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On Mars, deep-water diversity has stood the test of time, meteorites show

 Mars as seen from orbit by NASA’s Viking mission.
Mars as seen from orbit by NASA’s Viking mission.
(Image: © NASA/JPL)

The water buried deep within Mars likely came from at least two very different sources long ago, a new study suggests.

"These two different sources of water in Mars' interior might be telling us something about the kinds of objects that were available to coalesce into the inner, rocky planets," Jessica Barnes, an assistant professor of planetary sciences in the University of Arizona Lunar and Planetary Laboratory, said in a statement.

"This context is also important for understanding the past habitability and astrobiology of Mars," Barnes added.

Related: What is Mars made of?

The famed Mars meteorite ALH84001.  (Image credit: NASA)

Barnes and her colleagues analyzed two Mars meteorites: Northwest Africa (NWA) 7034, also known as Black Beauty, and Allan Hills 84001 (ALH84001), probably the most famous Red Planet rock of all time. In the mid-1990s, a team of researchers announced that they'd found compelling evidence of Martian life in ALH84001. Most other scientists were not convinced, and the claim remains controversial, and debated, to this day. 

Barnes and her team determined the hydrogen-isotope compositions of Black Beauty and ALH84001, which interacted with water in the Martian crust about 1.5 billion years ago and 3.9 billion years ago, respectively. 

Isotopes are versions of an element that have different numbers of neutrons in their atomic nuclei. For example, the hydrogen in "normal" water has no neutrons in its nucleus, whereas the hydrogen in deuterium, or "heavy water," has one.

Studies of Mars meteorites over the years have found a wide variety of hydrogen-isotope ratios. But, in  the new study, which was published online Monday (March 30) in the journal Nature Geoscience, Barnes and her team found that Black Beauty and ALH84001 have very similar amounts of normal versus heavy hydrogen.

That ratio is roughly the same as that observed for much younger rocks, suggesting that not much has changed, hydrogen-isotope wise, for the past 4r billion years or so on Mars. In addition, the isotope ratio is about halfway between that seen in crustal rocks here on Earth and the ratio observed in the Martian atmosphere, which has a lot more heavy hydrogen.

The Martian atmosphere is so "fractionated" because charged particles from the sun have preferentially blown the lighter normal hydrogen into space, scientists say.

"We thought, OK, this is interesting, but also kind of weird," Barnes said of the new results. "How do we explain this dichotomy where the Martian atmosphere is being fractionated, but the crust is basically staying the same over geological time?"

Researchers have long thought that Mars' mantle — the thick rock layer beneath the thin crust — sports a similar hydrogen-isotope ratio to that of Earth. So, the variability in isotope ratios seen in Mars meteorites was chalked up to terrestrial contamination or alteration by the Red Planet's atmosphere as the rocks, blasted free by powerful impacts, barreled outward toward space, Barnes said. (Crustal rocks should be similar to mantle rocks, after all; the crust is made of material from the interior that made it to the surface, cooled and solidified.)

But the presumed mantle similarities between Mars and Earth are inferred from one study of a meteorite thought to originate in the Red Planet's mantle, the researchers said. And it may be time for a rethink.

"Martian meteorites basically plot all over the place, and so trying to figure out what these samples are actually telling us about water in the mantle of Mars has historically been a challenge," Barnes said. "The fact that our data for the crust was so different prompted us to go back through the scientific literature and scrutinize the data." 

This analysis revealed that two different types of Martian volcanic rocks, enriched shergottites and depleted shergottites, have different hydrogen-isotope ratios. These rocks probably represent different source material, the researchers said — different reservoirs of water in the Martian interior.

"Consequently, these features may have been inherited from the primary building blocks that constructed Mars, implying that the Martian mantle has almost always been heterogeneous because it was poorly mixed during accretion, differentiation and its subsequent thermochemical evolution," the researchers wrote in the new study.

If this is true, then, unlike Earth and the moon, Mars probably did not feature a global ocean of liquid rock shortly after its birth. A global magma ocean would have mixed everything together, erasing the distinct isotope signatures, the researchers said.

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook

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  • dfjchem721
    I do not recall hydrogen forming any stable minerals, which it would have to do in order to survive for billions of years on a given rock. If the source is not mineral, than it might be from mineral hydrates, i.e. minerals which are capable of forming loose bonds with water. These or any other source of these isotopes would have to come from deep inside the rocks, and trapped so it could not escape. Water and hydrates would outgas very quickly unless they were locked inside.

    Anybody know where this hydrogen and deuterium is coming from in these rocks?
    Reply
  • Torbjorn Larsson
    Admin said:
    The water buried deep within Mars likely came from at least two very different sources long ago, a new study suggests.

    On Mars, deep-water diversity has stood the test of time, meteorites show : Read more

    That is slightly different than the inner planets, where Venus us believed to have had or still has a basal magma ocean https://www.essoar.org/doi/10.1002/essoar.10501095.2 ] and Mercury had one 4 billion years ago. (Or at least it was believed that Mercury had one, in 2013 https://www.space.com/19911-mercury-volcanic-magma-messenger.html ] and still in 2016 https://www.newscientist.com/article/2079634-mercury-once-had-a-graphite-crust-floating-on-a-sea-of-magma/ ].)

