Ok, excuse me for asking but HTF do they know it is from Mars?
As no Human has (to our knowledge and with Dr Who's assistants excluded) set foot on the Red Planet how can they really 100000000% know that it came from Mars?
Boffins have dated a piece of Martian meteorite they reckon is the oldest bit of the planet ever collected. Martian meteorite NWA7533 Radioactive analysis of the zircons in NWA7533, a meteorite found in Northwest Africa, have revealed that the rock is around 4.4 billion years old - so it was formed just 100 million years …
But its a good question: The only thing we can compare those rocks to, is current data from our mars rovers, or recent meteorites with trajectories confirmed to be originating from mars. Also of course atmospheric measurements by either orbiters, rovers or spectral analysis from earth or earth orbit, but they can't confirm what mars' composition was 4.x billion years ago, afaik.
It's down to overall ratios of trace elements compared to known (current) ratios on the various planets.
It's never 100% conclusive because some other solar body could just happen to have a make up, or region, that is similar enough to Mars that a bit of it knocked off that happens to land on Earth could match.
The formation and type of rock can help determine if it came from a planet or not (formation under gravity is very different to under low or none) and trapped atmospheric gasses are also indicative of where a rock initially formed. Where there are trapped atmospheric gasses the list of sources is quite short (few planets / planetoids have ever had or have an atmosphere) and they are quite different from each other.
How can you be 100000000% certain the rocks in your backyard came from Earth? How can you be 1000000000% certain the rocks in your backyard are really there? :)
They determined the rock was from Mars by comparing its composition to the data we have about Martian rocks. The rock obviously isn't from Earth and its composition matches that of Mars so well that any other possibility becomes enormously more unlikely with every hypothesis.
You start getting into weird theoretical situations like comets colliding and the resultant debris from both are amalgamated under the enormous force of the impact and part of it lands on Earth and just happens to be the same composition as Martian rocks. Like I said, weird and extraordinarily unlikely.
Humans have set plenty of little scurrying robot feet on Mars, and plenty of orbiters. Between the two, we're getting a pretty good map of Martian minerals, isotope ratios, and general geology. "Black Beauty" has a chemical and isotopic composition quite similar to the southern highlands of Mars and nothing quite like Earth.
The end result is some room for doubt, not 100000000% certain, but it is certain that rock ain't from around here.
Often it's done by comparing the composition of gas bubbles within the rock to measurements of the Martian atmosphere obtained from Soviet and American Mars landers. If that can't be done then it's a process of elimination. Isotope ratios will not match those of the Earth and Moon. The mineralogy is often quite evolved and can incorporate hydrous minerals, and will not match that of the Moon or regular stony meteorites. They have a *relatively* young crystallisation age determined by radio dating will not match that of stony meteorites, and the effect of cosmic ray bombardment usually shows they have been in space for only a few million years.
Good point. We do not know where it came from. They do not know. In any case the Martian surface they now explore is a highly altered "new surface" a few million to 1 or 2 billion years old, not a 4.5 billion year old pristine surface of an asteroid. We cannot compare the NOW surface chemistry of Mars with the chemistry of a meteorite fro who knows where.
Chris Landau (geologist) November 22, 2013
Yep, bits of Earth could be on other planets and moons (especially Luna). It's a bit more challenging to get bits of Earth elsewhere, owing to Earth's higher escape velocity, but I'm sure it's happened.
I mean, in the extreme, current theories of Luna's formation say it's a big gobbet of Earth-Theia crust and mantle. So the moon is kind of a big bit of Earth.
Bits of planets on other planets would tend to be found further in towards the Sun than the planet they came from. Although not impossible, the amount of Earth material on Mars would be minuscule compared to the reciprocal case. One could assume Mars material on Venus and Mercury, a lot less* Earth material on Venus and Mercury (see comments above about escape velocity), etc.
*notwithstanding the theorised cataclysm that created the Moon
"In addition, incompatible element abundances in clast-laden impact melt rocks and interclast matrix provide a geochemical estimate of the average thickness of the Martian crust (50 kilometres) comparable to that estimated geophysically" is what the abstract says. Fascinating though that sounds, I am not about to spend 22 on the whole article.
There are a couple of things that worry me about all the meteorites coming out of North West Africa. I know it is a desert, but there are other deserts for preservation purposes. I worry about scams going on. Secondly, It is extremely rare to find a rock on Earth older than about 4.0 billion years old. Just as Earth's surface has been reworked, so has the surface of Mars. What we look at today is younger rocks. It is laughable to think that you can tell the exact chemistry, mineralogy or age from a vast suite of rocks by scanning from satellite. The scan of Vesta in 2012 is a case in point. (HED meteorites from Vesta very vague). I had enough trouble telling one mineral from another under a petrographic microscope and to sweep large sections of the planet Mars as the same smacks of huge arrogance or stupidity or both. Mineralogy varies from one meter to another and age dates from one millimeter to another depending on the crystals and degree of melting.
We do not know where this rock came from. They do not know. In any case the Martian surface they now explore is a highly altered "new surface" a few million to 1 or 2 billion years old, not a 4.5 billion year old pristine surface of an asteroid. We cannot compare the NOW surface chemistry of Mars with the chemistry of a meteorite fro who knows where.
It could have come from our Earth, our moon, our other planets or their moons, our asteroid belt or any one of the unseen Kuiper belt bodies.
We also do no have bubbles of gas in vesicles in rock to date from Mars as we have no definitive samples from Mars or any where else.
Chris Landau (geologist) November 22, 2013
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