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The Mohs scale of mineral hardness is a qualitative ordinal scale characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. Now there is a standard table where minerals are given standard Mohs hardness value. Talc has Mohs hardness value 1 that means it is the least hard mineral while diamond has Mohs hardness value 10 which means it is the hardest mineral. Most of the minerals found on Earth have been assigned a Mohs value. But is this scale applicable for minerals found on other terrestrial planets i.e. Mercury, Venus, Mars?

  1. Mercury contains various silicate minerals of magnesium, aluminum and calcium and iron. (source)
  2. Venus contains pyrite, magnetite, anhydrite found in surface basalts and rhyolites. See more examples here
  3. Mars is a mineral rich planet. The dust that covers the surface of Mars is fine like talcum powder. Beneath the layer of dust, the Martian crust consists mostly of volcanic basalt rock. See Composition of mars for more information.

Can these rocks and minerals be assigned a Mohs Hardness value? Why/Why not?

Loosely related questions:

  1. How sharp are the bits of sand and rock on mars?
  2. Are rocks on Mars or the Moon the same as rocks on Earth?
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    $\begingroup$ Great question! As currently worded, you just ask if it can be applied, to which the answer is as expected: "Sure, why not; rocks are rocks." I recommend you ask a follow-up question which may have more interesting answers; "Has the hardness of a mineral ever been measured in space? If so, was it reported using Mohs scale of mineral hardness?" For example, lots of Moon rocks were brought back to Earth for study, but you can ask if they'd made the measurements at all while still on the Moon. The next follow-up question can ask about robotic missions. $\endgroup$ – uhoh Oct 11 '20 at 15:22
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    $\begingroup$ Mohr or less, if you will forgive a little bit of humor. $\endgroup$ – bmargulies Oct 11 '20 at 23:44
  • $\begingroup$ @uhoh indeed great follow-up question. Since, it came in your mind, I suggest you to ask this question. You'll have my upvote ;-) $\endgroup$ – Nilay Ghosh Oct 12 '20 at 4:27
  • $\begingroup$ This is probably better asked on EarthScience.SE instead of Space Exploration, for waht it's worth. $\endgroup$ – TylerH Oct 12 '20 at 14:09
  • $\begingroup$ @NilayGhosh The name "Earth Science" does not mean "science only as it applies on Earth", though it's easy to see how one might think that. By "Earth" science, it means "natural" science about the physical world. Specifically, the entire subjects of geology, meteorology, oceanography, and environmental sciences are on-topic there. Those subjects cover all space, not just the bit of space that Earth exists within :-). $\endgroup$ – TylerH Oct 12 '20 at 14:28
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Yes, because for the vast majority of minerals you can find in space, they are the same as those you can find (or synthesize) on Earth. A pyrite in space is a pyrite just like on Earth. A wollastonite in space is no different than a wollastonite on Earth.

Nearly all space minerals are present on Earth, but the converse is not at all true--plate tectonics, biological chemistry, and, very recently, human activity have produced a wealth of natural minerals seen nowhere else in the universe. A particular paper (poorly, imo) called this "mineral evolution", insofar as the mineral assemblage on Earth has diversified alongside biological life.

But to get back to your question, minerals are minerals no matter where they are formed, and hardness is an intrinsic material property--you can assign your shirt a place on the Mohs hardness scale, so you can assign any space-found material a place as well.

Because I forgot to mention it, the study of extraterrestrial geology (which is what the Apollo missions, all the Mars rovers, etc. were all about), is called Planetary Geology.

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    $\begingroup$ Is this strictly true? Or might there be minerals out there that can not exist under normal earth preasures or temperatures? Thus making then incomparable to many earth minerals. $\endgroup$ – lijat Oct 11 '20 at 16:05
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    $\begingroup$ Other things are further out--is neutron star material a mineral? I'd lean towards "no", but at that point, it's less geology & more philosophy. $\endgroup$ – Anton Hengst Oct 11 '20 at 16:31
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    $\begingroup$ @AntonHengst - sorry, but I'm about to show my ignorance here. One of the points you list defining a mineral is "inorganic" - in chemistry, doesn't "inorganic" mean "relating to or denoting compounds which are not organic (broadly, compounds not containing carbon)"? If that's the case, doesn't that rule out all carbon-containing minerals from being, well, minerals? I know my interpretation must be wrong, but if you could give me a brief pointer as to why, I'd be very grateful. $\endgroup$ – Spratty Oct 12 '20 at 8:02
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    $\begingroup$ @Spratty "Organic" technically refers to carbon-hydrogen bonds; this rules out carbide materials (e.g. silicon carbide. Coal counts as an "organic mineral" by this definition, but is a product of life. There have been some "organic" compounds found in space, and Titan is mostly covered in organic methane. $\endgroup$ – pjc50 Oct 12 '20 at 9:00
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    $\begingroup$ @AntonHengst We are in agreement. I was illustrating by example how trying to get the "correct" or more and more specific term rapidly gets silly. $\endgroup$ – Schwern Oct 13 '20 at 19:33
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Yes, but with a couple modifications.

  1. On some planets, notably Venus within our Solar Systen, different conditions from those on Earth, especially temperature, can affect mineral hardness. Pyrite is indeed pyrite, but at 400+°C on the surface of Venus it may have a different hardness versus 20°C on Earth. Thus a distinction must be made between hardness of a Venusian mineral on Venus versus hardness under Earth-ambient conditions.

