It may be some sort of cosmic joke that the only possible evidence of Martian life we’ve ever found has been sitting around on Earth for the past 13,000 years, hiding inside a baked-potato-sized chunk of rock.

AN UNLIKELY SPACESHIP
Actually, there’s nothing ordinary about this rock. Found in Antarctica in 1984 by a team of American meteorite hunters sponsored by the National Science Foundation, it was labeled ALH84001 for the Allen Hills area where it was collected.

Weighing a little less than 5 pounds (1.93 kilograms), it’s greenish-brown on the inside and covered with a telltale "fusion crust" on the outside—a thin, glassy black glaze that forms on meteorites as they burn through the Earth’s atmosphere.

Hundreds of meteorites have been found in the pristine Antarctic terrain, and for a while, ALH84001 was considered just one among many. But when analyzed more closely in 1994, it was found to contain traces of gases specific to the Martian atmosphere. That made it one of only twelve known meteorites thought to have come from Mars. And of those twelve, it’s the oldest by far—billions of years older than any of the others.

Scientists think ALH84001 began its journey 4 billion years ago as a chunk of molten lava spewed from a volcano on Mars. The rock cooled, crystallized, and then, over the millennia, was heated and deformed by subsequent shocks to the planet’s surface. At some point, about 3.6 billion years ago, some sort of liquid seeped into the rock, depositing tiny balls of carbonate minerals inside. Then, 16 million years ago, a final assault—the impact of an asteroid, perhaps—is thought to have hurled ALH84001 off the Martian surface, where gravity is much weaker than on Earth, and out into space.

Based on evidence of cosmic ray exposure, scientists think ALH84001 orbited the sun like so many other bits of rocky debris until it was caught by the Earth’s gravitational field 13,000 years ago and crashed to the ground near the South Pole.

TEENY, TINY ASTRONAUTS
Once ALH84001 was recognized for what it was, a group of scientists led by Dr. David McKay of NASA’s Johnson Space Center studied the meteorite and announced their show-stopping findings.

What, you ask, could be so amazing about a homely hunk of rock?

The Martians inside it.

Remarkably, by 1996, McKay and his team had identified three characteristics of ALH84001 that might prove life once existed on Mars. Within fractures in the rock were microscopic grains of carbonate minerals (chemical compounds often associated with life) that were similar to those produced by bacteria on Earth. The fractures also contained minute amounts of oily PAHs (polycyclic aromatic hydrocarbons), byproducts of decaying bacteria on Earth. And—most wondrous!—they identified the microscopic segmented shapes of what may actually be fossilized bacteria from Mars. One, in particular, stood out. Here’s what it looks like:

When McKay’s team announced its findings in Science magazine, they postulated that certain minerals in the meteorite must have been deposited by liquid water—a prerequisite for life . . . at least on Earth.

Then-NASA Director Daniel S. Goldin agreed that the evidence was "exciting, even compelling," but not conclusive. Any number of scenarios still had to be considered, from Earthly contamination to the possibility that evidence that seemed to be biological may have come from chemical reactions instead. There were also questions about the size of the bacterium-shaped object itself. At 200 nanometers long and 20 nanometers wide, it’s submicroscopic, about 1/100th the size of any bacteria known on Earth. A cell membrane alone would take up most of the organism’s width. Could the processes of life have taken place inside an organism so small? Despite cautious interest, most scientists don’t think these "fossil bacteria" will prove to be evidence for life on Mars.

WOULD THE REAL MAN FROM MARS PLEASE STAND UP?
Understandably, when you’ve got a fossilized something from another planet that’s billions of years old, there’s always room for doubt—from scientists and the public alike. For instance, if that lumpy little bacterium-like squiggle is a real, (once-)live Martian, then who the heck is this?

In 1976, twenty years before the Martian bacteria were tentatively identified, Earthlings fixated on another possibility of life in space. A series of images showing the Cydonia region of Mars was transmitted to Earth from the Viking 1 orbiter as it photographed potential sites for the lander to set down. Among those photos was one that seemed to show a mile-wide "sculpture" of a human head.
Of course, the image caused a sensation. After centuries of watching the skies, here, finally, seemed proof that we shared the universe—either now or sometime in the past—with other intelligent beings. Enthusiasts all over the world scrutinized the "sculpture," studying its structure, analyzing the mathematical relationships of its nostrils to its hairline, and speculating on its tight-lipped, foreboding expression. Some compared its characteristics with those of the Sphinx, and nearby geological features in the photo were identified as artificially made cities, building complexes, and pyramids.

