When Curiosity used its powerful laser to zap the fist-sized “Coronation Rock” last Sunday, the resulting spark of vaporized minerals revealed the rock was made of basalt (ba-SALT). Basalt is a common volcanic rock found both on Mars and Earth. Just this morning I tromped around on grey, fine-grained basalt pebbles along the beach in Duluth’s downtown.
Basalt is formed by rapid cooling of lavas on the surface of a moon or planet. Fast-cooling lavas are made of tiny crystals, which is why basalt pebbles are a flat, uniform gray and pleasingly smooth to the touch.
Molten rock beneath the surface of the Earth is called magma. Magmas take a longer time to cool and grow bigger crystals, forming rocks like granite and gabbro. Both magma and lava cool and solidify to produce igneous rocks. I apologize for the Geology 101 lesson, but it will be helpful when we look at the makeup of Mars rocks.
As of July 2012, 63 Martian meteorites have been found on Earth. All are igneous rocks similar to the one Curiosity sampled. Some have been altered by water flowing over them, creating patches of minerals called carbonates, but most are a Martian version of basalt similar to Earth’s igneous rocks.
How do we know they’re from Mars? Thanks to the two Viking spacecraft that landed on Mars in 1976, scientists were able to precisely measure the composition of the Mars’ atmosphere. Those same gases in the same proportions were found trapped within a meteorite called Elephant Moraine 79001 recovered in Antarctica in the early 80’s and in other meteorites since. Elephant Moraine has other elemental and mineral peculiarities shared by a small group of space rocks called “Snick” or SNC meteorites that scientists have positively linked to Mars.
Getting here took a lot of energy. Meteorite and asteroid impacts on Mars millions of years ago sent chunks of Mars’ crust flying into space. Over time, Earth intercepted some of this material which landed as meteorites. The most recent meteorite to land on Earth from the Red Planet fell in the early morning hours of July 18, 2011 in a rugged desert near Tissint(TEE-sint), Morocco. Based on a detailed study of its composition, the Tissint meteorite is believed to have formed in a lava flow on Mars 460 million years ago and likely launched by impact 1.1 million years ago along with 11 other similar meteorites.
We know the impact that excavated the rocks was powerful in the extreme because a number of Mars meteorites, including Tissint, contain dark “melt pockets” and shock veins where heat and pressure melted the minerals into a black glass. It’s in those dark glasses that scientists find precious samples of Mars atmosphere sealed up waiting to be released and analyzed by re-heating.
Going to Mars to study rocks as Curiosity is doing will help us understand the context for the “free delivery” samples we already have here on Earth. We may even be able to nail down just where our earthly Martians originated. Scientists think the currently known 63 were launched from a half dozen different sites rather than 63 separate ones. Enough are similar to one another it’s believed they’re different parts of a single large lava flow.
The various Mars rover missions have uncovered lots of Mars rocks that are different from the samples bequeathed to us by chance encounter. Only one is a good match, a rock called “Bounce” examined by the Opportunity Rover and found to have a composition very similar to some of the Mars meteorites. No doubt we’re missing many Mars rocks from our collection. Think of the diversity of types on Earth.
If we had only 63 rocks from our planet we’d hardly get a complete picture of Earth’s garden of rocky delights. Scientists would love to stumble across a Martian sedimentary meteorite blasted from an ancient river bed. A better sampling of extremely ancient rocks would also be appreciated. Most of the samples we have are relatively young with formation ages measured in millions rather than billions of years.
That’s why we keep returning again and again the Red Planet – to wrest the secrets the rocks hold onto so tightly. To learn more about Mars meteorites, click HERE or check out a current list of Martians HERE.