NASA Satellite Measures Earth's Carbon Metabolism 141
Roland Piquepaille writes "To celebrate Earth Day, the NASA Earth Observatory recently revealed global measurements of the Earth's metabolism. 'Combining space-based measurements of a range of plant properties collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) with a suite of other satellite and surface-based measurements, NASA scientists produce composite maps of our world's 'net primary production' every 8 days. This new measurement is called net production because it indicates how much carbon dioxide is taken in by vegetation during photosynthesis minus how much is given off during respiration.' Check this column for a summary including the usefulness of such measurements. You'll also find maps showing the seasonal variation of Earth's net primary production."
Re:I don't entirely buy this... (Score:2, Informative)
Re:These kinds of studies... (Score:1, Informative)
Even if Mars is leaking gases, it takes a long time. Maybe we'd have to keep vaporizing water ice in the atmosphere to keep replenishing it. There is a lot of ice out there, most visibly in comets and the rings of Saturn.
Re:Interesting (Score:5, Informative)
Re:Interesting (Score:3, Informative)
However, this only shows there are a lot of plants hard at work on the atmosphere there. It does not show what is happening to the carbon in dead trees, soil, mud flowing down the river, carbon entering from the mountains upstream...
Re:Coniferous forests - some numbers (Score:1, Informative)
(10^12 m^2) (kg(C)/m^2/yr)
.tropical forests 24.5 0.83
temperate forests 12.0 0.56
boreal forests 12.0 0.36
woodland and shrubland 8.0 0.27
savanna 15.0 0.32
grassland 9.0 0.23
tundra and alpine meadow 8.0 0.065
desert scrub 18.0 0.032
rock, ice, and sand 24.0 0.015
cultivated land 14.0 0.29
swamp and marsh 2.0 1.13
lake and stream 2.5 0.23
open ocean 332.0 0.057
upwelling zones 0.4 0.23
continental shelf 26.6 0.16
algal bed and reef 0.6 0.90
estuaries 1.4 0.81
From Harte's "Consider a Spherical Cow" pg257 via
Earth's surface as a 20 x 25 Megameter rectangle
http://www.vendian.org/envelope/dir2/f
Note the npp units. kgC/_square_meter_/yr.
Not per square kilometer.
That would make leaf raking sooo much easier...
And sorted by npp...
kgC/m^2/yr Mm^2
swamp and marsh 1.13 3.6
algal bed and reef 0.90 2.9
tropical forests 0.83 2.6
estuaries 0.81 2.6
temperate forests 0.56 1.8
boreal forests 0.36 1.1
savanna 0.32 1.0
cultivated land 0.29 0.92
woodland and shurbland 0.27 0.86
grassland 0.23 0.73
lake and stream 0.23 0.73
upwelling zones 0.23 0.73
continental shelf 0.16 0.51
tundra and alpine meadow 0.065 0.21
open ocean 0.057 0.18
desert scrub 0.032 0.10
rock, ice, and sand 0.015 0.048
Re:These kinds of studies... (Score:2, Informative)
Venus is not just slightly smaller than the Earth. The Earth is around 20% more massive than Venus, being somewhat denser, so notwithstanding the relatively small difference in radius, the surface gravity on Venus is only about .91g. Mars is not nearly as dense as the earth and is quite a bit smaller, which accounts for its .38g surface gravity.
What you say is true, though. Venus holds a very dense atmosphere even with its lower gravity.
Re:These kinds of studies... (Score:4, Informative)
Well, that depends on how long you want to hold the gases. At any given temperature, the molecules of a gas at thermal equilibrium (or practically speaking, anywhere close to it) will have some distribution of speeds. Some molecules will travel faster, some slower. The mathematical expression characterizing this range of speeds is the Maxwell distribution. Here's [wolfram.com] a mathematical treatment of the Maxwell distribution; this page [davidson.edu] presents a nifty Java applet showing how this equilibrium takes place.
Note that a plot of population vs. speed, the Maxwell distribution tails off at higher velocities, but never actually goes to zero. In an atmosphere, this means that a small number of molecules will periodically get kicked up to above escape velocity through collisions with other molecules in the gas. If they happen to be heading the right direction, then they will escape into space.
Each molecule in a gas (on average) has roughly the same amount of kinetic energy. Earth's atmosphere contains very little hydrogen and helium because these light elements travel faster for a given amount of kinetic energy and escape more readily. A good part of the velocity distribution for these species is above escape velocity. Oxygen and nitrogen (not to mention water vapour and carbon dioxide) are significantly heavier, and bleed off at a much lower rate.
Moving to Mars. The surface gravity is only about forty percent that on Earth, if I remember correctly. It's a much shallower gravity well, and escape velocity is much lower (5 km/s on Mars vs. 11 km/s for Earth). Since kinetic energy is a function of the square of velocity, it takes a significantly smaller push to move a molecule out of Mars' hold. Nevertheless, there actually is still only a very small tail of the Maxwell distribution that sits above Mars' escape velocity.
I should also mention that there are sputtering processes that remove gas from the Martian atmosphere. Lacking a strong magnetic field to deflect the solar wind, a significant amount of gas is lost to sputtering, as well.
Nevertheless, even the most pessimistic estimates suggest that an atmosphere similar to Earth's would last tens of thousands on years on Mars. A short lifespan in terms of planetary evolution--a long time for human beings. Even the Moon would take from one to ten thousand years (depending upon who you ask) to bleed off an Earth-like atmosphere. Recall that Mars has surface features strongly suggestive of flowing surface water. (Liquid water requires an appreciable atmosphere, otherwise it just boils off.) That sort of erosion takes a long time to happen, which further supports the notion that Mars can hold on to an atmosphere, at least for a few million years at a time.