According to NASA, "The Face," as it came to be called, was nothing more than a curiosity—an artifact of image processing combined with chance combinations of light and shadow. This is the explanation they released with the photo:

NASA's Viking 1 Orbiter spacecraft photographed this region in the northern latitudes of Mars on July 25, 1976 while searching for a landing site for the Viking 2 Lander. The speckled appearance of the image is due to missing data, called bit errors, caused by problems in transmission of the photographic data from Mars to Earth. Bit errors comprise part of one of the "eyes" and "nostrils" on the eroded rock that resembles a human face near the center of the image. Shadows in the rock formation give the illusion of a nose and mouth. Planetary geologists attribute the origin of the formation to purely natural processes. The feature is 1.5 kilometers (one mile) across, with the sun angle at approximately 20 degrees. The picture was taken from a range of 1,873 kilometers (1,162 miles).

Where NASA saw accidental arrangements of hills, rocks, and ridges in the scarred Martian landscape, believers saw evidence of long-sought-after extraterrestrial life. Accusations were rife that the U.S. government knew the site had been constructed by intelligent life, and theories of a cover-up plagued NASA from the moment the image appeared.

In answer, NASA tried to photograph the area again. But when the Mars Observer mission was lost in 1993, demonstrators gathered outside the Jet Propulsion Laboratory in Pasadena to protest what they saw as an ongoing effort by the government to keep the public from seeing proof of intelligent life on Mars. Finally, in 1998, the Mars Global Surveyor satellite took this picture of the Cydonia area from a new angle, showing a better view of eroded ridges and valleys spread out along a raised escarpment.

NASA even offered its own "Happy Face" crater, pointing out how easy it was to create pictures from random patterns.

But not all believers were amused. Nor were they satisfied with the new Cydonia photos. Some accused NASA of tampering with the images to hide evidence of The Face; others simply "corrected" for the altered angle and identified additional features, including eyebrows and teeth, to prove their point. No amount of proof, it seemed, could sway the believers.

LOST IN THE SHUFFLE
While some people were mesmerized by the photos from the orbiter, others were paying attention to the more esoteric data coming in from the Viking 1 lander. Onboard were three biology experiments designed to test the Martian soil for the presence of organic molecules.

Just hours after the experiments had begun, two of the three sent back totally unexpected information. In one, there was a burst of oxygen; in another, a sharp spike of carbon.

On Earth, both results would have indicated the presence of biological processes, and for a while, mission scientists were beside themselves with joy. But it didn’t last long. The results had clearly been too much, too soon, and it wasn’t long before researchers came up with an alternative explanation for the oxygen. The surface of Mars, they decided, must be covered with oxidants—chemically active substances that (in the experiment) reacted with water by fizzing like Alka-Seltzer tablets; the same sort of chemicals currently used to boost cleaning power in your washing machine.

The experiments were run until May 1977 with similar results from both Viking 1 and Viking 2 landers. In the end, the official report admitted "No conclusions were reached concerning the existence of life on Mars." There was no way to tell if the reactions had come from the biology or the chemistry of the land. It was a disappointing moment, made all the more frustrating by a history of anticipation and expectation.

FUZZY DATA
In August 2003, Mars came within 35 million miles of the Earth. That’s as close as it’s come in the past 60 thousand years. But even when it’s practically at our doorstep, Mars is still small and hard to see. Early astronomers using unsophisticated instruments were at a particular disadvantage, but that didn’t stop them from trying to understand what they saw.

While all important observations, these familiar characteristics (a 24-hour day, white polar caps, changing seasons, an atmosphere, and so on) seemed to support the popular notion that Mars, like all the known planets, was very Earthlike. And like the Earth, it must be home to a variety of life forms. In 1688, for instance, Bernard de Fontenelle, Secretary of the French Academy of Sciences, noticed "phosphorescent materials" shining from the Martian surface. One possibility, he suggested, were huge flocks of luminous birds. "How do we know that Mars does not have a great number of these birds which, when night comes, scatter on all sides and make a new day?" Almost two hundred years later, the idea of different life forms on Mars was still viable. In 1873, for example, French astronomer Camille Flammarion suggested that Mars was red because of a red-colored vegetation that covered its surface.

