Tuesday, 31 December 2013

Earth or Not Earth

Earth or Not Earth
Earth or Not Earth is a new Android app for testing your knowledge of planetary features.
Image Credit: NASA with annotations by Cosmoquest.
Earth or Not Earth Game
The first game in the Earth or Not Earth app has you decide if a featured picture is from Earth or Not.
Image Credit: NASA with annotations by Cosmoquest.
This year, I got a really interesting present for Christmas. It's an app called "Earth or Not Earth" and it's available for Android phones and tablets from the Google Play Store. Brought to you by the CosmoQuest project, this app lets you test your knowledge of planetary features by (you guessed it) identifying which features are from Earth and which aren't.

Actually, there are three "games" in this app. The first one is where you have to figure out which features are from Earth. The app shows you a picture of a planetary feature, and you have to click the big "Earth" button if you think this is a feature from Earth, or the "Not" button if you think this is a picture of another planet. It's actually quite fun, and a bit harder than you think. But this is labelled as the "Easy" game.

The next "game" is labelled as medium in difficulty. Here, the app shows you a picture and you have to choose which of the 4 multiple choice answers best fits the image. The choices are usually either planets or feature types, so you need to either identify which planet the image comes from, or which kind of feature the image is showing. It's multiple choice, so it can't be that hard, right?

The final game is labelled as hard. And it is. You need to pick out two pictures that come from the same planet out of a set of 12 images. This is actually a timed activity, and you need to see how fast you can match up all the pairs in a group of 32 images.

Matching Game         Pick Two Game         Learning Cards
The second game has you pick which planet or feature type is being showcased in the image.
Image Credit: NASA with annotations by Cosmoquest.
        The third game has you pick two pictures from the same planet in a set of 12 images.
Image Credit: NASA with annotations by Cosmoquest.
        Features are described in the Learn section. (Click the "Hide" or "Show" buttons to remove or restore the text).
Image Credit: NASA, annotations by Cosmoquest, text by Irene Antonenko.
So, the app is lots of fun. But, if you find you are sucking at this, don't despair. There is a Learning section in the app that lets you explore the image set and learn more about the worlds and features in these images. Navigating either by planet or feature type, you can read about each image, what it's showing, and where it's is located. You can also "Hide" the text so you can examine the full image in more detail.    

Now, here is where I have to make a confession. You see, about a year ago, I created a set of Planetary Surface Feature "Vocabulary Cards" for the CosmoQuest project. Each card showed an image of a planetary feature and had a description of the feature and the planet it came from provided on the back. I was pretty proud of these cards, but nothing much ever seemed to come from them. Then CosmoQuest team member Joseph Moore took these cards and created this really amazing and fun app with them. So, for me, the best Christmas present was seeing my planetary surface feature cards used in this really creative and interesting way!!

If you think this app sounds like fun, why don't you give it a try. It costs only $1.99 USD and all proceeds go towards supporting CosmoQuest science research and outreach initiatives.

Earth or Not Earth Android App, Google Play Store, Dec. 10, 2013.

CosmoQuest, Would you like to play a game with us? Google+, Dec 11, 2013.

Sanders, App Review: Earth or Not Earth, Universe Today, Dec. 20, 2013.

Saturday, 30 November 2013

Sinus Iridum - China's Next Stomping Grounds

Near side of the Moon with Sinus Iridum pointed out
Clementine mosaic of the near side of the Moon, with arrow pointing to Sinus Iridum.
Image Credit: NASA/JPL/USGS, Annotation by Irene Antonenko
Earlier this week, China's National Space Administration (CNSA) held a press briefing to provide more details about their Chang'e 3 lunar lander, which is scheduled to launch early Monday morning on Dec 2, 2013 around 1:30 am Beijing Standard Time.  That should put the lander on the Moon some time by the middle of December, making it the first spacecraft to execute a soft landing on the Moon in over 37 years. The last man-made lunar visitor to do so was the Soviet Luna 24, which landed in August of 1976.

The Chang'e 3 lander will also deliver a rover, named Yutu, the legendary "Jade Rabbit" companion of the goddess Chang'e.  The last man-made vehicle to roam the Moon's surface was the Soviet Lunakhod 2, which landed in January of 1973 and operated on the surface for 4 months. China's Yutu is expected to roam around the Moon for 90 days and cover about 5 square kilometers of terrain.

The all important landing spot for the Chang'e 3 lander and Yutu rover is to be in Sinus Iridum, or the Bay of Rainbows, on the near side of the Moon.  No spacecraft has ever landed there before. The Soviet Luna 17, which delivered the rover Lunakhod 1, came close, landing over 200 km to the south, on the edge of Mare Imbrium (the Sea of Rains). But even that was a long time ago and over 40 years have passed since then. It is expected that technological advances since that time will make it possible to obtain far better data, allowing us to study the lunar surface in unprecedented detail.

Change3 & Yutu
Artist's conception of the Chang'e 3 lander and its accompanying rover Yutu.
Image Credit: Beijing Institute of Spacecraft System Engineering
But, it is also important to study the landing site as much as possible before arrival. Most obviously, this is necessary to ensure a safe site, both for landing and operating on the surface. Secondly, it is important to select a site that is scientifically useful. We have been to the Moon before, so to get the most bang for our buck, we want to make sure we are seeing something different from what we have seen before.  And finally, we want know the landing area as well as possible, so that we can quickly understand and interpret the information we see when we get there. This allows scientists to make critical decisions about where to send the rover next without wasting precious surface time on elementary data analysis.  With that in mind, here is a short primer on the Chang'e 3 landing site.  

Topography of Sinus Iridum
Topography of the Sinus Iridium region. Lunar Reconnaissance Orbiter Camera (LROC) data has been processed to determine the topography of the surface. Colours represent elevation, with yellows and oranges representing high topography and blues indicating low lying surfaces. The topography data has been overlain on top of LROM wide angle camera mosaic imagery, helping to highlight the difference between high, rough terrain, and smooth low-lying areas.
     Feel free to explore this region in more detail at the LROC Act-React Quick Map on-line web tool.
Image Credit: NASA/GSFC/Arizona State University
Sinus Iridum is a small lava filled impact crater about 250 km in diameter, which sits beside, and opens into, the much bigger Mare Imbrium, which is also a lava-filled impact structure (up to 1800 km in diameter). The Iridum crater sits on top of an uplift structure (called an inner ring) inside the Imbrium basin and, therefore, must have formed after the Imbrium impact event. The topography of the pre-existing Imbrium impact basin seems to have controlled how the later Iridum crater formed, so that the parts of the Iridum crater that face the centre of Imbrium (to the southeast) are overall lower than the parts that face away. This fact played an important role when both the structures were flooded with basaltic lava, as the topographically lower portions of the Iridum crater were completely covered over, while the higher portions in the northwest remained untouched.

