For scientists interested in habitability and life, Titan and Enceladus are obvious targets for exploration. The Cassini spacecraft, which is nearing the end of its thirteen year exploration of the Saturn system, discovered that both worlds have salty water oceans beneath icy crusts. Titan also has a rich stew of organic molecules in its atmosphere that are deposited on its surface and into its methane-ethane surface seas. Enceladus conveniently has plumes jetting samples of its ocean into space. Within the plumes, Cassini’s instruments have found organic molecules and trace minerals suggestive of hydrothermal water-rock interactions that could provide a habitat for microbes.

The three proposed missions to Enceladus and Titan would employ relatively simple spacecraft with just two to three instruments. They stand in contrast to NASA’s planned mission to explore the habitability of Europa, another ocean world. The Europa mission will bristle with nine instruments, several of which will produce floods of data that require a high-powered communications system to return the data to Earth.

A key difference between the Europa and the Saturn moons missions is the prior history of exploration. Europa was explored by the Galileo spacecraft built with 1970s technology that had a crippled communications system. As a result, NASA’s new Europa mission must conduct its own comprehensive global study of this world. At Saturn, this initial investigation of Enceladus and Titan has been conducted by the highly capable Cassini spacecraft built with 1990s technology. As a result, the next mission to these moons can focus on a few specific questions answerable with two to three instruments.

(For those of you who recall that the Galileo spacecraft explored Jupiter’s system in the 1990s, the completed spacecraft sat on the ground for almost a decade due to launch delays.)

Two of the E2T spacecraft’s instrument would focus on in situ composition measurements. At Enceladus, the spacecraft can fly through the plumes and directly taste the ocean’s contents. At Titan, complex organic molecules are carried to the outermost fringes of the atmosphere where the spacecraft can sample the atmosphere’s chemistry.

The E2T spacecraft’s two mass spectrometers would sample material during each passage and determine their composition by “weighing” their constituent molecules. The distribution of the weights of the different molecules can be interpreted to determine the composition of the original material. The spacecraft’s ion and neutral gas mass spectrometer would determine the composition of gases while the Enceladus Icy Jet Analyzer would determine the composition of ice, salt, and dust particles in Enceladus’ plumes. The two mass spectrometers will measure the nature of the organic chemicals, the pattern of carbon isotopes, the relative abundances of noble gases, and search for amino acids and abnormal isotope rations in organic molecules that suggest a biological origin.

The Cassini spacecraft carried versions of both these instruments, but the E2T instruments would have a forty to fifty times improvement in resolution (the ability to distinguish similar molecules) and sensitivity (the ability to measure minute amounts of a substance) over their predecessors. (NASA’s Europa mission would carry similar instruments to E2T’s as will Europe’s JUICE mission to Jupiter and its moon Ganymede.)

The E2T team has several key questions that the two mass spectrometers would address. For Enceladus, are the materials in the plume most likely from its formation or from current geological or biological processes? What does the composition reveal about the nature of the liquid reservoir (currently believed to be a global ocean beneath an icy cap) and its potential as a habitat for life? For Titan, what are the sources of its volatiles and how they have been subsequently processed? Does the atmospheric composition suggest that the current atmosphere is refreshed by material reaching the surface from the deep water ocean?

The E2T’s spacecraft’s third instrument would image Titan’s surface and the sources of Enceladus’ plumes (the so called ‘tiger stripes’) in the near- and short-wave infrared. (The instrument’s name spells out TIGER for the Titan Imaging and Geology, Enceladus Reconnaissance camera.) Titan is perpetually shrouded in atmospheric haze, hiding the surface from cameras that image in most wavelengths. However, spectral windows at 1.3, 2, and 5 microns allow a camera to image the surface at several times finer resolution than a similar instrument on the Cassini spacecraft (and also at better resolution than Cassini’s radar images). The images returned by the camera will address several key questions such as: To what degree are sediments produced and transported by fluvial and aeolian processes? How have the rivers and seas of liquid methane and ethane modified the surface? How does the composition of the surface, revealed by the three ‘colors’ of the spectral bands, vary?

If the E2T mission is selected as a finalist in ESA’s medium class competition, the team will investigate whether a radio science experiment to study the two moons’ gravitational field could be added. If it is, this experiment will investigate the thickness and mechanical properties of the ice shell at Enceladus’ southern pole where the plumes originate. The gravity measurements would also be used to investigate Titan’s ice shell and the properties of its internal ocean.