Unfortunately, no human geologist has been to Mars. So the best information that we have about the planet's beginnings 4.6 billion years ago comes from images taken by orbiters and landers, Martian meteorites, and comparisons with its planetary peers (Mercury, Venus, Earth and Earth's moon). The current theory goes like this:
- Mars formed from clumping or accretion of small objects in the early solar system.
- However, unlike Earth and Venus, Mars finished forming within 2-4 million years and never grew beyond the planetary embryo stage.
- Possibly, aluminum 26 decay turned the planet into a magma ocean.
- After cooling, there was a period of intense bombardment from meteors.
- The hot mantle pushed through and lifted portions of the crust.
- One or more periods of intense volcanic activity and lava flows followed.
- The planet cooled and the atmosphere thinned.
Let's look at these steps in more detail.
Mars was created by the accretion of small objects in the early solar system, which took about 2-4 million years. Mars grew and developed a larger gravity field, which attracted more bodies. These bodies would fall into Mars, impact and generate heat. Some models suggest that such heating would not have been enough to bring about large-scale melting on Mars; rather, because the planet formed so quickly, it might have gobbled up enough of the aluminum 26 nuclide, which has a half-life of only 717,000 years, to melt from radioactive decay. Gradually, the material sorted itself out into a core, mantle and crust. Gases released from the cooling formed a primitive atmosphere [source: Dauphas and Pourmand].
But as an embryo planet formed in the solar system's chaotic early days, Mars couldn't catch a break. It was heavily bombarded by meteors in the inner solar system. These bombardments produced craters and multi-ring basins all over the planet, like the 1,400-mile- (2,300-kilometer-) wide Hellas Planitia impact crater in the planet's southern hemisphere. Some geologists think that a huge impact occurred that thinned the crust of the northern hemisphere. Similar impacts occurred on Earth and our moon at this same time. On Earth, the craters were eroded by wind and water. On the moon, the evidence of these great collisions is still visible.
Now imagine Mars is a soft-boiled egg; the inside is hot as the shell cools. If the shell is weak in spots, the egg will crack and the cooked yolk will protrude. A similar occurrence happened with the Tharsis region, a continent-sized land mass in the southern hemisphere. The hot mantle bulged out, pushing up the crust and fracturing the surrounding lava plains (forming Valles Marineris, a network of canyons). In other spots, the mantle pushed through the crust, giving rise to the region's many volcanoes, such as Olympus Mons. (We'll talk about all these Martian landmarks next.)
During this period, there were widespread volcanic eruptions. Lava flowed from volcanoes and filled the low-lying basins. Eruptions released gas that contributed to a thick atmosphere, which could have supported liquid water. Therefore, there might have been rain, flooding and erosion. The erosion would produce sedimentary rocks in the basins and plains, and form channels in the rock. More than one period of widespread volcanic eruptions may have occurred during Mars' history, but eventually the volcanoes stopped rumbling as much.
The bulges that caused the crustal uplifts and the widespread volcanic activity released vast amounts of heat from the inside of Mars. Since Mars isn't as large as the Earth, it cooled much faster, and the surface temperature cooled with it. Water and carbon dioxide from the atmosphere began to freeze and fall to the surface in vast quantities. This freezing removed large amounts of gas from the atmosphere, causing it to thin. In addition, any surface water may have frozen into the ground, forming permafrost layers. Intermittent volcanic eruptions would release more heat that would melt more water ice and cause flooding. The flooding would erode channels and carry more material down to the surrounding plains.
As for the rest of Mars' atmosphere, it was likely blown off under the assault of solar wind. Earth's magnetic field protects us from the worst of such effects, but the Mars equivalent shut down around 4 billion years ago, possibly due to a series of massive asteroid impacts that threw off the temperature gradient powering the planetary electric dynamo [source: Than].
While this is the current theory about the origin of Mars, it needs more data to back it up.