Why can't we detect water on Mars in the same way as in exoplanets? Often I see news articles claiming astronomers found exoplanets with water. If we are capable of detecting water on such distant objects, why don't we employ similar methods to find water on Mars or Titan moon?
We can look up for water in the atmosphere of the planet, not at the planet itself. We can do that when a planet passes in front of a star. That tehnique is called transmission spectroscopy.