2. spectral absorption features caused by molecular oxygen in Earth's atmosphere. The amount of O2 in our atmosphere is many orders of magnitude greater than is found on any other planet in the Solar System. An oxygen-rich atmosphere is a curiosity because oxygen slowly combines with rocks on the earth's surface. Maintaining the oxygen content requires some replenishing mechanism, in this case photosynthesis by plants -- the action of life.

3. infrared spectral lines caused by methane in the atmosphere. Although the amount of methane Galileo saw was miniscule -- about 1 part per million -- it is still important. In a oxygen-rich atmosphere like Earth's, methane should rapidly oxidize into water and CO2. Not a single molecule of methane would remain in equilibrium. Biological action such as bacterial metabolism in bogs replenishes the supply.

4. modulated narrowband radio transmissions. These emissions look nothing like natural sources of radio waves like lightning and plasma instabilities in Earth's magnetosphere. They are clear signs of a technological civilization.

Galileo's flyby of Earth was just the beginning of the first-ever control experiment in astrobiological remote sensing. The second part happened two years later, in 1992, when Galileo returned for a flyby of the moon.

Right: The false-color image of the Moon was taken in 1992 by the Galileo spacecraft enroute to Jupiter. The Sea of Tranquillity (Mare Tranquillitatis) is the blue area at right, the Ocean of Storms (Oceanus Procellarum) is the extensive blue and orange area on the left, and white lines radiate from the crater Tycho at bottom center. Three filters were used to make three separate exposures, combined in an exaggerated color scheme to emphasi
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Contact: Linda Porter
Linda.Porter@msfc.nasa.gov
256-544-7588
NASA/Marshall Space Flight Center--Space Sciences Laboratory
21-May-1999