What kind of atmosphere does Mars have?

Certainly, quite inhospitable. Thin Martian atmosphere is composed of 95% carbon dioxide, 2.7% nitrogen, 1.6% argon and traces of several other gases, including 0.15% oxygen. The abundance of CO2 is simply forbidding. Besides, surface atmospheric pressure is less than 1% of the Earth’s atmospheric pressure, though it does vary by season because most of the atmosphere becomes frozen at the planet’s polar caps during the winters. A jet plane flying at about 1,00,000 feet (30,500 meters) from the Earth’s surface would encounter an atmospheric pressure similar to the surface of the red planet Mars.
Temperatures on Mars vary widely between day and night, winter and summer, mainly because it is about 80 million kilometers farther from the Sun than the Earth. In the northern hemisphere, summer temperatures range from a low of -88 degrees Celsius just before dawn to a high of -12 degrees Celsius in the mid afternoon. Polar temperatures during the winter drop as low as -140 degrees Celsius.
If an astronaut stepped from his spacecraft onto the Martian surface without protection, he would suffocate from lack of oxygen after only half a minute or so — much before he would freeze from the cold or would get scorched by cosmic radiation, the influx of which is about 100 times as intense as that on Earth.

Additional reading:
Atmosphere of Mars (Wikipedia)

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