The "key" to unlock extraterrestrial life! How to detect life in icy bodies?

In the solar system, icy bodies represented by Europa and Enceladus have outstanding life potential and may answer humanity's ultimate question about extraterrestrial life in the 21st century. However, all current space probes are unable to enter the water for detection. Exo-AUV can perform life detection missions in potential areas such as ice crusts, ice-water interfaces, and the seabed, and is highly expected by space agencies of many countries. Where, based on what methods, what kind of Exo-AUV equipment and technology to develop, and what goals to achieve are important issues in future icy life detection missions.

Wang Bin and Qin Hongde from the National Key Laboratory of Intelligent Ocean Vehicles, School of Naval Architecture, Harbin Engineering University, took Europa as a hypothetical target, discussed the scientific objectives, detectable objects, potential areas and biogenic analysis of the icy life detection mission, and proposed a method for icy life detection based on Exo-AUV; analyzed the key conditions of Exo-AUV in different operating scenarios, and proposed basic technical requirements for hull, payload and autonomy; introduced the research background, research status and existing problems of Exo-AUV, and proposed a set of Exo-AUV concept development technology routes and a multi-Exo-AUV system operation concept (ConOps for MEAS). The system will help planetary scientists and astrobiologists explore icy bodies, search for strong biogenic signals, and even living life and pre-life chemical systems.

This research is an intersection of marine robotics in ship and ocean engineering with planetary science and astrobiology, and the relevant results were published in "Science China: Earth Sciences" 2024 Issue 11. Icy bodies such as Europa and Enceladus have the basic conditions for the survival of microorganisms. Life detection in areas with high life potential such as the ice, ice-water interface and seabed of icy bodies is likely to discover strong biogenic signals, living life, and even answer the question of the birth of life. However, all current space probes are unable to enter the water for detection. The Extraterrestrial Autonomous Underwater Vehicle (Exo-AUV) can autonomously and efficiently realize in-situ, multi-object, multi-scale, and multi-dimensional detection, and will be a key equipment for planetary scientists and astrobiologists to explore icy bodies and search for extraterrestrial life.

Taking Europa as an example, this study proposes that life potential be used as the scientific goal of life detection in icy bodies, because life potential not only meets the hypothetical connotation of the detection mission, but also avoids binary conclusions, and can be used to discover life signals, living life and pre-life chemical systems. The speculation, evaluation and verification of life potential require a large number of perceptible environmental variables and parameters, some of which may serve as evidence of life and are called life signals. Even on Earth, there are areas where life is thriving and life is scarce. Detecting the life potential of Europa should give priority to those areas with high life potential and life signal potential. Based on the similar environment hypothesis and ecological theory, this study proposes an inference of potential areas and believes that the life potential and life signal potential in areas such as ice, ice-water interface and seabed are the most prominent. However, the current mainstream detection methods generally focus on biogenic analysis, but ignore how to collect high biogenic signals. In oligotrophic systems, the distribution of life is sparse and heterogeneous. Even in areas with theoretically high life potential such as under the ice or on the seabed, if only weak biogenic signals are collected, it is difficult for both the binary diagnostic framework and the Bayesian method to obtain highly reliable analysis results.

It is worth emphasizing that the study believes that a complete exploration mission should at least include four links: hypothesis, sampling, analysis and verification. Exo-AUV and its ice-penetrating vehicle can reach almost any potential area below the ice surface, and realize in-situ, multi-object, multi-scale and multi-dimensional data collection and analysis by carrying a variety of scientific payloads. Based on the idea of ​​ecological niche, this study proposed a new method for life detection in icy bodies (Figure 1). Using this method, Exo-AUV can autonomously and efficiently infer the local micro-areas with the most prominent life potential, collect more mutually orthogonal strong biogenic signals and even living life; in addition, it can also use the measured data on icy bodies to verify, falsify or correct the data model established on Earth. This method takes advantage of Exo-AUV in underwater detection, avoids the shortcomings of relying solely on passively collected data for biogenic analysis, and integrates the four links of hypothesis, collection, analysis and verification to form a complete, closed-loop, and autonomously evolving method for life detection in icy bodies. This method will help Exo-AUV discover strong biogenic signals and even living life and pre-life chemical reactions in Europa's hundreds of kilometers thick, globally covering, oligotrophic ice and water layer under limited energy reserves, material supplies and human intervention, ultimately verifying the potential for life.

