Because of the proximity and likeness of Mars to Earth, scientists have long speculated about the possibility of life on Mars. As early as the mid-17th century, astronomers observed polar ice caps on Mars, and by the mid-19th century, scientists discovered other similarities to Earth, including the length of day and axial tilt. But in 1965, photos taken by the Mariner 4 probe revealed a Mars without rivers, oceans or signs of life. And in the 1990s, it was discovered that Mars, unlike Earth, no longer possessed a substantial global magnetic field, allowing celestial radiation to reach the planet's surface and solar wind to eliminate much of Mars's atmosphere over the course of several billion years. 

More recent probes have focused on whether there was once water on Mars. Some scientists believe that this question is definitively answered by the presence of certain geological landforms. Others posit that different explanations, such as wind erosion or carbon dioxide oceans, may be responsible for these formations. Mars rovers Opportunity and Spirit, which have been exploring the surface of Mars since 2004, have both discovered geological evidence of past water activity. These findings substantially bolster claims that there was once life on Mars.

In the first paragraph, the author most likely mentions the discovery of polar ice caps to suggest that