ROCKS FROM MARS
© Mickey Law
 

Before 1983, planetary scientists could not positively identify any meteorite as being from Mars. Meteorites that most planetary scientists believed were from Mars were well known, but there was no way to prove any of them had a Martian origin because there were no samples known to be from Mars for comparison.

As the map above shows, there have been eight successful landings on Mars to gather data and information about Mars. However, none of these missions was a sample return mission. To date, no samples have been collected on Mars and returned to Earth for a full laboratory analysis.

The instrument capabilities on the Mars landers and rovers are greatly reduced by the necessity of significant reductions of their size, weight, and power demands. Instruments sent to Mars can only examine sample targets that are 1 to 2 centimeters in size. Scientists working in Earth labs use instruments that analyze the chemistry of individual mineral grains that are only a few microns in size. That is a four orders of magnitude difference!

Instead of analyzing the chemistry of one carefully selected mineral grain in an Earth lab, instruments on Mars are analyzing the total combined chemistry from as many as 10,000 different mineral grains. Only the meteorology instruments sent to Mars are capable of analyzing the Martian atmosphere without any loss of capability. These instruments simply count the number of elemental isotopes per cubic centimeter of Martian atmosphere.

Before 1983, the meteorites that many planetary scientists believed were from Mars were referred to as SNC METEORITES. The SNC abbreviation was from the first letter of the name of the first three types of meteorites thought to be from Mars. These meteorite names were: Shergotty, Nakhla, and Chassigny. SNC meteorites were classified by which of the three types their chemistry most resembled: SHERGOTTITES, NAKHLITES, or CHASSIGNITES.

The first three SNC meteorites fell over a hundred years ago. That was long before these meteorites were the subject of serious scientific research with sophisticated instruments, so it is quite fortunate these meteorites were observed to fall as “shooting stars”. Otherwise, it is very unlikely these rocks would have been preserved in museums for future generations to study. SEE FIGURE 1

When the first missions to Mars were scheduled in the 1970s, meteorite researchers began serious attempts to determine if SNC meteorites were actually from Mars and where to search for any analogs on Mars. One of the first observations was that SNC meteorites were ALL IGNEOUS VOLCANIC ROCKS. This meant that ALL of these rocks originated on a parent body that was large enough and hot enough to support volcanism. This fact eliminated asteroids and chondritic bodies as a potential source.

Another observation was that the SNC meteorites had, geologically speaking, ALL COOLED AND CRYSTALIZED ON THEIR PARENT BODY QUITE RECENTLY. They ALL had a crystallization age of between 1.3 billion years ago and 180 million years ago. This fact eliminated Mercury, the Moon, or any large failed planets like Ceres as potential parent bodies because they had no recent volcanic activity. SEE FIGURE 2

An absolutely essential fact was then determined by the combined research from many different laboratories. It turns out that all meteorites have a very unique oxygen isotope signature, and the relative percentage of oxygen isotopes for ALL the SNC meteorites is identical, and does not match any other sampled solar system body. This means that SNC meteorites, regardless of their type, ALL HAVE THE SAME PARENT BODY! SEE FIGURE 3

Mars has suffered the same history of impacts from asteroid bombardment as Earth and the Moon. These impacts and other geological processes left many rocks within a few kilometers of the surface of Mars with very small, almost microscopic, vein like cracks throughout these rocks. When these small cracks connect with the surface, they are filled with Martian atmosphere.

Asteroids can also impact the surface of Mars with such an incredible force they eject part of the surface rock at escape velocity and those rocks become meteorites. That same impact also produces an enormous shock wave moving through any vein like cracks with enough energy to melt the walls of those cracks into glass. In theory, Martian atmosphere could be trapped and preserved inside tiny bubbles in the melt glass filling the cracks. SEE FIGURE 4

Finally, in 1983, a brilliant discovery was made. An intact melt glass bubble was observed in a positively identified SNC meteorite collected in Antarctica. Researchers were thrilled because this glass bubble could contain a sample of the atmosphere from the meteorite’s parent body!

