Saturn's moon Titan may have only recently turned hazy, according to two new studies that could spell trouble for the chances of life on the mysterious world.
The only moon in the solar system with a substantial atmosphere, Titan has long intrigued astronomers searching for hints of life beyond our planet.
"When we look at places that we think might host exotic life-forms, we tend to look for three things: liquid water, organic molecules, and an energy source," said Sarah Hörst, a planetary scientist at the University of Colorado who wasn't involved in either study.
At -289 degrees Fahrenheit (-178 degrees Celsius), Titan's surface is far too cold for liquid water, but it does have lakes of liquid hydrocarbons, which could play similar biological roles to what water does on Earth.
For an energy source, heat from sunlight trapped by the atmosphere could fill that role on Titan, as on our planet.
But it's Hörst's third factor that really sets Titan apart: organic molecules, carbon-based substances associated with life as we know it.
"As far as organic molecules are concerned, Titan is the most interesting place in the solar system," Hörst said in an email.
The moon's smoggy shroud contains a complex brew of organics churned out by methane, the main component of natural gas on Earth—and the key indicator of Titan's atmospheric age in both new studies.
Although the reports took different approaches toward analyzing the methane's history, they reached almost the same conclusion: Titan's organic haze has been around for only a billion years or so—not quite a third of the moon's total lifetime.
And "if Titan hasn't always had this organic molecule factory in its atmosphere," Hörst said, "this could potentially affect the possibility of life on Titan."
Weighing In on Titan's Carbon
The two new studies, published April 20 in the Astrophysical Journal, were based on different scenarios for how methane entered Titan's atmosphere. The papers also used different data sets from the NASA/European Space Agency Cassini-Huygens mission.
The Huygens probe landed on Titan in 2005, and the Cassini orbiter has been touring Saturn and its moons since 2004. (See pictures from the Cassini mission in National Geographic magazine.)
Both study teams combed through the data for signs of a relatively rare, "heavy" form of carbon that occasionally gets incorporated into methane molecules.
Sunlight readily breaks down Titan's atmospheric methane, so that the molecule gets converted into more complex organic substances, some of which rain onto the surface. (See "Methane Rain Formed New Lake on Saturn Moon.")
Methane with the lighter, more common form of carbon gets converted a bit faster than the heavy version, so over time the relative concentration of heavy methane slowly increases.
Tracing how the ratios of light to heavy methane change, therefore, allows the scientists to model how long the molecule has been breaking down in Titan's atmosphere—and therefore how old the atmosphere itself might be.
For his work, researcher Conor Nixon, of the University of Maryland, College Park, examined the infrared signatures of methane detected by the Cassini orbiter and the Huygens lander, as well as direct sampling of Titan's lower atmosphere from Huygens.
Assuming methane had initially entered the atmosphere in a single, sudden release of interior gas—possibly triggered by a large impact—Nixon's study suggests that the moon has had its hazy atmosphere for no more than 1.6 billion years.
In the second, separate study, Kathy Mandt, a scientist at the Southwest Research Institute (SwRI) in San Antonio, Texas, used models based on Cassini's direct sampling of Titan's upper atmosphere.
Like Nixon's, her models assumed that the atmospheric methane originated underground. But Mandt's simulations looked at both a single outgassing event and at the possibility that the gas is constantly seeping out from subsurface pockets of methane-containing ices.
According to Mandt's data, even if Titan's methane is being constantly replenished, the moon couldn't have been producing complex organics for more than 940 million years—and possibly a lot less.
"If methane itself is escaping from the top of Titan's atmosphere"—a possibility suggested by previous studies—"then we both find that will actually change the relative amounts of methane very quickly," the University of Maryland's Nixon said.
"In that case, we found a time scale of ten million years [for the age of the organic haze]. I'm not sure how believable that is, but it is a case we have to consider."
Early Titan Too Cold for Life?
Ultimately, the origin of Titan's organic atmosphere "is really the million-dollar question," the University of Colorado's Hörst said.
"Unless we are really fundamentally missing something, Titan's atmosphere just hasn't existed in the form we see it now for the entire age of the solar system"—about 4.6 billion years. "For the reasons discussed in [Mandt and Nixon's] papers, it just isn't possible."
And without methane, early Titan most likely had no atmosphere to speak of, Hörst said.
"Methane is a greenhouse gas and therefore, much like Earth, the surface and atmosphere of Titan are warmer than they would be if methane wasn't there," she said.
Without the warming effect from methane, the moon's mostly nitrogen atmosphere would freeze and collapse. (Related: "Pluto's 'Twin' Has Frozen Atmosphere.")
This newly constrained atmospheric time frame, in turn, puts limits on how long Titan might have been friendly to even extreme forms of life.
"When we talk about the possibility of life on other planets, there is maybe a fourth thing to add to my earlier list, which is time," Hörst said.
"It took life on Earth quite a while to get going, and Earth was apparently the most favorable place for it to happen in the solar system. If [Titan] hasn't had as long, then it is maybe less likely that it has happened or could happen."
(Related: "Space Poison Helped Start Life on Earth?")
Still, SwRI's Mandt added in an email, "what makes Titan such an amazing place from an astrobiology perspective is that any life that may exist on Titan would be different from any life-forms with which we are familiar on Earth.
"It would be speculation beyond my area of expertise to say how long any potential life-forms would take to evolve. But Titan remains one of the most important places to explore, because of its unique environment and the potential for discovering life-forms, if they exist, that are so different from anything we know."