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The Mars Rock

Notes on ALH84001, by Lupe Lagasse

My name is Guadalupe Lagasse. Everybody calls me Lupe. I taught earth science at a high school here in Albuquerque for thirty-one years, and I have been collecting meteorites, in a small way, for about twenty of them. I retired last year. I thought that meant my former students would stop calling me with questions about rocks. Then on the evening of Tuesday, August 6, the news got out that NASA thought it might have found signs of life in a meteorite from Mars, and my telephone has not stopped since.

So I have written down what I have been telling them. I have tried to be careful. Scientists announced that they found evidence that may be the remains of very simple life that lived on Mars billions of years ago. They did not say they had proved it, and neither will I. If you take only one thing away from this page, take that.

On this page:
  What was announced
  The rock itself
  What a meteorite is, and how we know this one came from Mars
  The four lines of evidence
  Why many scientists are not convinced
  My own meteorites
  What happens next

What was announced

The research was supposed to appear in the August 16 issue of the journal Science. Word leaked out a day early, on Tuesday the 6th, and by that night it was on every newscast. On Wednesday, August 7, NASA held a press conference in Washington with the team that did the work: David McKay and Everett Gibson of NASA's Johnson Space Center in Houston, with colleagues at Stanford University and McGill University in Montreal. They spent about two years studying one small rock from Antarctica, and they believe that four different kinds of evidence inside it, taken together, are best explained by ancient life on Mars.

NASA, to its credit, also put a skeptic at the same table: J. William Schopf of UCLA, who studies the oldest fossils on Earth, and who said plainly that he was not yet convinced. I thought that was good science and good manners.

The same day, President Clinton spoke about it at the White House. He said that if the discovery holds up it will be one of the most remarkable things science has ever revealed about our universe, and that the space program will put its full effort into finding out whether it is true, starting with the two Mars missions launching this fall. He also said he would ask Vice President Gore to hold a meeting on the future of the space program before the end of the year. Notice the "if." The President was careful too.

The rock itself

ALH84001 at a glance

Name: ALH84001. "ALH" is for the Allan Hills in Antarctica, "84" for the 1984 field season, and "001" because it was the first rock from that season's collection to be described.
Found: December 27, 1984, on the blue ice of the Allan Hills, by a U.S. team searching Antarctica for meteorites.
Weight: 1.93 kilograms, about 4 1/4 pounds. Roughly the size of a large potato.
Kind of rock: an igneous rock made mostly of the mineral orthopyroxene. It cooled from molten rock inside Mars.
Age: about 4.5 billion years since it first hardened. The oldest Martian rock known.
Left Mars: about 16 million years ago, blasted off the surface when an asteroid or comet hit.
Fell in Antarctica: about 13,000 years ago.
Martian meteorites known: 12, counting this one.

Here is a part of the story I enjoy telling. When it was found, ALH84001 was thought to be a piece of an asteroid. It was put away with the collection at the Johnson Space Center and stayed there, mislabeled, for almost ten years, until a scientist looked again in 1993 and saw that its chemistry matched the meteorites from Mars. There may be other treasures sitting in drawers. There usually are.

Why Antarctica? Meteorites fall everywhere, evenly over the whole Earth. But a dark rock on white ice is easy to see, nothing else is up there to be confused with it, and the slow flow of the ice sheet carries meteorites along and piles them up where the ice meets the mountains and wears away. Teams have collected thousands of meteorites there since the 1970s. It is the best hunting ground in the world, if you don't mind the cold.

What a meteorite is, and how we know this one came from Mars

First, the words, which my students always mixed up:

Most meteorites are bits of asteroids. They come in three broad kinds: stones, most of them chondrites, full of tiny round grains called chondrules; irons, almost solid nickel-iron metal; and stony-irons, a mix of the two. A very few are pieces of the Moon or of Mars, knocked loose by big impacts and sent drifting through space until they happen to cross Earth's path.