Not all this lava flooded the area at once. It came in relative dribs and drabs. Geologists have used things like surface albedo (or brightness) and the crater counting to show that different parts of the sinus and mare were emplaced at different times, with younger flows covering over older flows in some places, but not in others. These resulting variations can be seen on geologic maps, where the different basalt units are represented by varying colours. In addition, remote sensing studies of the iron content of these basalts also shows that there is great variation between the different flows.

The flooded mare regions of both Imbrium and Iridum are crossed by mare ridges. These are long linear hills that extend for tens to hundreds of kilometers across the mare surface, often running parallell to impact rim and ring structures buried under the mare. It is believed they form when thick piles of basaltic lava cool and contract. This leaves an  upper-most "chill crust", which cooled much earlier and is now too large and loose for the underlying contracted pile, forcing it to bunch up and wrinkle like a table cloth. 

Geologic Map of Sinus Iridum
Geologic map of Sinus Iridium and the northwest parts of Mare Imbrium. Purple colours represent materials from the Iridum crater rim. Greens, yellows, and browns represent materials from various younger craters. And pink and grey areas show the different units of mare basalts.
   You can download this geologic map at Lunar and Planetary Institutes Resources portal.
Image Credit: USGS
Iron Map of Sinus Iridum
Map showing the iron content of surface materials in the Sinus Iridium region. Reds to yellows indicate relatively high iron content, while greens and blues show a lower iron content. The high iron regions (red) appear to correspond to the grey mare unit in the geologic map at left.
   You can explore the iron content of the Moon at the USGS Map-a-Planet Explorer web page.
Image Credit: USGS
The exact location where Chang'e 3 and Yutu are going to land is still not known. However, it is likely that these wrinkle ridges will play a role in the landing site. Wrinkle ridges may be able to provide some shade during parts of the lunar day, which can help to stabilize temperatures for the operating spacecraft. Temperatures on the lunar surface reach 390K (117 degrees Celcius) in the day time, which is higher than the temperature of boiling water, and drop  down to 110K (-163 degrees Celcius) at night. The spacecraft cannot function at either of these temperature extremes for prolonged periods of time, so landing in an area where shade is available, at least some of the time during the day, can be very useful for improving spacecraft life.

Personally, I can't wait to see what Chang'e 3 and rover Yutu will discover on the surface of Sinus Iridum. Here's hoping for a successful launch, uneventful journey, and a very happy landing!

Lakdawalla, 2013, Chang'e 3 may launch December 1 with Yutu rover, will not harm LADEE mission, The Planetary Society Blogs, Nov 27, 2013.

Schaber, 1969, Geologic Map of the Sinus Iridum Quadrangle of the Moon, I-602 (LAC-24), USGS.

Wang et al., 2013, The Chang E-3 Landing and Working Area Selecting: Based on the Lunar Digital Terrain Model, 21st International Conference on Geoinformatics, DOI: 10.1109/Geoinformatics.2013.6626076

Coming soon: China launches Chang’e-3 lunar probe, China.Org.Cn, Nov 29, 2013.

Wednesday, 13 November 2013

Why the Far Side has no "Man in the Moon"

Near and Far side of the Moon
Lunar Reconnaissance Orbiter Wide Angle Camera mosaic of the near side (left) and far side (right) of the Moon. Mare Moscoviense is the largest dark lava deposit on the far side of the Moon, where only a few small lava patches can be seen.
Image Credit: NASA/GSFC/Arizona State University
Looking up at the night sky, anyone can see that the near side of the Moon has many gigantic dark splotches. In many cultures, these splotches make up the "Man in the Moon". In others they form a rabbit. So, when the Soviet probe Luna 3 took the first pictures of the far side of the Moon in 1959, we were surprised to see no giant splotches, no Man (or rabbit) in the Moon. Scientists now know that those dark splotches represent large basins that formed when asteroids impacted the young Moon, and which were then filled with massive amounts of volcanic lava. But the question of why there are so few large basins and almost no lava deposits on the far side is still a source of some mystery.

Generally speaking, the distribution of impact basis should be fairly similar on the two hemispheres of the Moon, with both the near side and far side having the same amounts of large, medium, and small impact features.  The size of an impact structure is generally determined by measuring its diameter using the basin's edges, which are called rims. The problem for near side basins is that they are filled with lava, which can often hide important clues for determining where exactly the rim is located, making it hard to measure the basin's size. Also, during the final stages of the impact process, the basin sides collapse inwards due to gravity. For very large basins, this can result in multiple concentric ring structures, where it is not clear which, if any, of these ring structures represents the true basin diameter. So, the question is, are there more large impact basins on the near side because we have incorrectly measured their size?

To answer this question, scientists are using data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission. Launched in September 2011, the GRAIL mission consisted of two satellites, Ebb and Flow, which were named in a contest (school children from Montana submitted the winning entry). The two satellites measured the gravity of the Moon by tracking the changing speeds and distances between them.  When the GRAIL mission ended in December 2012, the two satellites were purposefully crashed into the Moon. But, during their year-long operating mission, the satellites collected a great deal of data, which scientists are still evaluating today.

Crustal Thickness Map of the Moon
Global near side (left) and far side (right) map showing the thickness of the Moon's crust, derived from gravity data obtained by NASA's GRAIL mission. Basins identified in this study are outlined by black circles.
Image Credit:  NASA/JPL-Caltech/S. Miljkovic

One interesting product that has been derived from the GRAIL data is a global map showing the thickness of the Moon's crust. Working together with colleagues from around the globe, Dr. Katarina Miljkovic from the Institut de Physique du Globe de Paris, France is using this crustal thickness map to study the sizes of lunar impact features.  Instead of measuring the diameter of rims to determine a basin's size, Dr. Miljkovic is using the diameter of thinned crust. During an impact, a great deal of material is excavated from the target surface, which then rebounds, allowing the mantle to push up under the crust. Together, this causes the crust to be much thinner under impact basins than in other parts of the crust. And this thinned crust can be used to represent a basin's size. The beauty of this technique is that it doesn't suffer from the same problems as the basin rim measurements. Because gravity measurements see into the interior of the crust, surface lava flows and ring structures are not an issue. The drawback of the crustal thickness technique is that it is generally limited to larger basins since the gravity signature of smaller basins can be difficult to resolve from other features.

Using this crustal thickness technique, Dr. Miljkovic and her colleagues measured all the lunar basins larger than 200 km in diameter.  The results of this work were published just last week in the journal Science, where the researchers reported that basins on the near side are truly larger than those on the far side. The problem is that the total number of craters on the two sides is the same, with just the distribution of their sizes being different. The probability of this occurring randomly is estimated to be less than 2% if both sides of the Moon were subjected to the same population of impacting asteroids. So, what could have happened to put all the big basins on the near side, leaving the far side devoid of large impacts?

