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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.

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    $\begingroup$ Related, but not a duplicate (the accepted answer is wrong): Is the curvature of Mars visible from the top of Olympus Mons? $\endgroup$ Dec 13 '21 at 9:32
  • $\begingroup$ 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. $\endgroup$ Dec 13 '21 at 11:14
  • $\begingroup$ @DavidHammen Perhaps you could provide references for your own claim? Just saying everyone else is wrong isn't very helpful :-) $\endgroup$ Dec 13 '21 at 13:10
  • $\begingroup$ 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 $\endgroup$ Dec 13 '21 at 13:14
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    $\begingroup$ @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.). $\endgroup$ Dec 13 '21 at 14:52
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Tharsis Tholus. Is one of the steepest shield volcanos with average flank slope of 10 degrees (followed by Ceraunius Tholus which is 10 percent less steep (average flank slope is 9 degrees) than the Tharsis Tholus) and has the steepest slope of angle 16 degrees near its summit. Ascraeus Mons and Elysium mons also have steep slopes compared to Tholus, But when it comes to average flank slope, Both are 33% less steep (average flank slope in 7 degress) than Tharsis Tholus.

Talking about the longest line of sight, as you mentioned in the question, due to the size and shallow slopes of Olympus Mons, an observer standing on the Martian surface would be unable to view the entire profile of the volcano, even from a great distance. The curvature of the planet and the volcano itself would obscure such a synoptic view, 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. Neither Ascraeus mons nor Elysium mons (Even Tharsis Tholus) will allow you to see an actual "horizon" because of the "low" steepness and the curvature of Mars. Yes, The longest line of site can be seen in Valles Marineris (Melas Chasma?). With help of some astrogeology and math, taking the reference site as Melas Chasma (the widest segment of Valles Marineris, you can actually see as far as 255-279 (+/- 8 percent) Kilometers.

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    $\begingroup$ That's begs the question: Is there any mountain/volcano more steeper than Tharsis Tholus (in other planet) having much greater slope value? $\endgroup$ Dec 21 '21 at 12:22
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    $\begingroup$ @NilayGhosh That needs some analysis. I will let you know. you can post this question in the question stream ! $\endgroup$ Dec 21 '21 at 12:26
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    $\begingroup$ @uhoh yes, I couldn't find single approximation but, I have given a range. $\endgroup$ Dec 21 '21 at 13:13
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    $\begingroup$ oh that's excellent, thanks! $\endgroup$
    – uhoh
    Dec 21 '21 at 15:07
  • $\begingroup$ @NilayGhosh only part of the mountain is steep... if we see by this way then there are mountains which have 170 - 175 degree steepness (probably the steepest). See more google.com/… $\endgroup$ Dec 22 '21 at 4:58

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