How can we explore Saturn’s moon, Enceladus, to include its surface and subsurface ocean, with the goal of potentially discovering life as we know it? This is what a recent study presented at the American Geophysical Union (AGU) 2024 Fall Meeting hopes to address as a team of students and researchers proposed the Thermal Investigation of Geothermal Regions of Enceladus (TIGRE) mission concept, which is designed to conduct in-depth exploration of Enceladus with an orbiter, lander, and drill, while laying the groundwork for future missions to icy moons throughout the solar system.
Here, Universe Today discusses this incredible mission concept with Prabhleen Kour, who is a senior at River Valley High School in Yuba City, CA, and lead author of the study, regarding the motivation behind TIGRE, how TIGRE can improve upon findings from NASA’s now-retired Cassini mission, potential landing sites on Enceladus, how TIGRE can improve missions to other icy moons, the next steps in making TIGRE a reality, and whether she thinks Enceladus has life. Therefore, what was the motivation behind TIGRE?
“TIGRE mission was born during our time with the NASA STEM Enhancement in Earth Science (SEES) program in collaboration with UT Austin’s Center for Space Research,” Kour tells Universe Today. “As part of our internship, our team was tasked to design a space mission within our solar system based on a few assigned parameters. The designed mission had to be aligned to current work being performed by NASA but separate from active missions such as the Europa Clipper. Similarly, the main subject of our mission, Enceladus, and our goals with it, had to be chosen in accordance with the Decadal Survey which dictates what missions and priorities space agencies have. In our case, we were driven to explore a celestial body that might hold the signs of life.”
The TIGRE mission concept comes more than seven years after NASA’s Cassini-Huygen mission ended by performing an intentional dive into Saturn, resulting in Cassini breaking apart in Saturn’s atmosphere. During its storied mission, Cassini spent more than 13 years conducting the most in-depth exploration of Saturn and its many moons, including Titan, Mimas, Atlas, Daphnis, Pandora, Iapetus, Rhea, Dione, Pan, Hyperion, and Enceladus.
Of these moons, Titan and Enceladus are the only two that exhibit potential conditions for life, as Titan is the only moon in the solar system with a dense atmosphere and contains lakes of liquid methane and ethane, while Enceladus boasts a large subsurface ocean that discharge geysers of liquid water from its large crevices in its south pole, dubbed Tiger Stripes. It is the geysers of Enceladus that Cassini not only discovered but flew through twice during its mission, identifying water, carbon dioxide, and a myriad of hydrocarbons and organic materials, the last of which exhibited density 20 times greater than predicted. Therefore, how does TIGRE improve upon findings from the Cassini mission?
“Though Cassini’s flyby was incredible and provided us with great information, TIGRE aims to get an incredibly close look at Enceladus’ secrets,” Kour tells Universe Today. “Since TIGRE is designed to go on the surface of Enceladus, it will get more of the ‘inside scoop’ than Cassini. Cassini has already helped us by identifying the organic molecules contained within the ocean, now we want to explore other factors that might make life possible on Enceladus. We are planning to locate any potential regions of interest and stability of habitable zones, analyze samples for organic/inorganic indicators of prebiotic lifeforms, and utilize our findings for future missions. The TIGRE mission contains a drill design, which will reach the subsurface ocean and collect water samples for elements such as CHONPS.”
Enceladus’ Tiger Stripes consist of four main features officially named Damascus Sulcus, Baghdad Sulcus, Cairo Sulcus, and Alexandria Sulcus, with a smaller feature branching off Alexandria called Camphor Sulcus (sulcus being plural for sulci and is an astrogeology term meaning parallel ridges), and are responsible for the geysers that discharge Enceladus’ interior ocean into space. The thickness of the ice in this region is estimated to be approximately 5 kilometers (3.1 miles). Since one of the primary goals of the TIGRE mission is to obtain drill samples of the ocean and identify potential signs of life, the team targeted the Tiger Stripes as potential landing sites for a craft to land and obtain samples of the ocean.
To accomplish this, the team outlined specific landing site criteria to maximize mission success, including landing on relatively flat terrain near a geyser, but not directly on a geyser, to avoid being damaged by uneven terrain or disrupted during geyser activity. Additionally, they determined a low-elevation region would be substantial to minimize the amount of ice the drill would have to penetrate to obtain samples. In the end, the team chose a primary landing site located near the Baghdad stripe that met their landing criteria, located approximately 6.4 kilometers (4 miles) from a geyser and a surface elevation of approximately 450 meters (1,476 feet), along with potential backup landing sites.
“Our decision to land near the Baghdad stripe was due to the following: Flat terrain to prevent lander damage, proximity to a geyser, and low elevation to minimize drilling distance,” Kour tells Universe Today. “Any other location that met these requirements were deemed as backups. We analyzed multiple different locations throughout the four stripes, and there were a few that met the requirements on the Cairo stripe. More specifically, one location of interest was between a large geyser and a smaller geyser on the Cario stripe. However, because the location on the Baghdad stripe was close to multiple other smaller geysers, we chose the Baghdad location.”
