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What are the highest and lowest elevations on Mars?
This question is difficult to answer due to the fact that the absence of oceans makes the definition of "sea level" meaningless. Instead, scientists use a term called "Mars datum surface" that refers to the average elevation on Mars.

The highest point is at the top of the volcano Olympus Mons and lies at an altitude of 27 km above the datum surface. The lowest point is at the bottom of the Hellas impact basin in the southern hemisphere and lies 4 km below the datum surface. That's a difference of nearly 31 km.

In comparison, the differential between the highest and lowest points on Earth (Mt. Everest and the bottom of the Mariana Trench in the Pacific) is only 20 km.

How long is a day and year on Mars?
The length of a Mars year is about 687 Earth days. A Mars day lasts for about 24 hours and 37 minutes.

What is the strength of the Martian gravity?
The gravity on Mars is only 38% of that found on Earth at sea level. That means that a 100 pound person on Earth would weigh only 38 pounds on Mars.