There is currently no satisfactory way to get all the large asteroids to target the near side, while the smaller asteroids veer to the far side. For this reason, Dr. Miljkovic and colleagues propose a different reason for the basin size dichotomy. They suggest that the basins on the near side were made by exactly the same kinds of asteroids as those on the far side, but that these asteroids literally made a bigger impact on the near side because it was much warmer than the far side.

 If you recall, the dark splotches on the near side of the Moon also represent the presence of volcanic lava.  Most of the lava is disproportionately located on the near side of the Moon, with 99% of the lava-covered surfaces being found on the near side. This volcanism dichotomy is thought to be a result of two things: 1) a high concentration of radiogenic, heat-producing elements on the near side and 2) a thinner crust on the near side.  Early in the Moon's history, radiogenic elements would have decayed, producing radiogenic heat (though why these elements were concentrated on the near side is still not clear). The resulting heat would have melted parts of the lunar mantle, creating magma which was able to erupt onto the surface because of the thinner crust on the near side.  So, the near side of the young Moon would have been much warmer than the far side, which had a thicker crust and fewer radiogenic elements. These different temperature states on the two hemispheres would have persisted for the whole time the large impact basins were being formed.

Effects of Crustal Temperature on Basin Size
Simplified diagram showing how the temperature of the crust affects impact basin formation. Warm crusts experience more rebound during basin formation, decreasing the amount of gravitational collapse. Cold crusts experience less rebound, allowing more gravitational collapse to occur.
Image Credit: Irene Antonenko
To study how the temperature and thickness of the crust affects impact basin sizes, Dr. Miljkovic and her coworkers ran computer simulations. Using exactly the same impactors, they studied how the size of the resulting basin differed for impacts into a warm thin crust (simulating the lunar near side) and those into a cold thick crust (simulating the lunar far side). They found that the crustal temperature had an effect during the last stages of basin formation, when a temporary "transient" basin cavity rebounds and collapses due to gravity. Impacts into a warm crust experienced more rebound, because warm material moves more easily.  The increased rebound means there is less difference in height between the rim and centre of the basin, so less material collapses into the interior due to gravity. This means that outer parts of the basin are not thickened as much by rim material collapsing inward, resulting in a larger diameter of thinned crust. In a cold crust, less rebound means that more rim material collapses into the interior, thickening the outer parts of the basin, making the diameter of thinned crust smaller. Dr. Miljkovic estimates that, for exactly the same impactor, basins in the warm crust can appear to be as much as 2 times larger than their counterparts in the cold crust.

Using this result, the researchers "corrected" the sizes of the near side impact basins, to reflect what they would have been if they had impacted into the cold, thick far side crust. After this correction, the distribution of basin sizes on the near and far sides of the Moon is more comparable, confirming that they were both bombarded by the same population of asteroids. 

Dr. Maria Zuber from the Massachusetts Institute of Technology in Cambridge is the principal investigator of the GRAIL mission. She sums up these findings very well, saying "GRAIL data indicate that both the near side and the far side of the moon were bombarded by similarly large impactors, but they reacted to them much differently.” So now we know why the near side looks so different from the far side. Early in the Moon's history, the near side was much warmer than the far side. This allowed very large basins to form, making huge bowls into which the volcanic lava flowed, so creating the big dark splotches we see today as the "Man in the Moon."

But only on the near side.
Hit the warm side if you want to make a bigger impact
Image Credit: Irene Antonenko

NASA's GRAIL Mission Puts a New Face on the Moon, NASA News Release, Nov 7, 2013.

Miljkovic et al., 2013, Assymmetric Distribution of Lunar Impact Basins Caused by Variations in Target Properties, Science, 342, p724-726, DOI: 10.1126/science.1243224

Thursday, 31 October 2013

The Bizarre Lakes of Titan

Bird's Eye View of the Land of Lakes
This image of the north polar region of Saturn's moon Titan was obtained on Sept 13, 2013, using the Cassini Imaging Science Subsystem (ISS). A number of seas and lakes, consisting of very cold hydrocarbons, show up as dark patches. The image spans about 2000 km from top to bottom, and has a resolution of about 500 m/pixel.
Click here to see an annotated version of the image.
Image Credit: NASA/JPL-Caltech/SSI/JHUAPL/Univ. of Arizona
Besides Earth, Saturn's moon Titan is the only other planetary body in our Solar System that we know has stable liquid seas and lakes at its surface. Planets like Mars may see liquids occasionally erupt from beneath their surfaces, but such liquids tend to quickly evaporate into the atmosphere, and so don't stay long on the surface. But on Titan, scientists have discovered a number of lakes and seas that appear to be stable over long geologic time periods.

The surface of Titan is believed to be made predominantly of water ice. But the temperature at Titan's surface is a very cold 90 Kelvin (about -300 degrees Fahrenheit or -180 degrees C). At that temperature, water ice does not easily melt and so acts more like the planet's bedrock. Titan's seas are, therefore, not made of water, but rather of hydrocarbons, like ethane and methane, which at these temperatures are liquid.

Scientists have been studying the ultra-cold hydrocarbon lakes of Titan for several years, most recently with the help of the Cassini mission to Saturn.  This past September, the spacecraft flew by Titan's north pole and obtained exciting new imagery of these intriguing liquid features using the near-infrared instrument on Cassini's Imaging Science Subsystem (ISS). These images show the lakes as dark patches with very distinctive shapes, having rounded scallop-like edges and steep sides. The surrounding material is also unusual, being much brighter than the rest of Titan's surface, which tends to be dark grey in colour.

The majority of Titan's seas and lakes are found at the north pole, with only a few lakes near the southern pole. It was originally thought that dark terrains at the equator were also liquid hydrocarbon seas. But Cassini images have shown that these are large plains covered in long, linear dunes. Thus, the polar lake areas are truly unusual on Titan. It is thought that their unique environment holds clues to how they were formed, but the exact process is not yet known. Scientists have suggested two possible scenarios: 1) they are Karst terrains, that were formed when the liquids dissolved the underlying rock (or water ice in this case), making surface holes and underground caves in the bedrock, or 2) they were formed by volcanic processes, where magma chambers, which were emptied by volcanic eruptions, collapsed leaving large holes at the surface. In both scenarios, the liquid hydrocarbons would simply have filled up the resulting holes.

