So now let's move elsewhere.
Let's migrate to check out these kaolinite, these aluminum clay layers.
So we're going to zoom in to these two craters.
And our crater of interest is this guy up here, but
we'll compare it to a nearby fresh one.
And when we do that topographically,
we notice that our crater of interest here is over a kilometer shallower.
And that's because it's been filled with sediments.
But it's not just been filled, it's also been eroded.
And you can see that the crater here with a breach channel out
the northern bit of it where water drained from there.
But erosion has exposed a nice area here that we can check out and
look at at high resolution in CRISM.
So here we're mapping out now iron magnesium minerals with aluminum minerals.
But zooming in still further, you can see here that our aluminum clay is a very,
very thin unit no more than a few meters thick above our iron magnesium
thick sequence of rocky, blocky sediments.
And here's our cartoon stratigraphy here, okay, so what's going on?
Well, a logical explanation for how you get these mineralogies
in this particular setting is that we had, in contrast to deep hydrothermal systems
flowing through the crust, here we have evidence for top-down weathering.
That is, starting with basaltic parent materials,
these are partially altered in some cases to iron magnesium smectite.
Perhaps in some cases underground, perhaps in some cases near the surface.
But then those materials sitting near the surface are subject to enhanced leaching.
Either from greater amount of water throughflow,
exposure to more acidic conditions.
And so they lose the calcium, magnesium, and iron ions out of the system,
leading to this residual Kaolin aluminum clay-rich material.
In contrast, where we have olivine, and we'll get to this serpentine in a moment.
Where we have olivine, the magnesium from the olivine reacts with atmospheric CO2.
And so we get these magnesium carbonates where olivine is at the top of
the stratigraphic section.
So top-down weathering with whatever the original lithology is, mafic or
ultramafic, exerting a control on what the alteration minerals are.
Okay, so we worked our way in the system
only through part of the stratigraphy here.
If we headed southward, we would also see the evidence of sulfates coming in just
beneath the Syrtis Major lava flows.
So you can repeat this exercise all over the planet to construct the columns that
we have here.
And there are certain trends that you can see.
The oldest, deepest layers have iron magnesium clays.
They're partially altered, some of them have no alteration at all.
We just have high and low-calcium pyroxene, olivine, primary minerals.
Toward the top of the section and
this line here connecting them indicates the approximate
location of the Milwaukeean Hesperian geologic boundary in each location.
Toward the top of the section, we have these aluminum phyllosilicates that
indicate this near-surface weathering and leaching.
And then you see that, in most places across the planet,
when we cross into the Hesperian, there are unaltered lavas or ashes on top.
There's very little sign of hydrated minerals.
There are a few sections though where this is not the case.
For example, Meridiani, where the rovers are now,
there are sulfates at the top of the stack.
And you'll hear it from John Grotzinger about what we're discovering at the bottom
of this section now, as the Curiosity rover climbs through Gale Crater.
So just as a difference here,
transitioning from these very early clays into the sulfates.
We'll actually jump over to the other side of the planet here and to Meridiani.
So reminder, Meridiani is this location here, our beacon of hematite.
So the rovers landed, an interplanetary hole-in-one here.
And they extensively traversed this crater,
leaving our tracks behind as we explored this outcrop.
And here, we're able to ground truth what we were able to see from orbit.
We're only able to do this in a few locations where we've sent rovers.
But in this case, so
the hematite signature was actually carried by these small concretions.
But you can see this interesting sedimentary rock here that has these
hematite concretions.
As well as these void spaces where there used to be crystals.
And one can use the Mössbauer instrument combined with the chemical
instrument on the rover to build the mineralogy.
Which is actually about 35% sulfate hematite in these concretions.
And then the remainder is our amorphous silica and
allophane alteration phases, with maybe a little bit of primary feldspar left.
The overall picture, based on the traverses of the rover,
is these sulfates that we're visiting in the late Noachian, early Hesperian
are basically small paleolakes that are in-between dune sediments that form
from ground water upwelling, becoming acidified as it nears the surface.
And then slightly changing groundwater chemistries over and
over again coming through the rock, flushing through.
Leaving and coming back with slightly different chemistries,
lead to the formation of this concretions.
As well as the development and dissolution of minerals that formed previously.
And it's only by landing at these sites that they we're able to discern these
really fine-scale details.
From orbit, our inferences are coarser.
So collectively, though,
we can now tie with this understanding from the ground from Meridiani here.
We can tie this together with similar mineralogy and
sedimentary deposits that we see from orbit.
And then we can start to model geophysically what's going on.
So this shows the correspondence between a groundwater
upwelling model developed by Jeff Andrews-Hanna.
And the fact that it correlates with many of the places where we see iron oxides and
sulfates together across the planet
in the interior layer deposits of Valles Marineris here.