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Charles E. Miller

Publications and source records attributed to Charles E. Miller.

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Mapping ammonia emission plumes using shortwave infrared imaging spectroscopy

Atmospheric ammonia emissions are harmful to ecosystems and human health. These emissions have traditionally been monitored using thermal infrared spectrometers, though such techniques are limited by thermal contrast requirements, the coarse spatial resolution of existing satellite sensors, and low measurement frequency of higher-resolution aerial surveys. Here, we show that ammonia emissions can be quantified using shortwave infrared imaging spectroscopy, circumventing these challenges by using reflected sunlight instead of thermal emission for signal and by enabling a large class of existing and future imaging spectrometers to enter the ammonia observing system. As a proof of concept for this newly discovered capability, we use Tanager-1 satellite data to quantify emissions from industrial point sources of ammonia in Pakistan and Uzbekistan.

physics.ao-ph

Hypotheses for near-surface exchange of methane on Mars

The Curiosity rover recently detected a background of 0.7 ppb and spikes of 7 ppb of methane on Mars. This in situ measurement reorients our understanding of the Martian environment and its potential for life, as the current theories do not entail any geological source or sink of methane that varies sub-annually. In particular, the 10-fold elevation during the southern winter indicates episodic sources of methane that are yet to be discovered. Here we suggest a near-surface reservoir could explain this variability. Using the temperature and humidity measurements from the rover, we find that perchlorate salts in the regolith deliquesce to form liquid solutions, and deliquescence progresses to deeper subsurface in the season of the methane spikes. We therefore formulate the following three testable hypotheses. The first scenario is that the regolith in Gale Crater adsorbs methane when dry and releases this methane to the atmosphere upon deliquescence. The adsorption energy needs to be 36 kJ/mol to explain the magnitude of the methane spikes, higher than existing laboratory measurements. The second scenario is that microorganisms convert organic matter in the soil to methane when they are in liquid solutions. This scenario does not require regolith adsorption, but entails extant life on Mars. The third scenario is that deep subsurface aquifers produce the bursts of methane. Continued in situ measurements of methane and water, as well as laboratory studies of adsorption and deliquescence, will test these hypotheses and inform the existence of the near-surface reservoir and its exchange with the atmosphere.

astro-ph.EP