The Dragonfly Gamma-Ray and Neutron Spectrometer (DraGNS): Elemental Composition Measurements on the Surface of Titan

The Dragonfly mission concept currently under study for NASA's New Frontier program has the overall goal of studying the complex carbon-rich chemistry on the surface of Saturn's moon Titan using multiple instruments on a landed rotorcraft vehicle. A top-level objective of the Dragonfly mission is to carry out compositional measurements in different geologic settings to understand how far organic chemistry has progressed on the surface of this ocean world. Key to this objective is the Dragonfly Gamma-Ray and Neutron Spectrometer (DraGNS). Gamma-ray and neutron spectroscopy, or more generally planetary nuclear spectroscopy, is a powerful technique widely used to make planetary composition measurements from orbital platforms. Typically, planetary nuclear spectroscopy makes use of galactic cosmic rays (GCRs) to generate the gamma rays and neutrons from which elemental compositions are derived. However, because of Titan's thick atmosphere, GCRs do not reach its surface. As a consequence, DraGNS would contain a Pulsed Neutron Generator (PNG) that will use 14 MeV neutrons to illuminate Titan's surface and generate the diagnostic gamma rays and neutrons.

In addition to the PNG, DraGNS would have two gamma-ray spectrometers (GRS) and two neutron spectrometers. The primary GRS is a cryocooled high-purity Ge (HPGe) sensor similar to the GRS sensors on the MESSENGER and Psyche missions. The HPGe sensor would be passively cooled to cryogenic temperatures using Titan's atmosphere instead of a mechanical cryocooler as is normally done for orbital missions. A CeBr₃-based GRS provides redundancy for most of the key threshold GRS measurements. Finally, two ³He neutron sensors would measure thermal and epithermal neutrons, which provide key compositional parameters.

With the PNG coupled to its suite of sensors, DraGNS would measure bulk abundances of C, N, H, and O, allowing rapid classification of the surface material at each of dozens of landing sites (e.g., ammonia-rich water ice, pure ice, and carbon-rich dune sands). DraGNS could also detect minor inorganic elements such as Na or S. DraGNS' rapid geochemical reconnaissance would provide compositional context and inform the Dragonfly science team as to the types of sampling and detailed chemical analysis that should be performed on a site-by-site basis.