A Schematic Overview
A. Basic facts in planetary sciences:
Fact #1 -- Most primitive bodies (e.g., comets, some asteroids) contain four general types of materials : silicates, oxides, water (ice), and organic materials.
Fact #2 -- Larger solid planetary bodies (Earth, Mars, Venus, Moon, Io, Europa, etc. ) and some asteroids have undergone igneous chemical differentiation. Some have undergone extensive surface geochemical evolution. Each planetary body has a unique chemical composition and geochemical history.
B. Basic questions in planetary science:
Question #1 ---- The history & evolution of planetary bodies : Fundamental to understanding the origin and evolution of a planet is detailed knowledge of its surface rocks and their component minerals. From such information, the planetary scientist learns the path and extent of igneous differentiation of the planet, the changes that are produced in igneous rocks by the action of water and gases, and from this the general nature of past environments on and near the planet’s surface.
Question #2 ---- The possibility that life originated: Past environments provide constraints on the probability that life might have developed on the planet. More directly, residual organic materials related to life and its origin can be sought. For Mars, the most promising planet besides Earth for the development of life, this level of understanding is being urgently and actively sought.
C. Earth, Moon, Mars -- Mineralogy, Petrology, and Lives :
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Geologists have detailed information on the histories of only two planetary objects: the Earth and the Moon. Early in its history, the Earth underwent extensive, internal chemical separations to produce a large metallic core, a thick silicate mantle, and a thin crust. Heat produced within the planet drives the continents to move about, as parts of lithospheric plates. The floors of the oceans are divided by volcanic centers known as ridges, where fresh lava pours out to produce new ocean floor. Water causes erosion of the continents and produces sedimentary rocks. Water also affected the chemical separations that produced the continental rocks. Earth chemistry is exceedingly complex, and has thus produced a wide variety of igneous, sedimentary, and metamorphic rocks. |
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The Moon also underwent extensive and complex chemical separations very early in its history, and those separations produced a mantle and a crust, but little or no metallic core. The Moon lost much of its internal heat early because it is much smaller than the Earth, only 1/80 of Earth’s mass. There are no lunar lithospheric plates and probably never were any. The youngest lunar lavas sampled are more than 3 billion years old. The Moon has no water, and thus no chemical erosion or chemical sediments. Its continents consist mainly of a single mineral, plagioclase feldspar. Rock types are far fewer than on Earth. |
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Mars is intermediate in size, about 1/11 of Earth’s mass. It has the largest volcanoes known, and erosion channels that are now dry. What types of rocks will we find on Mars? We can expect lavas, but what kinds? Just basalt, as on the Moon, or a wider variety, as on Earth? What are the main crustal rocks on Mars? Are they composed mainly of plagioclase feldspar, as on the Moon, or two types of feldspar and quartz, as on the Earth? Are there chemical sediments and hydrothermal minerals on Mars, as on Earth, or only igneous rocks and impactites, as on the Moon? |
D. Tasks for surface exploration of Mars Surveyor Missions:
Task #1 -- To characterize the minerals and thus to identify the Mars' surface rock types
Task #2 -- To seek the evidence of life-friendly environments, or to identify its residues if it existed
E. Why use Raman spectroscopy ? -- A comparison with other spectroscopic techniques used or proposed to use for planetary explorations
| Visible-near IR spectra of lunar mineral and lunar soils (after Pieters, 1978) -- The spectral peaks of major igneous minerals are wide. They overlap in the spectra of mineral mixtures. | mid-Infrared spectra of lunar minerals and lunar soil (after Nash et al., 1993) -- Each mineral phase has its characteristic infrared spectrum. The spectral peaks of major igneous minerals are broad. They overlap each other in the spectra of mineral mixtures. |
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| Mössbauer spectrum of a lunar soil 10084 (after Wang et al., 1995) -- The brown points are the Mossbauer spectrum. The other colors are the contributions to the overall spectrum from each of the Fe-bearing minerals present in the sample. Spectral peaks of these different mineral phases overlap, and phase identification requires curve fitting. | Raman spectra of lunar minerals and a lunar soil (after Wang et al., 1995) --The main peaks of major rock-forming minerals are sharp and well spread across a wide spectral range. They do not overlap in the spectra of mineral mixtures. |
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Comparison of transition mechanism, spectral assignment, phase information, and spectral features:
| Vis-near-IR spectroscopy* | Mössbauer spectroscopy* | mid-IR spectroscopy | Raman spectroscopy | |
transition mechanism |
d-electrons of Fe |
Fe Nucleus |
structural framework |
structural framework |
spectral assignment |
Fe2+ in structural sites |
Fe0 & Fe2+ in structural sites |
structures & compositions |
structures & compositions |
phase information from a mixture |
only Fe-bearing phases |
only Fe-bearing phases |
Yes |
Yes |
in a real spectrum of mixture, peaks of phases overlap ? |
yes, wide peaks in wide region |
yes, narrow peaks in narrow regions |
yes, wide peaks in wide region |
No, narrow peaks in wide region |
phase identification in a mixture: ** silicates ** oxides & glasses |
difficult |
need curve fitting |
difficult |
difficult for some |
phase quantification in a mixture: ** silicates ** oxides & glasses |
-- |
need calibration |
-- |
-- |
* --- only the aspects used for lunar mineralogy.
Reference: Wang A., Jolliff B.L., and Haskin L.A. "Raman spectroscopy as a method for mineral identification on lunar robotic exploration missions", Journal of Geophysical Research (1995), Vol. 100, p21189-21199.
F. The feasibility of building a Raman system suitable for planetary missions
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There are four basic functional units for a Raman system -- a light source, a sampling device, a spectrograph, and a detector. Components of these general types have already flown in space. |
Existing commercial Raman systems for industrial applications – similar systems can be made smaller and robust to the conditions of space flight and in situ planetary mineral analysis. |
