Clastic Detritus

I first saw this press release on the Arizona Geology blog a couple weeks ago and it caught my eye. Cycles in sediments — I love that topic. Who doesn’t?!

A study led by researchers from Caltech used high-resolution images of the surface of Mars (from HiRISE) to document and analyze ancient sedimentary layering exposed at the surface of the red planet. Before discussing the study and the paper, I just want to make a comment about press releases. As with most press release titles, I’m not a huge fan of this one:

Caltech Researchers Find Ancient Climate Cycles Recorded in Mars Rocks

Yes, I realize these titles are written to attract readers. And, yes, I realize that the real titles of papers are typically full of jargon … but, here’s the title of the paper:

Quasi-Periodic Bedding in the Sedimentary Rock Record of Mars

See the difference? The title for the actual paper directly fits the results of their study — what they found with the data they collected and analyzed. The press release title jumps immediately to an interpretation and implications of those results. Maybe you think I’m splitting hairs and/or being overly cranky, but there is a difference. Here is a statement in the abstract about the rhythmic bedding:

This repetition likely points to cyclicity in environmental conditions, possibly as a result of astronomical forcing.

The repetition in the bedding is what the study documents, a reasonable interpretation of the repetition is that it’s a function of cyclicity in the controls on bedding — and, finally a statement about the possibility of that cyclicity itself being forced by orbital geometry (e.g., Milankovitch) factors. This is what you do in a paper like this — report your results and then discuss interpretations. This is in contrast to the statement from the press release that jumps directly to the potential implications. The reason I bring this up is because I fear that the language in press releases can really throw some people off. I strongly urge those who typically read only press releases to track down the actual papers from time to time. I realize access is an issue for many … but at least read the abstract, which is almost always available for free.

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Okay, enough of that … this paper, which was published in Science by Lewis et al. in December 2008, is very cool. You can find the paper here, or read the press release here.

The study area for this work is an area on Mars called Arabia Terra where layered sedimentary rocks are beautifully preserved within craters. The image below, which is from the Mars Reconnaissance Orbiter (MRO) website, shows the nature of these exposures in Becquerel Crater.

credit: http://www.nasa.gov/mission_pages/MRO/multimedia/20081204a.html

The authors of the study used 1 m-resolution stereo images from the HiRISE equipment onboard the MRO to analyze the bedding. The images were integrated with a digital terrain model (DTM), which allowed correcting for structural dip. Spectral analyses* of both thickness and color (i.e., image grayscale/brightness values) revealed statistically significant quasi-periodicity of the layering. Their analysis also revealed multiple orders of layering — beds (meter scale) and ‘bundles’ of beds (10s of meters scale). To see the actual results and read about their methods please see the original paper.

The observation of rhythmic layering on Mars at multiple locations across the Arabia region constitutes evidence of cyclic variation in ancient surface conditions.

Credit: Topography, Caltech; HiRISE Images, NASA/JPL/University of Arizona (click on image to go to source)

The biggest question on my mind while reading this paper is what the depositional environment was for these sediments. Johnson et al. address this question directly when they state:

The nature of the sedimentation process that deposited the Arabia layers remains uncertain. However, the observation of regularly cyclic bedding rules out processes that occur in a purely stochastic manner, including volcanism and impact cratering. … the size-frequency distributions of many stochastic depositional processes are skewed toward smaller events and can be described in many cases by a power law. Such processes include turbidites, flood events, landslides, volcanic eruptions, and impacts …

If I had to wildly speculate provide an educated guess — I would say that these strata represent chemical sedimentation processes of some kind (e.g., precipitation out of solution). To get such rhythmically-bedded deposits that are so consistently thick across a large area usually requires relatively “quiet” conditions (i.e., not a lot of highly energetic processes like waves and currents moving sediment around). Perhaps in the interior of a large lake or similar body of water. But, this is just a hunch — it would be fun to chat with the authors to hear their speculation.

The paper then discusses the intriguing implication of these rhythmic deposits — that they were caused by climate cycles. If the observed cyclicity is not a function of stochastic depositional processes or internal dynamics (sometimes referred to as autogenic cycles) then external forcings are at play. In this case, the authors draw upon research that has been done on Earth regarding the signature of Milankovitch cycles in strata.

Based on a pattern of layers within layers measured at … Becquerel crater the scientists propose that each layer was formed over a period of about 100,000 years and that these layers were produced by the same cyclical climate changes [link].

How cool is that!? This is an intriguing study because if we can find more areas on Mars where this method can be applied then we’ll have a better chance at unraveling the planet’s climate history (at geological time scales), which will help us better understand the planet’s overall geological history. This is why these orbiters, landers, and rovers are so important — they are collecting so much information that researchers can do science with.

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Reading this paper and writing this post got me thinking about planetary science in general. An interesting area of research is investigating and testing various remote sensing tools and methods here on Earth where researchers can “ground truth” the results and comparing that with results from Mars (or other bodies). Obviously, there will always be specific aspects of other planetary bodies that are different from Earth — tests like that may not always return valuable results, especially for very specific questions. But I think it is valuable to do such studies to get a general sense of which methods work in different situations and why.

Within the context of the Lewis et al. study, it would be interesting to see if something like this has been done for a location on Earth — where rhythmic bedding was analyzed quantitatively by remote sensing methods and from direct measurement on the ground. How might they compare? How significant would the measurement error be between the methods? Etc.

It’s very exciting to see how methods of sedimentary research that have been utilized on Earth are influencing the investigation of sediments and sedimentary rocks on other planets.

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K. W. Lewis, O. Aharonson, J. P. Grotzinger, R. L. Kirk, A. S. McEwen, T.-A. Suer (2008). Quasi-Periodic Bedding in the Sedimentary Rock Record of Mars Science, 322 (5907), 1532-1535 DOI: 10.1126/science.1161870

* See this post from the climate science blog Open Mind for some general information about how researchers use statistical methods to extract information about cycles.

Note: this blog post is a very short summary of some of the aspects of this study and is not meant to be a thorough review — please read the original paper for more.

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