The Geography of Mars

The Shergotty-Nakhla-Chassigny Meteorites

This lab has the following objectives:

• to familiarize you with the "SNC" or "snick" meteorites on Earth thought to have come from Mars
• to build understanding of the evidence for and controversy around the possibility of biotic activity on these meteorites

Background

Meteoroids are small chunks of rock and/or metal travelling through space in their own orbits around the sun. They are smaller than asteroids, many in the gravel to pebble range. They are often the product of asteroids breaking up due to impacts with other asteroids or other impactors. Some of them are materials broken off from comets, the moon, or ... Mars, again due to impacts, impacts large enough to impart escape velocity to the debris.

Meteors are meteoroids that have entered the earth's (or another planet's) atmosphere, where their outer layers heat and ablate due to friction with the atmosphere, creating an incandescent or glowing streak in the night sky. Meteors are often called "shooting stars," and a concentration of them over a one or two night period is called a "meteor shower." Meteor showers are the result of Earth's orbit crossing the orbit of a comet, often one broken into a debris stream strewn along its orbit.

Meteorites are meteors that are large enough to survive the loss of sizable amounts of material during the trip through the atmosphere, the remnant crashing on the surface. Recovered meteorites on Earth have been classified into three basic types, based on their composition and presumed formation and history:

• iron meteorites, thought to be from the cores of asteroids, which had undergone melting and differentiation early in their histories (iron, nickel, with some small amounts of sulfide and carbide)
• stony-iron meteorites, roughly half and half.
  • Some, called pallasites, may have come from the interior of differentiated asteroids, from the zone of contact between the iron core and the olivine and silicate mantle above it (this theory is contested, however).
  • Others, called mesosiderites, are thought to result from the collision and intermixing of two asteroids, preserving some bits from either parent asteroid.
• stony meteorites are by far the most common (over 90%).
  • These include chondrites, the silicate materials in which have never actually melted during their histories. The resulting material is a grainy mix of silicate droplets along with grains of sulfides and iron-rich metals. These are the most primitive, oldest meteorites, forming 4.5-4.6 billion years ago at the beginning of the solar system.
  • Achondrites are the second type of stony meteorite. They are comprised of mineral melts, magmas, and are, essentially, igneous rocks. These clearly show signs of having come from a differentiated body, whether a melted and density-layered asteroid or the moon or Mars. They are, thus, younger than the chondrites.

This lab is about a distinctive type of achondrite, the "SNC" or "snick" meteorites. They are named for the three meteorite falls that defined their characteristics:

• Shergotty, which fell on Bihar State, India, on 25 August 1865 -- comprised of basalt, an extrusive igneous rock of mafic composition (sometimes it's more of a gabbro or intrusive igneous rock) rich in olivine and plagioclase, which can only form on a differentiated planet-sized body. Some local color: two of these were found near Los Angeles in 1999!
• Nakhla, which hit the El-Baharnya region in Egypt on 28 June 1911 and killed an unfortunate dog -- a clinopyroxenite dominated rock, another very mafic igneous rock type
• Chassigny, which struck Haute-Marne province in France on 3 October 1815 -- comprised of dunite, an ultramafic intrusive igneous rock typical of deep mantle rock, made up nearly totally of olivine.

These are known to be Martian in derivation. It's up to you to figure out why.

ALH 84001 is the martian meteorite that's been the focus of so much debate and speculation about the possibility of life on Mars. It was the first (001) meteorite found in 1984 by the Antarctican team picking up rocks in the Allen Hills area. It actually represents a completely new type of martian meteorite, being vastly older than the true SNCs and mineralogically quite different. It formed among the first surface rocks to cool and solidify on Mars, dating back to ~4.5 billion years ago and made up of orthopyroxenite (an ultramafic mineral that solidifies at very high temperatures early in the process of magma fractionation).

Your data

I've put together a table summarizing several attributes of sixteen well-characterized martian meteorites, including their ages of formation, the age of the impact that launched them out of Mars orbit, the time the rock stayed in space subject to alteration by cosmic ray exposure, and, if known, the amount of time the meteorite spend on Earth before it was discovered. You can download it here: https://home.csulb.edu/~rodrigue/geog441541/SNCages.ods.

As you look through this table, pay attention to:

• The ages of the various true SNC meteorites in comparison with the ages of the chondrites (above) and ALH 84001

• The age of Nakhla 1911 (when the material comprising it crystalized)

• The age of water exposure of Nakhla 1911 (clays)

• The age of ALH 84001

• The age of the carbonates depositied within veins of ALH 84001

• The cosmic-ray exposure for ALH 84001, which tells the amount of time it spent in space from the time of its launch to the time of its arrival on Earth (judged by helium [³He], neon [²¹Ne], and argon [³⁸Ar] isotopes)

Other relevant data:

• The gasses found in the SNC meteorites:
  • bubbles of gas trapped in the meteoroids during the impact shock that launched them, which entailed the solidification of impact melted glasses, together with the gasses that hadn't escaped the melt
  • not all SNCs show signs of impact shock, but they do share certain distinctive oxygen isotope (¹⁶O, ¹⁷O, and ¹⁸O) mixes

• The ratios of aluminum:silicon plotted against the ratios of magnesium:silicon for ultramafic Earth rocks, ultramafic Mars rocks sampled by Spirit and Opportunity, and various SNCs

• The escape velocity from the surface of Mars (5.4 km/sec) and of the earth (11.2 km/sec)

• Tunnel structures in the Nakhla 1911 meteorite

• Electron microscope images that look like Earth nanofossils of bacteria

• The relative sizes of Earth and Mars "nanofossils" -- http://www.nature.com/news/1998/000309/full/news000309-3.html

A few helpful general background resources:

• Lunar and Planetary Institute Technical discussions about Mars meteorites.

• Eric Weisstein's World of Astronomy article, SNC Meteorites

• John Bridges' martian meteorites article for the British Natural History Museum.

• NASA's Planetary Remote Sensing primer, Life on Mars?

Lab report

Answer the following questions on a separate sheet of paper with question numbers indicated (or deposit it in the BeachBoard digital dropbox under Lab: SNC). Each answer can be quite short on the order of bullet statements, though the last two require at least a paragraph or two each. Don't forget to autograph your work!

1. How old are most SNC meteorites? How is their age determined?

2. How old is the Nakhla 1911 meteorite (and don't tell me 111 years old!)

3. When in Nakhla's history was it exposed to water? Explain.

4. How old is the ALH 84001 meteorite?

5. When was ALH 84001 launched from Mars' surface? How can the launch time be estimated?

6. Is it likelier for a meteoroid to be launched from Earth and get to Mars (and, thus, maybe carry life there) or for it to be launched from Mars and get to Earth (hmmmm, and maybe carry life or protolife here)? Explain your choice.

7. Briefly describe the the tunnel structures in Nakhla 1911 (size, relationship to fractures in rock) and their significance in the ongoing debate about possible signs of life in the SNC meteorites?

8. Briefly describe the ALH 84001 structures that some argue are signs of microbes. What is their general chemical composition and size range and where in the meteorite are they found (portions of the meteorite are made up of carbonate globules and orthopyroxenites, together with fused black glass on the surface probably from the launch event -- the fossil-like bodies are found in which of these)?

9. How do these structures compare in size with Earth bacterial nanofossils?

10. What other peculiar chemical was found in association with the fossil-like structures, which some workers think makes even stronger evidence for these structures plausibly being fossils?

11. Briefly summarize the case for Martian microbial life evidence in ALH 84001 and Nakhla 1911.

12. Briefly summarize the arguments made to debunk this case.