Lasers discovered above Mars Pathfinder!

As Mars Pathfinder prepared to deploy its rover equipped with laser navigation, Mother Nature had beaten it to the punch : Natural lasers have been discovered on mars!

Discovery of CO2 Laser in Martian Atmosphere

NASA scientists directed a ground based telescope towards Chryse Planitia the landing site of the Mars Pathfinder and discovered naturally occurring laser radiation coming from the upper martian atmosphere. They used a sophisticated detector called the 'NASA Goddard Space Flight Center InfraRed heterodyne spectrometer' to scan the emissions. The incredible discovery was confirmed by several other groups around the world.

Quantum Description

Mars The small red circle centered on Chryse Planitia represents the region over which the laser emissions were detected. Solar radiation is responsible for pumping a population inversion in the carbon dioxide of the tenuous upper levels of the atmosphere of Mars (Mumma et al., 1981) and Venus (Deming et al., 1983). Population inversions have also been found in comets (Mumma, 1993). On mars, the solar pump intensity is strongest near the solar point, and falls off gradually towards the terminator. There is some locus where the inversion vanishes, but it is difficult to say exactly where that is. The R(8) transition at 10.33 microns in the infrared is produced from one vibrational quanta of asymmetric stretching to one quantum of symmetric stretching with a change of one quantum of rotational energy from J=8 to J=7. (mars image courtesy : Philip James, University of Toledo; Steven Lee, University of Colorado; and NASA Hubble Space Telescope)

CO2 Energy Levels Vibrational energy level diagram depicting the 10.6 micron infrared transition in the carbon dioxide molecule. (The nitrogen vibrational levels shown on the right are used to enhance lasing in laboratory lasers)

Due to the low densities of the lasing species in the mesosphere and thermosphere of Mars the gain is very low, about 10 percent, comparable to single-pass gains in some earth based CO2 lasers. The low gain is partly compensated by the extremely large volumes of active lasing medium. Over the very long distances scales, the exponential properties of amplified spontaneous emission produce a significant spectral signature at the lasing frequency. The laser amplification has been confirmed by several groups (Gordiets et al., Stepanova et al. and Dickinson et al.)

Martian CO2 Laser Line

Spectra of martian CO2 emission line as a function of frequency difference from line center (in MHz). Blue profile is the total emergent intensity in the absence of laser emission. Red profile is gaussian fit to laser emission line. Radiation is from a 1.7 arc second beam (half-power width) centered on Chryse Planitia (long +41 lat +23).
(Mumma et al., 1981)

This unusual infrared emission from CO2 was first observed by students of Charles Townes (Johnson et al., 1976), and later identified as a 'natural laser'. The lines are 100 million times brighter than what would be expected if thermodynamic equilibrium was established. The laser emission line can be used as a diagnostic probe of the temperature and wind patterns on Mars (Mumma, 1993) and Venus (Goldstein, 1991).

Bow and Arrow Analogy

The emission of a laser photon by an excited CO2 molecule (below) can be compared to the release of energy stored in a bow (above) : When the molecule vibrates or changes its geometry, it snaps back and emits a photon like a bow launching an arrow.
Image courtesy Dale Gustafson and the National Geographic Society ©

Mechanical Description

The Goddard infrared heterodyne spectrometer (Mumma et al., 1978) used to scan the laser emissions from mars is itself based on a carbon dioxide laser which produces beats when it is mixed with the incoming radiation. The process of mixing an external signal with a precise internal reference oscillator is called heterodyning. The interference produces two extra signals, one with the sum of both frequencies and another with their frequency difference. A filtering system rejects all frequencies except for the difference frequency. The signal to be measured can be seen as shifted from very high frequencies (10^14 Hz or terahertz) down to a much lower frequency (a few hundred MHz) where standard electronics can very easily measure the spectrum over a narrow wavelength region near the signal's center frequency. This has been used in radio receivers to downconvert high frequency signals into intermediate frequency signals that are easier to handle.

Goddard Space Flight Center IR heterodyne spectrometer. The CO2 laser beam from the bottom (vLO) is mixed with the input signal (vIR) coming in from the left. The sensitive IR photodetector produces an electronic signal at the upper right (vLO-vIR) which is further processed by an RF spectrometer (not shown). Kostiuk and Mumma (1983).

Practical Use

Thus a terrestrial CO2 laser detects a martian CO2 laser. Other astrophysical detectors that use a laser to detect another are laboratory microwave lasers attached to radio telescopes which are sometimes used to amplify natural microwave laser radiation.

There are proposals for realizing a planetary scale laser by placing mirrors in appropriate orbits about the planet to enhance the gain and produce oscillation. (Sherwood, 1988, 1992)

REFERENCES

  1. Mars Global Circulation Model group
  2. Michael J. Mumma's home page
  3. Center for Mars Exploration (NASA)
  4. Johnson,M., et al. : 1976, Astrophys.J., 208, L145. (online)
  5. Mumma,M.J., Buhl,D., Chin,G., Deming,D., Espenak,F., Kostiuk,T.: 1981, Science, 212, 45.
  6. Deming,D., Mumma,M.J.: 1983, Icarus, 55, 356. (see also NASA 83-30342)
  7. Gordiets,B.F., Panchenko,V.Ya.: 1983, Cosmic Res. (USA), 21, 725. (see also NASA 83-30341)
  8. Kostiuk,T., Mumma,M.J.: 1983, Applied Optics, 22, 2644.
  9. Stepanova,G.I., Shved,G.M.: 1985, Sov.Astron.Lett., 11, 390.
  10. Dickinson,R.E., Bougher,S.W.: 1986, J.Geophys.Res., 91, 70.
  11. Sherwood,B.: 1988, M.Sc. Thesis, NASA CR-180780, (NASA, Washington, DC) see also Optical SETI and also here for further references
  12. Goldstein,J.J.: 1991, Icarus 94, 45.
  13. Sherwood,B., et al.: 1992, NASA Conf. Proc. 3166, (National Aeronautics and Space Administration, Washington, DC), p.637-645.
  14. Mumma,M.J.: 1993, in Astrophysical Masers, Clegg,A.W., Nedoluha,G.E. (eds), (Lecture Notes No.412, Springer Verlag) p.455
  15. Espenak,F., Deming,D., Jennings,D., Kostiuk,T., Mumma,M.J., Zipoy,D.: 1983, Icarus, 55, 347. (see also NASA 83-29157) Observations of the 10-micron natural laser emission from the mesospheres of Mars and Venus
  16. Loperz-Valverde,M.A., Loperz-Puertas,M.: 1994, J.Geophys.Res., 99, No.E6, 13093. nLTE radiative transfer model for IR emissions in the atmosphere of Mars.
  17. Deming,D., Mumma,M.J., Espenak,F., Kostiuk,T., Zipoy,D.: 1986, Icarus, 66, 366. Nonthermal emission of 10.33 and 10.72 micron CO2 lines at 23 locations on mars.
  18. Mumma,M.J., Deming,D., Espenak,F., Kostiuk,T.: 1986, in NASA Washington Reports of Planetary Astronomy Non-thermal emissions from Mars, Jupiter and Comet Halley.
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Heterodyne Spectroscopy

This page was created by John Talbot and last modified March 1, 1998

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