After the Viking missions, it was discovered that
several of the rocks in the meteorite collections
matched Mars' atmospheric composition exactly.
Thus, it was determined that we had a small
sample of Mars rocks -- a dozen or two --
here on Earth.
This particular rock is called ALH84001 which means
it was discovered in Allan Hills (Antarctica) in
1984, and is the first rock from that season to be
analyzed.
This rock fell to Earth about 13,000 years
ago and
was
recovered
in Antarctica. The rock itself is probably
around 3.6 billion years old.
It took around 17 million years to travel to Earth.
(How do we know all that stuff
about this rock?)