3 moons and a planet that could have alien life

Recurring Slope Lineae (RSL) are dark streaks on Mars appearing on steep slopes – a critical characteristic is their repeatable annual appearance and disappearance. The coincidence of RSL with spring warming has raised the possibility of water being readily accessible on the surface of Mars at lower latitudes. Previous studies have shown that RSL appearance is associated with the hydration of salts. Recently, Chojnacki has reported numerous RSL found in the low latitude areas of Melas and Coprates Chasmata (part of Valles Marineris). However, because of their number and density, and their association with several different topographies, a near sub-surface source of water is difficult to explain. This raises the question of how much water is actually available, what is the source, and whether there would be sufficient water available for future human explorers.

Recurring slope lineae (RSL) are long, linear surface albedo features that darken on sunny slopes in local summer, extend downslope, and then fade in autumn, only to reappear in the next Mars year. The best hypothesis for these features is that they form due to moistening of the surface by a surface or sub-surface flow of water during the warm season. Mars Reconnaissance Orbiter has found that during the season when RSL grow, they contain hydrated perchlorate salts. Perchlorate salts provide a powerful freezing point depressant, which would keep briny water liquid for a greater portion of the day and for more days in the warm seasons. Perchlorates reduce the freezing point of water to as low as -78°C. The observed hydration of the salts is consistent with the probable presence of liquid water.

A primitive ocean on Mars held more water than Earth’s Arctic Ocean, according to NASA scientists, including members of the NAI’s Goddard Space Flight Center team, who used ground-based observatories (the VLT (Very Large Telescope), Keck, and IRTF (InfraRed Telescope Facility) to measure water signatures in the Red Planet’s atmosphere. These ground-based infrared telescopes on Earth were used to study the remaining water molecules in the Martian atmosphere. The results showed that a very large amount of heavy water (having the deuterium or D hydrogen) remains on Mars today, meaning that Mars lost a significant amount of normal water (having just hydrogen or H) over time. Today, Mars has only 13% of the water it once had, losing 87% of the original inventory. NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft is now looking into the processes that tell us how Mars lost its water.

The Curiosity rover has detected nitrogen-bearing compounds in scooped sand and drilled sedimentary rock within Gale crater, supporting the equivalent of 110–300 ppm of nitrate in the sand samples, and 70–1,100 ppm of nitrate in the mudstone deposits. Discovery of indigenous martian nitrogen in Mars surface materials has important implications for habitability and, specifically, for the potential evolution of a nitrogen cycle at some point in martian history. Fixed nitrogen (such as nitrates) could have facilitated the development of a primitive nitrogen cycle on the surface of ancient Mars, potentially providing a biochemically accessible source of nitrogen.

Aqueous salt solutions are critical for understanding the potential for liquid water to form on icy worlds and the presence of liquid water in the past. Salty solutions can form potentially habitable environments by depressing the freezing point of water down to temperatures typical of Mars’ surface or the interiors of Europa or Enceladus. A recent paper has been published investigating such low-temperature aqueous environments by experimentally measuring the low temperature properties of salt solutions and developing thermodynamic models to predict salt precipitation sequences during either freezing or evaporation. These models, and the experimental data generated are being applied to understand the conditions under which water can form, the properties of that water, and what crystalline salts indicate about environmental conditions such as pH, temperature, pressure, and salinity.

The deep-sea hydrothermal vent habitat hosts a diverse community of archaea and bacteria that withstand extreme fluctuations in environmental conditions. A new study, funded in part by the NAI, reveals that viruses lend a surprisingly helpful hand to these microbes. When they infect the vent’s resident bacteria and archaea, the viruses mix and match the single-celled creatures’ genes. As a result, the microbes can benefit from possessing a wide range of genes in a way that broadens their repertoire of responses to the quick-changing, harsh conditions of the vent environment. These results offer insight into mechanisms useful for life on Earth as well as beyond Earth in the extreme conditions on Enceladus and Europa, the icy moons of Saturn and Jupiter, which are also believed to have active hydrothermal vents.

The debates regarding the sites on planetary bodies where life can arise now always includes the mid-ocean ridges where hydrothermal vents pump energy and nutrients into the deep ocean on Earth. Cassini discovered the first evidence for ongoing seafloor hydrothermal activity on a body other than Earth. Similar activity is observed around mid-Atlantic seafloor vents, where some extreme life forms reside. This new finding opens the possibility for prebiotic or even biotic chemical mixtures to “slow-cook” inside Saturn’s moon Enceladus, where the ocean meets hot rock. This raises the potential for habitable environments beneath the ice crust of this small active moon. Silica nanoparticles were captured by Cassini’s Cosmic Dust Analyzer (CDA). Analysis revealed these particles came form Enceladus’ seafloor. Furthermore, laboratory experiments indicate that these dust particles must have formed on the seafloor at temperatures above 90C (194F). This is a much hotter environment than scientists thought existed inside the icy moon, further supporting the idea that seafloor hydrothermal activity is occurring. This result shows that Enceladus’ plume activity is an eruptive process that begins in its core and is not limited to the near-surface.

New data from the Cassini spacecraft have been used to model the ocean water on Enceladus to estimate the pH of its ocean, answering a fundamental question in determining whether Saturn’s icy moon Enceladus could support life. Mass spectra observations of the plume gas made from the Cosmic Dust Analyzer (CDA) onboard the Cassini spacecraft indicate that the ocean is a sodium-chloride-carbonate solution with an alkaline pH of ~11-12. The dominance of NaCl is similar to oceans on the Earth, but the dissolved Na2CO3 concentrations mean that the ocean composition is similar to that of soda lakes on Earth (e.g., Mono Lake in California). The alkaline pH results from serpentinization, a geochemical process in which iron- and/or magnesium-rich rocks interact with water to produce hydrogen, a geochemical fuel that can support both abiotic and biological synthesis of organic molecules such as those that have been detected in Enceladus’ plume from Cassini. Serpentinization has happened in many places throughout the solar system, but it is not known whether serpentinization is taking place on Enceladus today, or whether Enceladus’ rocky core has been completely altered by past hydrothermal activity. The detection of native hydrogen gas in the plume today would indicate current serpentinization, and thus a source of energy for possible life.

Although it is a moon of Saturn and not a planet, Titan is a particularly notable Solar System body because of its thick atmosphere, thought to be similar in some ways to that of early Earth. Its origin has been shrouded in mystery for over a half century. A decade after landing on Titan, data from the Cassini Huygens probe is helping scientists understand how the atmosphere of Saturn’s moon was formed. The data may help to resolve the source of Titan’s atmosphere: was it accumulated from solar nebula gas or via implantation of solar wind particles into the building blocks of Titan? Glein, a former member of the NAI’s Arizona State University team, proposes that the noble gases in Titan’s atmosphere may have been released from its rocky core.

The lack craters on Titan at mid-to-high latitudes where the elevation is lower has been perplexing, but a comparison to Earth environments could provide a potential answer. After observing the unequal crater distribution on Titan, the team examined seven scenarios that could account for this correlation with elevation. The explanation that was deemed most reasonable was that comets and other solar system debris that impact Saturn’s moon Titan splash into wetlands, leaving only subtle traces in a marine environment. Similar to terrestrial submarine impacts, on Titan many impacts could have fallen on extensive wetlands or a global sea that could have existed on Titan in the last few hundred million years. Such wetlands would be fed by an aquifer of liquid methane or ethane. Since the inventory of these hydrocarbons likely fluctuated over time, the preservation of features in some impacts at lower elevations would also be an expected feature.