more nuanced mathematical model (but were omitted because of their complexity).

Atmosphere

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.

Particulates in the Martian atmosphere

Dust is a huge part of the Martian environment on a local and global scale! Particulate matter in the atmosphere can have a dramatic impact on the amount of sunlight reaching the Martian surface. For a vivid example, look at the decreasing visibility in this surface view from the NASA Opportunity rover amidst a brewing dust storm. Were this kind of disruption sustained for too long, any settlement or machine wholly dependent on solar energy would fail.

FIGURE: Dust storm cloudy skies http://www.nasa.gov/images/content/182691main_mer-20070719. jpg ; caption http://www.nasa.gov/mission_pages/mer/images/20070720.html τ (greek tau) stands for optical density, which is a measure of how transparent the atmosphere is. τ = 0 represents perfect transparency

Excluding the effect of regional storms, which interfere with sunlight in a temporary but unpredictable way, there are other dust dynamics we can consider as more constant.

Using ground measurements taken by NASA rovers over time, we know that the visible optical depth of the Martian atmosphere is τ = 0.9 (Lemmon 2004). Put another way, this means that typically around 10^(-0.9) or 13%, of incoming sunlight is not scattered or absorbed before it hits the Martian surface.

Recent data gathered from satellites such as the Mars Reconnaissance Orbiter (MRO) have begun to shed more light on the distribution and layering of dust. In particular, the MRO is able to take vertical profiles in addition to top-down images, offering a three-dimensional view.

FIGURE McCleese 2010: cross sections of dust opacity, with latitude on the X axis and altitude on the Y (measured in pressure); each image is a different time of year

From satellite imagery, researchers have also compiled maps showing the global distribution of dust over a sample two-week span. This begins to illustrate how unified and variable the dynamics of atmospheric dust.

FIGURE GIF Caption: Images from http://tes.asu.edu/dust/, animation created by team

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

Estimating the extent of energy conversion