Asteroid Bennu’s Rugged Surface Baffled NASA, We Finally Know Why

In one of the biggest surprises of NASA's OSIRIS-REx mission, its target asteroid, Bennu, turned out to be a jagged, rugged world covered in large boulders, with few of the smooth patches that earlier observations from Earth-based instruments had indicated.

"When OSIRIS-REx got to Bennu in 2018, we were surprised by what we saw," said Andrew Ryan, a scientist with the University of Arizona's Lunar and Planetary Laboratory in Tucson, who led the mission's sample physical and thermal analysis working group. "We expected some boulders, but we anticipated at least some large regions with smoother, finer regolith that would be easy to collect. Instead, it looked like it was all boulders, and we were scratching our heads for a while.

Particularly puzzling were observations made in 2007 by NASA's Spitzer Space Telescope, which measured low thermal inertia, indicative of an asteroid whose surface heats up and cools down rapidly as it rotates into and out of sunlight, like a sandy beach on Earth. This was at odds with the many large boulders that OSIRIS-REx found upon arrival, which should act more like blocks of concrete, shedding heat long after the Sun has set.