Those baddeleyite grains proved to be keystones for Moser’s subsequent investigations, which found them to be only hundreds of millions of years old. This was consistent with previous studies, but proponents of the idea that shergottites are billions of years old have long argued that the youthful baddeleyite grains were generated by the great heat and stress of an ejecting impact and were thus not representative of the rock’s true age.
Using a sophisticated scanning electron microscope, Moser and his colleagues first mapped the baddeleyite grains within a thin cross section of NWA 5298, then closely studied each one for clues about its past. Many of the grains bore concentric bands of material indicative of gradual growth, which in turn suggested they formed in slow-cooling magma. Subsequent uranium–lead radiogenic isotope dating showed their age to be approximately 187 million years. Even closer examination revealed that the grains’ original crystalline structure had been obliterated by the passage of a sudden, strong shock wave. The only remaining crystalline minerals, in fact, were vanishingly thin rims of silica-rich zircon that had flowed around and flash-frozen onto the baddeleyite grains—a sign of shock melting followed by cooling in the frigid vacuum of space. Uranium and lead isotopes extracted from grains with those rims yielded an estimated age of less than 22 million years. After correcting for sources of lead contamination originating on Earth, such as the burning of leaded gasoline, measurements of lead isotopes from the bulk mineral matrix around the baddeleyite grains yielded a date in excess of four billion years. That bulk mineral matrix also showed a strange preponderance of inert Martian lead, a sign that the region deep within Mars where the rock first formed had stopped mixing with its surroundings a long time ago.
“What this means is that both the young and old age estimates are ‘right,’ but they mean different things,” says Irving, who was not involved in the study. The young baddeleyite formed nearly 200 million years ago when the shergottites’ source rock crystallized on or near the Martian surface. The far older lead, he says, is an artifact from a time about four billion years ago when most of the deep Martian interior had already cooled and ceased convecting.
As for why all that lead was there in the first place, Moser and his colleagues suggest its presence may be related to one or more truly giant impacts more than four billion years ago that struck with such disruptive force they permanently altered the chemistry of the Martian interior. Tentative evidence for such massive impacts can be seen in hemisphere-spanning differences in elevation and crustal thickness on Mars.
Thus, from a single, small meteorite, a rather epic narrative suggests itself: Before four billion years ago Mars was already dying, rapidly losing its interior heat to space, when a planetoid leftover from the solar system’s formation may have slammed into it, melting some of the surface and sweeping away a significant portion of the atmosphere. The great scar from the impact gradually froze over and healed, and Mars settled into senescence. Much later, some 187 years million years ago, as dinosaurs walked the Earth, a slowly upwelling magma plume breached the Martian surface, pouring out young lavas imprinted with the signature of the ancient impact; these slowly crystallized and formed the baddeleyites. At least another 165 million years passed without incident, until a mountain-size asteroid fell out of the sky. The impact launched partially melted fragments of the crystallized lava up through the atmosphere and into space, glossing the baddeleyite grains with rims of molten zircon. One of those fragments drifted and spun alone in the dark for millions of years more, until some tens of millennia ago it crossed paths with a neighboring planet, falling to Earth in a fireball that landed in what is now the western Sahara desert of southern Morocco. It languished there, just another brown weathered rock in the sand, until, by chance, a passing person picked it up in 2008.