R. Amils | Geology Reports

USGS2008

TandEM: Titan and Enceladus mission

TandEM was proposed as an L-class (large) mission in response to ESA’s Cosmic Vision 2015–2025 Call, and accepted for further studies, with the goal of exploring Titan and Enceladus. The mission concept is to perform in situ investigations of two worlds tied together by location and properties, whose remarkable natures have been partly revealed by the ongoing Cassini–Huygens mission. These bodies still hold mysteries requiring a complete exploration using a variety of vehicles and instruments. TandEM is an ambitious mission because its targets are two of the most exciting and challenging bodies in the Solar System. It is designed to build on but exceed the scientific and technological accomplishments of the Cassini–Huygens mission, exploring Titan and Enceladus in ways that are not currently possible (full close-up and in situ coverage over long periods of time). In the current mission architecture, TandEM proposes to deliver two medium-sized spacecraft to the Saturnian system. One spacecraft would be an orbiter with a large host of instruments which would perform several Enceladus flybys and deliver penetrators to its surface before going into a dedicated orbit around Titan alone, while the other spacecraft would carry the Titan in situ investigation components, i.e. a hot-air balloon (Montgolfière) and possibly several landing probes to be delivered through the atmosphere.

Experimental Astronomy↗

USGS2008

Photoreduction fuels biogeochemical cycling of iron in Spain's acid rivers

A number of investigations have shown that photoreduction of Fe(III) causes midday accumulations of dissolved Fe(II) in rivers and lakes, leading to large diel (24-h) fluctuations in the concentration and speciation of total dissolved iron. Less well appreciated is the importance of photoreduction in providing chemical energy for bacteria to thrive in low pH waters. Diel variations in water chemistry from the highly acidic (pH 2.3 to 3.1) Río Tinto, Río Odiel, and Río Agrio of southwestern Spain (Iberian Pyrite Belt) resulted in daytime increases in Fe(II) concentration of 15 to 66 µM at four diel sampling locations. Dissolved Fe(II) concentrations increased with solar radiation, and one of the stream sites showed an antithetic relationship between dissolved Fe(II) and Fe(III) concentrations; both results are consistent with photoreduction. The diel data were used to estimate rates of microbially catalyzed Fe(II) oxidation (1 to 3 nmol L − 1 s − 1 ) and maximum rates of Fe(III) photoreduction (1.7 to 4.3 nmol L − 1 s − 1 ). Bioenergetic calculations indicate that the latter rates are sufficient to build up a population of Fe-oxidizing bacteria to the levels observed in the Río Tinto in about 30 days. We conclude that photoreduction plays an important role in the bioenergetics of the bacterial communities of these acidic rivers, which have previously been shown to be dominated by autotrophic Fe(II)-oxidizers such as Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans . Given the possibility of the previous existence of acidic, Fe(III)-rich water on Mars, photoreduction may be an important process on other planets, a fact that could have implications to astrobiological research.

Chemical Geology↗