The largest liquid body on Titan is Kraken Mare at the north pole.  A sea on planetary bodies is referred to as a mare, the latin word for sea. This dates back to the time of Galileo, who thought the large dark areas on the Moon were seas. We now know that the lunar maria (plural for mare) are not liquid at all, but are solid basalt rocks. However, the name stuck and has been passed on to the liquid seas of Titan. Kraken Mare is quite large by terrestrial standards, spanning 400,000 square kilometers. This is roughly equivalent to the combined size of the Caspian Sea and Lake Superior on Earth, a truly spectacular size!
Footprint of Ontario Lacus
This radar image of Ontario Lacus, the largest lake in Titan's southern hemisphere, was obtained on Jan. 12, 2010. The lake is about 15,000 square kilometers (6,000 square miles) in size, which is slightly smaller than its terrestrial namesake, Lake Ontario in North America.
Image Credit:  NASA/JPL-Caltech/ASI

Prior to the most recent flybys of Titan, the north polar region hadn't been imaged very well, with only distant, oblique, or partial views being obtained. Part of the problem was that when Cassini arrived at Saturn 9 years ago Titan was experiencing a northern winter, so the north pole was in complete darkness. Since then, summer has been approaching and the northern pole is finally receiving sunlight.  A number of factors have lined up to make the most recent flybys particularly conducive to collecting very good imagery. For one, the sunlight and flyover trajectory have provide a much improved viewing geometry over previous opportunities. Also, with the approach of summer, the thick cap of winter haze that hung over Titan's north pole has dissipated.  And finally, Titan's weather has been unusually cooperative, providing almost cloudless and rain-free skies.

These conditions have also allowed scientists to collect data using the visual and infrared mapping spectrometer (VMS) on board Cassini. By analyzing data collected at a variety of wavelengths, the composition of the surface materials can be inferred. While most of Titan's surface seems to be composed of water ice, sections of the north polar region appear to contain materials that are interpreted to be evaporites. On Earth, evaporites form when shallow seas evaporate, leaving thick deposits of salts behind. Titan's evaporites are thought to consist of haze particles. Liquid methane in the atmosphere dissolves the atmospheric haze particles, which are then rained down to the surface and left behind when the shallow methane lakes evaporate.

We have been referring to these interesting methane lakes and seas as liquid. However, it should be noted that these bodies may not be liquid quite the way that terrestrial seas and lakes are liquid. Radar imagery of these features shows that they are extremely smooth, even at the millimeter scale. This means that they have no waves on their surfaces, not even small ripples. But, scientists calculate that even the slightest breeze should produce substantial waves, because the mixtures of ethane and methane that make up these bodies are less viscous than water.  So, it may be that these lakes also contain other hydrocarbons which make the ethane/methane mixture much more viscous, giving it a thick consistency, like that of tar or mud.

Bizarre lakes, indeed!

JPL's Cassini Featured Image, 2013, Bird's Eye View of the Land of Lakes.

JPL's Photojournal, 2013, Titan's Northern Lakes: Salt Flats?

Hecht, 2011, Ethane lakes in a red haze: Titan's uncanny moonscape, NewScientist, 2820.

Lorenz, 2010, Winds of Change on Titan, Science,  V329 (5991), 519–20, doi:10.1126/science.1192840.

Thursday, 17 October 2013

The Fun of Geologic Maps!

USGS Geologic Map Excerpt
Excerpt from the geologic map of the western Winston-Salem area in
North Carolina, Virginia, and Tennessee. See the full map below, or
download it from the USGS  National Geologic Map Database.
Image Credit: USGS
Friday October 18th, 2013 is Geologic Map Day. No, really, such a thing does exist. It was founded by the US Geological Survey (USGS), the American Association of State Geologists, and the American Geosciences Institute in order to raise public awareness of the significant contributions geologic maps make in science, business, and public policy. To learn more, check out the Geologic Map Day website, where they have links to lots of neat stuff, such as geologic maps (of course), FAQs, and activities. Warning, this site is very US-centric. However, most countries have their own geologic branches of the government, which often provide on-line access to geologic maps. These can usually be found with a quick Google search. For example, the Geological Survey of India has links to a number of geologic maps throughout that country.

I think geologic maps are fun because they are so colourful. Unlike road maps, which link places with a network of lines, geologic maps look at areas. Each area is defined by the type of rock that is found there, and this rock type is shown on the map by a specific colour. This way, it is easy to tell at a glance which areas of a map have the same rock types.... and which ones don't. A legend is used to tell the map user what kind of rock each colour represents. Other geologic information, such as where faults are found, is represented by symbols, which are also explained in the legend. Some maps even come with cross sections, which show you a side view of the map at certain points, as if you had sliced the earth open like a cake and taken a look from the side. And some maps also have several paragraphs of text, explaining what happened in the mapped region, from a geological point of view.
USGS Geologic Map
Geologic map from the western Winston-Salem area in North Carolina, Virginia, and Tennessee, prepared by Rankin, Espenshade, and 
Neuman in 1972. The full map can be downloaded from the USGS  National Geologic Map Database.
Image Credit: USGS

Most people think of maps as something we make for places on the Earth. But we have been studying planets long enough that we have a fabulous assortment of maps, including geologic maps, for the other planets. The Lunar and Planetary Institute lists a bunch of links to planetary maps (and images) for the Moon, Mars, Venus, and Mercury on their Resources page.   My favourite is the Geologic Atlas of the Moon, which has links to on-line versions of every geologic map of the Moon published by the US Geological Survey.  Here you can find geologic maps for the Apollo landing sites, other regions of interest, and the entire Moon, divided up onto smaller segments.

This geologic map from 1971 (top) shows the Hadley Rille region of the Moon, where the Apollo 15 mission landed. The red lines overlaid on the map show the traverses that the astronauts undertook (determined from recent image data). This kind of information tells us that the astronauts saw a variety of geological regions on their traverse. They started out in flat mare terrain. One of their traverses skirted the ejecta of a young impact crater (olive green). Another traverse crossed the debris slopes (olive brown) of the Apennine Mountains (brown) to venture into the hills themselves. The third traverse cut through a crater field (pink), which was most likely formed by the ejecta from a much bigger crater well off the map, and then headed into the Apennine Mountains (brown) again.  In contrast, the Lunar Reconnaissance Orbiter Camera (LROC) image (bottom) does not provide this much information. The Apollo 15 traverses, shown in red, were determined from very high-resolution LROC images.
View the full map, with legend and explanations at the Lunar and Planetary Institute's Hadley Rille map page.
Explore this area of the Moon in more detail using the ActReact QuickMap Web Interface.
Examine the Apollo 15 traverses for yourself at the LROC Apollo 15 Traverse Page.
Image Credit: USGS (Map), NASA/Goddard/Arizona State University (Image and Apollo 15 traverse), and Irene Antonenko (compositing).

You can also find geologic maps for other planetary bodies in our solar system. Many haven't been studied long enough to have geologic maps made of the entire surface, but there are sections that have been mapped. Again, a quick Google search can find you lots of interesting tidbits. On a whim, I searched for "geologic map of Titan", which is one of Saturn's moons, and found an amazing little geologic map and article on the Selk crater of Titan, from The Planetary Society. Seriously, you should go check it out!

