Mooning away Tuesday

NASA / USGSYes, that was a silly title, but it was a good day filled with Lunar science. (Posting delayed by too much fun recording content).
The very first talk I saw this morning was so cool that instead of writing it up, I’m just going to interview the guy who gave the talk: Larry Copper of The University of Tennessee. Teaser: You can melt lunar dust with microwave ovens, and it is possible to use microwave tools to melt lunar dust into things like parabolic dishes, and landing pads. Check out the audio: [mp3].

From there it went to the story of the lunar poles. Presented by P. Lucey. A lot of focus is going into trying to understand the Moon’s poles as we gear up for new (and more lasting) manned missions to the Moon. The most often stated reason in the media for going to the poles is the possible water frozen in the crater areas that are in constant shadow. The truth is, I learned today, that there is also a lot of really cool science that can come out of this, and it is simply easier for humans and machines to function in the constant environment of the pole.

At the equator of the moon, a given plot of land or crater floor will go through a month long day to night to day cycle, with temperatures fluctuating 100s of degrees. These large temperature swings are not good things. At the poles, which are only inclined 1.5 degrees relative to the sun (compared to Earth’s 23 degree tip that leads to our seasons). This small inclination means that temperatures are pretty constant. In the areas that are in constant shadow, things maintain a temperature of 25K, which while extremely cold is at least constant. At the same time, by just sticking something straight up out of a crater, you can put it in constant sunlight. In this way, we could keep our equipment cool in the dark and feed it energy from solar panels placed in perpetual day light.

In thus very cold, very constant environment, it is possible to study gases in a variety of way – yes I said study gases on the moon. Sources of volatiles (a fancy word for gases that like to escape when warm but can be frozen) include comets, asteroids that formed with in the icy boundary (wet asteroids), and even interplanetary dust particles. These volatiles can help us understand the things as varied as Giant Molecular Clouds (like Bok Globules) and space weathering (the process of the space environment altering the chemistry of soils via many mechanisms, including solar wind bombardment, cosmic rays, and others). The Molecular Clouds are perhaps one of the more fascinating applications there has been some talk in the literature (the references given were Talbot & Newman 1977 and Pavlov et al. 2005) that the Earth regularly passes through molecular clouds and this changes the ability of light to make it from the Sun to the Earth, casting us in an interplanetary fog. This has been termed the “Snowball Earth,” and has been linked as a possible (but much debated) cause of past extinctions. It is possible evidence of these interactions is trapped on the moon. More likely, we’ll fund hydrogen trapped in silicates and different organics that formed in the extremely cold “space weather” of the shadows of the polar crater walls. There is a lot to learn, and right now all we can really do is speculate at what might be there waiting. An actual robotic or manned mission will be needed to make measurements.

From the poles, the topic changed to Craters. Put simply, the older the surface, the more craters it has, and the younger the surface the less craters it has. By counting craters you can to get the relative ages of different regions (radiocarbon dating is required to get years attached to our understanding of A is older than B). (silly note: the person next to me looked at me like a crazy person for needing to write down how crater aging works). Here are the steps: 1) Map homogeneneous areas (in terms of composition as determined from Clementine data), 2) Determine the number of primary craters as a function of size (how many big, how many medium, how many small, etc), 3) determine cumulative crater frequency for craters >= 1km (that means you make a plot and fit a line), 4) fit local distribution to lunar standard distribution (do you have more or less than average, and by how much). This provides relative age. Bonus step: Use coorelation between radiometric ages of returned lunar samples (Apollo moon rocks) and crater numbers to derive age for areas containing Apollo missions. This gets you a model age for different areas.

The reason ages of different areas on the moon vary is volcanism. The moon had active volcanism for roughly 3 billion years. There is a clear evolutionary motion of where things formed. This indicates that heat producing elements (things inside the moon radioactively decaying) allowed volcanism to exist until recently (which to a geologist means until 1 to 2 billion years ago). This volcanism peaked 3.5 billion years ago in the model ages presented.

The craters can also be used to address basalt thickness (basalt is what they are calling the stuff the volcanos spewed – I really really need to read a geology book). The measure the thickness by matching theoretical crater ditributions to determine what is not seen. For instance, are the tiny craters missing in an areas that has a lot of larger craters? Based on what is erased, they can determin the depth of erasing material – the Basalt that flowed from fissures to fill in the craters. They estimate the Basalt is generally 30-60 m deep.

While the Basalt was formed over several billion years, and it is observed to come in both titanium rich and titanium poor types, maps created by Lucey et al 2000 correlated with the presented crater counts didn’t show that the composition of the Basalt changed with time. Titanium rich and poor basalts erupted at same times and across the whole formation period. The region studied was limited to a region of the nearside Mare Basalts, and LROC data will allow this study to expand and address more regions on the moon. Since the moon doesn’t suffer wind, rain, and platetechtonics, it is possible to imagine one day having a map of “This region formed when…” for the lunar surface.

