http://www.nytimes.com/2004/11/07/science/07mars.html?ex=1100876165&ei=1&en=b1c55b27760922a2

There is also a more tangible piece of New York on Mars. On
each rover, a curved piece of metal the size of a credit
card and adorned with the American flag was cut out of
debris from the World Trade Center.

The Honeybee engineers obtained a box of twisted metal from
the city. Mr. Bartlett recalled the smell - "that acrid
electrical fire smell" - when they opened it.

Tom Myrick, Honeybee's chief engineer, flew down to Texas
with a piece of aluminum from the box. At the machine shop
they employ there, the metal was turned into shields to
protect cables on the drilling mechanisms.

Until now, that knowledge has remained within the mission
team.

"It was intended to be a quiet tribute," Mr. Gorevan said.
"Enough time has passed. We want the families to know."