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

Mar 19, 2020
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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?
 
Jan 4, 2020
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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.
 
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Jan 4, 2020
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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[OH,F,Cl]) 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.
 
Mar 19, 2020
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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............



 
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