The Moon is a predictable friend in our night (and daytime) sky. It has inspired human stories and legends for countless generations, and it’s the only planetary surface upon which human have trod other than our home planet, Earth. And yet… and YET, it still manages to reveal new secrets to us. The Moon made the space headlines twice this week with new science.
First, there is observational evidence of the impact that first created the Moon, lovingly called “The Big Whack.” This is currently the most plausible formation scenario for a moon with a diameter that is one forth the diameter of the Earth. (Typically, moons in our Solar System are much, much smaller than their planetary parents.) It’s too large to have been captured, and it’s not likely to have formed alongside the Earth for various reasons, so the winning hypothesis for some time has been a large impact between the proto-Earth and a Mars-sized body referred to as Theia.
In this scenario, much of the Moon would be a slightly different composition than the Earth, more like the original Theia, or, more accurately, reformed from the smashed remains of Theia and Earth’s crust and mantle. We already know that the Moon’s overall density is lower than the overall density of Earth, and that lines up with the giant impact hypothesis. A new, detailed analysis of Moon samples from the Apollo missions shows the different isotope ratios in some lunar samples that indicate their origin is not from Earth, but from the mysterious (and now nonexistent) Theia. These isotopes, or atoms with different numbers of neutrons, show that the Moon is probably about 50% Theia and 50% Earth material. This also goes to show just how valuable the continued preservation of Apollo moon rock samples are to current science.
The next bit of news covers when this LITERALLY Earth-shattering event occurred. This also looks at isotopes, but this time in Earth rocks, or quartz crystals to be exact. And we’re looking at isotopes of xenon now, a noble gas present in trace amounts in the Earth’s atmosphere. Well, in the giant impact scenario just described, Earth’s atmosphere would certainly not have survived. The quartz crystals that formed 2.7 billion and 3.5 billion years ago have measurable isotope levels of xenon, and when compared with today’s atmosphere, indicate that the atmosphere of the Earth was in place 40 billion years after the formation of the Solar System, not the 100 million years after that date that it was thought to be. So the collision between Earth and Theia that formed the Moon must have happened within that 40 million year time space, 60 million years earlier that previously calculated. It’s not a huge difference in the grant scheme of 4.5 billion years, but it does help firm up our understanding of the timeline of our early Solar System.
But it’s okay, Earth and Moon. You are only as old as you FEEL.
You all should feel free to explore the Moon in detail, and specifically the Apollo 15 landing site, with MoonMappers as you help us continue to unravel the secrets of our planet’s natural satellite.