37th DPS Meeting, 4-9 September 2005
Session 57 Moon, Mercury and Venus
Poster, Thursday, September 8, 2005, 6:00-7:15pm, Music Recital Room

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[57.04] Spatial Distribution of Sodium on Mercury

A. E. Potter (National Solar Observatory, Tucson, Arizona), R.M. Killen (University of Maryland, College Park, Maryland)

If sodium vapor were uniformly distributed over the surface of Mercury, images of sodium emission would show limb brightening, with the emission fading to minimum values along the terminator. The north-south brightness distribution would be symmetric about the equator. However, actual images of Mercury sodium emission do not always correspond to this expectation. Some images show bright emission near the dawn terminator, and some show excess emission in either the northern or southern hemisphere. These anomalous effects hold clues to the source and loss processes for sodium. We have searched for spatial anomalies in a collection of about a thousand images of Mercury sodium emission, measured using the McMath-Pierce solar telescope at Kitt Peak, Arizona. Our data set covers true anomaly angles ranging from 0 to 360 degrees, with a few small gaps in the region between 180 and 360 degrees. About a third of them show either northern or southern hemisphere enhancement, with no obvious correlation with geography or true anomaly angle. We suggest that these high-latitude enhancements are the result of solar wind sputtering in the northern or southern hemisphere, depending on conditions of the IMF. Dawn enhancement of sodium emission may result from evaporation of condensed sodium vapor as the cold surface is warmed at sunrise. We explored this effect by computing ratios of eastern to western hemisphere intensities for images where the dawn terminator was in view. Dawn enhancement was evident for true anomaly angles less than about 120 degrees, but disappeared for larger true anomaly angles. We have no explanation for this effect. Possibilities include temperature effects and preferential precipitation of sodium ions on the dawn hemisphere.

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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.