The Geology of Jezero Crater, Observations & Findings |
The Geology of Jezero Crater, Observations & Findings |
Feb 24 2021, 01:41 AM
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Merciless Robot Group: Admin Posts: 8784 Joined: 8-December 05 From: Los Angeles Member No.: 602 |
This thread is for those rockhounds among us to discuss the new terrain we'll see as Perseverance scoots around her new home. Let's get dirty & technical!
-------------------- A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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Feb 27 2021, 12:00 AM
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Senior Member Group: Members Posts: 1045 Joined: 17-February 09 Member No.: 4605 |
Good post Andreas. The isolated hills have been nominated by some as delta remnants although I haven't been able to find confirmation that their minerology matches the delta. If they are remnants then the erosion between them and the delta must have occurred in the (geologically short) gap between the crater drying and the lava flood. The attached image shows that the lava embayed the delta and I have marked what may be an example of embayed delta material eroding down below the lava level. But the lava would have covered the delta bottom sets and part of the foresets, preventing further erosion of the lighter materials. Jezero is a big crater and aeolian eroded material from the delta would not be particularly evident.
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Feb 27 2021, 12:41 AM
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Member Group: Members Posts: 684 Joined: 24-July 15 Member No.: 7619 |
If they are remnants then the erosion between them and the delta must have occurred in the (geologically short) gap between the crater drying and the lava flood. Question- why couldn't that be later aeolian weathering? Similar to what roto-rootered away hundreds of meters / yards of sediment at Gale crater? Is there a consensus about whether the deposit is ash, (dry then flooded, or deposited in a lake) or extrusive lava? |
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Feb 27 2021, 02:20 AM
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Member Group: Members Posts: 248 Joined: 25-February 21 From: Waltham, Massachussetts, U.S.A. Member No.: 8974 |
Question- why couldn't that be later aeolian weathering? Similar to what roto-rootered away hundreds of meters / yards of sediment at Gale crater? Is there a consensus about whether the deposit is ash, (dry then flooded, or deposited in a lake) or extrusive lava? Well, I admit I did not take a detailed look at the latest geologic map, linked above, before. But it more or less confirms a few of my assumptions. The outliers are indeed mapped as the same unit (NHjf2, Jezero fan unit 2) as the main delta, and it explains that the Jezero floor unit (Njf) is covered near the delta by a smooth layer: "Dark, smooth surface texture near unit NHjf2 is due to a dark mantle deposit or erosional lag derived from erosion of unit NHjf2.". The Jezero floor unit itself is revised from lava flood flow to ash and/or aeolian airfall deposits (not sure if that was by consens), and is older than the delta, but still seen as lacustrine, so possibly deeper water deposits. In addition there should also be clastic, distal flan floor deposits. These may have been mostly muddy and as such more easily blown away once the lake bed was exposed as they are not to be found. The floor unit then embays and sits above the Lower etched unit, also seen as volcanic ash falling into the lake. Thanks, serpens, for pointing out that nice contact. It looks to me like an erosional window into that deeper unit which is also lacustrine. Delta erosion could have occurred by water in the manner of retreating waterfalls which is geologically efficient and does not take much geologic time (cm to m per year if water is regularly flowing). [Still strange to me looking at preserved billions of years old units, possible on a planet without plate tectonics which just sits there.] The geologic map shows hills or outliers about 2km to the _east_ of the rover, as Jezero fan unit 2, farther from the delta as I first thought. This would mean that the delta was perhaps about double the radial length, or 4 times the area (and volume and mass ignoring changes in thickness), at an earlier point than what remains today. Hard to see that all that eroded material could have been transported very far away but it looks like it could have been. perhaps even out of the crater into the regional sink, Isidis Planitia ? The nice lobes of the delta as seen today could have been preserved by preferential erosion preserving orginally larger lobes. Perhaps the outliers are remnants of just the larger lobes reaching out into the basin. -------------------- --
Andreas Plesch, andreasplesch at gmail dot com |
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