Astrobiologists are engaged in the search for signs of extraterrestrial life in all forms, known as biosignatures, as well as specific signs of extraterrestrial technology, known as technosignatures. The search for technosignatures and biosignatures attempts to identify characteristic evidence of life on other planets that could be detected using astronomical methods. The first scientific searches for technosignatures began in the 1960s, which used radio telescopes to examine nearby star systems for evidence of narrowband transmissions used for communication. The search for extraterrestrial intelligence has continued to search for anomalous radio and optical signals that would indicate intentional or unintentional extraterrestrial communication. Advances in ground- and space-based spectroscopy are also beginning to enable searches for technosignatures in exoplanetary systems such as atmospheric pollution, city lights, large-scale surface structures, and orbiting satellites. Some technosignature searches also attempt to search for nonterrestrial artifacts within the solar system on planetary bodies or in stable orbits. All of these technosignature concepts use known technology on Earth as a starting point for thinking about technology that could be plausible and detectable in extraterrestrial systems.
Technology is a relatively recent phenomenon in the history of life on Earth, so the search for technosignatures also employs methods from futures studies to explore numerous trajectories for extensions of known technology. The range of possibilities considered by technosignature science can include any known or plausible technology that could be remotely detected and would not violate any known physical laws. Megastructures are examples of theoretical large-scale planetary engineering or astroengineering projects that could be detectable in exoplanetary systems through infrared excesses or gravitational effects. Many other technosignatures remain possible, even if they do not draw upon Earth projections, but most astrobiological study of technosignatures focuses on predictions that could be tested with current or near-future missions. The positive discovery of extraterrestrial technology could be of great significance to humanity, but technosignature searches that yield negative results still provide value by placing qualitative upper limits on the prevalence of certain types of extraterrestrial technology.
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Technosignatures and Astrobiology
Jacob Haqq-Misra
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Experimental Studies of Condensation in the Solar Nebula and Circumstellar Outflows
Aki Takigawa
Characteristics of minerals in primitive chondrites, micrometeorites, and interplanetary dust particles (IDPs) such as chemical composition, crystal structures, textures, size, and shape indicate that solids and gases hardly reached equilibrium in the solar nebula. They may record a part of physicochemical conditions where dust formed or altered in the solar nebula or their parent bodies. Even the presence or absence of the minerals constrain the conditions in which they can survive or disappear. On the basis of the thermodynamical equilibrium models, which succeeded in predicting minerals stable in each temperature and pressure condition, laboratory experiments have played crucial roles in understanding kinetically controlled processes, such as evaporation, condensation (nucleation and growth), and chemical reactions, and deducing formation and alteration conditions in the solar nebula and their parent bodies from observations of primitive extraterrestrial materials.
In laboratories, it is impossible to reproduce physicochemical conditions in the solar nebula mainly because of the limited laboratory timescales. Therefore, each experimental work focuses on a single process or reproduction of certain mineralogical characteristics observed in meteorites and IDPs. The kinetically controlled reactions of abundant minerals such as forsterite were examined by laboratory experiments of evaporation, gas–solid reaction, and condensation. Evaporation and condensation coefficients were determined based on the Hertz–Knudsen equation and nucleation theory, which are important parameters controlling timescales of reaction, temperature dependences, grain size or reaction volume, and chemical fractionation occurring in a limited timescale. In addition, chemical compositions and textures of amorphous metastable materials were systematically investigated by condensation experiments of nanoparticles. Various types of laboratory experiments and theoretical studies are complementary to each other for understanding the mineralogy of extraterrestrial materials and dust formation and evolution in the solar nebula.
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Infrared Remote Sensing of the Martian Atmosphere
Anna Fedorova and Oleg Korablev
The atmosphere of Mars, like most planetary atmospheres, consists of molecules absorbing and emitting in the infrared (IR) and of particles (dust or clouds) that also interact with the IR radiation. This makes the IR spectral range highly effective for the study of the atmospheric composition and thermal structure. Since the first missions to Mars, infrared spectrometers have been used to study the atmosphere. Thermal IR instruments, which sense the emission from the surface and the atmosphere of Mars, as well as near-IR spectrometers, sensitive to the reflected solar radiation, deliver the three-dimensional structure of the atmosphere and permit monitoring of the CO2, H2O, CO, and aerosol cycles over Mars’s seasons. IR spectroscopy at high spectral resolution from the ground or from orbit is the most commonly used method to search for unknown species and to monitor the known minor components of the Martian atmosphere. It is also used to study isotopic ratios essential for understanding the volatile evolution on the planet.
