Planet Four Talk

How much of the surface of Mars is Martian?

  • wassock by wassock moderator

    I-playered Horizon on meteors, a mash up of old programs and some new bits about our recent close encounter. One of the things that caught my eye was the amount of space rock we collect each year. Wide variation in numbers quoted for this but 50,000 tons a year looks like a good number to play with, give it an average density of 5g a cm3 or 5 ton a cubic meter and thats 10000 cubic meters a year. Spread out over all five hundred an some million square kilometers of earth and thats not much, and the surface is churning all the time, and lots of its sea, so not much accumulation to see.
    Now look at Mars, covered in dust and rocks but still showing the marks of liquid water that has been absent fo 2 or three billion years. Thus any debris from space fron the last 2thousand million years ought to be still lying around. Mars is smaller with less gravity than Earth so wont catch as much as Earth, but the atmosphere is thinner so more of it should reach the ground.

    So how much of the surface material is extramartian? Set yourself some arbitary numbers and do the math, I got a 2cm layer all over from my numbers, but didn't take my socks off so the sums may be a bit shakey 😃

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  • Kitharode by Kitharode moderator

    Hi wassock. I'm totally happy with your description of the scenario and 50,000 tons/yr of meteoritic dust (space dust) is fine by me. Estimates vary wildly, as you know, but for now I think your numbers and calculations are realistic enough to give a meaningful answer - on Earth. But I can't find a number for Mars' space dust anywhere. Lots of stuff about dust settling rates on solar panels and such, but they all measure everyday 'local' martian dust.

    "Mars is smaller with less gravity than Earth so wont catch as much as Earth, but the atmosphere is thinner so more of it should reach the ground". Absolutely right - gonna need a calculator I think.

    One potentially interesting item: Scientists appear to have 'proved' that most of Earth's space dust comes from a region within the asteroid belt. To get from the asteroid belt to Earth this dust must cross the orbit of Mars. There is an implication therefore that similar amounts of space dust are available for Mars.

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  • wassock by wassock moderator

    I just factored the value for earth as a tonnage (milligramage) per square kilometre and used that times the surface area of Mars as a first approximation. Bottom line is that whatever number you come up with you are gonna multiply it by 2 billion.

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  • Kitharode by Kitharode moderator in response to wassock's comment.

    I'm not sure I've totally got that. Can you tell me again a different way? Ta.

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  • wassock by wassock moderator

    If you assume that theres as much rock floating about in each neighbourhood then the amount picked up is a factor of the size and gravity of each planet. Too much physics to go there so I just decided that the average per unit of surface area would be the same for each planet. So divide earths annual tonnage by surface area then times that number by the surface area of Mars to get an annual tonnage for Mars (ish)

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  • mschwamb by mschwamb scientist, translator

    This is an interesting thought experiment. There are a couple of other things to think about. The Earth's atmosphere slows down the impact velocity, so on Mars, meteors will traveling faster when they hit, so more likely to create impact craters where the impactor vaporizes and mixes with the ejecta. Also I would guess that with a thinner atmosphere there is less break-up, so more likely to stay in one piece and make a crater and get mixed into the ejecta.

    Also there is a size distribution of particles and there will many more small things that although that hit the Martian atmosphere. Most of the mass in the distribution is in small particles and dust. So many will burn up in the Martian atmosphere (less than compared to the flux at Earth). So it may be a very small amount mixed into the Martian dust.

    Also probably better to assume a density of 3 g/cc which is rock instead of a density for iron.

    Cheers,
    ~Meg

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  • wassock by wassock moderator

    Picked 5g/cm3 to make the sums easier:-)

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  • Kitharode by Kitharode moderator

    Meg. The experiment is indeed interesting and so was your input - Thanks.

    Wassock. Thanks a lot, that's totally clear to me now. Need a brew and a calculator I suppose......

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  • Kitharode by Kitharode moderator

    In paragraph two, I've assumed that 1 m/b.y. means 1metre/billion yrs.

    An assessment of the meteoritic contribution to the Martian soil (Abstract)

    George J. Flynn David S. McKay.
    Article first published online: 19 DEC 2012

    The addition of meteoritic material to the Mars soils should perturb their chemical compositions, as has been detected for soils on the Moon [Anders et al., 1973] and sediments on Earth [Kyte and Wasson, 1986]. Using the measured mass influx at Earth and estimates of the Mars/Earth flux ratio, we estimate the continuous, planet-wide meteoritic mass influx on Mars to be between 2700 and 59,000 t/yr.

