Indirect sunlight

Since the sky of Mars scatters light, the sunlight comes from a range of angles, rather than in a straight line from the sun. During a relatively clear day, the indirect (scattered) component is relatively low (e.g., 30% of the total sunlight is indirect for optical depth 0.4.) When the optical depth is high, however, over 99% of the total sunlight reaching the surface is indirect.

The implication of this for solar cell selection is that concentration technologies (such as mirrors or lenses) will be much less effective than planar technologies which accept light from a wide range of angles: the efficiency of concentration devices will be worst at the highest dust loadings and this is when efficiency is most critical. It also means that physical spectrum-splitting devices, such as prisms or gratings, will probably be not be effective on Mars, since they generally require collimated light to function.

And the indirect component of the light is also significantly spectrally shifted toward the red from the direct sunlight.

In practical applications, solar cells do not operate under standard conditions. The most important effect that must be allowed for are due to the variable temperature. Solar arrays typically operate at temperatures between 50 and 100 Celsius. For this reason, a cell technology is usually selected to have a low coefficient of temperature degradation. On Mars, however, the temperature is lower than the standard temperature therefore a high temperature coefficient is in fact desirable as it has a higher efficiency. This shifts the technology choice toward a material with a lower band gap rather than high band gap materials, such as silicon. Furthermore, the most significant is that the temperature dependence of the voltage decreases with increasing temperature. In a silicon cell, the voltage decrease typically at a rate of 2.3 mV per °C. Since Mars environmental factors tend to favour lower band gap cells, the result will be that cells designed for Air-Mass zero conditions will tend to become current-limited by the top (highest band gap) of the cell.

The dust on Mars deposits out of the atmosphere and onto any flat surface; the time scale for this settling has been measured to be on the order of 100 days. On the solar arrays, a measurement on the Pathfinder mission indicated dust coverage rate of 0.3% power loss per day. This atmospheric dust will have several effects on the use of photovoltaic power systems on the surface, including decreasing the amount of sunlight on the surface and shifting the spectrum of the available sunlight, so some techniques must be developed to periodically remove the dust. Dust is expected to adhere to the array by Van der Waals adhesive forces. These forces are quite strong at the dust particle sizes. If the array surface is insulating, it is possible that we may also find electrostatic static cling, which is extremely strong. Dust-removal methods must overcome this force.

A technique combined electrostatic and Mechanical removal can remove those dusts. The array could be charged by putting incorporating a transparent conductor on the surface, and temporarily charging the array with a high-voltage supply, the dusts will be repelled from the array. And then rotate the arrays into a vertical orientation for dust to fall. This could be done with the motors used to deploy the arrays.

Radiation environment different from Earth orbit

The radiation environment includes ultraviolet (UV), primarily high-energy proton radiation and electron radiation). The radiation environment of Mars is quite good since the Mars has no trapped radiation belts and the shielding provided by the Mars atmosphere can protect the surface from such radiation.

Atmospheric pressure

The atmosphere of Mars consists of primarily carbon dioxide, the pressure is slightly lower than 1% of the pressure at the Earth's surface, but varying with landing site elevation and season. This atmospheric pressure is close to the Paschen minimum for plasma breakdown, and thus sets a significant limit to the maximum voltage (about 400V) which can be applied to any exposed conductors