The Other Mars Meteorite

alh_meteorite
The ALH Meteorite, about the size of a softball and one of more than two dozen Mars samples available for study on Earth today.
Image Credit: NASA/ Johnson Space Center

Perhaps the most famous of the martian meteorites was discovered at Allen Hills, Antarctica, and spawned a controversy about fossil-shapes and whether Mars could once have supported more favorable conditions than today.

"Most Mars meteorites studied in labs on earth," said Arizona scientist, painter, and writerDr. William Hartmann, now working with the Mars Global Surveyor image team, "have clear evidence of having been exposed to moisture and salty water. One (named Lafayette) has enough weathered minerals that they could be dated by two labs (California and Arizona) and the water exposure was found to have happened 670 million years ago."

Close-up of famous shapes measuring 20 to 200 nanometers across in Allen Hills meteorite [ALH84001], found at Allen Hills, Antarctica. Around 28 Mars meteorites have been identified so far. Credit: NASA/ESA/STScI, J. Hester and P. Scowen (ASU)

The Lafayette sample is named after Lafayette, Indiana, where in 1935 it was identified in a Purdue geological collection. The two pound (800 g) mass is shaped like a truncated cone that measures between four and five centimeters (about 2 in) across. The rock’s conical shape is consistent with its melting and solidification as it ablated upon entry through the Earth’s atmosphere. Parts of its surface began to flow into a smooth veinous crust. At first glance, the meteor might be mistaken for a mushroom cap [as shown in the banner image].

In 1992, the water found in Lafayette was determined to be extraterrestrial and still contained the most [0.387 %] water of any Martian meteor. More interestingly, its composition was enriched in heavy water, or deuterium, as measured when the rock was step-wise heated. After it formed initially as an iron-rich, volcanic rock, Lafayette was apparently altered by water.

The mineral olivine, which generally does not stay around unmodified for long when water is present, was also found to be the most altered of all the iron-rich minerals in the sample. When examined in thin sections, the meteor showed rusty-red grains surrounded by black veins, sometimes called a fibrous structure.

This volcanic makeup had changed remarkably while Lafayette was still on Mars–where many had presumed rock changes were not likely to be identified with water. The conclusion of mineralogists soon became surprisingly definitive. Many hundreds of millions of years ago, Lafayette had once resided in salt-water.

Computer modeling of half an hour after Giant Impact
Impact blasts seem to deliver martian fragments to Earth regularly, prompting comparisons to a kind of intra-solar-system ‘bus traffic’ of rocks.
Copyright William K. Hartmann

And about one part in 300 remained locked up as water still in the meteor.

The timeline proposed for Lafayette showed a lineage that began around 700 million years ago on Mars, when some saline began to seep into it and change the rock’s mineral content. About 11 million years ago, the fragment blasted off of Mars as debris and then landed on Earth [originally in Illinois] about 2,900 years ago. Or put another way, Lafayette arrived relatively recently on Earth, sometime after the Egyptian pyramids were completed.

In 2000, scientists concluded that the "salts" identified in the Lafayette alteration (based mainly on the mineral, iddingsite) formed by fractional evaporation of an acid brine on Mars. When combined with recent surface evidence for sulfur-rich brines once ‘soaking’ the Opportunity rover site, an intriguing picture begins to frame what earlier seemed to be surprising, but still isolated, Lafayette discoveries that otherwise might not have had much context to place it in a martian geological timeline.

These contextual questions will occupy comparisons of the world’s meteorite collections with what the Mars’ rovers are chemically analyzing on the surface this year. If Allen Hills is the oldest and Lafayette is the wettest, then in this remarkable meteor traffic between the red planet and Earth, our planetary neighbor seems to have left behind more than a few intriguing clues about its geological history.