Viewing a single comment thread. View all comments

Silk_Hope_Woodcraft t1_it0x2kc wrote

Reply to comment by Pluto_and_Charon in Curiosity Mars Rover Reaches Long-Awaited Salty Region by Pluto_and_Charon

Aren't layers sedimentary and not related to timeline?

5

Pluto_and_Charon OP t1_it10bd9 wrote

They are sedimentary, yes. In a lake bed setting, imagine clay particles settling out of suspension and forming a thin layer of mud ontop of the original rock. Now imagine that the lake persists for a million years, that deposit of mud on the lake floor has now grown hundreds of metres thick.

Billions of years later, the rover visits the site. The lake water is long dried up and that mud has turned into stone, which has been heavily eroded. The rover landed at the lowest point in the crater, where you can see where that very first layer of mud sitting ontop of the original rock. Therefore, we first encountered mud formed at the very beginning of the lake's history. In the years since, the rover has driven steadily uphill, climbing through sequentially higher mud layers and so 'further in time'. So, as the rover ascends upwards, we see progressively younger layers. We are now at the time when the lake dried up.

14

SenorTron t1_it2axsy wrote

What processes have eroded the deposited layers away since then, and why is it more eroded in the center of the crater?

2

Pluto_and_Charon OP t1_it2tte0 wrote

Good question! We believe at one point the sand dunes we're starting to see actually eventually completely buried the crater - which is several kilometres deep and 150km wide so quite a huge volume of sand! However, almost all of that sand has now been completely scoured away by wind over the past 3 billion years, exposing the ancient lake sediments underneath for us to study. The exact center of the crater is actually the least eroded, that's why the central peak mountain, Aeolis Mons (aeolis is latin for wind!) is so much taller than the crater floor, it's 5.5km high. It's this mountain that the rover is climbing up. In fact it's so tall its a bit of a puzzle, we're not sure why the sand formations didn't erode here.

4

Foosh718 t1_it1cmay wrote

Sedimentary layers are laid down sequentially, and so document the passage of time. In archeology, this is what makes stratigraphy useful as dating evidence.

6

Silk_Hope_Woodcraft t1_it4nq84 wrote

So, if there ever was water/liquid present, wouldn't the time passage of sequential laying of layers happen quickly? (Like ice rings don't count years, but changes in temperature) Aren't we assuming a lot to say these layers represent passage of time?

1

Foosh718 t1_it4usx3 wrote

As you say, under some conditions layers are laid down more quickly, and of different materials. Over time different materials also compress to different degrees. All of these conditions leave clues geologists can identify and interpret: "time passage of sequential" layers is still time passage, and additional information about context lets them interpret the scale of that time. (Similarly, tree rings grow at different rates under different conditions, but can be interpreted reliably.)

2