# Longest line of sight on Mars

It is said that the slope of Olympus Mons is so gradual that the peak cannot be seen from the plain; it is hidden by the planet's curvature. That provokes questions:

• How steep is the steepest of the great volcanic peaks?
• What is the longest line of sight between two points on the surface of Mars? (Perhaps it's between rims of Valles Marineris?)

I do not assume that the latter is known.

• Related, but not a duplicate (the accepted answer is wrong): Is the curvature of Mars visible from the top of Olympus Mons? Dec 13, 2021 at 9:32
• Re It is said that the slope of Olympus Mons is so gradual that the peak cannot be seen from the plain; it is hidden by the planet's curvature. The wikipedia article on Olympus Mons says that (or at least implies that). Said wikipedia article uses two bogus references to bolster that claim. Your eyes are not at your naval, for one thing. Olympus Mons is not flat, for another. Dec 13, 2021 at 11:14
• @DavidHammen Perhaps you could provide references for your own claim? Just saying everyone else is wrong isn't very helpful :-) Dec 13, 2021 at 13:10
• Sources like this one state the average slope is 5%, from which it should be simple to calculate sight lines given Olympus' peak height and the diameter of Mars. space.com/20133-olympus-mons-giant-mountain-of-mars.html Dec 13, 2021 at 13:14
• @CarlWitthoft The wikipedia article to which the accepted answer in the linked question refers says "Similarly, an observer near the summit would be unaware of standing on a very high mountain, as the slope of the volcano would extend far beyond the horizon, a mere 3 kilometers away.[23]" Reference 23 does not support that claim. That 3 km (actually, 3.5 km) assumes that Mars is a perfect sphere. Reference 23 goes on to refine that 3.5 km estimate upward. Way upward, by two orders of magnitude, assuming nothing is in the way. There most likely is something in the way (hills, etc.). Dec 13, 2021 at 14:52