One of the pre-MOLA hot topics was the supposed discovery of ancient Martiian shoreline features of various forms in, for example, Viking images. Now we have accurate height measurements, high quality MGS images, MOLA, and THEMIS data, and the whole subject has gone rather quiet, despite MEX images which are allegedly of glaciation and the remnants of ice floes.

I drove today from Glasgow to Gourock and back, and saw a 20 mile long raised beach; where are the Martian raised beaches?

Bob Shaw

One of my favorite all-time topics. Well, the idea of Mars oceans (viz., the putative Oceanus Borealis), really took a hit when the MOC data started rolling in, though, to be sure, there are still some die hards.

To give a flavor of the issue, see how the first nail was driven into the coffin here.

Of course, you may also wish to read this.

As you might guess, the literature is fairly extensive on the subject.

The case is not closed though. A report published last October provides further evidence that plate tectonics was once active on Mars:

New Map Provides More Evidence Mars Once Like Earth
10.12.05
"NASA scientists have discovered additional evidence that Mars once underwent plate tectonics, slow movement of the planet's crust, like the present-day Earth. A new map of Mars' magnetic field made by the Mars Global Surveyor spacecraft reveals a world whose history was shaped by great crustal plates being pulled apart or smashed together."
...
"This map lends support to and expands on the 1999 results," said Dr. Norman Ness of the Bartol Research Institute at the University of Delaware, Newark. “Where the earlier data showed a "striping" of the magnetic field in one region, the new map finds striping elsewhere. More importantly, the new map shows evidence of features, transform faults, that are a "tell-tale" of plate tectonics on Earth." Each stripe represents a magnetic field pointed in one direction­positive or negative­and the alternating stripes indicate a "flipping" of the direction of the magnetic field from one stripe to another.
"Scientists see similar stripes in the crustal magnetic field on Earth. Stripes form whenever two plates are being pushed apart by molten rock coming up from the mantle, such as along the Mid-Atlantic Ridge. As the plate spreads and cools, it becomes magnetized in the direction of the Earth’s strong global field. Since Earth’s global field changes direction a few times every million years, on average, a flow that cools in one period will be magnetized in a different direction than a later flow. As the new crust is pushed out and away from the ridge, stripes of alternating magnetic fields aligned with the ridge axis develop. Transform faults, identified by “shifts” in the magnetic pattern, occur only in association with spreading centers." [emphasis added]
http://www.nasa.gov/centers/goddard/news/t...mgs_plates.html

The theory of plate tectonics is closely allied to the idea of sea-floor spreading, where new crustal material is produced. See for example here:

Plate tectonics.
"Plate tectonic theory arose out of two separate geological observations: continental drift, noticed in the early 20th century, and seafloor spreading, noticed in the 1960s. The theory itself was developed during the late 1960s and has since almost universally been accepted by scientists and has revolutionized the earth sciences (akin in its unifying and explanatory power for diverse geological phenomena as the development of the periodic table was for chemistry, the discovery of the genetic code for genetics, evolution in biology, and the theory of relativity in physics)."
...
"Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge, and in the continental lithosphere by rift valleys such as the famous East African Great Rift Valley. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different massive transform faults occur. These are the fracture zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by linear features perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper (due to thermal contraction and subsidence).
"It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the sea-floor spreading hypothesis was found. Airborne geomagnetic surveys showed a strange pattern of symmetrical magnetic reversals on opposite sides of ridge centres. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Scientists had been studying polar reversals and the link was made. The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. The banding furnishes a map in time and space of both spreading rate and polar reversals."
http://en.wikipedia.org/wiki/Plate_tectonics

Seafloor spreading.
"Seafloor spreading is a part of the theory of plate tectonics. Sea floor spreading is the process by which continental drift occurs."
http://en.wikipedia.org/wiki/Seafloor_spreading

The full text of the article is available here:

GEOPHYSICS
Tectonic implications of Mars crustal magnetism.
Published online before print October 10, 2005.
PNAS | October 18, 2005 | vol. 102 | no. 42 | 14970-14975
OPEN ACCESS ARTICLE
http://www.pnas.org/cgi/content/full/102/42/14970?



