March 1997 Volume 6 Number 1
Mars Meteorites Give TES Geologists a Preview of the (Infra)Red Planet
Geology Mission. One of the ways geologists will use TES data is to determine what kinds of rocks are on the surface of Mars. In our laboratory at ASU, we have been looking at thermal infrared observations of minerals and rocks in order to understand what their spectral signatures look like, so that they can be identified in TES spectra of Mars. These spectra are somewhat like "fingerprints"-- each mineral has a unique infrared signal. Rocks are mixtures of (usually) several different minerals-- so we can identify rocks by looking for the "infrared fingerprints" of these minerals.
Chips off the Old Block. It is possible that humans may already have a few pieces of the surface of Mars in the form of meteorites. Meteorites are rocks (stony, metallic, or a combination of both) that travel through space and land on Earth. There are 12 meteorites that scientists think may have come from Mars. By studying the infrared spectra of these meteorites in our lab, we may be able to identify similar rocks on the surface of Mars, or even find the places that these 12 meteorites came from!
CLICK FOR FIGURE: Sample of Nakhla meteorite, thought to be from Mars. Circular cup in the photo is about 1.5 inches wide.
A Look at Their Fingerprints. Shown in the figure below are the thermal infrared spectra of two martian meteorites, named Nakhla and ALH77005. We have also been looking at two others (not shown here) called Zagami and EET79001. These four meteorites represent three different types of rock-- Zagami and EET79001 are the same kind. We know what these rocks are made of, but if we pretend that we don't know what kinds of rock they are, we can make some interesting observations. The shapes of the Zagami and EET79001 spectra are similar, suggesting that these two are probably very alike, while the other two rock spectra look very different. This observation suggests that we will have no problem distinguishing between major rock types on Mars. If these were spectra of Mars, we could label them Unit A (Nakhla), Unit B (ALH77005), and Unit C (Zagami and EET79001).
CLICK FOR FIGURE: Thermal infrared spectra of two martian meteorites compared with some minerals from Earth. Notice that the Nakhla meteorite spectrum looks similar to the augite (pyroxene) spectrum, while the ALH77005 meteorite spectrum looks similar to the forsterite (olivine) spectrum.
How We'll Identify Minerals on Mars. If we continue to pretend that we don't know what the meteorites are made of, the next step is to compare the spectra of the meteorites to the spectra of individual minerals in order to see which minerals are in the meteorites. Every mineral has a different spectral shape from every other mineral, therefore, we can usually compare mineral spectra to a rock spectrum and eliminate minerals that have spectral shapes that do not look similar to the rock spectrum. When we compare our research results to the research of scientists who know what the meteorites are made of, we find that we can very closely determine what minerals are in the meteorites. By performing many such studies, we can begin to understand what TES dta will be showing after Mars Global Surveyor goes into orbit on September 12th.
ALH77005 "lherzolite". This is a rock that is made mostly of a mineral called olivine, which is more commonly known as the birthstone, peridot. The second most common mineral in this meteorite is pyroxene, specifically the variety augite. The third most common mineral is plagioclase feldspar, and in this meteorite, this mineral exhibits shock effects that can be observed under a microscope-- it is essentially now a glass. Shock generally occurs when the rock is exposed to a very rapid high-pressure blow, such as when bedrock is impacted by a meteorite. Minerals vary in their strength (resistance to shock), so not all minerals in a sample will necessarily be shocked. These shock effects were probably induced by the impact event that ejected these rocks from the surface of Mars.
Nakhla "clinopyroxenite". This sample is dominated by the mineral augite. Augite is a type of pyroxene called a "clinopyroxene", which is where the rock type name comes from. The next most abundant mineral is olivine, and there is also plagioclase, but unlike ALH77005, it is not shocked.
Zagami and EET79001 "basalt". Zagami and EET79001 are very similar in composition. Both contain large amounts of two pyroxenes, augite, and pigeonite (pronounced like the bird). There is no olivine in these meteorites. The other primary mineral in these two samples is plagioclase feldspar, and in both meteorites, the plagioclase is shocked, as is the pyroxene. These rocks are classified as "basalt," which is a dark, iron-rich volcanic rock like those found in lava flows of Hawai'i or near Sunset Crater in Arizona.
Vicky Hamilton plans to complete a Geology Ph.D. in December 1997. Her research centers on the mineralogy of Mars and tectonic features of Venus. She has been examining pyroxene minerals and basaltic rocks in the thermal infrared, in preparation for the MGS TES mission. Her e-mail address is email@example.com.
TES News is published quarterly by the Arizona Mars K-12 Education
Program. This newsletter may be copied for EDUCATIONAL PURPOSES ONLY.
EDITED BY Kenneth S. Edgett, Arizona Mars K-12 Education Program,
Arizona State University, Tempe, Arizona, USA.