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A full sky aurora over Norway

Higher than the highest building, higher than the highest mountain, higher than the highest airplane, lies the realm of the aurora. Auroras rarely reach below 60 kilometers, and can range up to 1000 kilometers. Aurora light results from energetic electrons and protons striking molecules in the Earth’s atmosphere. Frequently, when viewed from space, a complete aurora will appear as a circle around one of the Earth’s magnetic poles. The above wide angle image, horizontally compressed, captured an unexpected auroral display that stretched across the sky over eastern Norway.

Image credit: Sebastian Voltmer

The Earth-like Mars

“Yet if hope has flown away
In a night, or in a day,
In a vision, or in none,
Is it therefore the less gone?
All that we see or seem
Is but a dream within a dream.”
“A Dream Within a Dream” (1849) by Edgar Allan Poe

Mars – a distant, extraterrestrial world, but it shares some surprising similarities with Earth. The rotation period is almost the same with 24 hours, 39 minutes and 21,67 seconds (as measured by astronomer William Herschel in 1777-1783), the planet possess an atmosphere and the surface shows periodic changes during a sequence of seasons.

First drawings of the Martian surface appeared in 1638, made by the Italian astronomer Francesco Fontana, and in 1645 the Dutch astronomer Christiaan Huygens produced the first “scientifically accurate” map, showing, so he believed, the large “hour glass”-ocean (today known as Syrtis Major, the southern highlands of Mars). In 1672 the Italian astronomer Giovanni Domenico Cassini (1625-1712) noted on the South Pole a white patch and in 1704/1719 a nephew of Cassini, Giacomo Filippo Maraldi (1665-1729), discovered also on the North Pole such a patch. It was William Herschel who suggested that these were polar ice caps and the extent of the glaciers varied during the Martian seasons. The melting polar ice caps suggested also that there was liquid water on Mars.

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Early Earth Turned Methane Haze On and Off?

Saturn‘s moon Titan is unique in our solar system, being the only natural satellite to boast a significant atmosphere, somewhat like Earth’s.

Also like Earth, Titan has bodies of liquid on its surface that support processes akin to our water cycle—the huge moon has clouds, spring rains, and fog—and even shows signs of a lake effect similar to the one seen over North America’s Great Lakes.

The key difference, of course, is that the liquid on chilly Titan is methane, a carbon-based chemical that, on our world, is the prime component in natural gas.

Still, Titan’s hydrocarbon haze is exciting to scientists who are hoping to get a glimpse of how life might have been sparked on Earth: Lab experiments, for example, suggest that the moon’s atmosphere may be flush with the building blocks of life, such as amino acids and DNA bases.

And now, for the first time, scientists say they have proof that early Earth had a very Titan-like atmosphere … at least periodically.

On today’s Earth, dry air contains roughly 78% nitrogen and 21% oxygen, with trace amounts of other gases. But ancient rocks show that, before about 2.5 billion years ago, atmospheric oxygen was a scarce commodity on our planet.

The widely held theory has been that before the so-called Great Oxygenation Event, Earth’s atmosphere was dominated by Titan-esque organics—but no one had yet found evidence for such a chemical makeup in the planet’s rocky history.

In a study published online yesterday in Nature Geoscience, Aubrey Zerkle of Newcastle University in the U.K. and colleagues report their analyses of rocks from South Africa that date to between 2.65 and 2.5 billion years ago.

Core samples from these rocks contain microbial mats, which show that some of the tiny creatures in shallow seas were producing oxygen long before the Great Oxygenation of our atmosphere.

The rocks also contain carbon and sulfur isotopes—chemicals that would have reacted with oxygen. The levels of the different kinds of isotopes present indicate that sometimes oxygen production was happening when the atmosphere was thick with methane—but other times the atmosphere must have been haze-free.

The clarity of the early atmosphere seems to flip flop roughly every few million years, Zerkle and co. report, hinting at a push and pull between microbes that generated oxygen and those that belched methane.

At last, though, *something* happened about 2.5 billion years ago to trigger the planet’s permanent oxygen high. (What that “something” was is still a mystery, although theories abound.)

Of course, this pattern has so far been seen only in the South African rocks, so more research on samples from around the world will be needed to truly tell whether Earth was once a Titan—atmospherically speaking.

Victoria Jaggard is a senior editor for National Geographic News, specializing in all things space. You can follow Victoria on Twitter @vmjaggard99.
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