Sampling Enceladus’ Subsurface Ocean with TIGRE Mission Concept
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.
In this article, Universe Today discusses this incredible mission concept with Prabhleen Kour, a senior at River Valley High School in Yuba City, CA, and lead author of the study. We explore 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 Kour thinks Enceladus has life. Therefore, what was the motivation behind TIGRE?
Motivation Behind the TIGRE Mission
“The 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 with 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-Huygens 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 the highlight of this report, Enceladus.
Enhancements Over Cassini’s Findings
Of these moons, Titan and Enceladus are the only two that exhibit potential conditions for life, as Titan possesses a dense atmosphere and contains lakes of liquid methane and ethane, while Enceladus boasts a large subsurface ocean that discharges geysers of liquid water from its large crevices in its south pole, dubbed Tiger Stripes. It was 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.
“Though Cassini’s flyby was incredible and provided us with valuable information, TIGRE aims to get an incredibly close look at Enceladus’ secrets,” Kour adds. “Since TIGRE is designed to land on the surface of Enceladus, it will gather more in-depth data compared to Cassini. Cassini already contributed by identifying the ocean's organic molecules; 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 life, and utilize our findings for future missions. The TIGRE mission contains a drill design that will reach the subsurface ocean and collect water samples for elements such as carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur (CHONPS).”
Landing Site Selection and Criteria
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 from 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:
- Landing on relatively flat terrain near a geyser, but not directly on a geyser, to avoid damage from uneven terrain or geyser activity.
- Choosing a low-elevation region 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 Sulcus that met their landing criteria, located approximately 6.4 kilometers (4 miles) from a geyser, at a surface elevation of approximately 450 meters (1,476 feet), along with potential backup landing sites.
Future Implications and Technology Advancements
As noted, Enceladus isn’t the only moon of Saturn that is deemed to potentially harbor life. Saturn’s largest moon, Titan, has a dense and hazy atmosphere caused by chemical reactions that scientists postulate existed on early Earth. Additionally, its lakes of liquid methane and ethane have also become prime targets for astrobiologists. Outside the Saturn system, other icy moons exist throughout the solar system that may have once supported life or could possess life today, including Jupiter’s moons, Europa and Ganymede, both of which offer 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, where 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 hosted liquid water long ago. Additionally, Neptune’s moon, Triton, which NASA’s Voyager 2 spacecraft observed active geysers on its surface made up of cryolava lakes, further entices future explorations.
The Technology Behind TIGRE
The TIGRE mission is designed to help advance remote sensing, orbiting, landing, and thermal drilling technologies, which will set a precedent for future exploration endeavors. The mission consists of three main components: the orbiter, lander, and drill. This design is not limited to Enceladus’ surface alone; rather, it is applicable to many other icy surfaces, including 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.
From Concept to Reality
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 stages and phases, including design, funding rounds, testing, re-designs, and re-testing, until finally, the mission is built and launched. After launching, it might take several years to travel to the destination, eventually collecting valuable scientific data.
For instance, the Cassini-Huygens mission was first proposed in 1982 but wasn’t launched until 1997; during that time, it endured years of studies and a decision process between NASA and the European Space Agency regarding a joint mission. After its launch in 1997, Cassini finally arrived at Saturn in July 2004, landing the Huygens probe on Titan in January 2005. Cassini would operate until 2017, providing invaluable images and data about Saturn and its many moons, even discovering some new moons along the way.
Next Steps for the TIGRE Mission
“One of the first steps in making TIGRE a reality is waiting for the completion of the Europa Clipper mission,” Kour mentions. “By observing the completion of the Europa Clipper mission, we can learn what worked and failed in gathering useful samples and navigating space's hostile environment as well. In the meantime, we can advocate for the significance of finding life to help enlarge NASA’s budget for active missions. This would open up resources for improving and testing our mission’s main components (orbiter, lander, and drill) against the extreme cold, ocean waters, and radiation.”
The Search for Life
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 shown on Earth, where liquid water leads to life as we know it. With a subsurface ocean, Enceladus presents a compelling case for the potential for extraterrestrial life. Kour elaborates, “It is not a stretch of reason to state Enceladus could harbor life. Previous evidence shows that it has the components for life through key elements and energy activity, making the possibility of life more plausible. Within the depths of its oceans, Enceladus may indeed have forms of life. However, there are many factors at play, such as its thin atmosphere and other chemicals that may not have been detected by Cassini.”
Kour continues, “If there is life and it is similar to Earth's life forms, we could expect it to be close relations to Archaea. These primitive unicellular organisms align with our hypothesis that Enceladus may harbor simple life forms capable of surviving harsh conditions, including extreme cold temperatures and radiation.”
The exploration of Enceladus through the proposed TIGRE mission promises to enhance our understanding of icy moons and their potential for life. As we embark on this voyage through the cosmos, our quest continues, one that seeks to unravel the deep mysteries of worlds beyond our own.
Conclusion
With future missions like TIGRE designed to scrutinize the icy enigma that is Enceladus, the exploration of extraterrestrial life becomes a tangible dream. This mission will focus on the subsurface ocean, sampling for organic and inorganic materials that can reveal the history of habitability on Enceladus and assess its potential for life. The advancements made through this mission can inform not only our pursuit of life in the solar system but also support ongoing explorations of other celestial bodies with similar environments.
