The original home of the world's most famous space rock, the Allen Hills Martian meteorite, has now been identified, thanks to data from the orbiting spacecraft Mars Global Surveyor and Mars Odyssey and a better understanding of cratering dynamics.
The rock, called ALH84001, has been the subject of intense study ever since 1996 when scientists from NASA's Johnson Space Center startled the world by reporting that fossilised microbial life might be embedded inside it. The rock, which formed at the very dawn of the solar system 4.5 billion years ago, was blasted from the surface of Mars around 17 million years ago by an impact and made its way to Earth, landing in Antarctica.
While the claim remains highly controversial, the JSC scientists say further study has bolstered the evidence for fossilised life in ALH84001. So the discovery of the rock's place of origin on Mars could make that spot a strong candidate for a future landing by robots or people searching for extraterrestrial life.
The analysis, based on the rock's mineral characteristics, was presented by Vicky Hamilton of the University of Hawaii at this week's meeting of the Meteoritical Society in Tennessee, US.
Perfect match
Hamilton looked for matches between the laboratory spectrum of the meteorite, a mix of orthopyroxene and basaltic minerals, and data from the Thermal Emission Spectrometer and other instruments on the orbiters. "There was only this one place, in all the places we can look that aren't too dusty, that had a composition that was consistent with the ALH84001," Hamilton told New Scientist.
The site is in the Eos Chasma, a branch of the enormous Valles Marineris canyon system. Hamilton has examined it further using topographic and thermal data and high-resolution imagery. "Putting this all together, it's all consistent with this being the source region," she said.
The pinpointed area is a “lobate flow”, the kind that occurs when an impacting object strikes a fluid-rich soil, as is the case with many Martian craters. There is a crater about 20 kilometres in diameter there, which cratering studies have now shown indicates a large enough impact to eject the rock out of Mars' gravitational field.
There is no single "smoking gun" piece of evidence that proves this was the place the meteorite came from, Hamilton concedes: "There isn't any piece of information that makes this a unique interpretation, other than the fact that we haven't seen any other suitable places."
Fascinating picture
Everett Gibson, one of the JSC scientists behind the original findings of possible signs of life in ALH84001 says the picture put together by Hamilton is "fascinating”. And like his work, there was “no single line of evidence that says, ‘ah, this is it’," he notes.
The meteorite is believed to have first formed deep beneath the Martian surface, and was later transported to the shallow depth from which it was propelled into space.
The Eos Chasma location fits that. The 4-kilometre-high cliffs bordering the canyon have exposed rocks from various ages in the Martian past, and impacts or erosion could have brought the rock to the canyon floor - along with rocks from a major portion of Mars' history.
That makes it a prime site for a landing mission someday, Hamilton said. It would be an opportunity to sample rocks from a wide variety of ages, all in one place. And, if it really is the launchpad from which ALH84001 began its interplanetary travel, it may also be the place where the mystery of whether the rock's unusual contents really are signs of ancient life is finally unravelled.