My first Magazine Sky & Telescope - 02.2019

Alien “Earth” Next Door We did not know that atmospheres were capable of producing such large molecules, especially so high in an atmosphere, where the pressure is very low and molecular collisions (which build larger compounds) are rare. If these molecules are made of the same atoms #5 Propane required for the small set of molecules that life as we know it here on Earth uses — car- bon, hydrogen, oxygen, and nitrogen (collectively dubbed “CHON”) — then these ions may be important for under- CHON: Life’s Four Building Blocks standing the origin and evolution of life on Earth. If life The discovery of these very large ions is incredibly important, exists on Titan, then they could be important for under- because it has changed our understanding of the chemistry standing its rise there, too. occurring in Titan’s atmosphere and of how its hydrocarbon Chemistry requires energy. The ionosphere is largely cre- hazes form. In doing so, it has also altered our ideas about ated by very high-energy photons from the Sun impinging how planetary atmospheres work in general. upon the atmosphere’s molecules and knocking electrons free. With so many photons ﬂooding into it, this region of Titan’s atmosphere has more energy avail- able to drive chemistry than regions deeper down, where few photons penetrate. These photons are important for breaking up N 2 , whose paired atoms share a triple bond that is one of the hardest to break; the #6 photons capable of breaking this bond Ethylene are only available very high up in the atmosphere. The amount of energy available matters for many of the questions we are asking, because it means the difference between nitrogen par- ticipating in the chemistry or not. We knew before Cassini that Titan’s atmospheric chemistry included substantial amounts of carbon and hydrogen, but INMS mea- surements have shown us that the ions in Ligeia Titan’s ionosphere also contain much more Mare nitrogen than we expected to ﬁnd there. If nitrogen is involved in the chemistry, then three of the four elements necessary for life Punga Mare are accounted for. One additional discovery made by CAPS shows that the ﬁnal element, oxygen, might be playing a role, too. Scientists didn’t know Kraken about the salty-water plumes of Enceladus Mare until spotting them in data from Cassini. It turns out that the water shooting out of that moon’s south pole spreads throughout the Saturn system, including to the top of Titan’s atmosphere. t NORTHERN LAKES This colorized mosaic of the moon’s north pole was constructed using radar-mapping data from the Cassini spacecraft; the edges extend down to 50°N latitude. Using Cassini’s observations, scientists discovered three seas (labeled) and hundreds of small lakes on Titan. Nearly all the lakes and seas are confined to a rectangle covering about 900 km by 1,800 km (600 by 1,100 mi) in the northern hemisphere; only 3% of the liquid detected on Titan falls outside of this area. The lakes might be seasonal features. 26 FE B RUA RY 2 019 • SK Y & TELESCOPE FE UNIV. HEMISPHERE: ecules consisting of six or seven heavy atoms in this region of Titan’s atmosphere, CAPS instead found ions made of 600 or 700 heavy atoms stuck together, the mass of small proteins on Earth! Unfortunately, CAPS was not designed to identify such heavy ions. So although we know they’re present high above Titan, we’ll need another mission to be able to identify them.

My first Magazine Sky & Telescope - 02.2019 | Page 28