How is there wind on Mars?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by wegs, Nov 29, 2022.

  1. wegs Matter and Pixie Dust Valued Senior Member

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    I realize that Mars has a fair amount of sandstorms, but where is the ''strength'' coming from to produce wind in a place with such low atmospheric pressure? So, would CAT 4 winds that occur on Earth during hurricane season for example, feel like a gentle breeze on Mars?
     
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  3. Seattle Valued Senior Member

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    My understanding is "yes" it would feel like a gentle breeze. The wind comes from temperature differences and circulation patterns. The sand is fine. I was in Spokane after Mt. St. Helens erupted and the ash was so fine I didn't feel it coming down and I was surprised when I went into the house and found that I was covered with white "powder". It was also darker outside due to the falling ash so I didn't realize it until I went inside.

    I think the sand on Mars is of similar constancy.
     
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  5. wegs Matter and Pixie Dust Valued Senior Member

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    Wow, that is so crazy. (that you were covered with ''powder'' but didn't feel it raining down on you) That's a helpful analogy.

    I need to find a ''recording'' of wind sounds on Mars, to hear if it sounds like Earth's. lol I know that sounds so silly maybe, but I have this idea in my mind that weather on other planets is so vastly different than ours, but it probably isn't.

    When you say ''temperature differences,'' what do you mean when Mars doesn't retain heat for very long and is dramatically cold?
     
    Last edited: Nov 29, 2022
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  7. billvon Valued Senior Member

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    Temperature differential, just like on Earth. There is less energy there overall but much less atmosphere, so wind speeds are higher. However, since the density is so low, you would not really feel it.
    Yes - but - there is one big difference. If you are in a 20kt wind here on Earth, and you get hit by a dust grain, it will be going 20kts. If you are in a wind that feels like a 20kt wind on Mars, and get hit by a dust grain, it will be going orders of magnitude faster. So sandblasting is more of a threat, even if the wind doesn't feel any stronger.
     
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  8. wegs Matter and Pixie Dust Valued Senior Member

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    You mean, a threat to a human observer? Even if the sand is super fine and could dissolve pretty quickly in water? It's not like Earth's sand (texture), right?
     
  9. Seattle Valued Senior Member

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    Wegs, I don't know (think) it dissolves in water. That flaky white ash that I was covered with (St Helens) for instance basically turned into cement when you added water.

    I think on Mars the danger is to sealed space suits and equipment.
     
  10. Janus58 Valued Senior Member

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    Sand isn't soluble. At best, it is suspended in water.
    Another St. Helens story: I lived on the other side of the mountain from Spokane, about 50 mi W. While the prevailing winds took the ash east during the main eruption, a week later they had shifted the other direction for the second, smaller, ash release.
    This carried it our way, and on that AM we had a light rain. I had promised to drive my mom to Portland to catch a flight that day, but first thought it would be a good idea to see what it would be like driving in it. I didn't get far. The ash mixed with the rain quickly smeared over the windshield, and I was looking through a small spot at the bottom before I got back after just a couple of miles. and as abrasive as the ash was, I'm not sure what state my windshield would have been in if I'd tried to go much further. ( As it turned out, flights out of Portland were canceled. so it would have been a wasted trip anyway.)
     
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  11. Seattle Valued Senior Member

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    That was a crazy time in Spokane. I was there visiting on a break from grad school. For the first week there was state of emergency and only police and fire vehicles could be on the road. A car would go by, the ash would go up in the air and it wouldn't clear for a few minutes.

    Police vehicles were failing due to the ash getting past the air filters and ruining the engines. People initially thought it would be a good idea to hose off their sidewalks and roofs. It just turned the ash into heavy "cement". We wore masks or bandanas when we went outside for the first week.

    It was interesting but I was glad to go back to Phoenix after 2 weeks.
     
  12. Sarkus Hippomonstrosesquippedalo phobe Valued Senior Member

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    Even the strongest of dust storms on Mars are only about 60 mph. But since the density of the atmosphere is only c.1.5% that on earth, the momentum from the wind of fine particles travelling at that speed is significantly less than you'd feel on earth. In fact, the most dangerous things about the storms are 1: the dust is rather fine and builds up electrostatic charge, meaning it sticks to things, and can cover everything sitting in its path with a layer of the dust, which can clog gears, interrupt electronics etc; and 2: it can reduce effectiveness of any solar panels simply by the atmospheric particles blocking out the sun, by up to 99%.
    So something like the dust storm experienced in "The Martian", for example, which stranded Watney, really is a fiction.

