Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

Stephen Mojzsis

Second Advisor

William Bottke

Third Advisor

Brian Hynek

Fourth Advisor

Joseph Smyth

Fifth Advisor

Shijie Zhong

Abstract

The epoch of habitability on a rocky world is dictated by the suite of geological events and processes it experiences. A planet may be habitable for a long stretch of its history, but that period has limits. When is too soon for life to arise on a planet, and when is it too late for it to continue? Limits for habitability on Earth-like planets can be defined by extracting Earth's history from the geochemical record to infer that of the rocky planets lying far beyond the reaches of our solar system.

The Late Veneer was the last recorded impact event capable of melting the Earth's crust--and extinguishing any extant life. Eoarchean ultramafic schists (metakomatiites) from Greenland and Canada show highly siderophile element depletions consistent with a deep mantle that had yet to be fully contaminated by Late Veneer material. Whether it was one impactor or multiple, it is clear that the iteration of life from which we originate must have arisen no earlier than the Late Veneer.

Planetary accretion stymies the emergence of life, but geophysics might provide its eventual demise. Plate tectonics operates on Earth as a globate climate moderator, keeping Earth's surface temperate. Geological activity is maintained largely by the long-lived, heat-producing radionuclides 40K, 232Th, 235U, and 238U, whose concentrations decline as Earth ages. Eventually, these isotopes will no longer be able to provide the heat required for mantle convection, and plate tectonics will shut down, calling into question Earth's ability to maintain habitability on a global scale.

Here, the Late Veneer is proposed for the first time as a hard constraint for when life could have arisen on Earth, and age is shown to be a key player in the long-term thermal regimes of conventionally defined Earth-like exoplanets. Provided with limits on the reign of life on Earth, the habitability of rocky exoplanets can be evaluated: those that are too young will still be experiencing surface-sterilizing impact events, while those that are too old will possess cool mantles incapable of sustaining the geological activity that supports the only life we know of in the Universe.

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