How can anyone tell that a rock came from Mars? Two good reasons:

  1. Trapped gas. The Viking landers measured the Martian atmosphere in 1976, and it has an unusual mix of gases unlike Earth's air. In the early 1980s, scientists found tiny pockets of gas sealed inside another meteorite from Antarctica, and the mix matched the Viking readings very closely. That rock had to come from Mars. The impact that knocked it loose sealed a little of the Martian air inside it.
  2. A chemical fingerprint. Oxygen comes in slightly different forms (isotopes), and every body in the solar system seems to have its own mix. The Martian meteorites all share one mix, different from Earth rocks and from the other meteorites. ALH84001 has it.

The four lines of evidence

None of these alone says "life." The team's argument is that all four turn up together, in the same tiny places inside the rock, and that ancient life is the simplest way to explain all of them at once.

  1. Organic molecules. The team found complex carbon compounds called polycyclic aromatic hydrocarbons, PAHs for short, inside the rock. On Earth, PAHs form when living things decay. They are also found in soot, in car exhaust and in some meteorites that never had anything to do with life. The team found more of them deep inside the rock than near its surface, which they take to mean they came from Mars and not from Antarctic contamination.

  2. Carbonate globules. Scattered through cracks in the rock are tiny globules of carbonate minerals, smaller than the period at the end of this sentence, orange-brown with dark rims. Carbonates usually form in water. So water once ran through this rock on Mars, and on Earth, bacteria sometimes help carbonates like these form.

  3. Magnetite grains. Inside the globules are very small crystals of magnetite (an iron oxide; it's the mineral in a lodestone) and iron sulfides. Some bacteria on Earth make magnetite crystals of about the same size and shape inside their own bodies.

  4. Tiny worm-like shapes. Under a scanning electron microscope, the team photographed egg-shaped and tube-shaped forms in the carbonate, some of them segmented like little worms. They are small: most about 20 to 100 nanometers long, which is far less than one hundredth the width of a human hair. The team suggests they may be fossils of bacteria-like organisms.

Why many scientists are not convinced

This is the part the evening news hurried past, so I will slow down.

I used to tell my classes that science is not a pile of facts, it is an argument that goes on until the evidence settles it. This one will go on for years. Other laboratories will cut their own pieces of the rock and test them their own ways. That isn't a failure. That is how it is supposed to work.

My own opinion, for what a retired teacher's opinion is worth: I don't know, and I'm delighted not to know. Whatever the answer, we have learned that Mars once had water running through its rocks, and that is something.

My own meteorites

I don't own a piece of Mars. Martian meteorites are rare and almost all of them are in museums and university collections, and the few pieces that change hands cost more than my truck. What I have is a modest shelf of the ordinary kind, bought over the years from dealers at rock shows, mostly at the big show in Tucson every February, where my wife Margaret has learned to bring a book.

Next month I hope to add a slice of an iron that has been cut and etched with acid to show the crisscross Widmanstätten pattern, which forms only when the metal cools extraordinarily slowly, over millions of years, deep inside an asteroid. Nobody can make it in a laboratory. There isn't time. My friend Tom Kiefer from the rock club has promised to show me how to etch one properly without taking off my fingers.

What happens next

Two American spacecraft leave for Mars this fall, and it is good timing:

Russia is sending its own spacecraft, Mars '96, in November as well. None of these missions is built to look for life; the Viking landers tested the soil for it twenty years ago and found nothing they could be sure of. But each will teach us more about where the water was and where it went, which is the first question you have to answer. The rocks that might really settle this argument will be the ones a future mission brings back from Mars on purpose.

Coming next: A Beginner's Guide to Collecting Meteorites: how to tell a real one from the slag and magnetite people bring me, where to buy, what to pay, and how to keep an iron from rusting. I'm writing it now.

Questions, corrections, and stories of your own rocks are welcome. Former students especially: write and tell me what you're doing now.
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Last updated Sunday, August 18, 1996