[meteorite-list] What Smacks Into Ceres Stays On Ceres

[Ron Baalke]

https://news.brown.edu/articles/2015/10/ceres

What smacks into Ceres stays on Ceres
Brown University
Contact: Kevin Stacey   401-863-3766
October 14, 2015   

Ceres, the largest object in the asteroid belt and closest dwarf planet 
to Earth, had been remarkable for its plain surface. New research suggests 
that most of the material that has struck Ceres in high-speed collisions 
has stuck - billions of years worth of meteorite material.

PROVIDENCE, R.I. [Brown University] - A new set of high-velocity impact 
experiments suggests that the dwarf planet Ceres may be something of a 
cosmic dartboard: Projectiles that slam into it  tend to stick.

The experiments, performed using the Vertical Gun Range at NASA's Ames 
Research Center, suggest that when asteroids and other impactors hit Ceres, 
much of the impact material remains on the surface instead of bouncing 
off into space. The findings suggest the surface of Ceres could consist 
largely of a mish-mash of meteoritic material collected over billions 
of years of bombardment.

The research, by Terik Daly and Peter Schultz of Brown University, is 
published in Geophysical Research Letters .

Ceres is the largest object in the asteroid belt and the nearest dwarf 
planet to Earth. Until the recent arrival of the Dawn spacecraft, all 
that was known about Ceres came from telescopic observations. The observations 
showed Ceres to be mysteriously low in density, suggesting it is made 
either of very porous silicate material, or perhaps contains a large layer 
of water ice. Observations of its surface were remarkable as well - largely 
for being unremarkable.

"It's really bland in the telescopic observations," said Daly, a Ph.D. 
student at Brown and the study's lead author. "It's like someone took 
a single color of spray paint and sprayed the whole thing. When we think 
about what might have caused this homogeneous surface, our thoughts turn 
to impact processes."

And to understand impact processes, the researchers turned to NASA's Vertical 
Gun Range, a cannon with a 14-foot barrel that can launch projectiles 
at up to 16,000 miles per hour. For this work, Daly and Schultz wanted 
to simulate impacts into low-density surfaces that mimic the two broad 
possibilities for the composition of Ceres' surface: porous silicate 
or icy.

"The idea was to look at those two end-member cases, because we really 
don't know yet exactly what Ceres is like," Daly said.

For the porous silicate case, the researchers launched impactors into 
a powdered pumice. For the icy case, they used two targets: snow, and 
snow covered by a thin veneer of fluffy silicate material, simulating 
the possibility the Ceres' ice sits below a silicate layer. They then 
blasted these targets with pebble-sized bits of basalt and aluminum, simulating 
both stony and metallic meteorites.

The study showed that in all cases, large proportions of the impact material 
remained in and around the impact crater. This was especially true in 
the icy case, Daly said.

"We show that when you have a vertical impact into snow - an analog for 
the porous ice we think might be just beneath the surface of Ceres - you 
can have about 77 percent of the impactor's mass stay in or near the crater."

The results were a bit of a surprise, said Schultz, who has studied impact 
processes for many years as professor of earth, environmental, and planetary 
sciences at Brown.

"This is really contrary to previous estimates for small bodies," Schultz 
said. "The thought was that you'd eject more material that you'd collect, 
but we show you can really deliver a ton of material."

The impact speeds used in the experiments were similar to speeds thought 
to be common in asteroid belt collisions. The findings suggest that a 
majority of impacts on porous bodies like Ceres cause an accumulation 
of impact material on the surface.

"People have thought that perhaps if an impact was unusually slow, then 
you could deliver this much material," Schultz said. "But what we're saying 
is that for a typical, average-speed impact in the asteroid belt, you're 
delivering a ton of material."

Over billions of years of such impacts, Ceres may have accumulated quite 
a bit of non-native material, Daly and Schultz said, much of it mixing 
together to create the relatively nondescript surface seen from telescopes. 
The researchers are hopeful that as the Dawn spacecraft scans the surface 
at much higher resolution, it might be able to pick out individual patches 
of this delivered material. That would help confirm the relevance of these 
experiments to celestial bodies, the researchers say.

The results have implications for missions that aim to return asteroid 
samples to Earth. Unless the landing sites are carefully chosen, the researchers 
say, those missions could end up with samples that aren't representative 
of the object's original material. To get that, it might be necessary 
to find an area where there has been a relatively recent impact.

"You can't do this like the old claw crane from the arcade," Schultz said. 
"You can't just reach down and grab whatever's there. You may need to 
find a fresh impact where perhaps the native stuff has been churned up."