Here's why:
* Hellas Basin: This is a massive impact crater, the largest known impact crater on Mars. Its immense depth creates a natural "depression" in the Martian surface.
* Gravity and Atmosphere: The gravity at the bottom of Hellas Basin is slightly stronger due to the increased mass of the surrounding terrain. This slightly higher gravity pulls in more of the thin Martian atmosphere, resulting in a slightly higher atmospheric pressure.
It's important to note that even at the bottom of Hellas Basin, the atmospheric pressure on Mars is still incredibly low compared to Earth's, only about 1% of Earth's sea-level pressure.
