Bally - So, yours is an interesting question, and there's actually more than one answer. On Mars, the speed of sound is expressed in Martian values. And on Earth (obviously) Mach is calculated based on the speed of sound at atmospheric densities on Earth. So, then the question becomes, if "Mach" speeds are a concern, which value do you use to design a parachute which will be used on Mars, but has to be designed and constructed within the confines of Earth and it's atmosphere? So, it's an excellent question actually.
Turns out, "Mach" is just a reference point, a computed value based on air density (and to a certain extent, humidity values) with relation to Mean Sea Level here on Earth. You are correct, all these values change on Mars, for a whole variety of reasons. So, what values matter more and which ones matter less on Mars? It boils down to velocity versus air density, regardless of what 'Mach' value that velocity computes to in either Martian, or Earthly, terms (values). The actual speed of "sound" is irrelevant in these equations, so 'Mach' numbers don't really mean anything, in terms of parachute designs. In re-reading my earlier post, I see where I kind of inadvertently made a somewhat misleading statement. I should have said..."the ideal atmospheric reentry conditions on Mars equate to a main chute deployment above about 'Mach' 1.5...in Earth terms." (And that's just off the top of my head. I'd have to go back and check some of the math to see if that's more or less accurate, honestly. But I wanted to respond to your question, because it was a good one).
In all reality, I should have refrained from using 'Mach' terminology and values altogether when speaking to velocities of spacecraft during reentry on the planet Mars.
Edit - If a reader was concerned with attempting to calculate velocities (visually), Doppler effects, and various other things...while standing on the surface of Mars...then 'Mach' numbers would be relevant.
Edit II - Your question prompted another thought. I wonder what a 'Sonic Boom' really sounds like on Mars?? I never actually contemplated this question before now. My first reaction to this question is, a Sonic Boom would be far less prominent on Mars, compared to Earth. But, would it even be notable? (Don't know this answer, and may have to look into).
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(next post...reply...(hint, hint, hint, this is a PITA)
On just a side note, I became interested in Mars landing dynamics after doing a bunch of research into Venus and landing on Venus. Venus is yet another challenge. On Venus you have some things which are easier, and other considerations which are MUCH harder. If you can picture this, the conditions on the surface of Venus are extreme, to say the least. Temperatures at the surface often exceed 900F, and even worse the atmospheric pressure at the surface on Venus is equivalent to traveling 1,000 feet below Sea Level. Forgetting things like extreme sulfur and CO2 levels and other bad ju-ju in the 'air', the conditions on the surface are pretty dang hostile indeed. Much of the surface conditions have been known about in varying degrees since the many Russian missions to Venus in the 70's. But, the conditions between space and the surface were much less understood.
Turns out in the Venetian atmosphere there is a region / "zone" high above the surface which is surprisingly hospitable. Temperatures from 30-80F and pressures similar to that of Sea Level here on Earth (Okay, it drops to -226F at night, but we won't worry about that right now!)
Now, while keeping the landing dynamics of Mars in mind, consider the crazy atmospheric dynamics while trying to land in an apocalyptic and hostile environment like THIS one!
And just to put things in perspective, Russia managed to have a successful landing on Venus (Venera 9) where the craft continued to operate after landing. But imagine your liquid Oxygen cooled, titanium bodied, camera "melting' inside of 53 minutes after landing on the surface!!! THAT...is Venus...for anyone wondering why we don't go back very often.
Turns out, "Mach" is just a reference point, a computed value based on air density (and to a certain extent, humidity values) with relation to Mean Sea Level here on Earth. You are correct, all these values change on Mars, for a whole variety of reasons. So, what values matter more and which ones matter less on Mars? It boils down to velocity versus air density, regardless of what 'Mach' value that velocity computes to in either Martian, or Earthly, terms (values). The actual speed of "sound" is irrelevant in these equations, so 'Mach' numbers don't really mean anything, in terms of parachute designs. In re-reading my earlier post, I see where I kind of inadvertently made a somewhat misleading statement. I should have said..."the ideal atmospheric reentry conditions on Mars equate to a main chute deployment above about 'Mach' 1.5...in Earth terms." (And that's just off the top of my head. I'd have to go back and check some of the math to see if that's more or less accurate, honestly. But I wanted to respond to your question, because it was a good one).
In all reality, I should have refrained from using 'Mach' terminology and values altogether when speaking to velocities of spacecraft during reentry on the planet Mars.
Edit - If a reader was concerned with attempting to calculate velocities (visually), Doppler effects, and various other things...while standing on the surface of Mars...then 'Mach' numbers would be relevant.
Edit II - Your question prompted another thought. I wonder what a 'Sonic Boom' really sounds like on Mars?? I never actually contemplated this question before now. My first reaction to this question is, a Sonic Boom would be far less prominent on Mars, compared to Earth. But, would it even be notable? (Don't know this answer, and may have to look into).
---------------------------------------------------------------------------------------------------------------
(next post...reply...(hint, hint, hint, this is a PITA)
On just a side note, I became interested in Mars landing dynamics after doing a bunch of research into Venus and landing on Venus. Venus is yet another challenge. On Venus you have some things which are easier, and other considerations which are MUCH harder. If you can picture this, the conditions on the surface of Venus are extreme, to say the least. Temperatures at the surface often exceed 900F, and even worse the atmospheric pressure at the surface on Venus is equivalent to traveling 1,000 feet below Sea Level. Forgetting things like extreme sulfur and CO2 levels and other bad ju-ju in the 'air', the conditions on the surface are pretty dang hostile indeed. Much of the surface conditions have been known about in varying degrees since the many Russian missions to Venus in the 70's. But, the conditions between space and the surface were much less understood.
Turns out in the Venetian atmosphere there is a region / "zone" high above the surface which is surprisingly hospitable. Temperatures from 30-80F and pressures similar to that of Sea Level here on Earth (Okay, it drops to -226F at night, but we won't worry about that right now!)
Now, while keeping the landing dynamics of Mars in mind, consider the crazy atmospheric dynamics while trying to land in an apocalyptic and hostile environment like THIS one!
And just to put things in perspective, Russia managed to have a successful landing on Venus (Venera 9) where the craft continued to operate after landing. But imagine your liquid Oxygen cooled, titanium bodied, camera "melting' inside of 53 minutes after landing on the surface!!! THAT...is Venus...for anyone wondering why we don't go back very often.