Credit: ESA/CNRS Verrieres
On July 3, 2003, thirty-one days after launch and from a distance of about 7 million km, the SPICAM instrument on board Mars Express was turned toward Earth. The main scientific objective of SPICAM is to observe both ozone and water vapor in the atmosphere of Mars. This test was to see how well SPICAM could detect ozone on Earth. Here, ozone forms a natural screen that protects life on Earth from harmful ultraviolet (UV) solar radiation. However, on Mars, the quantity of ozone is about 100 times less than that on Earth, making survival on the surface of Mars very difficult for any lifeforms.
In this first graph, above, the red line shows the simulated results that would be expected from SPICAM if the Earth's atmosphere had no ozone; i.e., if the atmosphere was not able to absorb ultraviolet radiation. The blue line is the simulated results with a realistic Earth atmosphere containing ozone.
The second graph shows the actual results obtained by SPICAM. The blue line indicates the spectrum (the intensity of light as a function of wavelength) of the Earth in ultraviolet light. This light comes from solar light scattered by the atmosphere of the Earth back to outer space. Only the peak just above 300 nm is significantly above the level of "noise." The red line, on the other hand, is the spectrum of the Sun in ultraviolet light. This is the amount of ultraviolet light the Earth would receive if the Earth didn't have the thick atmosphere with ozone that it has. (Conversely, the red line indicates the amount of ultraviolet radiation the moon does receive as, of course, the moon has no atmosphere to protect it.) The difference between the two lines, then, is the amount of ultraviolet radiation the Earth's atmosphere absorbs. As you can see, the ozone layer absorbs most of the ultraviolet radiation below 300 nm, preventing harmful effects on the DNA molecules of all living species exposed to the Sun.
As Jean-Loup Bertaux, principal investigator from Service d'Aéronomie/IPSL (the agency responsible for SPICAM), drolly said, "Together with the OMEGA infrared spectrometer detection of water vapor and oxygen, the detection of copious amounts of ozone in the atmosphere indicates that this planet that we call Earth could sustain life."