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Planetary Spectroscopy
Alian Wang
Planetary spectroscopy uses physical methods to study the chemical properties of the geological materials on the planetary bodies in our solar system. This article will present twelve types of spectroscopy frequently used in planetary explorations. Their energy (or wavelength) varies from γ-ray (keV) to far-infrared (μm), which involves the transitions of nuclei, atoms, ions, and molecules in planetary materials. The article will cover the basic concept of the transition for each of the twelve types of spectroscopy, along with their legendary science discoveries made during the past planetary exploration missions by the international planetary science and engineering community.
The broad application of spectroscopy in planetary exploration is built upon the fact that only limited extraterrestrial materials were collected (meteorites, cosmic dust, and the returned samples by missions) that enabled the detailed investigations of their properties in laboratories, while spectroscopic measurements can be made on the objects of our solar system remotely and robotically, such as during the flyby, orbiting, lander, and rover missions. In this sense, the knowledge obtained by planetary spectroscopy has contributed to a major portion of planetary sciences.
In the coming era of space explorations, more powerful spacecraft will be sent out by mankind, go to deep space, and explore exotic places. Generations of new planetary science payloads, including planetary spectrometers, will be created and will fly. New sciences will be revealed.
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Terrestrial Analogs to Planetary Volcanic Phenomena
Peter J. Mouginis-Mark and Lionel Wilson
More than 50 years of solar system exploration have revealed the great diversity of volcanic landscapes beyond Earth, be they formed by molten rock, liquid water, or other volatile species. Classic examples of giant shield volcanoes, solidified lava flows, extensive ash deposits, and volcanic vents can all be identified, but except for eruptions seen on the Jovian moon Io, no planetary volcanoes have been observed in eruption. Consequently, the details of the processes that created these landscapes must be inferred from the available spacecraft data. Despite the increasing improvement in the spatial, temporal, compositional, and topographic characteristics of the data for planetary volcanoes, details of the way they formed are not clear. However, terrestrial eruptions can provide numerous insights into planetary eruptions, whether they are effusive eruptions resulting in the emplacement of lava flows or explosive eruptions due to either volatiles in the magma or the interaction between hot lava and water or ice. In recent decades, growing attention has been placed on the use of terrestrial analogs to help interpret volcanic landforms and processes on the rocky planets (Mercury, Venus, the Moon, and Mars) and in the outer solar system (the moons of Jupiter and Saturn, and the larger asteroids). In addition, terrestrial analogs not only provide insights into the geologic processes associated with volcanism but also can serve as test sites for the development of instrumentation to be sent to other worlds, as well as provide a training ground for crewed and uncrewed missions seeking to better understand volcanism throughout the solar system.
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Impact Crater Densities as a Tool for Dating Planetary Surfaces
William K. Hartmann
The use of impact crater densities to estimate the ages of planetary surfaces began in the 1960s. Some predictive successes have been confirmed with radiometric dating of sites on the Moon and Mars. The method is highly dependent on our understanding of the rate of crater formation on different worlds, and, more importantly, on the history of that rate, starting with intense cratering during planetary formation 4.5 Ga ago. The system is thus calibrated by obtaining radiometric dates from samples of relatively homogeneous geologic units on various worlds.
Crater chronometry is still in its infancy. Future sample-returns and in situ measurements, obtained by missions from collaborating nations to various worlds, will provide ever-increasing improvements in the system in coming decades. Such data can lead to at least two-significant-figure measurements, not only of the ages of broad geologic provinces on solar system worlds, but of the characteristic survival times of various-sized smaller craters. Such data, in turn, clarify the rates of turnover of surface materials and the production rates of gravel-like regolith and megaregolith in the surface layers. Better measurements of the impact rate at various times, in turn, support better modeling of the accretion and fragmentation processes among early planetesimals as well as contemporary asteroids, in various parts of the solar system. Once the crater chronometry system is calibrated for various planetary bodies, important chronological information about those various planetary bodies can be obtained by orbital missions, without the need for expensive sample-return or lander missions on each individual surface.
