Abstract
The problem with speculation about extraterrestrial life is that if such life exists, it is extraterrestrial, making it hard to study from here on earth. The best that we can do from our terrestrial vantage point is to look for signs of life on other planets. These signs can fall into two categories: 1) evidence that conditions essential for life are present and 2) direct evidence of the activity of living things. Recently there has been much excitement over data collected by the Mars Odyssey spacecraft that seems to indicate hydrogen on the surface of Mars. This hydrogen is thought to be in water molecules and water is essential for life. If water is present on Mars, one condition for life is met, but it does not prove that all conditions necessary for life are present on Mars.