"Pale orange haze" conjures up some unpleasant imagery, from chemical warfare to Sriracha plants to 19th century England. But hazy orange days didn't always go hand-in-hand with hazmat suits. Two and a half billion years ago, a tangerine tinge in the air might have kept you alive.
Earth didn't always have an ozone layer, and a longstanding question among geobiologists is how life could have survived before that planetary radiation shield was in place. Now, we might have an answer. According to research presented today at the American Astronomical Society's Division of Planetary Sciences meeting, a haze of methane and complex hydrocarbons might have absorbed UV light and shielded Earth's first colonizers eons ago.
If correct, the hazy hypothesis would not only answer a key question about how life got started on Earth, it could aid us in the search for alien biospheres.
"Hazy worlds seem common both in our solar system and in the population of exoplanets we've characterised so far," said Giada Arney, lead author on the new study, which has been submitted to the journal Astrobiology. "Thinking about Earth with a global haze allows us to put our home planet into the context of these other worlds, and in this case, the haze may even be a sign of life itself."
In the not-too-distant future, new telescopes will give us our first glimpses into the atmospheres of planets beyond our solar system. When that happens, scientists will be able to directly search for the fingerprints of life on other worlds. In anticipation of this, astronomers are building catalogues of so-called "biosignatures" -- clues that could indicate biology.
One type of biosignature we'll look for is chemical; for instance, a mixture of oxygen, water vapour, and methane. A planet's colour is another telltale sign. The verdant glow of Earth's vegetation could be visible to alien astronomers far away. Likewise, it may be possible for us to infer the presence life on another world -- even microbial life -- based on that planet's pale hue.
Arney's work highlights a new alien fingerprint we may want to add to our catalogue. Prior research showed that Archaean Earth (4-2.5 billion years ago) was intermittently shrouded in an orange haze; the result of light breaking down atmospheric methane into more complex hydrocarbons. But it wasn't clear whether such hazes were the result of methane-producing microbes, geologic processes, or some combination of factors.
The new models suggest that orange hazes are very likely to indicate biology -- at least for rocky planets with atmospheres similar to our own. This, Arney says, makes them "a novel type of biosignature," perhaps indicating a planet not so different from Earth before the rise of oxygen and formation of an ozone layer.
"Based on evidence from the geological and biological record, biology was making methane at this time," Arney told Gizmodo in an email. "Of course, there were also geological methane sources, but it's likely that the biological source outweighed the geological sources."
Voyager's pale blue dot image has become our poster child for habitable worlds. But the more we learn about the Earth's own history, the more likely it seems that other life-bearing rocks -- should they exist -- will be brimming with colours.
In other words, let's not judge a planet just because it's orange.
Top: Artist's concept of a red dwarf star surrounded by three exoplanets, via Wikimedia