27-3 Meteorites from Mars have been scrutinized for life-forms

While spacecraft can carry biological experiments to other worlds such as Mars, many astrobiologists look forward to the day when a spacecraft will return Martian samples to laboratories on Earth. Until that day arrives, we have the next best thing: More than a hundred meteorites that appear to have formed on Mars have been found at a variety of locations on Earth.

The feature that identifies meteorites as having come from Mars is the chemical composition of trace amounts of gas trapped within them. This composition is very different from that of Earth’s atmosphere, but is a nearly perfect match to the composition of the Martian atmosphere found by the Viking Landers.

How could a rock have traveled from Mars to Earth? When an asteroid collides with a planet’s surface and forms an impact crater, most of the material thrown upward by the impact falls back onto the planet’s surface. But some extraordinarily powerful impacts have produced large craters on Mars—roughly 100 km in diameter or larger. These tremendous impacts eject some rocks with such speed that they escape the planet’s gravitational attraction and fly off into space.

There are numerous large craters on Mars, so a good number of Martian rocks have probably been blasted into space over the planet’s history. These ejected rocks then go into elliptical orbits around the Sun. Many such rocks will have orbits that put them on a collision course with Earth, and these are the ones that scientists find as meteorites from Mars. In fact, estimates are that several tons of Martian rocks land on Earth each year (equaling around a cubic meter of rock).

Using the radioactive age-dating technique (see Section 8-3), scientists find that most meteorites from Mars are between 200 million and 1.3 billion years old, much younger than the 4.56-billion-year age of the solar system. But one meteorite from Mars, denoted by the serial number ALH 84001 and found in Antarctica in 1984, was found to be 4.5 billion years old (Figure 27-10a). Thus, ALH 84001 is a truly ancient piece of Mars. Further analysis of its radio-activity suggests that ALH 84001 was ejected from Mars by an impact about 16 million years ago and landed in Antarctica a mere 13,000 years ago.

Figure 27-10: R I V U X G
A Meteorite from Mars (a) This 1.9-kg meteorite, known as ALH 84001, formed on Mars some 4.5 billion years ago. About 16 million years ago a massive impact blasted it into space, where it drifted in orbit around the Sun until landing in Antarctica 13,000 years ago. The small cube at lower right is 1 cm (0.4 in.) across. (b) This electron microscope image, magnified some 100,000 times, shows tubular structures about 100 nanometers (10^-7 m) in length found within the Martian meteorite ALH 84001. One controversial interpretation is that these are the fossils of microorganisms that lived on Mars billions of years ago.

(a: NASA Johnson Space Center; b: Science, NASA)

ALH 84001 was on Mars during the era when liquid water existed on the planet’s surface. Scientists have therefore investigated this rock carefully for clues about Martian water and possible life. One such clue is the presence of rounded grains of minerals called carbonates. Analysis of these carbonates indicates that they formed in liquid water (at a comfortable temperature of about 64°F).

In 1996, David McKay and Everett Gibson of the NASA Johnson Space Center, along with several collaborators, began reporting the results from studies of the carbonate grains in ALH 84001. They provided several pieces of evidence that life may have once existed within the rock’s cracks while it was still on Mars:

1. There are large numbers of elongated, tubelike structures around the carbonate grains that could be fossilized microorganisms (Figure 27-10b).
2. Microscopic cracks contain organic molecules—consistent with what is expected from the decay of microorganisms.
3. The carbonate grains contain small magnetic crystals called magnetite, which are similar to magnetite produced by bacteria on Earth.

Are McKay and Gibson’s conclusions correct? Their claims of ancient life on Mars are extraordinary, and they require extraordinary proof. Further studies found that the organic compounds detected are not a very good match for decayed organisms. Also, it was later found that the tubelike structures and magnetite crystals found in ALH 84001, while consistent with life, could have been formed through geological processes on Mars. Thus, while some of the evidence is consistent with Martian life, it is far from compelling.

Possible Microbial Tunnels

The most recent indications of possible Martian microbial remnants involve microscopic tunnels found in meteorites from Mars. In 2006, Martin Fisk from Oregon State University led a team that discovered small tunnels in a Martian meteorite named Nakhla. These tunnels are similar to structures found in rocks on Earth that are thought to be produced by rock-eating microbes. The Nakhla meteorite is not the only one with tunnels; they are also found in ALH 84001, and a 2013 study at the Johnson Space Center led by Lauren White finds them in the Martian meteorite Y000593 (Figure 27-11).

Figure 27-11: R I V U X G
Microscopic Tunnels in Martian Meteorite This figure shows microtunnels in the Martian meteorite Y000593. Although not common, very similar microscopic tunnels are found in rocks on Earth that contain DNA and are thought to be produced by rock-eating microbes. Even though Y000593 sat on Earth for thousands of years before being collected, the tunnels themselves were likely created on Mars: the tunnels in Y000593 are filled with Martian clay as determined by an analysis of the clay’s composition. The microtunnels emanate from larger fractures in the rock and are about 1–5 micrometers long.

(Lauren White)

Could the tunnels have been formed after the meteorites hit Earth? That is possible for ALH 84001, and Y000593, which sat on Earth for thousands of years before being collected. But the Nakhla meteorite was observed falling on June 28, 1911, and its debris was collected too quickly for microbes on Earth to have created its tunnels. Are the microtunnels produced by Martian microbes? As in the case of the tubelike structures and magnetite crystals from ALH 84001, features that appear biological in origin might simply occur naturally through Martian geology. Thus, the Martian tunnels lead us back to Earth, where we must determine if these tunnels can instead be created through geological processes. For now, the origin of these tunnels is uncertain. Though they only provide weak evidence at this early stage of analysis, these microscopic tunnels open up a whole new line of inquiry in the search for Martian life.