Researchers claim Mojave Crater on Mars is source of Mars rocks found on Earth

Mar 07, 2014 by Bob Yirka report
Mojave crater showing its well-preserved morphology and ray pattern. The direction of the red arrow indicates distant examples of secondary crater clusters shown in Figure 1B in the manuscript. Credit: Science/AAAS

(Phys.org) —A trio of researchers, two from France and one from Norway, has published a paper in the journal Science where they claim to have found sufficient evidence to identify a specific crater on Mars as the origin of Mars rocks found on the Earth's surface. In their paper Stephanie Werner, Anouck Ody and François Poulet describe their extensive research and how they came to their conclusions.

Scientists have known for many years that some of the meteorites that strike the Earth came from Mars. Such rocks are ejected from the surface of Mars when struck by meteorites themselves. Mars rocks have been classified into three main categories depending on their chemical makeup: chassignites, nakhlites and shergottites. The latter make up roughly three quarters of all such rocks found and have been studied extensively over the years. In this new effort, the researchers contend that all shergottites come from a single source on Mars: Mojave Crater.

To establish Mojave Crater as the source, the researchers combined several types of data, each of which suggest the crater as a likely suspect, though there is one, the estimated age of the rocks, that is not quite as clear-cut as the others.

MRO-CTX image mosaic of Mojave’s crater interior. Credit: Science/AAAS

The researchers began by analyzing imagery of Mars surface taken by various spacecraft over the years—they were looking for reasonably recent formation, a large size and rays leading away from the crater indicating a blast capable of sending rocky debris into space. Mojave Crater stood out as one of the best candidates for further study. The team next examined data gathered by spacecraft that have orbited the Red Planet over the years, specifically those that had taken mineral scans (measures of wavelengths of light bounced back off of them) of the Mojave Crater and the area around it. Analysis showed that the mineral composition of rocky material on the lip of the crater matched closely with the Mars rocks found on Earth. The team also sought to determine the age of the crater by studying the surface area around it—the number of craters around it and their size allows for a model to be made based on techniques developed with the Apollo moon project. Their study showed that the was likely formed approximately three million years ago. Finally, prior research has shown that the Martian rocks likely were in transit on average less than five million years, based on cosmic ray exposure.

THEMIS daytime image mosaic overlain by MOLA color-coded topography. Craters were counted for the plateau units (brown), channel units (blue-grey), and the continuous ejecta unit of Mojave crater (red line). Credit: Science/AAAS

Prior research has shown that the plateau in which Mojave Crater exists is approximately 4.3 billion years, which suggests that rocks blasted from its surface should be near the same age. Unfortunately, that's where things don't match as well. Most studies of Mars rocks have found them to be only 150 to 600 million years old. The researchers suggest this anomaly can be explained by events (shock waves, etc.) that transpired on Mars that set their age clock back.

Explore further: NASA Mars orbiter examines dramatic new crater

More information: The Source Crater of Martian Shergottite Meteorites, Science DOI: 10.1126/science.1247282

ABSTRACT
Absolute ages for planetary surfaces are often inferred by crater densities and only indirectly constrained by the ages of meteorites. We show that the <5 million-year-old and 55-km-wide Mojave Crater is the ejection source for the meteorites classified as shergottites. Shergottites and this crater are linked by their coinciding meteorite ejection ages and the crater formation age, and mineralogical constraints. Because Mojave formed on 4.3 billion year old terrain, the original crystallization ages of shergottites are old, as inferred by Pb-Pb isotope ratios, and the much-quoted <600 million years shergottite ages are due to resetting. Thus, the cratering-based age determination method for Mars is now calibrated in situ, and shifts the absolute age of the oldest terrains of Mars backward by 200 million years.

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Caliban
5 / 5 (1) Mar 07, 2014
Prior research has shown that the plateau in which Mojave Crater exists is approximately 4.3 billion years, which suggests that rocks blasted from its surface should be near the same age. Unfortunately, that's where things don't match as well. Most studies of Mars rocks have found them to be only 150 to 600 million years old. The researchers suggest this anomaly can be explained by events (shock waves, etc.) that transpired on Mars that set their age clock back


All well and good, but will only hold up if the meteorites themselves show metamorphism sufficient to have caused the radiologic reset --but no mention of whether or not this is the case.
24volts
not rated yet Mar 08, 2014
Is there any reason why the rocks we have here couldn't have also possibly come from the meteor itself that hit to make Mars to make the crater? With bits of both that got mixed up in the blast when it hit?

Captain Stumpy
not rated yet Mar 08, 2014
but no mention of whether or not this is the case

@Caliban
I found some stuff in the article on AAAS: let me know if this helps
To reconcile the apparently young (and often discordant) crystallization ages of shergottites with the very old age of Xanthe Terra, we propose that the original crystallization ages could have been underestimated for shergottites due to processes such as impact melting or shock-induced resetting (26), as well as aqueous alteration processes (27). The lobate-shaped ejecta blanket, the pitted crater floor, and the fluvial landforms in the crater interior are evidence for substantial amounts of subsurface water (ice) present at the impact site before and heated during the cratering event.

continued in next post...
Captain Stumpy
not rated yet Mar 08, 2014
@Caliban
continued from last post
The magnetic carrier pyrrhotite (iron sulphide), is present in many shergottites, basaltic shergottites being magnetic enough to account for the observed crustal remanent magnetisation on Mars (28). This carrier is a host for rare earth elements used for dating and prone to thermal and shock-released heat alteration, and can explain the moderate magnetization of the crust surrounding Mojave impact site.


does that help?
there is more, plus a PDF with supplemental material

I dont know if you can access it or not, though...
Caliban
5 / 5 (1) Mar 08, 2014
Thanks, Cap'n --can you provide a link?

Since this is such an obvious factor to be accounted for, I'm surprised they didn't address it more explicitly.
Captain Stumpy
not rated yet Mar 08, 2014
Thanks, Cap'n --can you provide a link?

@Caliban
Sure:
Abstract is here:
http://www.scienc....1247282

the full text is here:
http://www.scienc....1247282.full

supplementary material is here:
http://www.scienc...7282.DC1

I dont know if it is pay-walled at ASSS/Science Mag
it might be
Caliban
5 / 5 (1) Mar 08, 2014
Thanks, Cap'n --can you provide a link?

@Caliban
Sure:
Abstract is here:
http://www.scienc....1247282

I dont know if it is pay-walled at ASSS/Science Mag
it might be


Thanks.
Sadly, article is paywalled, and notes were no help.
I'll have to search elsewhere, it appears...
Captain Stumpy
5 / 5 (1) Mar 08, 2014
Thanks.
Sadly, article is paywalled, and notes were no help.
I'll have to search elsewhere, it appears...

@Caliban
if you ever end up on Universe Today (in their forums) look up my profile and send your e-mail