A pair of mysterious meteorites discovered in Antarctica is baffling scientists who are struggling to determine the origin of the space rocks.

The meteorites, dubbed GRA 06128 and GRA 06129, were found in the Graves Nunataks region of Antarctica in 2006 (see an interactive map of Antarctica).

GRA 06129 meteorite
A meteorite dubbed GRA 06129 is one of two space objects that has scientists stymied. Experts initially thought the meteorites shared their origins with the moon or perhaps Venus, but those theories have now been discounted.

The rocks were oddly rusty and salty and smelled like rotten eggs, its discoverers said.

Initially, a team at the University of New Mexico (UNM) caused a stir when its analysis hinted that the pair may hail from Venus or the moon.

But other teams then hurried to get pieces of the space rocks for analyses of their own - and for the most part, they disagree.

GRA 06128 looks like rocks retrieved from the lunar highlands by the Apollo 16 astronauts in 1972, but it contains much more sodium, research has shown.

The rock is also much older than the Venusian surface, according to the newer analysis, thereby eliminating that possibility.

The identity of the meteorites' source remains exciting and mysterious, said Allan Treiman, a scientist with the Lunar and Planetary Institute in Houston who led one of the recent investigations of the rocks.

"From what has been reported so far, it's pretty clear that the meteorite is not from the Earth, or the moon, or Venus, or any of the common sources of meteorites," he said. "It's much harder to know where it is from."

Both teams - along with three others - are presenting findings about the meteorites at the 39th Lunar and Planetary Science Conference in Houston this week.

Not of This World?

GRA 06128 and its mate are slab-shaped, gray rocks containing bits of black glass.

The researchers analyzed chemical signatures called isotopes and initially determined that the meteorites came from Earth or the moon, a position they've since retooled.

"Although initial oxygen isotopic compositions are consistent with an origin in the Earth-Moon system, numerous observations appear to eliminate both bodies," Shearer and his team write in an abstract presented at this week's meeting.

Doug Rumble from the Carnegie Institution and his colleagues will also present an analysis of GRA 06128 and 06129 at the conference.

He said the New Mexico researchers initially studied a "tiny crumb that fell off the outside" of GRA 06129, but they didn't wash the outer layer to remove any Earthly weathering agents that might alter the findings.

"The UNM analytical results reflect only imperfectly the actual extraterrestrial composition of the meteorite, because UNM also analyzed rust that formed as the meteorite lay exposed to Earth's atmosphere," he said.

Rumble's own analysis puts the meteorites "right in the range of values expected for rare meteorites called brachinites," he said.

Brachinites are believed to be the remnants of planets that once orbited between Mars and Jupiter (see an interactive map of the solar system).

"So now we are all meeting in Houston to hang our heads a little and admit to one another that maybe we were a little too hasty," Rumble said, "and got too excited over an unwashed meteorite."

Unsettled Debate

David Kring, a visiting scientist at Houston's Lunar and Planetary Institute and a co-author on Treiman's paper, said there's still work to be done to nail down the exact nature of the meteorite pair.

The chemical and mineral compositions of the rocks are similar to brachinites and another type of meteorite called chondrites, he said.

Chondrites, leftovers from the formation of asteroids, are the most common meteorites. But the pair doesn't match either category exactly, Kring said.

The rocks are partially melted, which is not characteristic of chondrites.

And they contain high levels of the mineral feldspar, which is not typically associated with the short-lived planets between Mars and Jupiter.

"Thus there are two mysteries," Kring said. "What is the parent planet for the rock, and what type of geologic activity on that planet produced the unusual mineral assemblage?"

There is no shortage of new ideas to account for the disparity.

"My preferred working hypothesis is that [the meteorites] actually represent a piece of crust from an asteroidal body - the surface of another planet, if you like," offered James Day, a geologist at the University of Maryland.

Day has joined the other researchers at this week's meeting to puzzle over the rocks.

Treiman added that it would take a very large asteroid or a dwarf planet to cause the melting seen in the meteorites.

Whether or not its home is discovered, GRA 06128 is a "pretty puzzle, and will help us understand how asteroids form and how they evolve to become planets," he said.