So, I hope this article has piqued your interest and inspired you to go check out some geologic maps, whether they are of Earth or any other planetary body, and celebrate Geological Map Day.

Saturday, 28 September 2013

Mining Astroids: Not Just Any Asteroid Will Do

The other day, I stumbled across an article about plans to mine asteroids by a company called Planetary Resources. This got me thinking about the feasibility of asteroid mining and I decided to look into it in more detail.

Asteroid Vesta
Asteroid Vesta was the recent target of much investigation by the Dawn mission. This research confirms that Vesta is composed of basaltic rocks and so is not a good candidate for mining.
Image Credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA
Scientists have been seriously considering the question of asteroid mining for over 20 years. In the 1990's Dr. Jeff Kargel, who was working for the US Geological Survey at the time, conducted a detailed study on asteroid mining and found that it would be economically worthwhile. Since then, other researchers have looked at this issue and all generally agree that mining the right asteroids could provide enormous benefit to society and the economy.

It is important to note that not just any asteroids can be mined profitably, because not all asteroids are alike. Roughly speaking, there are three main types of asteroids; C-Type, S-Type, and M-Type. C-Type asteroids, also known as Carbonaceous Chondrites, often contain lots of water (as much as 20%) and other organic compounds. They are thought to be very similar to the building blocks of our Solar System. S-Type or Stony asteroids contain mostly rocky materials. And M-Type asteroids, which are also called Metallic or Iron asteroids, are made up mostly of iron and nickel.

Iron asteroids are thought to come from the metal-rich cores of disrupted planetesimals. Early in the Solar System, orbiting material would have accreted into small planetary bodies. The largest of these would have differentiated, with the heavy metals sinking into an iron-rich core and the lighter silicates left behind to form a rocky mantle around the core. Some of these differentiated bodies would then have been broken up by impacts, creating smaller asteroids of metallic and stony compositions.
Differentiated Planetesimal
Large planetesimals will differentiate so that the heavy metals sink to form a core, leaving the lighter materials surrounding the core in a stony mantle. When these differentiated planetesimals are broken up by a large impact, chunks from the core end up as metallic asteroids, while bits from the mantle make a type of stony asteroids.
Image Credit: NASA/Goddard/Arizona State University and Irene Antonenko

 The iron asteroids seem to be attractive targets for mining, because they are made up almost entirely of iron and nickel. Scientists have estimated that one metallic asteroid with a diameter of 1 km could provide enough iron to supply the entire world's demand for 15 years (plus about 1000 years worth of nickel). This sounds wonderful until you realize that getting this metal to the Earth's surface, where it will be used, is a big problem. If we wanted to supply all the world's iron needs from asteroid metal, we would have to bring over 1.2 million tonnes of iron to the surface on a daily basis. Dr. Kargel points out that when this material entered the atmosphere, it would release a great deal of energy, effectively equivalent to a 10 megaton nuclear explosion, each day! We currently don't know how to safely dissipate this much energy in the atmosphere without causing significant environmental damage, not to mention addressing the risks of catastrophic accidents.

Fortunately, metallic asteroids also contain various precious metals (such as gold and platinum) that were dragged along with the iron when the original core was formed. The amount of precious metal in the richest iron asteroids is about 350 parts per million (ppm). This may seem like a very small amount, until you consider that on Earth, gold is mined at concentrations as low as 5 ppm. Dr. Kargel estimates that at 350 ppm, a 1 km diameter asteroid would contain about 400,000 tonnes of precious metals, worth many trillions of dollars.

If an iron asteroid was mined and processed in space, and only the precious metals returned to Earth, this would be much easier to manage. Spreading the recovery of these metals over a 20 year period would deliver only 55 tonnes of material per day, which is much less than the rate at which cosmic dust naturally falls on the Earth (about 130 tonnes per day). Dr. Kargel anticipates that this would have a negligible environmental impact, possibly even smaller than what traditional mining does to the Earth today.

Iron Meteorite
Iron meteorites are thought to come from iron asteroids. This iron meteorite has been sliced in half to show off the spectacular metal grains inside. It is thought that the insides of iron asteroids look like this, too.
Image Credit:  Carl Allen, NASA JSC
Of course, not all metallic asteroids are alike. In fact, the precious metal content of the iron asteroids is extremely variable, ranging from rich (350 ppm) to poor (less than 100 ppm). Clearly, the rich asteroids are much more attractive as mining targets, all other factors being equal. So, a detailed exploration of the potential metallic asteroids needs to be conducted, to determine which are the best mining candidates.

 But it's not just the metallic asteroids that make good mining targets. Dr. Kargel points out that some stony asteroids could also be mined profitably. This seems counterintuitive, considering that the process of planetesimal differentiation, which concentrated metals in the iron asteroids, would leave the stony asteroids depleted in metals. However, not all stony asteroids have their origins in differentiation. Some stony asteroids come from undifferentiated planetesimals, so all their metals are still dispersed among the stony parts.

Again, not all undifferentiated stony asteroids are alike. Dr. Kargel believes that the most likely mining candidates are a group of asteroids called the ordinary LL chondrites. The LL stands for low iron and low metal, but ironically these have the highest amounts of precious metals, because in undifferentiated stony asteroids the amount of precious metals is inversely proportionate to the amount of iron and nickel (so iron-poor asteroids have more precious metals, while the iron-rich ones have less). As a result, the LL chondrites contain only about 5% iron and nickel, but have as much as 200 ppm of precious metals, which is more than some metallic asteroids.

The advantage of mining stony asteroids is that their surfaces are covered by a thick regolith of impact-pulverized rock. Constant bombardment by other asteroids, of all sizes, effectively grinds the stony materials at the surface, until they are only about 25 micrometers (or 0.025 millimeters) in diameter, which is roughly the size of baking flour. At this size, most of the regolith particles would be monominerallic (meaning they contain only one mineral) making it possible to pick out only the minerals you want (the precious metals) with something like a finely tuned electromagnetic rake. This process is much easier than having to separate out precious metals from a solid body, as would need to be done for the iron asteroids, which are too strong and ductile for impacts to produce this kind of pulverized regolith.

So, how much precious metal could you get from the regolith of a stony asteroid? It is thought that most stony asteroids have regolith layers that are at least 100 m thick. Based on this, Dr. Kargel estimates that an LL chondrite asteroid of about 3 km in diameter would yield on the order of 200,000 tonnes of precious metal.

It seems both metallic asteroids and LL chondrite-type stony asteroids have the potential to provide very large amounts of valuable precious metals. The LL chondrites may be easier to mine, but the metallic asteroids have a larger amount of the desired materials. In other words, both have their advantages. So, now all we need to do is go prospect some asteroids and pick out the best ones to mine!

