First, authors review the instrumentation for in situ detection of biosignatures in the solar system.
(1) Nanopore sequencing with tunneling electrode-based detectors. The biopolymer sequencing by taking into consideration and incorporating the quantum-mechanical phenomenon of electron tunneling combined with nanopores can provide information to detect ancestrally related life or agnostic life unambiguously. Wherein, nanopore systems allow both nucleic acids and peptides to pass directly through a nanometer-scale pore for sequencing by detecting the current that arises when the specific base or residue passes through the nanopore. Meanwhile, novel techniques such as using tunneling electrode-based detectors as integrating sensors are being explored as alternative technologies due to their size and shape adjustability, faster sequencing time, and high sensitivity and durability, which may be ideal for space missions that have weight and size constraints.
(2) Integrating nuclear magnetic resonance (NMR) spectroscopy with x-ray diffraction (XRD). Inspired by the numerous integrative techniques used in structural biology, an integrative approach applied to life detection, i.e., hybrid NMR and XRD analysis, gives a more quantitative description of conformational atomic or physical structures of a sample than obtained by either technique alone. XRD in geology provides information like structure, size, orientation, stress, strain, and defects of sample crystals, while NMR spectroscopy is used in biochemistry to identify proteins, nucleic acids, and other macromolecular complex organic compounds while providing detailed structural and dynamic information. Both techniques are complementary to each other with their own strengths and weaknesses, making these techniques a perfect combination for integrative use in astrobiology.
(3) Digital holographic microscopy (DHM) (see Fig. 2.). An approach to work with basic locomotive properties is suggested: a combination of holographic optical tweezers with DHM. Optical tweezers can be used as light beams that capture and hold biological objects, and DHM is a method of imaging with portability and relatively low cost. Thus, such a combination would enable simultaneous tracking (DHM) and trapping (optical tweezers).
(4) Raman spectroscopy. Raman spectroscopy is often used in analysis of inorganic and organic systems, and is widely used for in vivo analysis in medicine. In particular, the Perseverance rover, launched in 2020, have contained a total of seven instruments, with Raman spectroscopy a part of many of these instruments. Further development of novel Raman spectroscopy-based life detection techniques can help to increase its utility in future space-related studies.
(5) Laser-induced breakdown spectroscopy (LIBS). LIBS has been successfully used to detect the presence of water in simulated Martian rocks, and recent advances in liquid-phase LIBS analyses may also lead LIBS to be a promising in situ technique for oceanography applications. These suggest LIBS as a candidate for future development as a broad-spectrum life detection tool.
(6) Gas chromatography (GC). GC has become a powerful and efficient method to detect organic and inorganic chemicals on Earth. Developing new and more improved GC instruments are required in future biosignature detection missions.
Fig. 2. Two forms of digital holography microscopy.
Second, authors review techniques applied to remote detection or analysis of biosignatures in exoplanets. (1) Atmospheric composition changes as a biosignature. On Earth, the atmosphere is studied from a climatological perspective to determine the effects of human life both in the past and in the present (see Fig. 3.). As is done on Earth, searching for life on other planets may be possible by studying atmospheric compositions of the past and looking for indicators of life. (2) Atmospheric disequilibrium of gases as biosignatures. Another such potential life detection technique that takes Earth as a planetary analog would be to measure the atmospheric disequilibrium of gases on the planet, including the coexistence of one or more gases. The presence of a mixture of several gases on a planet (i.e., atmospheric disequilibrium) coupled with modeling of the feasibility of such a mixture from abiotic or biogenic sources could be one approach for life detection. (3) Oceanic and surface detection of planets using reflective observations. Understanding and detection of oceans on other planets may be key to life detection. The orbital variation within the reflected starlight of a planet can be used to detect surface oceans on other planets. Another model following ocean light detection via polarization and phase-dependent observation is based on using multiphase wavelength to infer longitudinal maps and detect planetary surfaces.
Fig. 3. Global atmospheric carbon dioxide concentrations in parts per million for the past 800 years.
Third, authors review technosignatures as the new frontier of life detection. In order to support life detection missions including the techniques introduced above, as well as for accurate detection of technosignatures, it is necessary to increase the instantaneous data rates and data volumes attached to such missions, which call for an increase in telecommunications capacity. To increase radio frequency (RF) capacity, one may introduce higher-bandwidth radio spectrum usage, but this requires increasing the size of the antennae and radio transmitter power, which has other implications in payload design and weight limits. One alternative mechanism that has been proposed to communicate with distant spacecraft or for intra-satellite communication is laser communication (also known as optical communication), which promises higher transmitted data volumes. Laser communications use data that are encoded in photons and beamed over laser light rather than radio or microwaves, resulting in an increase in the data transmission and encoding rate due to its higher frequency and providing lower diffraction losses, better directivity, and greater transmission efficiency.
Finally, authors draw the conclusion. Some of the physical and biological analysis techniques introduced here have yet to be applied to life detection technologies, while others have been used in the life detection field regularly. In both cases, more focused development and optimization of these techniques could lead to application to life detection missions or more efficacious use in such missions. While each technique introduced focuses primarily on a disparate aspect that may or may not be currently used in life detection, astrobiology is necessarily an interdisciplinary field, and such disparate techniques likely have use in life detection applications with the correct focus. Although we have only introduced a small number of such potential applications, there are likely to be a litany of other techniques that can be applied as life detection technologies. In particular, we expect technosignatures to be a significant topic of interest in the near future, and thus, improvement of related life detection technologies should be a priority for the field. As such, we call upon the entire field, as well as those who are not directly participating in astrobiology research, to contribute techniques in your own fields of expertise to the cause of the search for extraterrestrial life.
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