NASA’s Dawn spacecraft has taken thousands of photos of the dwarf planet Ceres, but these may be some of the weirdest. They were made on April 29, when Ceres was directly opposite the sun as seen from the spacecraft. In your mind’s eye picture a neat lineup in this order: sun – Dawn – Ceres.
The same situation happens right here on Earth once a month at full moon, when the moon is opposite the sun in the sky. As soon as the sun sets in the west, the full moon rises in the east. From our perspective in the middle, sunlight shines directly at the moon, creating a virtually shadowless landscape. Shadows reveal edges, contours and countless hills, crater rims and valleys, but without them, those things blend together into bland regions of light and dark.
Mission specialists had carefully maneuvered Dawn into a special orbit so that the spacecraft could view Occator Crater, which contains those amazing bright spots, from this new perspective.
A new movie shows these opposition images, with contrast enhanced to highlight brightness differences. The bright spots of Occator stand out particularly well on an otherwise relatively bland surface because they’re patches of sodium carbonate, a kind of salt found on Earth in hydrothermal environments. Salt is a good reflector of light. Astronomers think the salt was once mixed with water that got pushed up from below the surface possibly in geysers or in short-lived lakes. After the water vaporized, the salt remained.
Based on what we know about shadow-hiding at opposition, scientists correctly predicted that Ceres would appear brighter at that time. This increase in brightness, or “surge,” tells us more about the size of the grains of material on the surface, as well as the porosity of those materials. In particular, scientists want to know if Ceres’ bright “eyes” are powdery or stiff and crusty. To read further about the opposition surge and the science behind it, check out the March issue of the Dawn Journal blog.
Just by changing our angle of view on a solar system object, we can gain new insights about the smallest things on its surface.