Kargel, 1994, metalliferous asteroids as potential sources of precious metals, Journal of Geophys. Res., V99(E10), 21,129-21,141, DOI: 10.1029/94JE02141.

Wednesday, 28 August 2013

The Amazing Eruptions of Io

Voyager Io Plume
Volcanic explosions on Io appear to be relatively frequent occurrences. This Voyager 1 image taken on March 4, 1979, shows a tall plume of lava exploding 100 miles above Io's surface.  The source of the plume is Loki, one of the most volcanically active regions on Io.
Image Credit: NASA/JPL
Summer is almost over, the kids will be back at school soon, but I just couldn't wait and had to find some time to write about the recent discovery of a new volcanic eruption on Jupiter's moon Io. 
On Aug 15, 2013 Dr. Imke dePater of the University of California at Berkley, was observing Io with the Keck II telescope on Mauna Kea in Hawaii, when she saw a massive volcanic eruption. This eruption turns out to be one of the top 10 most powerful eruptions observed on Io to date. Dr. dePater and her team have been able to pinpoint the location of this eruption to the Rarog Patera region, which is named for a Czech fire deity.

The Rarog Patera region was first identified in Voyager 1 imagery, where lava deposits appear to flow away from a broad, irregular depression. This region was also identified as a definite hot spot by Dr. Julie Rathbun and her co-workers, using data from the photopolarimeter-radiometer on board the Galileo space craft. However, the noted temperatures in this region are more indicative of an old, cooler lava flow, rather than an active region capable of producing the kind of eruption observed by Dr. dePater. That said, it is possible that Rarog Patera has been active prior to the Galileo mission. In August of 1998, Dr. Rathbun and her colleague Dr. Spencer observed an eruption using NASA's ground-based Infrared Telescope Facility on Mauna Kea in Hawaii. Rarog Patera is one of three possible source candidates for this eruption.   

Io Mosaic
This mosaic of Io was produced with data from both the Voyager 1 and Galileo missions. The colours are similar to what you would see with the naked eye, but enhanced to accentuate the many volcanic structures that are found on the surface of Io. The black arrow points to the Rarog Patera region, where a powerful volcanic eruption was observed on Aug. 15, 2013. 
Image Credit: NASA/JPL/USGS, with annotation by Irene Antonenko

Rarog Patera
The black arrow points to the Rarog Patera volcanic region. In this segment of Voyager 1 image PIA01485, lava flows can clearly be seen originating from what looks to be a broad, irregular depression.
Image Credit: NASA/JPL/USGS, with annotation by Irene Antonenko

Loki Lava Lake
This 200 km U-shaped lava lake in the Loki volcanic region is thought to be responsible for the episodic eruptions that occur here. This Voyager 1 image shows gray spots in the dark lake, which may be due to "icebergs" of solid sulfur in a liquid sulfur lake.
Image Credit: NASA/JPL

If Rarog Patera has erupted in the past, that would make it one of the periodically erupting volcanoes on Io. The most famous such volcano is Loki, which has an eruption cycle of about 540 days, with the actual eruption event lasting on the order of 230 days. This periodicity is thought to result from the cyclical overturning of a lava lake. Quiet times represent periods where the lava lake is covered by a solid crust. When this crust eventually sinks into the liquid lava underneath, it produces a long-lasting eruption event. Most volcanic eruptions on Io, however, are not periodic and are much less short lived than Loki, lasting on the order of days. As more research comes in, it will be interesting to see what kind of eruption Rarog Patera is, and what that tells us about the style of volcanism at this region.

No imagery of the current Rarog Patera eruption has been released yet. But, Io is one of the most volcanically active objects in the Solar System and changes on the surface due to volcanism have been recorded before. In the late 1990's, the Galileo spacecraft acquired multiple images of the Pillan Patera region, showing significant changes over a time span of about 2 years, with new deposits from volcanic eruptions clearly visible. Hopefully, future missions will be able to image the Rarog Patera region, to show us what kinds of changes this very massive eruption has wrought. 

Changes at Pillan Patera
Dramatic changes can be seen in this series of Galileo images of the Pillan Patera region of Io. Taken on April 1997 (left), September 1997 (centre), and July 1999 (right), these images show that a massive dark deposit about 400 km in diameter (upper right) was produced by a volcanic explosion at Pillan Patera sometime between April and September 1997.  Later, an unnamed volcano to the right of Pillan erupted, depositing a smaller patch of dark material, surrounded by a yellow ring.
Image Credit: NASA/JPL/University of Arizona
Major Volcanic Eruption Seen on Jupiter’s Moon Io, Universe Today, Aug 23, 2013. 
Huge lava fountains seen gushing from Jupiter moon, New Scientist, Aug 20, 2013.
USGS Geologic map of Io.
Rathbun and Spencer, 2010, Ground-based observations of time variability in multiple active volcanoes on Io, Icarus, V209, pp625-630, DOI 10.1016/j.icarus.2010.05.019.
Rathbun et al., 2004, Mapping of Io's thermal radiation by the Galileo photopolarimeter–radiometer (PPR) instrument, Icarus, V169, pp127-139, DOI 10.1016/j.icarus.2003.12.02.

Friday, 19 July 2013

How to Look for Hidden Mare

Mare Humorum Area
Red dots show the locations of basalt spectra on this map of the western limb of the Moon, while the background colouring gives information about the iron content of the surface. In general, mare are iron-rich (green to red), while the highlands are iron-poor (blue).  However, these data show that things can be more complex than expected.
Image Credit: Irene Antonenko
Well, it's summer. The kids are out of school and my home office is not the most peaceful place to work right now. In other words, it has been hard to find quiet time in which to write.  All this is my complex way of apologizing for not getting this blog post out earlier. But, on this very hot evening, I have a few calm hours to tell you more about my research on the Moon.

My last blog post talked about how I use spectra to identify complexity on the Moon's surface. The fascinating thing is, this complexity often reveals that interesting things are happening below the surface as well.

In order to study what's going on below the Moon's surface, or the lunar stratigraphy, I look for basalt spectra in areas that are otherwise highland-like in their signature. These kinds of basalt spectra are often associated with impact craters. What's happening here is that impacts are excavating hidden basalt units and distributing them in an ejecta deposit around the resulting crater, thus exposing them on the surface.

Basalt-excavating Crater
The presence of basalt spectra (red dots) on the rim of this crater, along with higher iron compositions (green-yellow) than in the surroundings (blue), shows that basalt materials have been excavated from deep below the surface.
Image Credit: Irene Antonenko

So, what are these hidden basalt units and how did they form? The most established theory is that these units represent ancient hidden mare deposits. 

