This week it was possible to observe the light of a star in the Orion constellation though the plumes of
Enceladus and NASA positioned Cassini to do that.
The star is Epsilon Orionis, the middle star in the belt of Orion. Such an alignment is called a stellar occultation. Usually “occultation” refers to an event where one object blocks and observer’s view of a second object.
In this case, the observer’s view isn’t blocked, its filtered through the water and gas of Enceladus’ plumes. Cassini has made four stellar and one solar occultations of the plumes over the last ten years.
From a distance of 535,000 miles from Enceladus NASA will use Cassini’s Ultraviolet Image Spectrograph (UVIS) to record the filtered starlight. The star itself is 2,000 light years from Enceladus.
Don’t think that the event will be akin to holding up a flashlight behind the plumes. The star will track at roughly a forty-five degree angle through approximately 80% of the plumes, starting at the top of the plumes and continuing downward until it disappears behind Enceladus itself. So NASA won‘t get definitive answers to all of its questions but will add a significant number of data points to its overall mosaic of Enceladus.
Observing starlight or sunlight through the plumes provides the best ultraviolet spectroscopy results.
“Gases absorb light at ultraviolet wavelengths. The wavelengths are unique to the gas, so absorption wave lengths are diagnostic of composition. “
Ultraviolet spectroscopy has a wide range of applications on Earth and is commonly used to detect organic compounds. While the data gathered from this observation will not provide direct evidence of life, it may help establish that the building blocks of life are present.
The plumes come from the ocean lying beneath the ice crust on the surface and therefore undoubtedly contain clues as to the composition of the ocean.
NASA is also hoping to gain a better understanding of what causes the plumes. The widely held belief is that hydrothermal vents create the pressure which forces the water through cracks in the ice crust but that has not yet been proven.
Cassini has observed the plumes with various instruments a number of times most recently in December of 2015. In October of 2015 Cassini actually flew through the plumes. NASA has not yet released extensive information about what was observed.
Whatever the outcome of this observation Enceladus remains an intriguing candidate for the existence of life. The discovery of organic compounds would certainly increase the pressure from Congress to mount a major effort to find out.