Essentially, Mars was considered to be a smaller, colder twin of Earth, so it made sense to compare the few features astronomers could see to similar-looking features on Earth. Early maps of Mars show the planet’s dark areas as "lakes" and "seas." "Continents" were marked, as were "islands," and "oases." All these familiar characteristics made it plausible for additional Earthlike possibilities to exist—including the famous Martian canals.

CHARTING CANALS
In 1858, Father Pietro Secchi, scientific advisor to the Pope, noticed what seemed to be dark streaks crisscrossing the red planet. Almost twenty years later, in 1877, Italian astronomer Giovanni Schiaparelli revisited Secchi’s discovery by mapping the lines in detail. He called the streaks canali, Italian for "channels," meaning only to identify their apparent shape. But the word already had a popular meaning. Since the Suez Canal had just been built (it opened in 1869), most people knew the word "canal" to mean an artificial system deliberately built to direct water from one place to another.

With no evidence to the contrary, it seemed perfectly reasonable to assume a similar meaning for the "canals" on Mars. Why couldn’t they have been built by Martians?

Though Schiaparelli never claimed his canali were anything but natural in origin, he didn’t dispute the possibility that they were artificial, either. In fact, he seemed to find the idea intriguing. "Their singular aspect," he wrote, "has led some to see in them the work of intelligent beings. I am very careful not to combat this suggestion, which contains nothing impossible."

If Schiaparelli popularized the idea—even unintentionally—then wealthy American diplomat-turned-astronomer Percival Lowell cemented it in the public’s imagination. Fascinated with the red planet, Lowell founded his own observatory in Flagstaff, Arizona, in 1894. From there, he made new maps of the complex systems of double and triple canals he thought he could see on Mars. He believed the canals were artificial, built by intelligent beings to bring water from the poles to irrigate the desert-dry lands.

To be fair, Lowell did do some important astronomical work in his lifetime (including the computations that led to the discovery of Pluto), but most scientists of the time strongly disagreed with his theories on Mars. Unfazed, by the early 1900s, Lowell was on the lecture circuit, convincing a fascinated public of the existence of intelligent life on Mars. Between 1898 and 1908, he wrote about his theories in three popular books, including Mars, The Abode of Life, and his ideas influenced generations.

HOME PHONE E.T.
Inspired by Lowell’s work, people proposed ingenious schemes to contact our Martian neighbors. One plan suggested setting giant fires in geometric patterns across the Sahara Desert. Another idea was to carve a gigantic proof of the Pythagorean theorem on the Siberian steppes. And in case the Martians were trying to contact us, we needed to listen, too. In 1899, electrical engineer Nikola Tesla thought he heard signals from Mars, as did Guglielmo Marconi, inventor of wireless communications, in 1921. An astronomy professor at Amherst College even convinced the U.S. government to turn off its high-powered transmitters at regular intervals for three days in August 1924 to see if he could pick up incoming signals from Mars. He couldn’t.

By the turn of the twentieth century, Mars was everyone’s favorite planet. In 1917, Edgar Rice Burroughs began publishing a long-running series of popular science fiction books about the adventures of John Carter on Mars. In 1938, Orson Welles’s night-before-Halloween broadcast of H. G. Wells’s 1898 novel War of the Worlds (originally inspired by Percival Lowell’s work)—convinced many panic-stricken listeners that the Martians were invading Earth. In 1910, when Thomas Edison produced the early four-minute film A Trip to Mars, he began a tradition of entertainment that made wild speculation about Martian life a staple of entertainment. Over the years, more than 100 movies—from Aelita, Queen of Mars (1924) to Horrors of the Red Planet (1979) to Brave Little Toaster Goes to Mars (1998)—have brought Mars to life.

CONNECTING THE DOTS
Today, we know that the canals Schiaparelli and Lowell saw aren’t real. They’re artifacts of perception; the human brain creating patterns where there are none. There are no continents, islands, or seas, as we know them. And so far, we’ve found no creatures—horrible or otherwise.

Are we alone—the only sentient beings in the universe? We’ve been watching the skies for millennia, reaching out farther and farther into space as the tools of science and technology improve, and we still have no definitive answer to that question.