Very early in the Moon's history (about 4.2-3.2 billion years ago), massive amounts of basalt flowed out onto the lunar surface. These solidified to form the mare deposits. However, during this time, occasional gigantic impacts were still hitting the Moon's surface. In some cases, the ejecta from such huge impacts was emplaced on top of an existing mare unit. Since these ejecta deposits could be quite thick (up 2 kilometers thick), they covered the underlying mare and hid it from view, forming a "cryptomare" (literally meaning hidden mare).  Subsequent impacts, if they were large enough, were able to penetrate through the overlying ejecta layer and into the cryptomare, excavating the hidden basalt material and emplacing it around the newly formed crater.
Cryptomare cross section
A cryptomare forms when ejecta from a large impact covers and obscures a mare deposit. Subsequent impacts into the region can penetrate the obscuring layer and excavate the underlying cryptomare material, exposing it on the surface.
Image Credit: Irene Antonenko
In my work, I search for craters that excavate hidden basalts. When I find them, I measure their diameter and use that to estimate their depth of excavation - that is the maximum depth from which this crater was able to excavate material. The smallest basalt-excavating craters in an area provide an estimate of the top of the cryptomare deposit, since smaller craters wouldn't penetrate through the overlying ejecta layer. The largest basalt-excavating craters in an area provide an estimate of the bottom of the cryptomare deposit, since larger craters would start to excavate the underlying highland crust and so obscure the basalt signature. The side boundaries of the cryptomare are estimated by where the basalt-excavating craters stop, since no craters of any size would excavate basalt where there is no hidden mare.  By using these techniques, I am able to estimate the size and shape of cryptomare deposits, allowing us to study them in more detail.

Schickard Area
Mapping of basalt (greed dots) and highland (blue dots) spectra suggests that there may be more than one cryptomare deposit in the Schickard crater area.
Image Credit: Irene Antonenko
However, things aren't always quite so simple. In one study area, the Schickard crater on the south-western limb of the Moon, my research suggests that there may be two layers of cryptomare present. If my interpretation is correct, this means that shortly after Schickard crater formed in the highland crust, it was flooded with basalts. Then, these basalts were obscured by a thick ejecta deposit, forming a deep cryptomare. Later, more volcanic material was deposited on top of the ejecta layer. Another impact then obscured this second mare unit, creating a shallow cryptomare. Finally, a small amount of volcanic flooding formed two small mare patches that can be seen at the surface today.  This kind of work is rather exciting, because it suggests that areas of the Moon may be quite complex below the surface, resembling an intricate multi-layer cake!

Thursday, 27 June 2013

The Spectra of a Complex Moon

Full Moon
Earth's Moon consists of bright highland areas and dark mare areas. 
Image Credit: NASA/JPL/USGS
As a planetary geologist, my work revolves around looking at gorgeous pictures of the Moon and then examining squiggly lined spectra at various locations in these pretty pictures. (To learn more about spectra and how they work, please check out my last blog post, The Colour of Squiggly Lines.) For me, the point of this exercise is to figure out what's going on with the surface of the Moon. From this kind of research, I and other researchers are learning that the Moon is far more complex than had originally been suspected.

Very basically, the Moon has two main types of terrains on its surface. There are the bright areas that make up the rugged and heavily cratered highlands and the dark areas (which Galileo named "mare", the Latin word for seas) that make up the flat and relatively smooth lowlands. The highlands represent the ancient crust of the Moon, the part that solidified from a lunar magma ocean about 4.5 billion years ago. The maria (plural form of mare) are massive deposits of solidified lava. Many of the maria are circular because the hot magma flowed into large impact basins, which had been excavated by bombarded during the early Solar System.

The highlands are made up mostly of a rock called anorthosite, whose main mineral is plagioclase. Plagioclase has a very non-descript spectra. The maria, as was already mentioned, are made up of solidified lava, which is to say basalt. The main mineral of lunar basalts is pyroxene, which unlike the highland plagioclase, has a very distinctive spectra.
Highland/Mare Spectra
The lunar highlands and mare can be distinguished using the spectra of their predominant minerals.
Image Credit: NASA (Images), USGS Spectral Library (Spectra), Irene Antonenko (arrangement)

Pyroxene contains two prominent absorption features, places where the spectra dips downwards. One such feature is located at a wavelength of around 1 micron, and another very broad feature is found at around 2 microns. This very pronounced difference between plagioclase and pyroxene spectra makes it possible to use them to distinguish between highland and basalt materials on the Moon. 

Clementine Spectra
Spectra from the Clementine mission, which only have 5 different wavelength bands, can still be used to separate highland from basalt spectra in many cases.
Image Credit: Irene Antonenko
The very first remotely sensed spectral data returned from the Moon was from the Clementine mission in 1994. The sensor on this mission was fairly minimal, providing only 5 bands in the Ultraviolet-Visible range of wavelengths. These 5-band spectra look quite different from laboratory spectra and contain much less information. Still, they have proven to be very effective at distinguishing between mare and highland minerals. And, although new sensors that have been flown since (such as the Moon Mineralogy Mapper on India's Chandrayaan-1 mission from 2009) have many more spectral bands, the Clemetine data set remains the only fully global data set and continues provide very useful information for studying the lunar surface.

My research uses Clementine spectra to map the locations of basalt minerals on the lunar surface. These studies have shown that basalt minerals are sometimes found in places that are very far away from any known maria. This is telling us that something much more complex is going on at these sites.
Mare Humorum Area
Locations of basalt spectra are shown as red dots on this map of the western limb of the Moon. The background colouring gives information about the iron content of the surface. In general, mare are iron-rich (green to red), while the highlands are iron-poor (blue).
Image Credit: Irene Antonenko

The current thinking is that such places represent locations where impact craters excavate hidden mare deposits or basalt dikes (intrusions of lava that didn't quite make it to the surface). This means that mapping the surface composition can also tell us about what is happening below the surface, allowing us to study the stratigraphy of the Moon.

In a later post, I will go into more depth on lunar stratigraphy and talk about how I use these techniques to search for hidden lunar mare deposits, called cryptomare.

Saturday, 15 June 2013

The Colour of Squiggly Lines

This week, I gave a talk about my lunar research to the Toronto chapter of KEGS, the Canadian Exploration Geophysical Society.  The interesting thing about my research is that whenever I want to go into detail about it, I generally have to do that using a bunch of squiggly lines called reflectance spectra. Since most people, even in the planetary field, are not reflectance spectroscopists (yes, that is what we are called), I usually have to explain what that means. And so I found myself explaining reflectance spectra to a group of geophysicists last Tuesday. That got me thinking that it was time to write a blog post about these squiggly lines.