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    Wind speeds aren't necessarily higher. Viking lander, for example, recorded speeds up to c.70 mph in a dust-storm, and typical wind only upto about 18 mph (what we might see as a "moderate breeze" on earth). Most sites seem to suggest 60-70 mph being typical of even the strongest dust-storms.
    While it's true in principle that the particle velocity will be significantly higher (c.8x) for the same dynamic pressure (i.e. an c.160kt wind on Mars will feel like a 20kt wind on earth), the wind speed is highly unlikely to get up that high on Mars (160kt = 280 km/h - so more than double the apparent highest expected speed) for sandblasting to be anything but a long-term consideration for equipment in the open.
    I don't know what particle velocity you'd need for sandblasting to be a concern, so maybe I'm wrong, but 60-70mph doesn't strike me as sufficient?
     
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  13. billvon Valued Senior Member

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    Not really. More a threat to (for example) optics. Particle speeds are far higher - but particle sizes are far lower.

    We have seen the erosion caused by this sandblasting when looking at exposed rocks on Mars, so we have some idea of how fast it works. It's not fast enough to put a human observer at risk, other than perhaps over a long period making his visor a bit less clear.
     
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  14. billvon Valued Senior Member

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    Agreed. This will be a long term consideration only, and even then, only for more sensitive surfaces (lenses, solar arrays.)
     
  15. DaveC426913 Valued Senior Member

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    Well, for one, sand doesn't dissolve in water. But for two the particles will not
    And, of course, dirty pots and pans.

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  16. foghorn Valued Senior Member

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  17. wegs Matter and Pixie Dust Valued Senior Member

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    Interesting that there isn't enough pressure to cause gale-force winds, yet Mars still experiences significant wind erosion. Those beautiful sand dunes and ripples must have taken forever to form. lol

    So, the only reason wind occurs on Earth or Mars, is because of atmospheric pressure? Suppose we're dealing with a planet that has no atmosphere like Mercury; would that mean a wind speed of zero/no wind to record at all?
     
    Last edited: Nov 29, 2022
  18. Sarkus Hippomonstrosesquippedalo phobe Valued Senior Member

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    Maybe you can explain why you think particle speeds are far higher, when the wind speeds are really no bigger, and with regard large storms are much lower? Not saying you're wrong, but it might help if you could explain why you think it is the case?

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  19. DaveC426913 Valued Senior Member

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    It's been eroding for a jillion years. Mars has neither vegetation to discourage erosion nor volcanic/tectonic activity to up-thrust new rock formations.

    Or, say, the Moon.

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    This print is expected to remain pristine for zillions of years.
     
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  20. billvon Valued Senior Member

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    Sort of. There are always gas molecules, but below a certain pressure they stop acting like a gas (with characteristics like pressure, windspeed, temperature etc) and start acting like individual molecules zipping around and only occasionally hitting something.
     
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  21. billvon Valued Senior Member

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    The thread that led up to this comment was:

    Me: "If you are in a wind that feels like a 20kt wind on Mars, and get hit by a dust grain, it will be going orders of magnitude faster. So sandblasting is more of a threat, even if the wind doesn't feel any stronger."
    Wegs: "You mean, a threat to a human observer?"
    Me: "Not really. More a threat to (for example) optics. Particle speeds are far higher - but particle sizes are far lower."

    So if you were in a wind that FELT like a 20kt wind on Mars (i.e. that's what an airspeed indicator would say, and that's how much pressure you'd feel) then particle speeds would be far higher than the same 20kt feeling wind on Earth.
     
  22. Sarkus Hippomonstrosesquippedalo phobe Valued Senior Member

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    The principle is fine, just the specific example of feeling an earthly 20kt wind on mars is fiction, it seems.

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    On the issue of sandblasting, though, is a 60mph wind on Mars going to be any different in that regard to a 60mph wind on Earth?
     
  23. billvon Valued Senior Member

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    That may be true. I don't think we have enough data to know for sure yet. As an example, most weather stations on Earth miss hurricanes since they are pretty localized.
    If the true airspeed is the same in both cases (Earth and Mars) then the damage done by suspended sand/dust is going to be the same - but there will be far less sand/dust in the air to begin with since there density is so low. So overall you are better off on Mars.

    If the indicated airspeed is the same in both cases (i.e. they feel the same) then the damage done by suspended sand/dust is going to be far worse on Mars, since the particle velocities will be much higher, and the damage they do is related to their energy, which comes from speed squared.
     

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