Episode 176 A notably red slab of rock with some even more notable features has been the target of intense investigation for the past two weeks. Now Perseverance has dug into it with its abrading tool and opened up a deeper level of intrigue.
The rusted rock implies oxygen?
I’m not a geologist, but from what I understand the red colour is a good indicator of iron and oxygen at the time the sediments were laid down.
I’m already committing the cardinal sin of discussing redox states on social media, Paul, so forgive me for adding this note:
With all the groundwater that seemingly flowed within the rocks of this region, oxygen needn’t have been present at the time of deposition. Alteration/diagenesis seems to be pretty damned important here. (Further aside - non-geologists are always shocked to learn that oxygen is part of so many minerals and rocks to begin with. Maybe it’s easier to talk about free oxygen, the kind that isn’t already attached to the iron or magnesium of whatever…)
Love it when knowledgeable folk join the chat :) It takes these posts to a whole new level :)
Very late reply, Paul, hope you don’t mind:
I’d love to contribute here a lot more - I’ve been planning to do so for a while now - but I tend to write very long-winded posts (see above, again) which maybe doesn’t work on social media, and I’m also not one-tenth the geologist Steve Ruff is. If you think my somewhat inexpert posts are OK, though, I’m happy to oblige when I actually manage to find the time.
I personally don’t care about long winded posts, I think you’ll find those that are seeking a deeper understanding will not care either. There will always be those who will want a TLDR post, but you can leave that to others to condense :)
Look forward to your posts (whenever you have time) :)
Not necessarily. Here comes another episode of Wide World of Iron Minerals…
The mineral that Prof. Ruff refers to - hematite - contains ferric iron, as opposed to the other kind, ferrous iron. The difference between the two is simple - ferric iron is missing 3 electrons, whereas ferrous is only missing 2. Some process has to strip the ferrous iron of that extra electron - it requires noticeably more energy to make ferric than ferrous. Mars has plenty of the ferrous kind, like you find in the rocks on the Jezero crater floor; it’s what you’d generally expect to find in the planet’s hard rock. So you want to pay attention when you get the ferric kind - especially when you find it in the “soft rock”, like Percy is exploring now. One way of making ferric is exposing it to free atmospheric oxygen and moisture, as on modern Earth, producing various “oxidized” minerals, which some casually call “rust”. But there are other ways for oxygen to do the job, as well - say, when it’s dissolved in groundwater. And this Neretva Vallis site evidently had plenty of groundwater. The oxygen content of that groundwater, however, is kind of a big question.
Thing of it is, hematite can also be produced without water and oxygen, purely by volcanic action, too. So hematite has a lot to say either way, it’s one of those minerals to watch.
The phenomenon of iron minerals on Mars has been a big deal, and will continue to be. Opportunity’s landing site was chosen because the variety of hematite that satellites detected there was unusual, and that led to the discovery of sandstone laid down by massive amounts of water - the first sedimentary rock ever discovered off Earth. Without that discovery, I’m not sure that Percy gets sent to Mars. And I haven’t even started to talk about other sources of ferric iron, like you find in the dust, or all the weird stuff that happens when sulfur and iron get together and have a baby…
EDITED to talk about hard and soft rocks. Don’t giggle, we’re geologists.
Thank you for the excellent write up, I have so many questions!
Wouldn’t we expect all the ground water to have no dissolved oxygen? Because there is no oxygen atmosphere to replenish it… I’m shaky on how dissolved oxygen can exist in water, but I thought it was due to churn with gaseous oxygen in the atmosphere mixing with waves or rivers or any turbulent water.
Groundwater on Mars wouldn’t have access to that, or at least for a very very long time
Wouldn’t we expect all the ground water to have no dissolved oxygen?
Very late reply - but your question is totally fair, so I hope you don’t mind:
On the face of it, you’d expect Martian groundwater to be pretty damned poor in dissolved oxygen, yes, and groundwater on Earth does get its oxygen almost entirely from the atmosphere, as you mentioned. (This would be easier on Earth than Mars due to the greater atmospheric pressure, among other things.) However:
If you’ve heard anything about recent discoveries of “dark oxygen” being generated on Earth’s deep seafloor, you might agree with me that nature often finds a way to create chemical niches where interesting stuff happens. In the just-discovered terrestrial case, metals on the seafloor are essentially acting as batteries, zapping water and splitting the oxygen off from the hydrogen. Obviously I can’t expect that this process was occurring at the Jezero Delta, but I’m cautious about saying that the groundwater there never had any dissolved oxygen, especially when we know that hot water can break down minerals and release the oxygen within.
So again, the question is a good one, but it’s already been partially answered by Curiosity, which found the following on the floor of Gale Crater:
Trace amounts of the element manganese typically exist in basalt. To get a rock with as much manganese as Caribou has, the manganese needs to be concentrated somehow. The rock has to be dissolved in liquid water that also has oxygen dissolved in it.
If conditions are right, the manganese liberated from the rock can then precipitate as manganese oxide minerals. On Earth, dissolved oxygen in groundwater comes from our atmosphere. We’ve known for some time now that Mars once had vast oceans, lakes and streams. If we could peer onto Mars millions of years ago, we’d see a very wet world. Yet we didn’t think Mars ever had enough oxygen to concentrate manganese—and that’s why we thought the data from Caribou must have been an error.
In the Earth’s geological record, the appearance of high concentrations of manganese marks a major shift in our atmosphere’s composition, from relatively low oxygen abundances to the oxygen-rich atmosphere we see today. The presence of the same types of materials on Mars suggests that something similar happened there. If that’s the case, what formed that oxygen-rich environment?
Good article to read if you have the time…