As noted, Enceladus isn’t the only moon of Saturn that is deemed to potentially have life, as its largest moon, Titan, has a dense and hazy atmosphere caused by specific chemical reactions that scientists have hypothesized existed on early Earth. Additionally, its lakes of liquid methane and ethane have also become prime targets for astrobiologists. Outside of the Saturn system, other icy moons exist throughout the solar system that potentially once had life or could have life today, including Jupiter’s moons, Europa and Ganymede, with both presenting evidence of subsurface oceans circulating beneath their icy crusts.
Venturing closer to the Sun and inside the main asteroid belt orbits the dwarf planet Ceres, which NASA’s Dawn spacecraft identified frozen salts caused by a process known as cryovolcanism. Current models debate the interior structure of Ceres, but it is hypothesized that it once had liquid water long ago. Finally, venturing to the outer portions of the solar system orbits Neptune’s moon, Triton, which NASA’s Voyager 2 spacecraft identified active geysers on its surface comprised of cryolava lakes. Since one of the primary mission objectives of TIGRE is to improve future missions to icy moons, how will it accomplish this?
“The mission will help advance remote sensing, orbiting, landing, and thermal drilling technologies, setting a precedent for future exploration,” Kour tells Universe Today. “TIGRE consists of three main components: the orbiter, lander, and drill. This design is not limited to Enceladus’ surface alone. Instead, this design can be applicable to many other icy surfaces, including those on Earth like Antarctica and other icy moons. Data from the lander’s sampling devices, thermal drill, and the orbiter’s remote sensing will provide comprehensive insights into the composition and formation of Enceladus’s subsurface ocean. These findings could also inform our understanding of other icy moons, broadening our knowledge of potentially habitable environments in the outer Solar System.”
As Universe Today recently discussed with the VATMOS-SR mission concept, it can take anywhere from years to decades for a space mission to go from a concept to reality, involving a myriad of steps and phases, including design, funding rounds, testing, re-designs, re-testing, until it’s finally built and launched. This is followed by several years of traveling to the destination, arriving, and finally collecting science.
For example, the Cassini-Huygens mission was first proposed in 1982 and wasn’t launched until 1997, during which time it endured several years of studies and swapped between a solo NASA mission or a joint NASA-European Space Agency mission, the latter of which was settled upon. After launching in 1997, Cassini finally arrived at Saturn in July 2004, landing the Huygens probe on Titan in January 2005, and spent until 2017 obtaining treasure troves of images and data about Saturn and its many moons, even discovering a few moons along the way and diving through Enceladus’ plumes. Given the journey that Cassini endured, what are the next steps in making TIGRE a reality?
“One of the first steps in making TIGRE a reality is waiting for the completion of the Europa Clipper mission,” Kour tells Universe Today. “In waiting for the mission’s completion, we will be able to see what worked and failed to gather useful samples and what failed to navigate space’s harsh environment. In the meantime, we can advocate for the significance of finding life to enlarge NASA’s budget for active missions. This itself would be a step towards launching the TIGRE mission by opening the resources for improving and testing our mission’s main components (the orbiter, lander, and drill) against the extreme cold, ocean waters, and radiation.”
As noted, Enceladus is a prime target for astrobiologists in the search for life beyond Earth due to its vast subsurface ocean circulating beneath its icy shell. As demonstrated here on Earth, liquid water leads to life as we know it, so Enceladus having a liquid water ocean, even a subsurface ocean, is a strong indicator that it could potentially also have life as we know it, too.
The hydrocarbons discovered by Cassini when the spacecraft flew through Enceladus’ plumes included carbon-bearing molecules like formaldehyde, acetylene, propane, and methane, which is evidence for hydrothermal activity occurring on the ocean floor of Enceladus, much like hydrothermal activity exists on the ocean floors of Earth, specifically regarding the water-rock interactions that occur here, as well. Therefore, in Kour’s opinion, does Enceladus have life and what kinds of life does she foresee finding within their potential TIGRE samples?
“It is not a stretch of reason to state Enceladus could harbor life,” Kour tells Universe Today. “As previously mentioned, Enceladus has the components for life through key elements and has the energy activity to make the possibility of life more plausible. Within the depths of its oceans, Enceladus may very well have life. However, we do not want to explicitly state that there is something there, as there are so many factors at play – thin atmosphere, other chemicals that were potentially not detected by Cassini, and environmental conditions. If there is life and it is similar to the one on Earth, we could expect it to be one of close relations to Archaea. The representatives of this domain are quite primitive and unicellular, which aligns with our hypothesis of Enceladus being able to harbor a simple life form. However, it can also survive harsh conditions – such as extreme cold temperatures on the moon and radiation.”
How will TIGRE help scientists better understand Enceladus and potentially other icy moons throughout the solar system in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!