Scientists studying the impact on Comet Tempel 1 say the material which burst from the collision site was extremely fine, much like talcum powder.
The researchers on the Deep Impact mission say this suggests the icy body probably built up over a long time.
Scientists continue to analyse the data gathered when a 370kg Nasa probe was deliberately slammed into the comet.
The 4 July event was pictured from the passing main spacecraft and a range of space and Earth-bound telescopes.
They caught the vast and dramatic plume of debris that spread out into space.
Images of this material, taken at different wavelengths, should give researchers the best information yet on how comets are put together.
These giant balls of ice, dust and rock are thought to contain the pristine, leftover building blocks of our Solar System, formed when a huge cloud of gas and dust collapsed about 4.6 billion years ago.
The best yet
The Deep Impact science team continues to wade through gigabytes of data collected during the encounter with the comet, which measures 5km wide by 11km long.
"The major surprise was the opacity of the plume the impactor created and the light it gave off," said Deep Impact Principal Investigator Dr Michael A'Hearn, of the University of Maryland, College Park.
"That suggests the dust excavated from the comet's surface was extremely fine, more like talcum powder than beach sand. And the surface is definitely not what most people think of when they think of comets - an ice cube."
DEEP IMPACT: 4 JULY
The explosive moment of impact on Comet Tempel 1
The impactor and flyby craft returned approximately 4,500 images.
"We are looking at everything from the last moments of the impactor to the final look-back images taken hours later, and everything in between," added A'Hearn.
"Watching the last moments of the impactor's life is remarkable. We can pick up such fine surface detail that objects that are only four metres in diameter can be made out. That is nearly a factor of 10 better than any previous comet mission."
The Deep Impact team says the impactor hit at an approximately 25 degree oblique angle relative to the comet's surface.
The ensuing debris plume expanded rapidly above the impact site at about 5km/s.
Scientists are still trying to get a good match on the size of the crater. They believe it was at the high end of original predictions, which ranged up to 250m wide.
Researchers using Europe's XMM-Newton space telescope report that Comet Tempel 1 is a very weak source of X-rays.
The team is not sure why - although such emissions have been observed in comets before.
It is possible the X-rays arise from an interaction between the high-speed, ionised particles streaming from the Sun (the solar wind) and the dust particles in the comet's diffuse halo, or coma.
Or, it could just be those coma particles are simply scattering X-rays coming from the Sun.
The University of Maryland in the US is responsible for overall Deep Impact mission science, and project management is managed by the Nasa's Jet Propulsion Laboratory in California.