ISOTOPIC RATIOS
Nitrogen on Titan

Isotopes of Nitrogen: Here, There, and Everywhere

Extraterrestrial chemistry in action

Around Christmas day, the European Space Agency's Hygens probe detached itself from the NASA Cassini orbiter which is exploring the planet Saturn and its rings and moons. The Hygens probe is especially interesting. On January 14, it will descend through the atmosphere of Saturn's moon, Titan, and transmit chemical composition data (among other information) back to the Cassini orbiter for subsequent transmission to us Earthlings.

Titan is among the most remarkable objects in our solar system. It is the second-largest moon (Jupiter's Ganymede is largest), and Titan is almost as large as Mars. It has a remarkable atmosphere, and the Hygens probe will, if all goes well, provide exciting new information on it. Titan's atmosphere is HUGE! Of course, it is cold on Titan - the surface temperature is a bit below 100 K - but the atmosphere is notably warmer, around 160 K, but still pretty darn cold. Most of the atmosphere is nitrogen gas, (N₂), as is our atmosphere. But it has been known for some years from terrestrial observations and other planetary missions that there are many gaseous hydrocarbons mixed in as well, such as methane, CH₄, the simplest hydrocarbon. The moon is covered in a dense haze of hydrocarbon clouds, and it is suspected that pools or "oceans" of liquid hydrocarbons may be found on the surface. (It has been said that "it rains crude oil" on Titan. Hygens may be able to prove or disprove that claim!)

The figure below (from the NASA/JPL web site on the entire mission, which is loaded with stunning images already) compares the Earth and Titan atmospheres. Note the altitude scales: Titan's atmosphere goes WAY out from the surface!

NASA Diagram

The picture below, taken on December 16, 2004 by the Cassini orbiter, shows a view of the many layers of Titan's atmosphere. This is an ultraviolet wavelength image of the side of Titan facing away from the sun (the so-called "night side limb") which has been colorized to look like true color. Scattered sunlight from the other side of Titan has reached this "night side," and thus it seems to glow.

NASA photo

As mentioned above, the major component of Titan's atmosphere is nitrogen, good ol' ordinary N₂ gas. What is remarkable, though, is that measurements taken on October 27, 2004, by Cassini's mass spectrometer (see page 51 in the text - Cassini uses a very different type of instrument, however) showed Titan's atmospheric nitrogen to be unusual.

It is known that nitrogen has only two isotopes that are stable: ¹⁴N and ¹⁵N. On Earth, 99.632% of all N atoms have atomic mass number 14 (and a mass of 14.003074007 amu). The remaining 0.368% has mass number 15 (and a mass of 15.00010897). Thus, the 15 isotope is scarce, but not impossibly so; there are methods available to separate these two isotopes (one says the 15 isotope is enriched), and enriched samples of nitrogen have a variety of research uses, especially in biochemical studies. (Nitrogen is an essential element for life, as I'm sure you know!)

On Earth, we say that nitrogen has an average mass (an "atomic weight," the quantity that is shown in most Periodic Tables) given by

(See the comparable calculation on carbon on page 52 in the text.)

On Titan, the heavier mass 15 isotope appears to be significantly enriched, however, more so than on any other place in the Solar System that has been checked:

NASA diagram

(The y axis in this diagram is "parts per thousand ¹⁵N.") Mars has a little bit larger ratio, but Titan is way out of whack! Some possible reasons have been advanced to explain this observation (can you think of any?), but there is no clear-cut explaination available now.

The bottom line is that isotope ratios such as these are important clues into the chemical and physical processes that have evolved over time across the Universe. On Earth, isotopic ratios have been used to provide dates to archaeological samples and to locate items geographically (see the story of elephant poachers on page 52 in the text). Dartmouth's Department of Earth Sciences has several researchers who are experts in this area. You can learn more about their work and facilities on their web site.