Has evidence for fossil life been found on Mars?

On Tuesday, August 6, 1996, a team of scientists at Johnson Space Center led by David S. McKay, a veteran of studying lunar rocks from the Apollo program, described their evidence that microscopic fossils, resembling ancient Earth bacteria were found in a meteorite recovered from Antarctica back in 1984. Daniel Goldin, the head of NASA, is quoted in the Washington Post ( Wednesday, August 7, page A4) as saying that "NASA has made a startling discovery" calling it "exciting, compelling but not as yet conclusive". The full technical document was published in the prestigious journal Science a week later.

What is all the excitement about?

In a hastily convened press conference at 1 PM on Wednesday at NASA headquarters in Washington, D.C., and in the press releases I have read, the story goes something like this.

In 1984 a meteorite was recovered from Antarctica, designated ALH84001. It was a 4.5 pound, 1.9 kilogram stone, whose age has been estimated to be near 4.6 billion years. Isotopic analysis later established that, along with 11 other rocks found in Antarctica, ALH84001 was from Mars. It is the oldest of the 12 meteorites, the youngest of which is only a few 100 million years old. ALH84001 was ejected from Mars perhaps 16 million years ago, and about 13,000 years ago found its way to Earth and Antarctica. It was discovered in 1984, classified and archived for later research.

During the last 2.5 years, scientists at NASA's Johnson Space Center meticulously analyzed a small piece from this meteorite, performing an intensive chemical, mineralogical and microscopic assay on this sample. They found that there were orange 'carbonaceous globules' embedded in the matrix of the rock with sizes of about 250 microns. This was not a new discovery, and a description of these globules has appeared several times in the technical journals already. The team states that globules such as these have not been found in other meteoritic samples, so they are of themselves very unusual and warrant investigation for this reason alone.

Sophisticated mass spectroscopy of a portion of the meteorite, shaved micron thin and including several of these globules, were then performed. The first new result is that these globules were found to coincide with enhanced traces of Polycyclic Aromatic Hydrocarbon molecules. These 'PAH's are organic molecules, however, there are many inorganic processes that produce them in the interstellar medium and in the atmospheres of cool stars. Astronomers have detected PAH lines in the infrared from many different, dusty, sites in the Milky Way. The association of the PAH molecules with the carbonaceous globules is, however, rather peculiar.

The orange globules have what appears to be an outer surface that alternates black-white-black. A mineralogic assay of this surface reveals the presence of magnetite, gregite and pyrrhotite minerals. The gregite and pyrrhotite minerals are iron sulfites. The shapes of the magnetite and pyrrhotite mineral 'crystals' bare a striking resemblance to organically-produced magnetite and pyrrhotite crystals which are either cubic or tear drop shaped.

The globules were photographed using a scanning electron microscope, and it is this evidence that is visually the most intriguing. The globules appeared to be covered by a fine surface with many elongated bodies exposed, or partially embedded, in the carbonate-rich material. The longest of these are 200 nano-meters long, and the smallest are about 100 times smaller than the smallest known Earth micro-fossils recovered from 3.8 billion years old Australian rocks. They resemble, in appearance, images of Earth nano-bacteria on calcium carbonate. The basaltic deposits taken from core samples in the Columbia River basin also show sub-surface bacterial colonies from depths of 2 kilometers that resemble the purported martial micro-fossils.

The team admits up front that they have no confirming evidence of cell walls, or a chemistry for the micro-fossils that is uniquely biologic in origin. They argue, however, that the same careful criteria used to confirm the existence of ancient Earth micro-fossils, when applied to the Mars sample, leaves open the possibility that what they are seeing is the following scenario:

About 4 billion years ago, there was abundant water on Mars as we can see from geologic features imaged by the Viking Orbiter. The oceans, perhaps up to several hundred meters deep, were rich in calcium carbonate, and this material precipitated out as the globules we see in ALH84001. These globules lodged themselves in fissures and crevasses within an older rock that had solidified about 0.8 billion years earlier. At about the same time, there were martian nano-bacteria present, swarming over the condensing globules, and in many instances becoming entombed in the globules as they grew. Then after billions of years, a body impacted Mars and ejected some of the rock in which the globules were encased. This happened about 16 million years ago, and the sample finally impacted the Earth about 13,000 years ago.

In the news conference, UCLAs Prof. Bill Schopf provided a rebuttle to these claims by emphasizing that "extraordinary claims require extraordinary evidence" as Carl Sagan often states.

1. I previously mentioned that the detection of carbon-rich PAHs is not a unique indicator of biogenic processes because PAHs are seen in many locals in the universe, even in the atmospheres of cool stars! So this is neither supportive or contradictory to the idea that there are micro-fossils in the rock.
2. Schopf notes that the aledged martian fossils are 100 times smaller than the smallest fossil bacteria ever seen in ancient terrestrial rocks dating from 3.8 billion years ago.
3. Schopf agrees that it is probably very likely that ALH84001 came from Mars, and that the dating is consistent with a time when Mars may have been more hospitable to life.
4. Schopf points out that there are published papers that propose that the globules in ALH84001 were formed under high temperature conditions near 450 C, and that the fissures in which the globules have apparently been found, could have been produced by the impact that ejected the fragment from Mars. There is substantial disagreement over whether the globules were formed under the benign low-temperature conditions required for life to have emerged.