As our tale of two planets continues, Earth and Mars--born rather as "twins" in the solar system--reveal differences in their developments.
As time passed, the twins evolved away from each other and grew to be two distinctly different worlds. Heat from the Sun stirred their atmospheres. Earth's moderate gravity and the heat from the Sun allowed lighter gases like hydrogen, methane, and ammonia to leak away into space. Our planet retained nitrogen and oxygen with a trace of other gases.
Photo. On Earth, the third closest planet to the Sun, temperatures can range from nearly 130 degrees Fahrenheit below zero to over 130 degrees F above zero. Courtesy of NASA.
Mars is smaller than Earth, and thus its gravity is weaker. Some of the oxygen in Mars's atmosphere combined chemically with iron in the soil to form iron oxide--rust--giving Mars its red color. Mars lost most of the rest of its atmosphere. Most of it leaked away into space except for the relatively heavy gas carbon dioxide which today comprises most of Mars's sparse atmosphere.
When the atmosphere that surrounded Mars leaked away into space, the absence of ozone left the planet vulnerable to deadly ultraviolet rays from the Sun. This sterilizing radiation made it extremely difficult for us to imagine that life can exist any longer on the surface of Mars.
With little atmosphere, the surface pressure was so low that the water on the ground disappeared. It evaporated. It leaked away into space. Some of it in the colder regions became locked in the polar ice caps. Some may have become trapped underground. It may have frozen beneath the surface, forming a permanent layer of ice, like the permafrost in Siberia.
Photo. This view of Mars was taken by the Hubble Space Telescope in March 1997 and represented the clearest view of the planet as seen by the observatory, which has orbited Earth since its 1990 placement. The image here was captured by the Space Telescope's second "Wide Field Planetary Camera" with each pixel (dot in the picture) at original resolution representing about 13 miles on the Martian surface. This particular photograph gives a clear indication of the contrasting dark and light areas that first interested astronomers in learning about Mars. Courtesy of Space Telescope Science Institute, David Crisp, WFPC2 Science Team (Jet Propulsion Laboratory/California Institute of Technology), NASA.
The stronger gravity of Earth retained a denser atmosphere. Our atmosphere holds in heat from the Sun. Winds distribute that heat. The atmosphere prevents our planet from becoming too cold at night. This keeps temperatures moderate enough for liquid water--and thus life--to exist.
Today it is hard to imagine that the barren, rocky, rusty, red planet was ever kin to the flourishing Earth. But images from the Mariner and Viking orbiters in the 1970s revealed surface features that look like long river channels, stream beds, and tributary channels on Mars, confirming that liquid water once flowed there. Did life ever exist there as well? Does it still exist today? We are compelled to find out.
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