    If distributed uniformly into a soil with a mean planetary production rate of 1 m/b.y., consistent with radar estimates of the soil depth overlaying a bouldered terrain in the Tharsis region [Christensen, 1986], our estimated mass influx would produce a meteoritic concentration in the Mars soil ranging from 2 to 29% by mass. Analysis of the Viking X ray fluorescence data indicates that the Mars soil composition is inconsistent with typical basaltic rock fragments but can be fit by a mixture of 60% basaltic rock fragments and 40% meteoritic material [Clark and Baird, 1979]. The meteoritic influx we calculate is sufficient to provide most or all of the material required by the Clark and Baird [1979] model.

    Particles in the mass range from 10^−7 to 10^−3 g, about 60–200 μm in diameter, contribute 80% of the total mass flux of meteoritic material in the 10^−13 to 10^6 g mass range at Earth [Hughes, 1978]. On Earth atmospheric entry all but the smallest particles (generally ≤ 50 μm in diameter) in the 10^−7 to 10^−3 g mass range are heated sufficiently to melt or vaporize. Mars, because of its lower escape velocity and larger atmospheric scale height, is a much more favorable site for unmelted survival of micrometeorites on atmospheric deceleration. We calculate that a significant fraction of particles throughout the 60–1200 μm diameter range will survive Mars atmospheric entry unmelted. Thus returned Mars soils may offer a resource for sampling micrometeorites in a size range which is not collectable in unaltered form at Earth.

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  • wassock by wassock moderator in response to Kitharode's comment.

    That's good stuff Kitharode. So what happens to the stuff that vapourises in atmosphere? Surely for something like iron the resultant molecules are heavy and will still get to ground.

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  • Kitharode by Kitharode moderator in response to wassock's comment.

    Good question. Don't know the answer, but I've got it on my 'things to look out for' list. The article says that a 'significant fraction' of stuff gets to the surface unmelted, some stuff gets there melted (so it'll still be solid) and all that in addition to vapourised stuff. So yes, there's some heavy stuff in there.

    I've read somewhere that although Mars has no magnetic field like Earth, there is some (much?) magnetised material on Mars. I'm not yet clear if there's any magnetism around that would attract particles to the surface, or if its magnetised rock buried underground.

    Good stuff again wassock

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  • wassock by wassock moderator

    i'd read that the magnetic field may have been lost following a, large, impact - inferring that magnetised rock may have been laid down afore that. but thats magnetised in the way that rock either side of the north atlantic ridge shows the switching of direction of the earths field, not boulders with iron filings sticking to them.

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  • Kitharode by Kitharode moderator

    Yes indeed. Magnetism is probably not a major player, but it's something else to read up on (as if we don't have enough to read about already).

    Although we haven't nailed down any precise numbers yet, I think a possible answer to your original question of 'how much of the surface of Mars is martian' might be; Not a lot. It'd be great if we could put some numbers to that though. I'm trying to learn more about that region of the asteroid belt where, apparently, most of Earth's spacedust comes from. I'm wondering about the possibilty of a 'comets tail' effect from that region being the source of the material, as opposed to a 'belt' of dust that drifts towards the Earth.

    On the (very) remote chance that this was the case, then as I said before it must cross the orbit of Mars to get into Earth's orbital path, so this might introduce a periodic influx of spacedust onto Mars in similar fashion to our meteor showers as we pass through 'comet tail debris'. I admit my ideas are a bit nebulous (like the asteroidal comets tail) but it's probably not my daftest idea to date 😃

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  • Kitharode by Kitharode moderator

    This doesn't help to answer your original question, but I found this dusty little snippet very interesting:

    "Piqueux and Christensen (2008) calculate from the fraction of dust coverage of the cryptic region at Ls = 210 deg that the mass of 'fines', 4 x 10^13 kg, moved in the seasonal cycle every year, is equivalent to 100 times the mass of dust in a global dust storm".

    'Fines' are the dust particles from the venting. Page 285 here: https://docs.google.com/viewer?a=v&q=cache:_iS_YyD3yfYJ:www.lpl.arizona.edu/~shane/publications/hansen_etal_icarus_2010.pdf+&hl=en&gl=uk&pid=bl&srcid=ADGEESh5DV0zqdOdYmDBG17CUAp5ZG-wwuBIEYPt7jQOBgciXYD6ZuwWqfdj0OP0vZG7qfu3ufb2o5UFK8p3x0OGdLyXl8z42scVLKqzJ5DBN8jlYFBQYIT0AGtuWTn61jd46ItPFa0e&sig=AHIEtbS9i4ku-URXDEMaBiAqbeIMXx5j1A

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