[asa] Thinking about the age of Mars craters

Hello,
I have always been interested in the various missions to Mars. The current rovers, Opportunity and Spirit have been especially interesting to me. Part of my morning routine is to go to the JPL site where they post new images daily of these two rovers treks across the surface of Mars. (you can find those updates here: http://marsrovers.jpl.nasa.gov/gallery/all/) I have looked at 10s of thousands of images the past two years. In particular I have been following the Opportunity rover and I’ve been trying to work out the geological history of what I see. I find the landscape to be utterly fascinating. I also find it interesting that I have been able to find no discussion of the images by YEC types. I think the geology of Mars presents some unique challenges to the YEC worldview. I have used Mars geology and the question of life on Mars as my introductory lectures in a non majors biology class for several years. In that class we look at Mars meteorites on earth as a way to talk bout chemistry (showing the gases in the rocks are similar to gases found on Mars) and various aspects of the scientific process.

I am planning to use some aspects of Mars geology in a course I am teaching this fall to a group of honors students (mostly non-science majors) that will examine the scientific process and will ask questions about the age of the earth. Our first discussions will be about the history of Mars. I find it helpful to use an example not from the earth so that the Flood answer become more difficult. There are many wonderful examples of pictures from Opportunity that can be used to stimulate students to think about length of time and order of events. The layers of sediment (water or wind blown sedimentary processes) in the crater walls, sand dunes, examples of craters within craters, obvious youthful craters verses very ancient craters, iron meteorites sitting on the surface (http://marsrovers.jpl.nasa.gov/gallery/press/opportunity/20050119a/Sol339B_P2581_L456-B352R1_br.jpg) and subsurface cracks that have altered the sand dunes above are just a few things I can entice the students to discuss. Looking at the traverse map of Opportunity one can see that it has examined a number of craters that are clearly of different ages.
http://marsrovers.jpl.nasa.gov/mission/tm-opportunity/index.html (traverse map)

One thing that has continued to perplex me is the scenes around the edges of the craters. I am wondering if I don’t understand something about geological processes because I always come back to the same question: Where is the ejecta debris around the craters?

Here are a couple of crater pictures:
http://marsrovers.jpl.nasa.gov/gallery/press/opportunity/20040916a/site_b28_navcam_360_cyl-B218R1_br2.jpg
This first one is of the largest crater visited so far “Endurance”. What amazes me is that although here is a raised lip to this 100 meter wide crater the edges are smoothed off and the surface around the crater is nearly flat cobblestones. There are no signs of rocks sticking up anyway. My assumption is that at one time there must have been some ejected rocks on the surrounding surface that have simply eroded in the wind. Does this seem like a likely scenario or is it possible for an impact to simply compress the rock (maybe softer sediments at the time?) without a great amount of ejected material? I can help but wonder how much time must have passed for this crater to have come to this appearance if wind erosion has been the only force in action. I don’t have time to show pictures or talk about it extensively but it even appears that the extensive sand dunes are likely very ancient features and that very little or no sand dune building/shifting is going on today.

http://marsrovers.jpl.nasa.gov/gallery/press/opportunity/20040421a/1NN085ILF14CYL07P1983L000M2-B086R1_br2.jpg
This second much small crater appears to be much fresher. I like this image because you can see Endurance crater off in the distance. Here is see some rocks ejects or at least broken up that have not been completely smoothed off.

http://marsrovers.jpl.nasa.gov/gallery/all/1/n/896/1N207728298EFF74YKP0666R0M1.JPG

This third image is from just a few days ago and is of Beagle crater which seems to be quite young as well (young is a very relative term because even this crater doesn’t look like it happened yesterday). This crater has many jumbled rocks surrounding them but even they look quite worn down and so this crater could be quite ancient.

This last crate though can’t be older than the large crater it sits next to. That crater, Victoria is the next destination of Opportunity (if it can wade through the sand dues on its flanks successfully) and is 600 meters wide and 60+meter deep and yet if you go look at the high resolution traverse map and the images that Opp is taking today that crater is surrounded by smooth sand dunes underlain by bedrock and there is a not a rock more than an inch or two in height in view. Truly an impressive amount of erosion of this crater has taken place unless my expectations of some sort of ejecta are way off base.

The bottom line is that opportunity has seen a number of different features that suggest a serious of discrete historical events occurring at different times. Unless God created these craters with the appearance of age I should think that these pictures should present a considerable challenge to a young age view. At a minimum I see the following events:

1) thick (at least 60 meters deep seen so far) sedimentary layers produced
2) an end to sedimentary processes followed by erosioin of some rock (ejecta and surrounding regions) to produce sand blown dunes
3) creation of impact craters. Some craters are completely filled with sand, some have little sand and jagged rock.
4) Some craters have craters inside of them that look more recent (Victoria has two craters on its flanks that would seem to be younger)

How old are all these features? Its hard to say but I can’t see any forces other than wind erosion to explain the erosion of the craters. Without any other forces it would seem obvious that very long periods of time would be necessary to result in the observed pattern of rock formations seen today.

Am I missing something fundamental in my geological picture here or is the basic logic OK?

Thanks,
Joel Duff
Akron OH

To unsubscribe, send a message to majordomo@calvin.edu with
"unsubscribe asa" (no quotes) as the body of the message.
Received on Wed Aug 9 11:39:01 2006