    Else it plays nicely with the new suggested assembly and timing of Earth and Mars, where Earth accreted within 5 Myrs and Mars had a prolonged crust formation between 10 and 20 Myrs, since each of the two sources could be related to that. The depleted shergottites sample a mantle reservoir that has roughly the same D/H as Earth average of inner and outer system mix, while the enriched shergottites sample a mantle reservoir with an outer system D/H.
    Reply
  • Torbjorn Larsson
    dfjchem721 said:
    I do not recall hydrogen forming any stable minerals, which it would have to do in order to survive for billions of years on a given rock. If the source is not mineral, than it might be from mineral hydrates, i.e. minerals which are capable of forming loose bonds with water. These or any other source of these isotopes would have to come from deep inside the rocks, and trapped so it could not escape. Water and hydrates would outgas very quickly unless they were locked inside.

    Anybody know where this hydrogen and deuterium is coming from in these rocks?

    A relevant concern. When Valley et al looks at oxygen isotope ratios to observe a 4 billion year old cold, global ocean on Earth they use zircons that are stable, even for a while if they are subducted and later form new crust.

    According to the paper the two initial meteorites that started their investigation shared apatite in common, so they used that, with several checks when they could.

    "The mineral apatite (Ca5(PO4)3) is the only hydrous mineral common to both samples; hence, it was used to assess the H2O con-tent and H-isotopic composition of the Martian crustal samples by nanoscale secondary ion mass spectrometry (Methods). Apatite grains within these samples display a similar spread in D/H ratios (between ~3.12 × 10−4 and 4.67 × 10−4) over a wide range of water contents (Fig. 1). The data for both samples are in agreement with H-isotopic data on other lithic clasts in NWA7034 and its pair NWA753315,16 and intercumulus apatite in ALH8400117. Our results are also consistent with D/H values reported for the Martian crust within the time span of 0.7 to 472 million years ago (Ma) (D/H ratio ~3.12 × 10−4 to 5.73 × 10−4)18 and analyses of Hesperian (~3 Ga) clays by the Sample Analysis at Mars (SAM) instrument on board the Mars Science Laboratory rover (D/H ratio of (4.67 ± 0.31) × 10−4)8. Combined, these results indicate that the Martian crust is characterized by D/H ratios that are depleted in D relative to the cur-rent Martian atmosphere (Fig. 2) over a time span of at least the past 3.9 Gyr with a bulk crustal D/H ratio ranging from 2.68 × 10−4to 5.73 × 10−4."

    I did not look further, but the remaining material is claimed to be described in the Supplementary Information, which is not paywalled.

    Apatites can be fairly stable (teeth, nuclear encapsulation) and the Moon rocks suggest they keep to some of their water for billions of years despite vacuum and impact gardening https://en.wikipedia.org/wiki/Apatite ]. But I guess the best evidence is in all those cross checks.
    Reply
  • dfjchem721
    Thanks for the post on apatite, T.

    The Wiki article on ALH84001 notes that "It contains polycyclic aromatic hydrocarbons (PAHs) concentrated in the regions containing the carbonate globules, and these have been shown to be indigenous."

    Moreover, "In October 2011 it was reported that isotopic analysis indicated that the carbonates in ALH84001 were precipitated at a temperature of 18 °C (64 °F) with water and carbon dioxide from the Martian atmosphere. The carbonate carbon and oxygen isotope ratios imply deposition of the carbonates from a gradually evaporating subsurface water body, probably a shallow aquifer meters or tens of meters below the surface."

    Since the concentration of PAHs increase with increasing penetration into the rock, it is unlikely a contaminant from earth. One wonders what the H/D ratios in the PAHs might be, and how it compares to the apatite. Since they only looked at this in the apatite, it would be interesting to have these numbers. PAHs are found throughout the universe, and their presence on Mars would not come as a surprise. However, where these hydrocarbons formed would play a significant role in evaluating their relationship to H/D ratios in Martian rocks.

    If the PAHs on Mars were formed elsewhere (quite possible), some of them could have degraded on an early Mars, and mixed with the non-organic H/D ratios originally present. Clearly not a simple study.

    Both the carbonates and apatites likely formed from aqueous intrusions. Since Wiki further notes that ALH84001 crystallized about 4 bya, it seems reasonable that an inherent porosity of the rock must have allowed such penetration, as the rock lay within a subsurface water source long after its formation. This would eliminate any requirement for the existence of PAHs or the minerals in the original melt. Now one wonders when this mineral/PAH intrusion occurred. Not sure if I missed such a proposition in all the stuff I have been reading............

    https://en.wikipedia.org/wiki/Allan_Hills_84001
    https://mars.nasa.gov/MPF/martianchronicle/martianchron7/signs.html
    https://en.wikipedia.org/wiki/Polycyclic_aromatic_hydrocarbon
    Reply