Of course it is still a topic of debate in terms of what did or did not exist on Mars.  Here is a brief background on the actual Mars atmosphere as noted from wikipedia:

The atmosphere of Mars is the layer of gases surrounding Mars. Like that of Venus, it is composed mostly of carbon dioxide, but is far thinner. The atmospheric pressure on the Martian surface averages 600 pascals (0.087 psi), about 0.6% of Earth’s mean sea level pressure of 101.3 kilopascals (14.69 psi) and only 0.0065% ofVenus‘s 9.2 megapascals (1,330 psi). It ranges from a low of 30 pascals (0.0044 psi) on Olympus Mons‘s peak to over 1,155 pascals (0.1675 psi) in the depths of Hellas Planitia. This pressure is well below the Armstrong limit for the unprotected human body. Mars’s atmospheric mass of 25 teratonnes compares to Earth’s 5148 teratonnes with a scale height of about 11 kilometres (6.8 mi) versus Earth’s 7 kilometres (4.3 mi).

The Martian atmosphere consists of approximately 96% carbon dioxide, 1.9% argon, 1.9% nitrogen, and traces of free oxygen, carbon monoxide, water and methane, among other gases,^[1] for a mean molar mass of 43.34 g/mol.^[2][3] There has been renewed interest in its composition since the detection of traces of methane in 2003^[4][5]that may indicate life but may also be produced by a geochemical process, volcanic or hydrothermal activity.^[6]

What do you think did Mars have a much more extensive atmosphere in the past?  And if so what do you think could have changed it?