Mars, Rainbow Planet
In fact, Mars is still our solar system’s beloved “Red Planet”, so-named for the abundance of iron oxide on its surface. And if held in your hand, Mars rocks most Mars rocks will appear rather similar to rocks from our home planet. However, also like rocks from Earth and other planetary bodies, very thin slices, or thin sections, of Martian terrain will look brilliantly colored using polarized light microscopy, a method that depends on how light bends through materials with varying optical properties, and used in identifying crystals and minerals .
We recently started imaging a new set of Martian and other extraterrestrial samples in the Astronomy & Astrophysics Research Lab, beginning with Martian rocks, shown below in brilliant color.
But first, how can we have samples of Mars, since we’ve never had a mission return with any rocks? The answer: meteorites. Out of more than 61,000 meteorites found on Earth, 132 are thought to be from Mars. Their origin is deduced primarily from their unique relative composition of trapped gases (Mars’ atmosphere is nearly 96% carbon dioxide, with nearly 2% each argon and nitrogen, and less than 1% oxygen and carbon monoxide), which we know to great accuracy from our missions to Mars. Some time in the past, these future meteorites were blasted off the surface of Mars from the impact of a large body, most likely an asteroid, then traveled in space for thousands, even millions of years, before reaching Earth.
Compare the two images below. The first shows the thin section from the Martian meteorite, Tissint, seen with our unaided eye. The second image below shows Tissint in polarized light, revealing the large olivine minerals which are colored due to the way the light bends as it passes through the complex crystalline structures.
The Tissint meteorite is classified as a shergottite, one of the four main subdivisions of meteorites from Mars, and representing nearly 3/4 of all Mars meteorites. Shergottites are igneous, meaning they formed from the cooling and solidification of lava during a time of active volcanism on Mars. The shergottite group is named after the first of these meteorite types, the Shergotty meteorite, which fell in Sherghati India in 1865. Tissint is an igneous basalt that is rich in the mineral olivine, large inclusions of which are readily evident in thin sections.
Unlike other types of meteorites that are very primitive solar system material, shergottites are most likely younger, with most age estimates ranging from less than one million to several million years old; however, the large degree of processing from magma and impacts make these samples more difficult to age than primitive, pristine samples. A 2014 (and controversial) paper in the journal Science suggested that the Mojave crater on Mars could be the source of the shergottite meteorites, and that the samples represent very ancient material — exceeding 4 billion years — that was ejected from the less-than 5 million year-old crater. The precise age of the shergottites remains comsochemistry’s key unanswered questions.
A fairly substantial body is thought to have been necessary to hit Mars hard enough to eject material from the surface in order to escape its gravity, which is about 38% that of Earth’s gravity. The large olivine crystals in Tissint show cracks that are the likely signatures from this impact event. Long black veins are also thought to be the result of shocking the rock, and the matrix (the material surrounding the olivine crystals) contains a glassy material called maskelynite plagioclase that formed during impact.
The images below show close-ups of two different olivine crystals in our sample, and their surrounding matrix. The left, middle, and right columns are 1/4, 1/2 and one-wavelength, which describes the different polarizer settings, each of which reveals various features including black veins of shocked glass in the matrix, and deep cracks in the olivine crystals.
Tissint was discovered by nomads in the small town of Tissint east of Tata, Morocco, on July 18, 2011. It is the fifth Martian meteorite seen to fall to Earth (as opposed to “finds”, or meteorites that are found some time after falling), and is scientifically more valuable than the former given that samples are less weathered, and the trajectory of the meteor can be studied as well. The sky reportedly shown bright yellow during the fall, and two sonic booms were heard.
We are grateful to Donald Cline, Director and CEO of the Pisgah Astronomical Research Institute, for providing the thin sections to the Museum for our work.
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I am an observational astronomer studying solar system origins from a different vantage point: outside of our solar system. I use the largest ground-based optical/infrared telescopes to study the chemistry of forming planetary systems in our Galaxy, and compare these data to the oldest material from our solar system, including meteorites and the Sun. My astronomical data directly connect to understanding unusual chemistry in the most primitive meteorites, which helps in understanding the earliest processes that influenced how planets formed here, and beyond the solar system. I am interested in expanding our meteorite studies in the Lab. You can visit some key meteorite specimens on display in the meteorite exhibit, adjacent to the Astronomy & Astrophysics Research Lab. Also, visit my Museum webpage and other research blogs if you would like to learn more about my research.