Is Titan older than Saturn? Gases in its atmosphere suggest the moon formed BEFORE its parent planet
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It's generally accepted that planets form before moons, gradually collecting their natural satellites over millions of billions of years in cosmic tugs of war.
But new research could be set to turn the world of planetary science on its head, as scientists have discovered Titan may have formed before its parent planet Saturn.
The incredible theory rules out the possibility that Titan formed in the disk of material that surrounded the infant Saturn - and instead if must have formed elsewhere.
Titan (pictured) is the largest moon of Saturn and, like other natural satellites in the solar system, it was thought to have been born out of a disk of material surrounding its host planet. New research, however, suggests its origins may actually pre-date that of Saturn, challenging our understanding of planetary science
The combined Nasa and Esa study was led by Kathleen Mandt of Southwest Research Institute in San Antonio.
TITAN: SATURN'S LARGEST MOON
With its thick atmosphere and organic-rich chemistry, Titan resembles a frozen version of Earth several billion years ago, before life began pumping oxygen into our atmosphere.
Because Titan is smaller than Earth, its gravity doesn't hold onto its gaseous envelope as tightly, so the atmosphere extends 370 miles (595 kilometres) into space.
As on Earth, the climate is driven mostly by changes in the amount of sunlight that comes with the seasons, although the seasons on Titan are about seven Earth years long.
Titan's 'water' is liquid methane, CH4, better known on Earth as natural gas. Regular Earth-water, H2O, would be frozen solid on Titan where the surface temperature is -180°C (-292°F).
With Titan's low gravity and dense atmosphere, methane raindrops could grow twice as large as Earth's raindrops. As well as this, they would fall more slowly, drifting down like snowflakes.
But scientists think it rains perhaps only every few decades.
The researchers found firm evidence that nitrogen in the atmosphere of Titan actually originated in conditions pre-dating Saturn.
In fact, Titan's nitrogen bears similarities to ancient comets found in the Oort cloud - a vast expanse of icy rocks surrounding the solar system.
The main implication of this new research is that Titan's building blocks formed early in the solar system's history, in the cold disk of gas and dust that formed the sun.
This was also the birthplace of many comets, which retain a primitive, or largely unchanged, composition today.
Nitrogen is also the main ingredient in the atmosphere of Earth (it makes up about 78 per cent), as well as on Titan.
The planet-sized moon of Saturn is frequently compared to an early version of Earth, locked in a deep freeze.
The research suggests that information about Titan's original building blocks is still present in the icy moon's atmosphere, allowing researchers to test different ideas about how the moon might have formed.
The formation of our solar system began 4.6 billion years ago from the collapse of a giant molecular cloud (illustration shown). While our sun took shape at its heart, further out in colder regions comets began to form. The new research suggests Titan, too, formed in these colder regions before Saturn
Mandt and colleagues demonstrated that a particular chemical hint as to the origin of Titan's nitrogen should be essentially the same today as when this moon formed, up to 4.6 billion years ago.
That hint is the ratio of one isotope (form) of nitrogen called nitrogen-14 to another isotope, nitrogen-15.
The small amount of change in this isotope ratio over long time periods makes it possible for researchers to compare Titan's original building blocks to other solar system objects in search of connections between them.
As planetary scientists investigate the mystery of how the solar system formed, isotope ratios are one of the most valuable types of clues they are able to collect.
In planetary atmospheres and surface materials, the specific amount of one form of an element, like nitrogen, relative to another form of that same element can be a powerful diagnostic tool because it is closely tied to the conditions under which materials form.
The team found that our solar system is not old enough for this nitrogen isotope ratio to have changed significantly. This is contrary to what scientists commonly assumed.
'When we looked closely at how this ratio could evolve with time, we found that it was impossible for it to change significantly,' explained Mandt.
'Titan's atmosphere contains so much nitrogen that no process can significantly modify this tracer even given more than four billion years of solar system history.'
Pictured is Saturn (background) and Titan (foreground), the latter of which is by far the largest of Saturn's 62 confirmed moons. Evidence for Titan's formation before Saturn stems from the ratio of two isotopes of nitrogen in its atmosphere, the ratio of which could not have come about if it formed after Saturn
The study also has implications for Earth - it supports the emerging view that ammonia ice from comets is not likely to be the primary source of Earth's nitrogen.
In the past, researchers assumed a connection between comets, Titan and Earth, and supposed the nitrogen isotope ratio in Titan's original atmosphere was the same as that ratio is on Earth today.
Measurements of the nitrogen isotope ratio at Titan by several instruments of the Nasa and Esa Cassini-Huygens mission showed that this is not the case - meaning this ratio is different on Titan and Earth - while measurements of the ratio in comets have borne out their connection to Titan.
This means the sources of Earth's and Titan's nitrogen must have been different.
Other researchers previously have shown that Earth's nitrogen isotope ratio likely has not changed significantly since our planet formed.
'Some have suggested that meteorites brought nitrogen to Earth, or that nitrogen was captured directly from the disk of gas that formed the Sun,' said Mandt.
'This is an interesting puzzle for future investigations,' Mandt said.
The ratio of nitrogen suggests Titan's origins bear a similarity to comets found in the Oort cloud, which is a vast expanse of the icy rocks thought to be surrounding the solar system at a distance of up to a light year (illustration shown). This would make Titan the first moon confirmed to be older than its parent planet
Mandt and colleagues are eager to see whether their findings are supported by data from Esa's Rosetta mission, when it studies comet 67P/ Churyumov-Gerasimenko beginning later this year.
If their analysis is correct, the comet should have a lower ratio of two isotopes - in this case of hydrogen in methane ice - than the ratio on Titan.
In essence, they believe this chemical ratio on Titan is more similar to Oort cloud comets than comets born in the Kuiper Belt, which begins near the orbit of Neptune (67P/ Churyumov-Gerasimenko is a Kuiper Belt comet).
'This exciting result is a key example of Cassini science informing our knowledge of the history of solar system and how Earth formed,' added Scott Edgington, Cassini deputy project scientist at Nasa's Jet Propulsion Laboratory, Pasadena, California.
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