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When Worlds Diverge

In recent decades, as scientists have learned more about the geology of Mars, they have been increasingly impressed by the planet's physical likeness to Earth. Mars rotates at roughly the same rate as Earth does, so one Martian day (or "sol") is approximately 24 hours long. Mars has polar ice caps, and it is tilted slightly on its axis, so the planet has seasons. There is clear evidence that Mars had volcanoes, although their activity ceased long ago. And now, there are strong signs that Mars had vast quantities of liquid, briny water.

For all the similarities, however, Mars does possess unique geophysical traitstraits that early on set Mars on a developmental path distinct from Earth's. "I like to think that Mars is the mirror of Earth," says Denton Ebel, a curator of meteorites with the American Museum of Natural History's Department of Earth and Planetary Sciences. "It was once a lot like Earth was once. It is now, perhaps, the way Earth might become in the distant future."


Olympus Mons Volcano

Mars Global Surveyor Project/MSSS/JPL/NASA

Mars, like the other terrestrial planets (Earth, Venus, and Mercury), formed about four billion years ago from the accretion of asteroid-like boulders and other rocky, protoplanetary debris. Mars, however, was forming near the gas giant Jupiter. With its strong gravitational pull, Jupiter managed to divert a great deal of rocky debris that might otherwise have become part of Mars. As a result, Mars is only about one-third the size of Earth.

That smallness made all the difference. Small bodies cool more quickly than larger ones; over its long history, Mars lost much of the internal heat from its metallic core. The core supplied enough heat to drive volcanic eruptions and the circulation of water within the ground, but not continental drift on the scale seen on Earth. (What are now the highlands of Mars, in the southern hemisphere, may once have been a supercontinent that "froze" in place when the planet's upper mantle and crust became immobile.) Then again, says Ebel, studies of several meteorites from Mars — five are on display at the American Museum of Natural History — indicate that Martian volcanoes were active as little as 150 million years ago. "That's pretty recent, in geological time."

Meanwhile, back on early Earth, tectonic activity was going strong. The larger planet retained its internal heat, and with it a hot core of partially molten metal. As Earth rotates, the molten core churns and acts as a dynamo, generating a strong magnetic field?something Mars lacks. Unshielded, the Martian surface receives a heavy dose of deadly cosmic rays and is buffeted by the solar wind, which over time has stripped away Mars's atmosphere. Had Earth formed nearer to Jupiter, conditions might have turned out less favorably for its inhabitants. "The evolution and habitability of the planet are direct consequences of its initial conditions — how big it is, and where it is in the Solar System," Ebel says.


In March 2004, the NASA rovers Spirit and Opportunity found evidence that, like Earth, Mars once possessed large bodies of briny water, perhaps even a sea, on its surface. The discovery of acid-sulphate salts on the Martian surface indicate that the briny sea evaporated at some point. But when that occurred, and where all the water went, is unclear. "What happened to the water?" Ebel muses. "We don't know. The key missing evidence is the timing."

One possibility is that the water wound up in the polar ice caps, both of which are now known to contain frozen water in addition to frozen carbon dioxide, or "dry ice". "There's a lot of water in there, more than was previously thought," Ebel says. Scientists are trying to model the Martian atmosphere, to determine how carbon dioxide and water move around the surface of Mars and, by working backward in time, where it came from and went. But the orbit of Mars is far more eccentric than Earth's, thanks to the gravitational influence of Jupiter. As a result, Ebel explains, "Figuring out which part of the planet was cooler and which warmer, and when in history, is a challenging puzzle."

There's also a strong possibility that water, frozen or liquid, may still exist beneath the surface of Mars. (The Martian climate is too cold for liquid water to exist aboveground.) The European Space Agency's Mars Express orbiter is equipped with ground-penetrating radar, which can detect signs of frozen water beneath the rocky Martian surface — if it's there to be found.

The existence of both volcanic activity and water on Mars raise the tantalizing possibility that the planet might have harbored life at one time in its history. "Water and heat are two very important ingredients for life," Ebel says. If life did exist on Mars, he adds, it likely thrived deep underground, protected from the freezing temperatures and deadly rain of cosmic particles aboveground. (Microbes-and those are the only fossilized life-forms scientists would expect to find on Mars-are known to live deep below Earth's surface.) Alas, fossil-hunting is not something the Spirit and Opportunity rovers are designed to do; the first astrobiology experiments won't arrive on Mars until sometime in 2010.

Even if Mars did harbor microbial life, Ebel says, it almost certainly didn't have enough of it for a long enough period of time to alter the planet's climate. On Earth, the emergence of photosynthetic, oxygen-producing microbes-organisms that could use sunlight to convert carbon dioxide into carbohydrates and oxygen-dramatically altered the early atmosphere. The rise of oxygen, between 2.4 billion and 2.2 billion years ago, in turn made possible the eventual evolution of more complex forms of life, including humans. In contrast, the Martian atmosphere today-what little of it remains-is still mostly carbon dioxide, as it likely was from the outset.

Many physical factors kept Mars from developing into the planet that Earth is today. But might Earth one day come to resemble Mars? "A lot would have to happen between now and then," Ebel says. First, Earth would have to cool considerably; its convective core would then stop churning. Its magnetic field would also fade, making the surface vulnerable to both the solar wind, which would eventually strip away the atmosphere, and cosmic rays, which would effectively sterilize the planet. "But don't worry," Ebel adds, "that future is still billions of years away. By then, who knows? Maybe humans will have colonized some other corner of the cosmos-maybe a quiet little planet, maybe someplace like Mars."

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