  2. Turning to the outer Solar System, we can find ices of materials that are normally fluid or gaseous on Earth. Such ices could have hardnesses well below that of talc, as with nitrogen ice on triton [1]. The Mohs scale would have to be extended accordingly.

Reference

1. M. R. Maughan, Z. Hacker, J. W. Leachman, J. W. Hartwig, "Solid Nitrogen Hardness Measurements at Triton Surface Conditions", 51st Lunar and Planetary Science Conference (2020), p 1690-1691.

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    $\begingroup$ "Thus a distinction must be made between hardness of a Venusian mineral on Venus versus hardness under Earth-ambient conditions." - This can be resolved by stipulating STP/NTP when performing hardness tests. Wonder if a concept of "standard gravity" would also be needed...does gravity affect hardness? $\endgroup$ – aroth Oct 12 '20 at 0:49
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    $\begingroup$ "Does gravity affect hardness?" How much? Stress from gravitational forces would be very small compared with the stress scale factors (e.g. tensile strength) of a typical mineral, but deep inside the planet it could get big. Compressed material deep inside could well be harder than what we find on a planetary surface. Mineral hardness at STP/NTP is not possible with a material like nitrogen and yet their hardness and strength are important for building geological features on outer solar system bodies. $\endgroup$ – Oscar Lanzi Oct 12 '20 at 1:06
  • $\begingroup$ I've just asked How hard is the hardest ice in the solar system? $\endgroup$ – uhoh Oct 12 '20 at 3:19
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    $\begingroup$ It is written here that: "certain types of rock is harder on the surface of the Moon or Mars than it is on Earth, whether it's because the minerals and pressure/temperature ranges are not found or because there are fewer mechanisms allowing deeper rock to rise to the surface". $\endgroup$ – Nilay Ghosh Oct 12 '20 at 4:38
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    $\begingroup$ Did I not distinguish between interiors and surfaces of planets in the.comment? $\endgroup$ – Oscar Lanzi Oct 12 '20 at 9:31
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Yes, for the simple reason that it is a scale of hardness rather than a classification of minerals: the minerals serve only as exemplars of hardness at various places on the scale.

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  • $\begingroup$ Indeed, the Mohs 'scale' is pretty qualitative. There are a number of other scales that could be used directly, such as Rockwell hardness - a well defined characterization technique. Given a Rockwell hardness tester on another planet, it can measure Rockwell hardness just fine. $\endgroup$ – Jon Custer Oct 13 '20 at 17:07
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Yes. If something softer than talc or harder than diamond shows up, the new low or high point changes the scale itself by extends the spectrum but the relative positions of everything else remain unchanged.

The answer would still be Yes if it were an absolute scale, such as temperature. The freezing and boiling points of water might vary in different conditions, but not because Newfoundium shows up in outer space.

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The way Mohs hardness scale works (in essence), is that if something can scratch talc (which has a hardness of 1), but not gypsum (which has a hardness of 2), then it will be assigned a number between 1 and 2. Since graphite has a hardness of 1.5, if the material you're testing is able to scratch graphite, then it will be assigned something between 1.5 and 2. If you want to be more precise, you can measure the scratch hardness using a sclerometer.

So you're asking whether or not a Mohs hardness assignment can be made to things like pyrite, magnetite, anhydrite which exist on Venus. Of course it can, in principle, because if pyrite is able to scratch something with a hardness rating of 5, but cannot scratch something with a hardness rating of 5.25, then it must be assigned a value between 5 and 5.25.

The main caveat has been pointed out by Oscar Lanzi, which is that the conditions on other planets are different from Earth (not just temperature and pressure due to gases in the atmosphere, but also due to gravity). To truly measure the hardness of "pyrite on Venus" relative to talc or gypsum "on Earth", you would need to probably do a computer simulation based on the known conditions on Venus.

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    $\begingroup$ "To truly measure the hardness of 'pyrite on Venus' relative to talc or gypsum 'on Earth'..." does indeed give one pause! Imagine if there was a crossover; a pair of standard samples which one was harder on Earth but due to some exotic conditions the other was harder on Planet X! A proper, standards-based Mohs scale would no longer be monotonic, yikes! So one could bring the Mohs specimens or carefully chosen proxies along to Planet X then use a simple Sclerometer on both the sample to be measured and all of our standards. $\endgroup$ – uhoh Oct 13 '20 at 9:23
  • $\begingroup$ (Sclerometer videos: 1, 2) Being an experimentalist I thought about how to collect data. I'm not sure yet how the proposed calculation would addres the interplanetary Mohs scale; does a computational hardness parameter already exist? If so, how is it compared to measurements? $\endgroup$ – uhoh Oct 13 '20 at 9:27
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    $\begingroup$ While I hope I'm wrong, and there's plenty of interesting research to be done on the matter, "exotic conditions" don't really seem to much exist, given that P-T changes are the primary drivers of changes in material behavior. When such P-T-driven changes in material do occur, it's typically as a polymorphic phase change. Suggesting that "whooo, exotic changes in hardness" due to alpha-quartz becoming cristobalite (or what have you) is as silly as saying "whooo, exotic changes in hardness" due to solid quartz becoming liquid quartz. $\endgroup$ – Anton Hengst Oct 13 '20 at 16:09
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    $\begingroup$ When a variation in conditions induces a change in physical structure, all properties are game to vary--not just hardness. At that point, might as well call it a new phase. But also worth nothing: hardness within a phase is also a function of P-T.... consider heating metal. $\endgroup$ – Anton Hengst Oct 13 '20 at 16:10

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