It turned out the glass bubble was indeed filled with gas, and researchers were able to determine the quantity and percentages of all the elements it contained. When the results from the SNC meteorite glass bubble were compared with meteorology data from both 1976 Viking landers, there was absolutely no doubt about the numbers being an exact match. The glass bubble was definitely filled with Mars atmosphere! This SNC meteorite was from Mars! SEE FIGURE 5

THIS SNC METEORITE WAS FROM MARS. IF ALL SNC METEORITES HAVE THE SAME PARENT BODY, THEN ALL SNC METEORITES ARE FROM MARS! This conclusive evidence was immediately accepted and embraced by the planetary science community; and after 1983, the proper name for these meteorites became Martian meteorites.

Robotic missions have well documented that the Martian surface has been weathered over billions of years by asteroid impacts, water, wind, volcanic eruptions, and radiation. Much of the Martian surface is covered by sandstone and a windblown REGOLITH comprised of bits and pieces of broken rock, mineral crystals, and very tiny bits of mineral material giving it the appearance and texture of dry sand. Mars regolith is analogous to Earth soil. However, the word soil implies the presence of biotic material which is not present on or near the surface of Mars.

Martian meteorites provide information about what is below the surface regolith, and reveal that Mars formed by the accretion of asteroids and other solar disk debris. Martian meteorites also demonstrate that the Mars mantle was considerably more molten than liquid, and its mantle did not experience a complete homogeneous magma mixing. This incomplete mixing left a heterogeneous Mars mantle with different zones of mineral concentration.

Mars magma not being liquid meant that it did not completely differentiate into a lighter ANORTHOSITE crust floating over a heavier BASALT mantle. However, there does tend to be a larger amount of the primary anorthosite mineral, PLAGIOCLASE, found nearer the surface in Shergottites. When a shock wave melts plagioclase, it forms MASKELYNITE glass as seen in the melt glass veins of some meteorites. Martian meteorites can be thought of as basalt rocks that may contain some amount of plagioclase. But, the different percentages and forms of the primary basalt minerals PYROXENE and OLIVINE are what define the classification type for a Martian meteorite.

It will not be possible to say with any certainty where on Mars the different Martian meteorite types originate until rock samples from known Mars locations are examined with instruments that have the capabilities of today's Earth instruments.

However, the differences between Martian meteorite types could provide some hints about where on Mars they might be found, and informed speculation can only lead to more serious research and understanding of the subject. With a few references and an elevation map of Mars at hand, anyone can closely examine a Martian meteorite specimen and imagine exploring the varied Martian terrain in search of where they might be more likely to find an outcrop of a similar rock.

Shergottites appear to be from shallow magma flows that erupted through fissures in the crust onto the surface and then cooled very quickly. They contain mostly low calcium pyroxene and plagioclase. Any large crater on the vast Martian plains would seem to be an excellent place to search for an outcrop of rock similar to a Shergottite. An outcrop should be more likely found beneath the sandstone layers near the bottom of the crater wall.

Nakhlites are thought to be cumulate basalts that cooled slowly in a magma chamber within a thick lava flow. They contain mostly high calcium pyroxene and olivine. The olivine present shows obvious contact with water, which could be from massive amounts of steam produced in a volcanic eruption. All of this suggests a thick lava flow near a Martian volcano could be a good place to hunt for a Nakhlite specimen.

Chassignites and Orthopyroxenites are cumulate basalts that seem to be from much deeper in the Martian crust. Chassignites contain over 90% olivine, and Orthopyroxenites contain over 90% orthopyproxene. These meteorites could possibly be from similar depths, but from different zones in a heterogeneous crust. Only an enormous impact could eject the extremely deep material that is apparently the source of these meteorites. So, it would seem collecting a sample like either of these two meteorite types would require searching the very lowest elevations of a deep Martian basin or valley.

No one can say for sure where on Mars future explorers will discover analogs to Martian meteorites, but anyone can examine a Martian meteorite and imagine where they might go on Mars to search for a rock just like it.

As yet, no human has actually gone to Mars. No human has bent down, picked up a Mars rock, and held it up to examine. But, Martian meteorites bring those rocks to Earth where anyone can examine a piece of Mars with a magnifying glass and feel like a Martian.