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Lecture Notes for the Midterm
History of Mars exploration
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History of Mars observation from Earth
- Spectral analysis: New toys, new Mars
- See Viewgraphs:
"Earth-based explorations by spectral analysis."
- Basic idea of spectroscopy
- Radiant energy is refracted into a spectrum, as when a prism or a
diffraction grating is placed in front of a light beam (Isaac Newton)
- Electromagnetic energy can be displayed by wavelengths or by frequency (or both), and a spectrum
can be represented as intensity readings by wavelength or frequency
- Wavelength is the distance from one crest of a wave to the next one (or from one wave
trough to the next). For radiant energy, this can range from gamma rays at ten quadrillionth of a meter long to long radio waves at 100,000 km long
- Frequency is the reciprocal of wavelength: It's how many wave crests pass a point in a given time interval, as in 60 hertz, meaning 60 of them pass by you in one second.
- A spectrum can be continuous across wavelengths or show variations where
the spectrum is less or more intense at one wavelength than at an adjacent
wavelength
- Spectroscopy is the study of radiation as it is emitted by radiant
objects and then absorbed, reflected, or scattered by substances
between it and a sensor. Absorption, actually, turns the absorber into a radiant object in its
own right re-emitting the absorbed energy at a longer wavelength.
- Hot, dense objects emit smoothly across a pretty uniform continuous spectrum
- Cooler, less dense objects emit uneven spectra with discontinuous higher
energy wavelengths or emission lines
- A hot, dense radiator with a cooler, more diffuse substance between it
and the sensor will show a continuous spectrum with discrete absorption
lines
at particular wavelengths characteristic of the intervening substances
- Wavelengths emitted or absorbed are diagnostic of particular elements,
compounds, or minerals, depending on such factors as resonances with chemical
bonds in a molecule, the lattice structure of crystals, the size of atomic
nuclei, movement of electrons out of
their orbitals, and photons released by electrons moving to lower orbitals in
an atom, and the wavelengths and energy level of the radiation involved
(X-rays, ultraviolet, visible, infrared, microwave, radio).
- Spectral libraries have been built up over many years to allow
classification of spectra. These may have been put together by systematic exposure of
standardized targets to various forms of energy, such as infrared, visible light, ultraviolet, or X-rays.
- Attempts to measure martian air pressure through spectral analysis
- In 1862, William Huggins tried to apply the general idea to use Mars
spectra to measure its atmospheric pressure
- Mars reflected sunlight, which wasn't too "illuminating"
- He and Pierre Jules Janssen in 1867 try to apply spectral analysis to
Mars to look for water and oxygen, which they thought they'd found. Much
later Huggins backed away from the claim.
- Lowell tried to apply spectral analysis to Higgins' problem in 1908
- He estimated it as 87% of Earth's, which we know is way off
- His method of measuring gas scattering in the atmosphere could have
gotten the right answer, but he wasn't correcting for other important
scatterers, such as the very abundant dust
- Even so and despite his increasing reputation as a bit eccentric, this
approach was a pioneering contribution to the science of Mars
- Other attempts to get at martian atmospheric pressure failed for 50 years
mainly because the composition of the martian atmosphere wasn't known
- Ironically, the first successful estimate of the martian atmospheric
pressure (around 5.16 mb or hPa) was done by Louise Young after spacecraft had
visited Mars and gotten the pressure directly: Her work showed that
Earth-based spectroscopy could do the job.
- Spectral analysis of martian temperatures
- This was more successful
- Any object that absorbs energy re-radiates it as thermal energy
- Measuring thermal emissions allows inference of temperature through
Wien's Displacement Law
- Temperature in Kelvins (degrees centrigrade above absolute 0) can be predicted if you know the peak radiation intensity wavelength of a radiant object's spectral emissions: T in Kelvins = 2897/wavelength in microns (millionths of a meter)
- You can predict what the peak radiation intensity wavelength should be if you know the object's temperature: wavelength (microns) = 2897/temperature in Kelvins
- Soooo, Earth's averaged out temperature is 15° C or 288 K -- where should Earth's radiation emissions peak?
- Mars' average is about 208 K, so what is its peak radiation intensity? (hint: It had better be bigger than Earth's because it's drastically colder!)
- What about Venus? That charming place has an average temperature of 737 K. Your answer should, therefore, be way smaller than for Earth and Mars.
- Lowell Observatory measurements back to the 1920s showed Mars was one
cold place, averaging -40° C (-40° F), whereas Earth averages 15°
C (59° F). The poles were about -70° C (-94° F), while the
"warmest" place along the equator was about 10° C (or 50° F). The
highest equatorial highs were pushed higher in 1954, around 25° C (77°
F).
- Life on Mars and spectral analysis
- There is a distinct wave of darkening of the planet that extends outward
from the polar caps in spring and eventually involves much of the planet
- Many folks thought that was vegetation activity
- In 1938, Peter Millman said that the spectra from this darkening wave is
not the same as any vegetation, at least on Earth, dealing that line of
speculation a serious blow
- In 1954, W.M. Sinton said he had collected spectra in the infrared that
resembled those of various organic compounds, perhaps the result of vegetation
after all
- He later withdrew his paper, saying that he and a colleague had collected
spectra for which he had not considered the contamination of heavy water in
the earth's atmosphere that had distorted the signals he was looking at.
- Audoin Dofus and Thomas McCord said that the darkening was not green:
That was an optical illusion. The dark areas were simply less bright areas
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![[ orthographic image of Mars on a black background ]](https://home.csulb.edu/~rodrigue/mars/marsblackbg.jpg)
![[ Olympus Mons seen at oblique angle that gives a 3-d sense ]](https://home.csulb.edu/~rodrigue/mars/olympusmons3d.jpg)
![[ Mars explorer ]](https://home.csulb.edu/~rodrigue/mars/marsexplorerpainting.jpg)
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![[ image of Mars ]](https://home.csulb.edu/~rodrigue/mars/marsatlas.gif)