Of all the icy moons of Saturn and Jupiter, Saturn's moon of Titan is thought to be the most like our very early earth. One reason is that because aside from earth, Titan has the richest organic (carbon-bearing) chemistry of any other known body in our solar system.
Whether such rich chemistry has already vectored towards some sort of subsurface life is up for debate. But Titan's surface is an oilman's dream. It's loaded with methane and ethane and all sorts of raw petroleum compounds.
Yet for now, Titan is arguably most fascinating for its potential in teaching us about our own planet's prebiotic history.
We know that there's thousands upon thousands of different complex molecules that are made by reactions mostly induced by sunlight on the methane, Conor Nixon, a planetary scientist at NASA Goddard Space Flight Center in Maryland, told me recently in Graz, Austria.
Titan actually has a diameter that's larger than our own moon by some 1500 km.
Titan is the only moon that we know of in our solar system that has a dense, unstable atmosphere, says Nixon. Titan may also have a water ocean deep inside that may provide an environment that is hospitable for life, he says.
Titan has plains, vast equatorial dunes and mountains that stretch over hundreds of kms and are thought to be composed of organic material transported to the surface from Titan's thick, orange atmosphere. Yet this moon is very different from earth.
Benzene, a 6-sided carbon ring, has already been detected on Titan, says Nixon. While it's not truly a 'molecule of life' it shows that ring molecules are possible on Titan, he says.
PANH molecules -- - polycyclic aromatic (nitrogen-containing) hydrocarbons -- - like pyridine and pyrimidine are one step more complex than benzene, because they include nitrogen as well as carbon in a 6-sided ring, says Nixon. PANH molecules are not definitively detected on Titan, but there are strong hints that they exist there, he says. These PANH molecules are very interesting to astrobiology, since they are one step closer to the nucleobases, the nitrogen-containing, biological building blocks of RNA and DNA, says Nixon.
Titan's manufacturing these complex molecules in the atmosphere that are very easy for us to look at because they're gaseous, says Nixon. Looking at the way these molecules react could also tell us about how those early reactions got started on earth, which then formed into the amino acids and peptides and polypeptides that became our DNA and RNA, he says.
How did Titan get its thick atmosphere?
Methane clouds in Titan's atmosphere evaporate off the surface from the icy moon's known frozen lakes and seas.
The methane that's evaporated rises until it gets about 10 to 15 km above the surface, it then cools and rains out in droplets, says Nixon.
But Nixon is still puzzled as to how Titan has managed to maintain such a thick atmosphere over long time periods.
If you evaporated all the methane out of the seas into the atmosphere, it's still not going to get you very far over long geological time periods, says Nixon. If you include everything that's in the seas and everything's in the atmosphere, it's only going to get you a few tens of millions of years, he says.
So, the question remains how Titan maintains its thick atmosphere over timescales of billions of years and why other such icy moons appear to only have thin exospheres. Could Titan's atmosphere be the key to the sort of rich chemistry that evolves into molecules that qualify as precursors of life? That's one of the main drivers for NASA's next robotic mission to the planet.
Titan is also the only moon of Saturn or Jupiter that humanity has actually touched down upon using a robotic probe. That's when the European Space Agency and NASA's Cassini Huygens mission sent ESA's Huygens probe to Titan's surface some 20 years ago.
But NASA is readying Dragonfly, a new golf-cart-sized rotorcraft that will actually use eight pairs of rotors to hop around Titan; taking data as it goes.
With a launch date of July 2028, Dragonfly is targeted to arrive at Titan in 2034 and once it begins its science operations will fly to dozens of promising locations on the icy moon, says NASA. It will specifically be looking for prebiotic chemical processes common on both Titan and the early earth before life developed, NASA notes.
Titan's atmosphere is thick and dense and a perfect environment for lifting off and flying, says Nixon. It will lift off, fly a couple of miles, land again and then do new measurements, he says.
But Dragonfly won't be capable of detecting life itself. However, it could detect nucleobases.
The Webb Space Telescope or other telescopes may never find nucleobases at significant levels floating in the atmosphere, says Nixon. But Dragonfly definitely has the capability to detect these on the surface, he says.
We yet don't know how large these chemical species on Titan get, says Nixon. My research is to find out where this organic chemistry leads and whether it leads to what we call 'precursors of life' or prebiotic chemistry, he says.