Planetary Atmosphere Composition
I would like to know how is the composition (what kind of
gases) of planets atmosphere is determined from the spaceship?
I believe the device used is called a spectrometer. When light travels
through a gas, There are certain very specific wavelengths which are
affected by the gas. Basically, every type of gas has a "thumbprint" which
can be read by a spectrometer looking at light that had to pass through the
Salvador- all I know is it is spectroscopy: Absorption vs. frequency, or
emission vs. frequency.
There are probably a bunch of different kinds of spectroscopy:
- sunlight reflected off the surface, transmitted through the atmosphere,
- sunlight grazing the horizon, transmitted through the atmosphere,
- night-side glow from UV and ionization of the top of the atmosphere,
- far-infrared thermal emissions as the night-side tries to cool into space,
- characteristic absorption-peaks at certain radar frequencies as they
go from satellite to ground and back,
- characteristic absorption-peaks of certain laser wavelengths when
trying to bounce them off the ground,
- faint back scattering of an intense pulsed laser beam, in a gas or
clear liquid or solid, happens with slight frequency shifts
called "Raman scattering". The shifts are characteristic of the
This is a rather powerful technique. It does not require any
surface-bouncing; it only needs clear enough gas for the laser-beam to reach the
point you want to measure.
In addition, any space-ship in low orbit is sailing through the upper
fringes of the atmosphere.
The ship can take some into an instrument an and can test that any way it
wants. You can usually tell the general type of atmosphere from that.
But you might not pick up on some heavy minority components present at
Plenty of room here for the Klingons to do it differently than the
Vulcans, in science-fiction stories.
In real life, some NASA space probes can do it right now, too, if the
atmosphere is clear enough.
And astronomers are always collecting spectrographic data from earth
telescopes and trying to fit that to atmospheric models.
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Update: June 2012