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?

love,
Nick
 

Answers and Replies

  • #2
diazona
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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.
 
  • #3
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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.
And of cource there is the carbon from the co2.
 
  • #4
DaveC426913
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Besides, the Earth is not a closed system. It exchanges matter in both directions with its environment.
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.
 
  • #5
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the earth does gain mass. about 35000000 kg per year. but this is due to debre from space being pulled into the earths atmosphere
 
  • #6
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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.
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.
 
  • #7
berkeman
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the earth does gain mass. about 35000000 kg per year. but this is due to debre from space being pulled into the earths atmosphere
Could you please post a source for that number? Thanks.
 
  • #8
D H
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Could you please post a source for that number? Thanks.
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 http://adsabs.harvard.edu/abs/1992A&A...263..361C

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 http://adsabs.harvard.edu/abs/1996A&A...311..329C

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) http://adsabs.harvard.edu/abs/1993Sci...262..550L
 
  • #9
berkeman
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Thanks DH. :smile:
 
  • #10
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Earth also loses masses from escape velocity high altitude atmosphere molecules....does it not?
No idea if its a significant amount though.
 
  • #11
pervect
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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.
 
  • #12
D H
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But, most of this is re-radiated
All (almost all) of it is re-radiated.
 
  • #13
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It's reradiated, but with a lower frequency, meaning less energy and so the Earth gained this energy difference as mass.
 
  • #14
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There are photons leaving the Earth, you know.
 
  • #15
pervect
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All (almost all) of it is re-radiated.
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.
 
  • #16
D H
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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.
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.
 
  • #17
DaveC426913
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It's reradiated, but with a lower frequency, meaning less energy
While true...
and so the Earth gained this energy difference as mass.
...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.
 
  • #18
D H
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...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.
... and if that were the case the Earth's surface would rather quickly warm to 5780 K or so.
 
  • #19
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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.
 
  • #20
Borek
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It's reradiated, but with a lower frequency, meaning less energy and so the Earth gained this energy difference as mass.
Frequency doesn't matter here, no idea what you are talking about.
 
  • #21
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Frequency doesn't matter here, no idea what you are talking about.
This is how I understand it: difference in frequency is the same as difference in energy level. And the difference is thought to have been converted to chemically stored energy (= mass).

But argument seems to be based on a "one in/one out" scenario, which - as DaveC426913 writes - is questionable.
 
  • #22
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Inside the sun, fusion takes place. Fusion is a process by which light elements combine to form heavier more tightly bound elements.

So basically:

A + B -> C + energy

But when we add the masses of A and B and then compare it to C, we see that C weighs less than the sum of (A+B). Where did the extra mass go? Well, it turns out that the mass lost is the same as the energy produced in that reaction divided by the square of the speed of light.

The mass lost by the sun is converted into energy that we see as sunlight.

When sunlight is absorbed by the earth, its mass increases by the corresponding factor. However, I have no idea what the scale of this mass increase is. Things like meteors falling into the earth probably far outweigh the increase in mass from this effect.

Photosynthesis - the mass you see as a pumpkin came from the carbon dioxide in the air, water and nutrients in the soil.

In short: yes there is mass lost by the sun and mass is gained by absorbing sunlight. But that is not related to photosynthesis.
 
  • #23
Drakkith
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Is it true that on the sun , mass is converted into energy.This energy is received by earth.On earth this energy is converted into mass.If this is so, then mass of earth must be continuously increasing.
Pretty much true, however this energy/mass is also lost as thermal radiation that is radiated away from the Earth constantly. The amount of energy absorbed is equal to the amount radiated away, resulting in no net gain or loss in mass.
 
  • #24
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Bhj_dk has it basically correct. But it does not have to be a plant that absorbers the photon of light. Soil or skin or clouds that capture a photon of light also convert the energy of that photon into mass. The question is how often does that excited electron reradiate the energy versus storing it in a more permanent chemical bond. Both occur. Yes capturing a photon does increase the mass of the earth and yes some of that captured energy/mass does get incorporated into the earths crust.
 
  • #25
Bobbywhy
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Photosynthesis does not convert sunlight into mass. It converts sunlight into chemical energy (sugars). If one wants to argue the Mass/Energy equivalence, OK, but any increase in the total mass of the earth, if true, would be miniscule compared to the gain in mass from impacts of meteorites and cosmic dust, as several members here have already mentioned.

"The mass of the earth is 5 983 000 zettagrams, and it gains another 40 Gg (gigagrams) every year from captured meteorites and cosmic dust."

http://www.essex1.com/people/speer/large.html [Broken]
 
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