Bob Clark

Those scottish raised beaches are Late Glacial or Early Holocene, i. e. about 10, 000 years old. Remember that any Martian ocean would only have existed quite early in Martian history while raised beaches and old shorelines are pretty short-lived features. I don't know of any older than the Pliocene here on Earth, but perhaps some of our geologist listmembers can think of older examples. Even on Mars I wouldn't expect to find any that are recognizable from orbit after 4 billion years, except in the very rare case where one was buried early on and just happened to be exhumed recently.
I think that in general we tend to underestimate how much Mars surface must have changed since the Noachian. Most of the surface we see has probably been cratered, buried and exhumed again and again since then.

Admittedly geologic processes work much faster here on Earth but it should be remembered how fast even large-scale geomorphic features erode away to nothing over geological time. Even major mountain-chains disappear in a few hundred million years. Admittedly there are places like Western Australia where Cretaceous river-valleys and Permian glacial topography are still recognizable, but this is very very unusual.

tty

Martian soreline are very hard to identify the borderline from any orbiter (above than 200 km) and I tought the same as tty since the most probably that Mars had undergone with great amount of water until the early Hesperian epochs, that is there water since 4.5-4.0 Billion (Noachian) and lasted it to around 3.5 Billion. During the Hesperien epochs there were lots of small metores impacts and some late mud flows. Then from around 3 billions until now, never had much amount of water except ones from impacted icy meteors .

The other thing, I am not sure that by observing an alterated rock with strated lines of water erosion are good ones to spot the soreline but the camera must not come from such high altitud -orbiter-. Maybe, the future rovers will be able to spot better any soreline of water when it roams around the probably sea lines from Vastitas Borealis.

Rodolfo

From what I understand, Erin Kraal et al. have a paper, Quantitative geomorphic modeling of Martian bedrock shorelines, that should be published online in JGR-Planets early next week. Here's the crux from the abstract: "In light of these results and the difficulty of maintaining warm, wet climates on Mars, it seems unlikely that the putative shoreline features identified in images are a result of water wave erosion, and alternative hypotheses, such as ice cover, should be entertained."

Floating ice can be a very powerful erosional agent. "Strandefladen", a characteristic bedrock platform along the Norwegian coast is thought to be largely due to ice erosion. Another characteristic glaciomarine erosion feature that might be worth keeping in mind on Mars is iceberg plowmarks which can be very large.

tty

Google throws up zip on that one - any pointers?




It's a bit more complicated than that, but yes, they're not very old. The raised beach I mentioned is generally agreed to have been created by isostatic uplift, post-glaciation, and is a dramatic 50 mile linear feature all along the Inverclyde coastline and down to Ayrshire. There are other Scottish raised beaches which have more specifically glacial origins.

Bob Shaw

The Kraal et al. paper was just published online.

Sorry, it should be "strandflaten", I mixed up Norwegian and Danish spelling.

Raised beaches aren't created by isostatic uplift. They are upraised by it. There are plenty of them here is Scandinavia too, thanks to the postglacial isostatic rebound. There are presumably plenty of glacial beaches elsewhere too, but the postglacial transgression has drowned them.

tty

I think we know what we mean re the raised beaches - and presumably whatever uplifted Tharsis might have left some such, somewhere.

As for the strandflaten, I'm off to dig through my geomorphology textbooks - I'd always had the impression that skerries etc were features of glacial deposition rather than erosion, and yes, the effects of grounded sea-ice/glaciers *would* be impressive agents of erosion! Mr Holmes, where *are* you hiding?

Bob Shaw

Come now, Watson, you know my methods -- simply apply them.

First, we observe rocks on the surface, which have obviously been eroded by aeolian forces for millions, if not billions of years. Second, we see massive erosion of landforms -- upon which sit rocks which seem not to have moved for millions, if not billions, of years.

By process of elimination, we can see that aeolian forces, even over billions of years, could not have been the sole erosive force acting upon Martian landforms.

Whatever remains, however unlikely, must be the truth...

-the other Doug

You see but you do not observe, Mr. Holmes. The complicating factor is that Noachian Mars almost certainly had an atmosphere tremendously denser than it now is -- maybe denser than present-day Earth's -- and thus capable of generating far stronger aeolian erosive forces than its current wispy air. The question is whether the far higher erosive rate which undoubtedly existed on Noachian days -- thousands of time faster than the current rate, judging from the comparative erosion patterns of craters on Noachian terrain as opposed to more recent terrain -- was due entirely to such stronger winds, or whether fluvial processes also played a major role.

And the pair of you jump to conclusions too fast! I was referring to ARTHUR Holmes, not Sherlock!

Now, must go and discover the structure of DNA, Crick and...

Bob Shaw

Just as long as you weren't referring to a certain John Holmes....