What is the highest point on Mercury?

Question

Continuation of "Gravity on Mercury's highest elevation?"

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Because when you Google that, you see "Caloris Montes" which is actually a mountain range consisting of mountains 1-2km tall from base to peak (Wikipedia). However, the previous question noted a value of 4.48 km "from what is considered a sea level". But mercury doesn't have sea, does it? Moreover, this value is confirmed here:

This new model reveals a variety of interesting topographic features, as shown in the animation above, including Mercury’s highest and lowest points. The highest point on Mercury is at 2.78 miles (4.48 km) above Mercury’s average elevation, located just south of the equator in some of Mercury’s oldest terrain.

A certain discrepancy is noted for our planet Earth. Mt. Everest is considered the tallest mountain (8848 m) when considered from sea-level. But, when we consider the height from base to peak, Mauna Kea/Loa is the highest mountain (of volcanic origin) (Total height=10.2 km, 4.2km above sea level). But, Mercury doesn't have "a sea level". So, why this discrepancy?

Questions:

1. What is the highest point of Mercury? Why is it "Calores Montes" even if it is a mountain range consisting of many mountains? Even if that peak of height 4.48 km is considered the highest point, why doesn't it have a name (since it is the tallest point, it should be notable and should have a name)?
2. What should be considered the base for planets which doesn't have a "sea" (sea level is only possible for Earth and Titan)? Wikipedia says " peak elevations above an equipotential surface or a reference ellipsoid could be used if enough data is available for the calculation, but this is often not the case". Where is it not applicable? Doesn't it lead to discrepancy like for instance, Mercury?

*This video says something called Mt. Hermes as the highest point of mercury.

Answer 1

There are a couple of components to your question, so rather than directly answering, I need to discuss a few things in a more narrative form.

Mercury was first spacecraft-imaged in the 1970s, with only about 45% coverage. From those images, some topography could be generated through stereogrammetry (two images taken with different viewing geometries that use parallax to reconstruct topography) and photoclinometry ("shape-from-chading").

The MESSENGER spacecraft orbited for several years, 2011–2015. Its orbit was designed, in part, for global stereogrammetric coverage and it had a laser altimeter that could operate at high- and mid-northern latitudes. As data were collected, early results were published.

The Wikipedia page you link to for Mercury's highest point, the Caloris Montes, was based on earlier results. It goes at least as far back as 2015, I didn't check further. Caloris Montes is named because it is an important feature on Mercury due to its association to the giant Caloris impact basin. It's not named because it's a high point, specifically. "Mons" is a singular mountain, while "montes" is used for a mountain range.

The article you link to is based on the full MESSENGER mission dataset, which was used to construct a new global mosaic and digital terrain model (available here). From the new, better, global model, there is a higher point in the southern hemisphere than the Caloris Montes. To me, it looks like a random point between some large craters, not anything especially important. Therefore, it doesn't have a name. Planetary nomenclature is used only when features are considered important and so need a name for easy referencing. So, instead of saying, "That really big basin on Mercury," I can say "Caloris" and people will know what I'm talking about. Similarly, as I noted above, the Caloris Montes are important as a mountain range, but a specific highest point among them is not.

As for a YouTube video saying the highest point is "Mt. Hermes," that sounds dubious to me with respect to anything official (for one, mountains on other bodies are "montes" after the Latin, if they have a formal name). A search shows no one else is using this name.

Regarding "sea level," it's what Wikipedia said: You use a reference ellipsoid, usually. The reference ellipsoid is made usually by fitting many different limb profiles to an ellipsoid, and/or via a laser altimeter. The best fit is the ellipsoid. Any deviations from that would be topography. Mercury has plenty-enough data to make a reference ellipsoid, indeed you linked to one in the comments above. Once these are published in the scientific literature, assuming they are generally accepted, the International Astronomical Union makes that a standard and publishes it in various reports. They don't often update their standards, though I think the last one was in 2015. So, it would not reflect the latest you found.

Practically all resolved solar system bodies have a reference ellipsoid that's used and published. How good it is, though, is another question. Asteroids, given their highly irregular shapes, for example, that were imaged by flyby missions would generally have a much more poorly constrained ellipsoid than, say, Earth's moon that has been observed through telescopes for over 400 years, orbited by numerous spacecraft, has retroreflectors on it, and has >8 billion laser altimeter points from the Lunar Reconnaissance Orbiter Laser Altimeter.