Which planet has polar ice caps and a thin atmosphere?

Mars is the correct choice because it is known for having polar ice caps composed primarily of water ice and dry ice (frozen carbon dioxide). These ice caps change with the Martian seasons, growing and shrinking as temperatures fluctuate. Additionally, Mars has a thin atmosphere, which is mostly carbon dioxide, with very little oxygen or other gases found in more substantial atmospheres like Earth's. This thin atmosphere contributes to the harsh and cold environment on Mars, where the surface temperature can drop significantly, particularly at the poles.

In contrast, while Earth does have polar ice caps and a much thicker atmosphere that supports life, it does not match the characteristics of having a thin atmosphere like Mars. Saturn and Neptune, being gas giants, do not feature solid surfaces with polar ice caps; instead, they have complex atmospheres dominated by hydrogen and helium. Thus, Mars is the only planet in the provided options that fits the description of having both polar ice caps and a thin atmosphere.