Earth Mass Increasing? Photons and Photosynthesis making more matter on earth?

  1. This is a question for whoever is out there.

    Is the earths mass ever increasing? I understand that under the Second Law of Thermodynamics, the Earth is a closed system, ie it exchanges energy with its environment, but not matter.

    So, my question is, if sunlight energy is constantly radiating onto the Earth, and plants transform that energy into matter, wouldn't it follow that the earth is constantly gaining mass?

    This idea is also informed by something I read from the food systems writer, Michael Pollan, called "Pumpkins Leave No Holes." It was about how, if you take 50 lbs. of soil and plant a pumpkin seed in it, and lets say the pumpkin grows to 50 lbs. also. If you took everything that grew out of the seed, the salk, the leaves, the stem and the pumpkin, and you weighed the soil, it would still weigh 50 lbs.

    What's more, if you ate the edible parts of the pumpkin, and composted the inedible parts, you would produce MORE soil, making this process more than sustainable. Its actually constantly growing.

    Is this correct? Is the earth gaining mass? Is our gravitational pull increasing slightly because of it? Does it matter?

  2. jcsd
  3. diazona

    diazona 2,155
    Homework Helper

    Plants don't actually transform the Sun's energy into matter. They transform it into a different kind of energy (chemical bonds). Besides, the Earth is not a closed system. It exchanges matter in both directions with its environment.

    What you've read from Michael Pollan seems like complete nonsense. I doubt that a pumpkin could grow in as little as 50 pounds of soil, but if it could, the weight of the pumpkin plus the final weight of the soil would equal the original 50 lb. plus the weight of any water or plant food etc. that you had added over the course of the pumpkin's growth.
  4. And of cource there is the carbon from the co2.
  5. DaveC426913

    DaveC426913 16,534
    Gold Member

    Ah but in a thought experiment, it could be.

    Let's posit a transparent massless shell that passes energy but not matter.

    Alas, it still wouldn't make Earth grow.

    Sunlight could enter and do lots of chemically interesting things, but ultimately it would be re-radiated in infrared.
  6. the earth does gain mass. about 35000000 kg per year. but this is due to debre from space being pulled into the earths atmosphere
  7. Actually, plants gets most of their mass from water and CO2, they just get a little ammount of nutrient from the soil. Jan Baptist van Helmont did this experiment long ago, weighing the soil before planting a tree, and a couple years later, the soil weighed almost exactly the same.

    Plants use Sun's energy to change all this H2O and CO2, and some Nitrogen and other nutrients (from soil), to the gazillions of organic compounds that make up a plant.
  8. berkeman

    Staff: Mentor

    Could you please post a source for that number? Thanks.
  9. D H

    Staff: Mentor

    Three recent estimates, listed in decreasing mass order:

    1.7×108 kg/year
    Ceplecha, Z. (1992), "Influx of interplanetary bodies onto earth", Astronomy and Astrophysics:263

    1.5×108 kg/year
    Ceplecha, Z. (1996), "Luminous efficiency based on photographic observations of the Lost City fireball and implications for the influx of interplanetary bodies onto Earth", Astronomy and Astrophysics:311

    40×106 kg/year
    Love, S. and Brownlee, D. (1993), "A Direct Measurement of the Terrestrial Mass Accretion Rate of Cosmic Dust", Science:262(5133)
  10. berkeman

    Staff: Mentor

    Thanks DH. :smile:
  11. Earth also loses masses from escape velocity high altitude atmosphere molecules....does it not?
    No idea if its a significant amount though.
  12. pervect

    pervect 8,155
    Staff Emeritus
    Science Advisor

    If you look at the solar constant of about 1400 watts/ m^2, and multiply it by the cross sectional area of the earth, pi r^2, you get about 10^17 watts or 2 kg/ second * c^2.

    But, most of this is re-radiated, though I"m not sure offhand to figure out how much. And in any event, it's still lower than the above figures being only 6*10^7 kg/year total influx.
  13. D H

    Staff: Mentor

    All (almost all) of it is re-radiated.
  14. It's reradiated, but with a lower frequency, meaning less energy and so the Earth gained this energy difference as mass.
  15. There are photons leaving the Earth, you know.
  16. pervect

    pervect 8,155
    Staff Emeritus
    Science Advisor

    I'd agree that almost all of it is re-radiated, but particularly in the case of photosynthesis, I think it's reasonable to assume some very small fraction is not re-radiated. I'm not sure how to estimate how much is not, unfortunately.
  17. D H

    Staff: Mentor

    Those plants will eventually die and decay. Compost piles can get quite warm because biomass decay is an exothermic process. A very, very tiny amount of that biomass will be sequestered in bogs, river deltas, etc. However, the same plate tectonic processes that truly sequester partially decayed biomass eventually re-expose that stuff to the environment. Sans human activity, the net has to be so extremely close to zero as to be immeasurable.

    If anything, I would argue that the amount of outgoing radiation slightly exceeds the amount of incoming radiation from the Sun. The Earth is still cooling from its initial formation 4.6 billion years ago and radioactive decay is still producing heat deep within the Earth. The Earth's surface is heated from above by sunlight and from below by the Earth's mantle and core. The Earth's surface temperature would observably increase were there any substantial imbalance between the total incoming heat transfer rate and the total outgoing heat transfer rate. The increase would continue until the Earth's surface reaches some equilibrium temperature where the heat balance is zero.
  18. DaveC426913

    DaveC426913 16,534
    Gold Member

    While true...
    ...this does not follow. Unless you plan on insisting that the exact number of photons being reradiated is no more than the exact number being absorbed.

    The Earth will reradiate as much energy as required for it to reach equilibrium. That could be lots more low-energy photons radiated than high energy photons absorbed.
  19. D H

    Staff: Mentor

    ... and if that were the case the Earth's surface would rather quickly warm to 5780 K or so.
  20. trying to sum this up:

    through photosynthesis plants convert some of the incoming photons to chemically bound energy (=> mass gain).

    plants get eaten by animals, insects, bacteria ect. This converts part of the stored chemical energy to heat which ultimately causes it to be re-radiated (=> mass loss).

    But does mass-gain equal mass-loss? or does some of the chemically bound energy (gained mass) get stored?
    The amounts of fossil fuels used over the last couple of centuries seems to suggest that some build up of chemically stored energy have taken place.

    However; in the very long term the general trend toward entropy suggests an overall loss of mass.
  21. Borek

    Staff: Mentor

    Frequency doesn't matter here, no idea what you are talking about.
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