AbSciCon 2015 Chicago
Jeremy Tregloan-Reed NASA Postdoctoral Research Fellow NASA Ames Research Center, Freund-Team
[email protected] www.astro.keele.ac.uk/~jtr/
A Tantalising Glimpse of the Future
Snellen et al., 2013
Does O2 or O3 = Life? Yes
No
Segura et al. (2005)
Selsis et al. (2002)
Kaltenegger et al. (2010)
Tian et al. (2014)
Snellen et al. (2013)
Wordsworth & Pierrehumbert (2014)
A New Abiotic Pathway to Planetary Oxidation: Peroxy Defects Peroxy ubiquitous in rocks from H2O-laden magmas cooled to below 700-800K. Peroxy are metastable, formed by a redox conversion of hydroxyl pairs such as: O3Si-OH HO-SiO3 O3Si-OO-SiO3 + H2 Peroxy defects introduce an oxidized form of oxygen into a planetary environment, even though the rocks still appear overall reduced (Freund & Freund 2015).
Thermal Activation of Peroxy Defects
Two stage break-up of the peroxy defect in MgO introduced by the redox conversion of Mg2+ vacancy-associated OH– pairs into peroxy anions, O22.
Step I 500–600K The spins of the two O– decouple.
Step II 800-900K The O– pair defect dissociates.
Heating High Purity MgO Crystals • Disassociated peroxy defects are positive hole charge carriers. • The charge carriers are physically, defect electrons in the oxygen anion sublattice, but are chemically O-.
Hydrolysis of Peroxy Defects Water will hydrolyse the peroxy defects during weathering, producing H2O2 O3Si/OO\SiO3 + 2H2O ⇒ O3 Si/OH HO/SiO3 + H2O2 Hydrogen peroxide then decomposes into: H2O2 ⇒ H2O + O Other reduced components released by weathering will react with H2O2 such as transition metal cations in aqueous solution, oxidising, for instance, ferrous iron, Fe2+, to ferric iron Fe3+.
How Much Oxygen can be Generated? Today’s global weathering rate is on the order of ~1013 kg yr-1 of rocks (Hilley & Porder 2008). Peroxy contents in the range of 1000±500 ppm had previously been determined for MgO through magnetic susceptibility measurements using a Vibrating Sample Magnetometer (Batllo et al. 1991). Ultramafic Rock weather faster than today's granitic-andesitic rock. This would lead to the release of ~1010 kg yr-1 of O2 – an order of magnitude more than what has been considered necessary, as a yearly average, to precipitate the Banded Iron Formations (BIFs) (Holland 1984; Kesler & Ohmoto 2006).
O2 Evolution from Heating Granite • Trapped Gases in Pores • Secondary Chemical Reactions • Complex System
Take a Step Back and Think Semiconductor Physics
Left: Anomaly in the thermal expansion coefficient of a high purity MgO. Middle: Two-step anomaly in the effective dielectric constant of high purity MgO. Right: Appearance of positive surface charge on MgO indicative of p-type response. (Freund et al. 1993)
The Hall and Seebeck Effects Hall Hole Lifetime
Hole Effective Mass
Hole Mobility
Seebeck Carrier Concentration
Quasi Fermi Energy Level
Preliminary Work
Experimental setup to measure the Seebeck Coefficient.
Two steps in the gradient of the surface potential, indicating a change in active charge carrier concentration.
Future Work Ascertain Peroxy Concentrations for rocks found in the Earth’s Crust.
That's the Easy Part! Construct a full abiotic oxidation model. • Crustal composition? • Atmospheric composition? • Plate tectonics? • 1D and 3D atmospheric chemical modelling.
The list goes on!
Does O2 or O3 = Life? Yes, maybe..... If we can detect O2 or O3 concentrations above the upper bound from ALL abiotic processes.
Thank You!
Earthrise Credit: NASA
