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NPR's Pluto Has White-Capped Mountains, But Not Because There's Snow includes the following:

"Initially, it seemed logical that this high-altitude frost could form like on the Earth," says Tanguy Bertrand, an astronomer at NASA's Ames Research Center in California, who was intrigued by how much Pluto's mountains resembled familiar landscapes here at home.

The mountains that he and his colleagues examined were observed by NASA's New Horizons mission, and they lie west of a big heart-shaped glacier at Pluto's equator. They're about two and a half miles tall.

"They are comparable to the Alps for Earth, but Pluto is a much smaller object," says Bertrand. "So for Pluto, the mountains are really tall."

A visitor to Pluto would see these dark, red and brown mountains looming above. That's because even though Pluto is, on average, about 40 times farther away from the Sun as the Earth, there's still enough daylight to take in the scenery.

The mountains are made of water ice, as temperatures on this dwarf planet can drop as low as minus 387 degrees Fahrenheit. "Water ice on Pluto is so cold that it's hard, just like rock on Earth," says Bertrand. "That's why you can make mountains of water ice on Pluto."


Answers to What forms of water ice have been observed and verified in the solar system? tell us that there are several.

Answers to Is Mohs scale of mineral hardness applicable for rocks and minerals of terrestrial planets other than Earth? (especially @OscarLanzi's) remind us that conditions in the solar system vary substantially from those on Earth's surface, so we need to think twice about materials we feel to be familliar.

Question: How hard is the hardest ice in the solar system? Is it still "softer than talc"?

While one link above addresses only water ice, solid forms of other materials we encounter as liquids or gasses also count e.g. ammonia, methane, argon, whatever the stuff that might be blowing around Titan's surface is: How quickly might a Titan rover or drone get covered in oil and dirt? Will it need windshield-wipers?.

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1. Diamond. Its hardness is legendary. That it appears in liquid form on Uranus or Neptune hasn't been directly measured (no recent probes), but lab measurements in 2009 and 2010 of diamond's phase diagram still haven't been contested to claim that diamond can't be liquid there. On the contrary, in 2017 a process was demonstrated of converting diamond seas into diamond "hailstones".

So that could count as an ice in at least a meteorological sense. It probably wouldn't in the context of planetary science, which calls compounds either volatile or refractory, and which calls a solid volatile an ice. But Wikipedia's claimed 100 K boundary between the two is informal. The boundary seems to be a convenient abbreviation to describe planetary structures, used by Fred Whipple to describe comets in the 1960's (p. 112 of "Comet" by Carl Sagan and Ann Druyan). The closest primary source definition of the boundary that I can find is a table of ices on p. 119, listing their freezing points from water (273 K) to nitrogen (63 K). I'd love to see a better source.

Our cousins in the chemistry SE also haven't found a formal definition for ice, beyond water ice.

2. Methane. At least it might be harder than nitrogen. That's a pretty weak claim, and even lacks a number, but it's the best that even a recent article in a prestigious journal could manage (DOI: 10.1126/science.aao2975):

CH4 ice retains hardness and rigidity under Pluto surface conditions, which is ideal for saltation and dune formation, whereas N2 ice is likely to be softer.

Tables of bulk properties of elements and compounds have gaps in this area.
I found one claim that CO2 has a Mohs hardness of 2, but that was probably measured close to its freezing point, not far colder where it might be much harder, just like H2O is, as stated in the quote from Tanguy Bertrand.

It may be that nobody has properly measured these things yet. We might have to resort to ab initio simulation.

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    $\begingroup$ Hmm... Hmm... :-) Okay. Diamond is the name of one of many solid forms of carbon, and the name of the lattice that silicon and germanium share as well. But when liquid, does it ever retain any "diamondness"? Some kind of extremely long short-range order? $\endgroup$
    – uhoh
    Oct 12 '20 at 5:48
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    $\begingroup$ I won't claim "water has memory" homeopathy :-). Maybe constrain your question to polyatomic compounds to avoid this crystalline corundum, ahem, condundrum? $\endgroup$ Oct 12 '20 at 6:27
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    $\begingroup$ Well I don't know about the corundum condundrum but carborundum is also quite a sic little semiconductor, and yet it's a binary ice so that's won't do. btw apparently neither diamond nor carborundum actually melt, so this gets curiouser and curiouser! $\endgroup$
    – uhoh
    Oct 12 '20 at 7:03
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    $\begingroup$ Wikipedia says: "The solid phases of several other [non-water] volatile substances are also referred to as ices...if its melting point lies above or around 100 K." Diamond melts only above 4000 K, so I doubt that solid allotropes of carbon, including diamond, are considered an "ice" by any scientific community. But, thanks for prompting me to read research on phase transitions in carbon! This paper ( doi.org/10.1103/PhysRevLett.82.4659 ) claims two liquid phases: one diamond-like (sp3) and one with triple bonds (sp). They say graphite-like (sp2) liquid is unstable. $\endgroup$ Oct 12 '20 at 16:17
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    $\begingroup$ @uhoh - to my knowledge, liquid carbon is not well studied. Both silicon and germanium go from the four-fold coordinated semiconducting diamond cubic solid structure to an 8- to 12-fold coordinated metallic liquid. $\endgroup$
    – Jon Custer
    Oct 14 '20 at 12:51

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