The meteorite NWA 7034 is among the world's most scientifically precious rocks, having been knocked off Mars. The discovery of a single shocked crystal within NWA 7034 has revealed Mars was still suffering heavy asteroid bombardment later than previously proposed, possibly narrowing the window in which life might have survived there.
Meteorites from Mars are rare compared to those that originated in the asteroid belt, since only a large impact could blast them from the Red Planet's surface into space in the first place. That makes them precious, and NWA 7034's size at 320 grams (11 ounces) and color led to it being nicknamed Black Beauty. Perseverance may be collecting Mars samples as we speak, but we're not scheduled to receive them until 2031, so rare Mars meteorites are all we have for now.
NWA 7034 is composed of material from diverse parts of Mars that became deposited in one spot and pressed together, providing an opportunity for planetary scientists to develop an unusually broad picture of the planet's geology. A new paper in Science Advances describes studying a variety of Black Beauty's crystals known as zircons that date to Mars's beginnings and finding one among them that displayed evidence of being in a particularly powerful impact.
Black Beauty contains many zircons formed between 4.48 and 4.43 billion years ago, not long after Mars condensed out of the protoplanetary disk. A 2019 study sampled a portion of these and concluded none showed signs of having suffered a major impact post-formation. This was taken to suggest the large bodies that were slamming into the Moon until around 3.9 billion years ago somehow had no counterpart on Mars.
Curtin University's Dr Aaron Cavosie told IFLScience he thought the zircons in Black Beauty were “So heterogeneous we needed a larger sample. One of these zircons could have a history nothing like its neighbor.” After almost doubling the zircons studied, Cavosie and lead author PhD student Morgan Cox found one grain that showed a sign of shock damage known as “twinning” seen on Earth in crystals that survived the largest impacts – those greater even than the one that ended the dinosaurs' reign.
“This grain is truly a one-off gift from the Red Planet,” Cox said in a statement. The crystal in question was dated using uranium/lead ratios to 4.45 billion years old, proving the impact must have occurred after that, but Cavosie told IFLScience there is no way to know how long thereafter.
It is thought impacts large enough to induce twinning might have destroyed habitable conditions – if life was present on Mars beforehand it might have needed to start again from scratch thereafter. A one-off impact of such size might not matter, but if it is indicative of an ongoing bombardment similar to the Moon's, the prospects for life at the time would have been grim.
This, Cavosie said, “Highlights the possibility that the habitability window may have occurred later than previously thought, perhaps coinciding with evidence for liquid water on Mars by 3.9 to 3.7 billion years ago.”
Cavosie acknowledged to IFLScience that considerable uncertainty remains on the topic. In the light of evidence the Chicxulub impact created a hydrothermal province beneath the crater that may have been better suited to life than the surface, such impacts could, he said, have been “A two-edged sword.”
Nevertheless, this single grain will force a rethink of the prospects of ancient life on Mars, and the possible reasons for its absence if we never find any.
Naturally, a larger sample would expand our understanding. Unfortunately, Cavosie told IFLScience that may have to wait, as the equipment required to detect such shocks is currently far too large to be put aboard Martian rovers, leaving us dependent on rare meteorites.