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The Atmosphere of Uranus
Leigh N. Fletcher
Uranus provides a unique laboratory to test current understanding of planetary atmospheres under extreme conditions. Multi-spectral observations from Voyager, ground-based observatories, and space telescopes have revealed a delicately banded atmosphere punctuated by storms, waves, and dark vortices, evolving slowly under the seasonal influence of Uranus’s extreme axial tilt. Condensables like methane and hydrogen sulphide play a crucial role in shaping circulation, clouds, and storm phenomena via latent heat release through condensation, strong equator-to-pole gradients suggestive of equatorial upwelling and polar subsidence, and the formation of stabilizing layers that may decouple different circulation and convective regimes as a function of depth. Phase transitions in the watery depths may also decouple Uranus’s atmosphere from motions within the interior. Weak vertical mixing and low atmospheric temperatures associated with Uranus’s negligible internal heat means that stratospheric methane photochemistry occurs in a unique high-pressure regime, decoupled from the influx of external oxygen. The low homopause also allows for the formation of an extensive ionosphere. Finally, the atmosphere provides a window on the bulk composition of Uranus—the ice-to-rock ratio, supersolar elemental and isotopic enrichments inferred from remote sensing, and future in situ measurements—providing key insights into its formation and subsequent migration.
As a cold, hydrogen-dominated, intermediate-sized, slowly rotating, and chemically enriched world, Uranus could be the closest and best example of atmospheric processes on a class of worlds that may dominate the census of planets beyond our own solar system. Future missions to the Uranian system must carry a suite of instrumentation capable of advancing knowledge of the time-variable circulation, composition, meteorology, chemistry, and clouds on this enigmatic “ice giant.”
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Lunar Exploration Missions and Environmental Discovery: Status and Progress
Kyeong J. Kim
Exploration of the Moon is currently one of the most important and interesting subjects. The Moon is considered not only a place to explore but also a place to live in preparation to explore planets beyond it. This opportunity has arisen due to a series of discoveries associated with water on the Moon during the past half century. Lunar exploration of the moon began with the flyby mission by the United States in 1959. Since then, scientific investigations of the Moon have increased understanding of the lunar geology and surface environment. Based on more than 70 lunar missions to date, a major goal is to explore how humans can live on the Moon for a long period of time to examine sustainability on the Moon. Consequently, the area of lunar science and technology is being employed to discover how in situ resources can be utilized for humans to live on the Moon and, eventually, Mars and beyond.
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Planetary Systems Around White Dwarfs
Dimitri Veras
White dwarf planetary science is a rapidly growing field of research featuring a diverse set of observations and theoretical explorations. Giant planets, minor planets, and debris discs have all been detected orbiting white dwarfs. The innards of broken-up minor planets are measured on an element-by-element basis, providing a unique probe of exoplanetary chemistry. Numerical simulations and analytical investigations trace the violent physical and dynamical history of these systems from astronomical unit (au)-scale distances to the immediate vicinity of the white dwarf, where minor planets are broken down into dust and gas and accrete onto the white dwarf photosphere. Current and upcoming ground-based and space-based instruments are likely to further accelerate the pace of discoveries.
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Detection and Characterization Methods of Exoplanets
Nuno C. Santos, Susana C.C. Barros, Olivier D.S. Demangeon, and João P. Faria
Is the Solar System unique, or are planets ubiquitous in the universe? The answer to this long-standing question implies the understanding of planet formation, but perhaps more relevant, the observational assessment of the existence of other worlds and their frequency in the galaxy.
The detection of planets orbiting other suns has always been a challenging task. Fortunately, technological progress together with significant development in data reduction and analysis processes allowed astronomers to finally succeed. The methods used so far are mostly based on indirect approaches, able to detect the influence of the planets on the stellar motion (dynamical methods) or the planet’s shadow as it crosses the stellar disk (transit method). For a growing number of favorable cases, direct imaging has also been successful. The combination of different methods also allowed probing planet interiors, composition, temperature, atmospheres, and orbital architecture. Overall, one can confidently state that planets are common around solar-type stars, low mass planets being the most frequent among them.
Despite all the progress, the discovery and characterization of temperate Earth-like worlds, similar to the Earth in both mass and composition and thus potential islands of life in the universe, is still a challenging task. Their low amplitude signals are difficult to detect and are often submerged by the noise produced by different instrumentation sources and astrophysical processes. However, the dawn of a new generation of ground and space-based instruments and missions is promising a new era in this domain.
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