Planetary exploration missions are journeys of epic endurance, stretching across many years and, more often than not, decades. Even after they are gone, their legacy carries on as their precious data gets reviewed, reanalyzed, and reinterpreted. In April last year, I mentioned how recent studies of the Voyager 2 data taken forty years ago revealed a strong possibility of Uranus' moon, Ariel, hosting a subsurface ocean. This month, I focus on another study looking back at past data from a completed mission. This time, the spacecraft was Cassini. And the planetary body, Enceladus.
The Cassini mission, conducted by NASA in collaboration with the European Space Agency, extensively explored Saturn and its icy satellites from 2004 until 2017, when it intentionally plunged into the planet's atmosphere. Considered the most ambitious of all flagship missions, Cassini returned vast amounts of valuable data on the Saturnian system, including from its tiny, yet, remarkable icy moon Enceladus. There, the spacecraft discovered the now famous geysers spewing water vapour and icy particles from large fissures, so-called tiger stripes, located at the south pole. As described in my first book, Cassini performed a total of 23 close flybys over 13 years, including one at an astonishing distance of only 25 km from the surface.
Thanks to these flybys, Cassini managed the incredible feat of flying through the water plumes a handful of times - something it had not been designed to do - and sampled its compounds with some of its instruments. This required a complete overhaul of various software and a reassessment of the capabilities of such instruments, and in particular the ion and neutral mass spectrometer (INMS). Following these passes, numerous compounds were detected including water, carbon dioxide, ammonia, methane, argon-40, hydrogen sulfide, methanol, salts such as sodium carbonate, and even silicate matter. Considering that we have partly characterized the ocean of liquid water hidden under Enceladus' ice shell is remarkable, and provides much insight into the ocean's ability to support life.
Yet, a problem persisted. Due to the inherent limitations of INMS (and more precisely, its low mass resolution), the identification of additional compounds detected by the instrument proved difficult as a large number of plausible models were compatible with the data. Recently, to resolve these ambiguities and effectively deal with the complexity of plume models, scientists have refined their statistical modelling and published a paper last year confirming that they have identified new compounds in the sampled plumes. The results of this paper were presented at the latest American Geophysical Union (AGU) meeting in December 2023. Thanks to these new models, the new compounds identified with very strong certainty include hydrogen cyanide (HCN), acetylene, propane, and alcohols.
While poisonous to most creatures on Earth, HCN could have played a key role in chemical reactions that created the ingredients that set the stage for the advent of life. Indeed, some biologists believe that it may have acted as a precursor to nucleic acids and amino acids. In addition, detecting acetylene and propane in the plume further confirms ongoing catalytic reactions driven by minerals within the ocean. The paper goes into further detail on the other compounds detected and the implications for the moon's habitability, so I recommend delving into it further. (See bottom for reference).
Once again, Enceladus has proven to be a truly unique and remarkable celestial body. Capturing our imagination of what might be, this tiny moon is a testament to the diversity and wonder of our solar system and serves as a reminder that the possibility of extraterrestrial life may exist beyond the confines of Earth.
As always, onwards and upwards. Bernard Reference: Detection of HCN and diverse redox chemistry in the plume of Enceladus by Jonah S. Peter, Tom A. Nordheim, Kevin P. Hand. https://arxiv.org/abs/2301.05259