Emily LakdawallaSep 05, 2014

Pretty pictures: Vesta's dark material

Ready for more eye candy from the second-largest member of the asteroid belt? Dawn left Vesta behind more than two years ago, but scientists and amateurs both are still hard at work processing all the data. Dawn's images of Vesta are hard for amateurs to work with, because Dawn's perspective on the lumpy asteroid changed continuously. It's difficult to assemble multiple images into color or high-resolution views because the landscape in view changed so rapidly with that shifting view of bumpy terrain. But Björn Jónsson has risen to that challenge, making global mosaics and high-resolution images of cool surface features. I posted his work on Aricia Tholus a while ago; here are two more interesting locations on Vesta. First up: Aelia crater.

Aelia crater, Vesta
Aelia crater, Vesta Aelia is a 4.3-kilometer-diameter crater 14 degrees south of Vesta's equator that shows intriguing variety in the albedo of its ejecta. Streaks of ejecta radiate in all directions, including down the slope of the adjacent Rufillia crater, which is 15.8 kilometers across. This photo is a mosaic of two images taken during the Low Altitude Mapping Orbit phase of Dawn's mission on January 10 and April 30, 2012, colorized with data taken during the High Altitude Mapping Orbit phase on on October 4, 2011.Image: NASA / JPL / UCLA / MPS / DLR / IDA / Björn Jónsson

There are several cool things about Aelia. The most obvious are the wispy streaks of dark material radiating out from it, onto the Vestan plains in some places and also down the slope of the older Rufillia crater. Let's zoom in to see what else is going on inside the crater:

Aelia crater, Vesta (detail)
Aelia crater, Vesta (detail) Image: NASA / JPL / UCLA / MPS / DLR / IDA / Björn Jónsson

There are patches of dark material cascading down the walls, but it is very patchy, with those patches located at different depths from the surface. You can also see that the crater has an asymmetrical shape; the southern two-thirds of the crater has a sharp edge and cascaded walls, while the northern third has a rounded edge of disorganized-looking material. That's a common shape for Vestan craters; it's because much of Vesta has very steep slopes. This is an impact that happened on such a slope; downhill is toward the top of the image. Material cascaded down from the high rim and flowed out and over the low rim.

Before I move on to the next spot, I thought I'd share a map of Vesta to help you locate these features. Aelia and Rufillia are near 300°E longitude, just a bit south of the equator.

Vesta's place names as of September 2014
Vesta's place names as of September 2014 This map is available in PDF format from the United States Geological Survey's Gazetteer of Planetary Nomenclature.Image: NASA / JPL / UCLA / MPS / DLR / IDA / USGS

Most of the dark material on Vesta is associated with impact craters, like Aelia. There are a few places, though, where it's harder to tell exactly what the dark material has to do with impact craters. Here's a spot where there's a spattering of dark material spread across the landscape near some craters. The crater at left is Fulvia; it's south and just a bit east of Aelia.

Dark spots on Vesta
Dark spots on Vesta One of the more intriguing aspects of Vesta as seen by Dawn is the "dark material" found across its surface. In most places, dark material is associated with craters. In this photo, the dark material shows up in the ejecta around two large impact craters, but the relationship between the dark stuff and the crater is not obvious. The left crater is the 17-kilometer-diameter Fulvia. In some places the dark material seems to be associated with small secondary craters, but in other places there is no obvious origin. This photo consists of three high-resolution images taken on December 20, 2011, March 13, 2012 and April 17, 2012, colorized with a lower-resolution image taken on September 30, 2011.Image: NASA / JPL / UCLA / MPS / DLR / IDA / Björn Jónsson

Did the formation of Fulvia splatter the dark material around? Or not? It's unclear. Let's look closer:

Dark spots on Vesta (detail)
Dark spots on Vesta (detail) Image: NASA / JPL / UCLA / MPS / DLR / IDA / Björn Jónsson

The dark material almost disappears when you look at it this close. It's weird and elusive and has been one of the bigger mysteries of the Dawn mission to Vesta. Some answers are starting to come out, though. An entire special issue of Icarus devoted to the bright and dark material on Vesta has just been published. I'm still digesting the papers, but I'll be writing something about them eventually!

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