The Mare Basalt is something I have to admit I haven’t thought a lot about before today. It is the dark stuff that makes up the rabbit or the old man in the moon, and it exists mostly on the near side of the moon (the side we get to see). I knew it was a type of lava flow, but I hadn’t understood all the things people study about it. In addition to studying the history of Basalt formation via cratering, work is also being done using Moon Meteors (chunks of moon that got knocked off in an impact and landed on Earth), to study the age of the moon and what formed when.

The Moon formed after the Earth, and is actually a blasted off chunk from a giant impact event (we think). This rather traumatic experience occurred 4.5 billion years ago. Over the years we have found more than 50 lunar meteors on our planet and ~10 (approximately means some are being argued over) are what are called Crystalline Basalts (the remaining are Feldsphathic and Basaltic). Translation – about 10 are cool lava rocks that we can date and the others are less useful rocks for this purpose, but are still cool. Of those 10, the youngest is NWA 773, which is 2.8 billion years old, and the oldest is Kalahori 009, which is 4.35 Billion years old. (There is also a possibly younger rock in the Apollo samples, that may be 2.4 years old).

The oldest rock, Kalahori 009, was found in Botswana in 1999 and is very large, weighing in at 13.5 kg! It contains phosphates that can be used for aging and both its mineralogy and oxygen isotopes indicate that its lunar (certain oxygen isotope ratios are particular to the Earth-Moon part of the solar system, and certain mineral ratios are typical for the moon and not the Earth.) By studying this rock and a lot of chemical processes that went into forming this rock, including the radioactive decay of Uranium into Lead and the Argon-Argon crystallization, they were able to identify its age and thermal history through multiple techniques. This allowed them to discover that the lunar highlands formed in two stages, including volcanic modification in a second stage of lunar crust formation.

How this volcanism worked is actually a fascinating mathematical story. L. Wilson gave a captivating to a geek presentation that in pure algebra, with variables identified with the occasional word, went through and demonstrated that the eruptions that deposited the Basalt were turbulent with a flow velocity of 2.5 m/s and an effusion rate of ~1×10^6 m^3/s. (for the uber geek: a fit to a laminar flow model gave a flow of 65m/s and a Reynolds number of 1.6×10^6!!!). The flow was most likely limited by the volume of material in a reservoir, with a 300 km flow taking a day and half to 4 days, depending on slope of fissure. He even estimated the size of the fissures based on observed dikes and flows. He estimates the volume of a dike to be not less than 10.8km^3 to 54km^3 depending on if it is long and skinny or, well, more rotund in shape. Dancing through the numbers, he showed how the densities of the magma, the crust and the lunar lithosphere all balance to point to specific flow rates and geometries, indicating that global compression of the lunar ground caused late stage volcanism to be extremely finely balanced between material lerupting through the lunar surface or instead tunneling through the subsurface forming intrusions. He suspects much more basalt exists in these underground intrusions than in the volume of lava we see.

On that wonderfully mathematical note, my brain was fully sated.

Nonetheless, I tried to cram one more talk into my mind and my notes on my laptop.

The very first talk on the new NASA GRAIL mission was the last talk I saw in the morning. This mission is designed to determine the internal structure of the moon using gravity modeling. While this is part of SELENE’s mission, the GRAIL mission will do it with higher precision – this is the most over designed mission ever. They are utilizing existing technologies that were designed for Earth studying (and other) projects, and it just happens that these technologies allow them to do the moral equivalent of measuring the distance between Houston and Florida with Vernier Calipers. Check out the mission page here.

I’m afraid that if I consume any more content at this moment I’ll have the mental equivalent of what happened to that guy on Monty Python who consumed just one last wafer thin mint. Therefore (warning: I may not post this until the end of the day), I’m going to go get lunch.

3 Comments

  1. Vagueofgodalming March 12, 2008 at 8:31 am #

    You touch on an interesting point (well, all your posts are interesting, but I don’t often have anything to say): that it seems astronomers are much more likely to get that there are people who don’t know what an emission nebula or an elliptical galaxy is, than (planetary) geologists are to get that there are people who don’t know what basalt or cratering chronology is.

  2. Astrogeek March 12, 2008 at 9:44 am #

    Hi Pamela;

    My geo classes where a while back, but I think I can help you out with Basalt…

    Basalt is an extrusive igneous rock: meaning it was extruded (flowed out onto the surface) from a volcano or other volcanic source. It is typified by it’s density (it’s a heavy rock) and it’s fine grained crystalline structure (since it cooled rapidly, being out on the surface, the internal grains are very small). This makes it different than, for example, the granites, which are an ‘intrusive’ rock… they both formed from magma, but one (basalt) reached the surface, flowed out as lava and cooled rapidly, and one (granite) did not reach the surface, but instead squeezed itself into other rock formations, and cooled slowly under pressure.

    The numbers you cite above indicate that the lava that formed the mare basalts was very thin and runny… which is not surprising since the moon is made up of much lower density material than the earth.

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