Figure 1 A method for detecting life in icy bodies

Taking Europa as an example, the study also analyzed the three typical operation scenarios of ice-penetrating detection, ice-water interface detection, and seabed detection into four types of scenario conditions: operating environment, measured object, key operations, and probe body. Combined with the relevant conditions in the previous links such as rocket launch, interplanetary flight, entering Europa's orbit, and landing on the ice surface, the main technical requirements for Exo-AUV and its ice-penetrating vehicle were proposed.

Europa's ice shell and sub-ice ocean are global, with a thickness of tens of kilometers. The deepest hydrostatic pressure on the seafloor is even twice that of the bottom of the Mariana Trench. The ice-penetrating vehicle can carry a small modular nuclear fission reactor (SMR) or a radioisotope thermal generator (RTG) power source and heat source, adopt a hot drilling hybrid ice-penetrating method and an energy-saving boat type, use sonar or synthetic aperture radar to assist navigation, use lateral nozzles or auxiliary heat to assist steering and obstacle avoidance, penetrate the ice shell, place the Exo-AUV in the water, and serve as an underwater base station to provide navigation, communication, data exchange and charging services for it. Exo-AUV can be made of pressure-resistant hull materials, equipped with RTG power supply and high-performance navigation and communication modules. It can navigate at different depths, large ranges, and for a long time at a constant depth, speed, and direction in the sea. It can achieve gliding through variable buoyancy design, approach and hover in local micro-areas with higher life potential, and live on the ice or on the seabed when necessary, covering sea areas of different scales from large to small.

In order to discover sparse, heterogeneously distributed strong biogenic signals and living life in the vast ice sea space, Exo-AUV and its ice-penetrating vehicle must carry a variety of scientific payloads, using acoustic, visual, spectral, electrochemical, analytical chemistry, cell biology and molecular biology instrument payloads to gradually focus on objects with different characteristic lengths from several kilometers to sub-micrometers, collect multi-dimensional information such as morphology, structure, composition, movement, distribution, physicochemical, etc. in situ, and complete ecological niche and biogenic analysis online. Europa is far away from the earth, and the payload of the launch vehicle is very limited. The surface above the ice is strongly radiated by Jupiter, and the demand for protective materials is high. Under the premise of ensuring detection capabilities, the micro-electro-mechanical system (MEMS) technology is applied to achieve miniaturization and lightweight of the payload.

The communication delay between Europa and the Earth is as long as 0.5 hours, and the communication bandwidth and window period are very narrow, which makes frequent human intervention and high-throughput data exchange impossible. Complex life detection missions will rely on the autonomy of the detector. First of all, Exo-AUV and its ice-penetrating vehicle should autonomously locate, navigate, and plan paths based on acoustic, optical and other perception methods, and control thrusters, steering gears and buoyancy to adjust speed and posture. In addition, based on the detection method proposed in this study, Exo-AUV should also achieve scientific autonomy, be able to autonomously infer and screen potential areas in different scales, plan detection missions, and autonomously call multiple scientific payloads to complete data collection and analysis directly or through airborne experiments, autonomously verify the hypothesis of life potential and life signal potential, autonomously update the computational model, and autonomously screen, summarize, sort and communicate important data.

