Speculation: winds from Bonnie obliterated most of the thin sheen throughout the area; but since then, sheen has had time to "reassemble" into observable layers that noticeably affect the sunglint on MODIS images, and the backscatter on radar, but may not look like much to folks out in the Gulf on vessels or in low-flying aircraft. That's our theory at this point. Chime in if you have other thoughts about what we're seeing on these images:
The MODIS / Aqua satellite image above, taken at 2 pm Central time on July 28, shows oil slicks and sheen (encircled with orange line) that we think are likely attributable to the BP / Deepwater Horizon oil spill, spread out across 11,832 square miles (30,644 km2) in the Gulf of Mexico. We've marked the eastern edge of a persistent ocean-color anomaly with a dashed line; this anomaly may simply be related to the Mississippi River discharge, or could indicate an area where ocean chemistry has been affected by oil, dispersant, and/or dissolved methane from the spill and cleanup response. Three small slicks attributable to natural oil and gas seeps are also marked.
RADARSAT-2 satellite image (black-and-white) taken July 28, 2010. RSAT-2 data courtesy CSTARS.
We overlayed the RADARSAT-2 image (black-and-white) taken at 6:48 pm Central time on the MODIS/Aqua image taken earlier that same day. The large dark area on the radar image is probably oil slick and sheen from the BP oil spill: wind conditions throughout the area were ideal for slick and sheen detection on radar satellite imagery, ranging from 2 to 8 meters per second with minimal gusts. Weather satellite images taken at about the same time showed few clouds in the area and very low chance of any rain in the vicinity.