1982; Sun et al. 2010). What microorganisms remain begin the colonization and
biomass building process again, in a cycle estimated to take 10,000 years (Sun and
Friedmann 1999). There is evidence that the endolithic microbial communities
recycle carbon within the rock on similar timescales, mixing “modern” fixed carbon
into the community at variable rates (Brady et al. 2018). The exfoliation process
contributes to the mineral soils which make up the valley floor, delivering carbon,
and microorganisms to the permafrost environment. Though the microorganisms
delivered to the valley floors are adapted to growth at sub-zero temperatures, the
conditions on the valley floor do not seem to be amenable to the same level of
metabolic activity, and cells must shift to other means of persistence. The context of
the rock habitat itself in University Valley is what lends itself to abundant and
active life.
6.3 Prospects for Life Beyond Earth in Our Solar System
Within our solar system, there are multiple promising planetary bodies being
explored for potential microbial life; these include the planet Mars and Enceladus,
Titan and Europa, the moons of Saturn and Jupiter. What these planetary bodies
share in common is the potential presence of a liquid solvent and cold temperatures.
Average surface temperatures are À60
C on Mars, À190
C on Enceladus, À160
C
on Europa and À 180
C on Titan. As a result of these frigid temperatures, any
potential life would be constrained by the presence of a liquid solvent, similar to how
life is constrained on Earth. Mars may have had vast oceans in its geologic history,
and sinuous channels indicative of prolific water flow are globally distributed (Zuber
2018; Head et al. 1998; Dohm et al. 2001; Davis et al. 2016). As recently as 5 Mya,
the conditions to melt ice-cemented permafrost would have been met in the Martian
North pole, where vast quantities of water-ice are currently located (McKay et al.
2013). If life once was present on Mars during these more clement conditions, could
it have survived in the subsurface permafrost in a state of dormancy or with low
levels of microbial activity? On present-day Mars, transient dark streaks are
observed on equatorial slopes in the Martian summer; these recurring slope linneae
(RSL) are potentially formed by liquid brines flowing through permafrost, though a
dry origin is also being considered (McEwen et al. 2011; Sun et al. 2010). Both
Enceladus and Europa, moons of Saturn and Jupiter, respectively, are ocean worlds,
with large bodies of salty water beneath km’s thick ice. On Titan, no liquid water is
known to be present, but lakes of liquid hydrocarbon can be found (Stofan et al.
2007). Could life at cold temperatures utilize other liquids as a solvent for lifesustaining metabolic processes? Cryoenvironments on Earth are the best analogs to
inform where, and how, we search for life on other cold planetary bodies.
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J. M. Goordial
biomass building process again, in a cycle estimated to take 10,000 years (Sun and
Friedmann 1999). There is evidence that the endolithic microbial communities
recycle carbon within the rock on similar timescales, mixing “modern” fixed carbon
into the community at variable rates (Brady et al. 2018). The exfoliation process
contributes to the mineral soils which make up the valley floor, delivering carbon,
and microorganisms to the permafrost environment. Though the microorganisms
delivered to the valley floors are adapted to growth at sub-zero temperatures, the
conditions on the valley floor do not seem to be amenable to the same level of
metabolic activity, and cells must shift to other means of persistence. The context of
the rock habitat itself in University Valley is what lends itself to abundant and
active life.
6.3 Prospects for Life Beyond Earth in Our Solar System
Within our solar system, there are multiple promising planetary bodies being
explored for potential microbial life; these include the planet Mars and Enceladus,
Titan and Europa, the moons of Saturn and Jupiter. What these planetary bodies
share in common is the potential presence of a liquid solvent and cold temperatures.
Average surface temperatures are À60
C on Mars, À190
C on Enceladus, À160
C
on Europa and À 180
C on Titan. As a result of these frigid temperatures, any
potential life would be constrained by the presence of a liquid solvent, similar to how
life is constrained on Earth. Mars may have had vast oceans in its geologic history,
and sinuous channels indicative of prolific water flow are globally distributed (Zuber
2018; Head et al. 1998; Dohm et al. 2001; Davis et al. 2016). As recently as 5 Mya,
the conditions to melt ice-cemented permafrost would have been met in the Martian
North pole, where vast quantities of water-ice are currently located (McKay et al.
2013). If life once was present on Mars during these more clement conditions, could
it have survived in the subsurface permafrost in a state of dormancy or with low
levels of microbial activity? On present-day Mars, transient dark streaks are
observed on equatorial slopes in the Martian summer; these recurring slope linneae
(RSL) are potentially formed by liquid brines flowing through permafrost, though a
dry origin is also being considered (McEwen et al. 2011; Sun et al. 2010). Both
Enceladus and Europa, moons of Saturn and Jupiter, respectively, are ocean worlds,
with large bodies of salty water beneath km’s thick ice. On Titan, no liquid water is
known to be present, but lakes of liquid hydrocarbon can be found (Stofan et al.
2007). Could life at cold temperatures utilize other liquids as a solvent for lifesustaining metabolic processes? Cryoenvironments on Earth are the best analogs to
inform where, and how, we search for life on other cold planetary bodies.
124
J. M. Goordial