This study reviewed the Exo-AUVs developed abroad and believed that most of the existing designs are not capable of handling complex life detection missions and are far from realizing the potential of the Exo-AUV platform. In order to prevent future icy life detection missions from repeating the mistakes of Viking 1 & 2, this study proposed a set of Exo-AUV concept development technology routes based on icy life detection missions. This route includes key elements that affect the Exo-AUV concept, helping developers to study potential areas and objects that can be detected by Exo-AUVs starting from scientific goals, design detection methods, summarize key scenario conditions, refine all technical requirements, and design or evaluate concepts from three aspects: hull, payload, and autonomy (Figure 2).

Figure 2. Exo-AUV concept development technology roadmap

Based on this technical route, the study also proposed a concept of operations for multiple Exo-AUV systems (ConOps for MEAS). A simplest MEAS system (Figure 3) includes an Exo-AUV Carrier (EAC), an Exo-AUV equipped with a survey module (Easy Exo-AUV), and an Exo-AUV equipped with an observation module (EAO). EAC is equipped with RTG or SMR power supply and heat source, and uses a hot drilling hybrid method to penetrate ice. EAS and EAO can be placed inside EAC. All three can achieve acoustic communication and data interconnection through optical fiber interfaces. EAS can use a foldable wing body hull and carry RTG power supply. It can sail for a long time, over a large range, at a constant depth, at a constant speed, and in a directional manner. It can also glide at full sea depth to perform detection tasks at the interface between ice and water and large-scale spaces and large characteristic length objects on the seabed. EAO can adopt a disc-shaped hull, a full-drive design, equipped with rechargeable batteries, and equipped with a variety of MEMS scientific payloads, which is suitable for the detection of objects with small characteristic lengths in local micro-areas. EAS can be mechanically connected to EAO in the water and then operated, or the two can operate separately. EAS can serve as a carrier for EAO and provide charging and data exchange services for it. Due to the large differences in scene conditions in different potential areas of Europa; there are many detectable objects, some with characteristic scales exceeding 1 km, and some less than 1 μm; the measurement scale also spans from the global ice sea space to local micro-areas. If many complex technical requirements are compatible with one Exo-AUV, it will cause the idleness of payloads in different scenarios, waste of hull space, weight and energy, and will also be a challenge to the limited transportation capacity and space of the rocket. The MEAS system proposed in this study can effectively solve the above problems, and can greatly improve individual maneuverability and robustness, system survivability and operation efficiency. If there are major discoveries, it is also possible to build a detection network covering the global ice sea space based on multiple MEAS systems through multiple launch missions and multi-point ice penetration.

Figure 3. Concept of operation of a multi-Exo-AUV system

In recent years, my country has developed rapidly in various fields, and a series of well-known cutting-edge equipment and technologies have been born; however, there are still few major discoveries in the field of basic science. The detection of extraterrestrial life will be highly dependent on major scientific research and major national projects of detection equipment and technology, which has extremely high scientific significance; its derivative value can also be integrated into social production and life, and promote more disruptive technologies that benefit the people's livelihood and enhance national competitiveness; it is in line with my country's basic national conditions, long-term national policies and international status. Although the United States and Europe started early in this field, they have not established an absolute advantage in the key link of Exo-AUV. my country can establish an organizational structure to manage the extraterrestrial life detection project at the top level, coordinate the proposals, discussions, decisions, development and task implementation of relevant grassroots units; cultivate a multidisciplinary research system, team and talents; demonstrate the target celestial body, scientific goals, project implementation and technology development route; carry out special research and task implementation in stages, levels and fields. At present, taking Exo-AUV as a breakthrough point to gradually develop equipment and technology for the detection of life in icy bodies and carry out special detection tasks is of great strategic significance to a major country like my country that shoulders the mission of national rejuvenation and the common destiny of mankind.

For more details, please read the full article

Wang B, Qin H.D. 2024. A method for detecting life in icy bodies based on an extraterrestrial autonomous underwater vehicle (Exo-AUV). Science China: Earth Sciences, 54(11): 3553–3573.