Scientists use their knowledge of Earth to search for evidence of past habitability on other planets like Mars. Students learned about this by comparing the atmospheres, hydrospheres, geospheres, and biospheres of Earth, Mercury, Venus, and Mars. Their analysis found that Mars is most similar to Earth, especially in its geosphere, as there is evidence volcanoes once erupted there and it has frozen water at its poles. This suggests Mars could be the best place in the solar system to search for signs of past life. The unit will have students analyze real Mars data to answer how scientists can search for evidence other planets were once habitable by applying what they know about the conditions needed for life from Earth.