Electro-magnetic spectrum
The electromagnetic spectrum includes many different kinds of radiation, such as gamma rays, X-rays, the visible colours of light, radio waves, and microwaves.  Each radiation type has a defining wavelength, which is the distance between two side-by-side crests in the wave that makes up that radiation.
Image Credit: Irene Antonenko
To understand spectra, you first have to understand light. Most people know that white light is made up of all the colours of the rainbow. Each colour represents a different wavelength of light, which is the distance between two crests of a light wave (since light travels in waves). But visible colours are only a small portion of all the possible wavelengths, which together make up the electromagnetic spectrum. At shorter wavelengths, we have X-rays, gamma rays, and ultraviolet light. At longer wavelength, we have infrared light, microwaves, and all the different frequencies of radio waves.

When light hits an object, not all of the different wavelengths (or colours) that make up that light will be reflected from the surface of the object.  The wavelengths that do get reflected are what gives that object its colour. For example, a green object looks green to us because that is the wavelength of light that it reflects. If no wavelengths are reflected, the object will look black. But if all of the wavelengths are reflected the object will look white.

Reflecting light
The colour of an object is determined by which wavelengths of light are reflected from the object. If no light at all is reflected, the object will look black. If all light is reflected, the object will look white. If only green wavelengths of light are reflected, the object will look green.
Image Credit: Irene Antonenko
Sample Spectra
When the percentage of light reflected from an object is plotted for each wavelength of light, the result is a squiggly line called a reflectance spectrum. The colours below this spectrum remind us of the corresponding colour of each wavelength.
Image Credit: Irene Antonenko


However, most objects are much more complex than this simple example. They reflect in many wavelengths, and they reflect some percentage at each of these wavelengths (it's not just all or nothing). So, when you plot the percentage of light that is reflected at each wavelength, you get a squiggly line that we spectroscopists call a reflectance spectrum (spectrum is the singular form of spectra). The thing to note is that most of these squiggly spectra have a number of peaks and valleys. There is no one wavelength to establish colour. Instead, the colour is determined by the strongest reflectors.

Leaf Spectra
The chlorophyll found in plants reflects most strongly at green wavelengths, giving them their green colour. Deciduous maple leaves lose their chlorophyll as the weather cools, allowing other compounds to dominate the leave's colour.
Image Credit: Irene Antonenko

Lunar Spectra
The three main minerals that are found on the Moon have spectra that can be distinguished relatively easily from each other. This allows us to study the composition of the lunar surface using remotely sensed spectral data.
Image Credit: Irene Antonenko
A great example to explain this is the leaf spectrum.  All vegetation reflects very strongly at infrared wavelengths (greater than 700 nanometers).  If our eyes could see infrared, all trees, leaves, etc. would look infrared to us. But our eyes don't see those wavelengths, so the next strongest reflected wavelengths in the range we do see (about 400 to 700 nm) produce the leaf's colour. Chlorophyll, which converts sunlight to energy in plants, reflects most strongly at green wavelengths. This is why most plants look green. But, in deciduous trees, such as maples, the leaves stop producing chlorophyll when the weather cools. As the amount of chlorophyll drops, the strong chlorophyll reflectance in green disappears. This allows other compounds, which reflect more strongly at red or yellow wavelengths (but not as strongly as the chlorophyll does in green), to be revealed. 

The important thing about spectra, which the leaf example demonstrates quite nicely, is that these squiggly lines can be used to identify components that make up the object you are observing. In the case of the leaf, its spectrum can tell you (among other things), whether or not chlorophyll is present in the leaves - though our eyes can tell us that almost as readily.

Which brings us to the planetary portion of the story. Reflectance spectra can be used to tell us which minerals make up the rocks on planetary surfaces. Each mineral has a very distinct spectrum, which can be used by experts and highly specialized software to identify it, allowing us to study the composition of planetary surfaces with remotely sensed spectral data.

My next post will build on this concept and talk about how we use spectral data to study the surface of the Moon...

Tuesday, 28 May 2013

A Volatile Moon

Last week, I attended the NASALunar Science Institute's Workshop without Walls on Lunar Volatiles. The really interesting thing about this workshop is that it was conducted completely on line. Presenters gave their talks using a web cam, and they and their slides were displayed in a split screen through a regular web browser. This meant the talks could be followed by anyone with a computer and internet access. I watched from home, but some people gathered in designated meeting hubs, to get more of a communal experience. Participants could also interact with the speakers or other participants through chat windows.  The best part of this kind of virtual workshop, is that all the talks were archived and are now available through the site's Schedule web page.  I would encourage you to go check it out!

Fictitious Moon Lake
There is a lot of water on the Moon, but not this much!
Image Credit: NASA/GSFC/Arizona State University and Irene Antonenko
In the very first talk of the workshop, Dr. Larry  Taylor from the University of Tennessee summed up the exciting turn-around that has happened in the study of lunar volatiles over the past few years.  Up until relatively recently, we thought the Moon was "bone dry", but now we know it is quite "wet" in a variety of ways.

Back in the Apollo era, the accepted wisdom was that the Moon contained effectively no water or other major volatiles, not only on the surface, but in the rocks themselves too. It was believed that all the volatiles would have evapourated during the Moon's very hot formation and any volatiles added later on wouldn't have stayed on the surface very long - volatiles being relatively light would have easily escaped the pull of the Moon's weak gravity. So, the Moon must be very dry.

This belief was so entrenched in the 1970's that when rust was found in samples brought back by the Apollo  missions, it was determined that water from the Earth must have contaminated the sample boxes and allowed the iron-rich lunar materials to rust.  Dr. Taylor himself pleads guilty to pushing this interpretation and confesses that "his big mouth" convinced people that this was just terrestrial contamination.

Moon Mineralogy Mapper Water Data
This data from Chandrayaan-1's Moon Mineralogy Mapper instrument shows the distribution of various materials on the Moon. Small amounts of water and OH molecules show up as blue, and are clearly concentrated at the poles, where low temperatures are more likely to trap them.
For more information on this image, check out the Moon Mineralogy Mapper Exploration Resources page.
 Image Credit: NASA/ISRO/Brown Univ.
However, in 2009 it was finally realized that there was water on the Moon, when data from the Clementine, Lunar Prospector, and Chandrayaan-1 orbital missions all showed evidence for its presence.  With so much data pointing to water on the Moon, the reality could no longer be ignored.  Dr. Taylor himself admits that he has completely changed his position on the topic!

With all this orbital evidence for water, a number of researchers took another look at the Apollo lunar samples. Analyzing basalt rocks with techniques that were not available in the 70's, they found a significant amount of water within the minerals that make up the rocks, in some cases as much as 1% of sampled minerals.  

The important thing about basalt is that it is solidified magma. As such, basalts provide a sample of the lunar interior, from where the magma originates. So, the presence of water in the basalt rocks means that there must have been water in the lunar interior at the time these basalts formed on the Moon's surface.
Thin Section of Apollo Basalt 14053 in Cross Polarized Light
A thin slice of Apollo basalt sample 14053 viewed magnified in cross-polarized light (xpl). Each type of mineral interacts differently with the polarized light, producing the various colours we see.
You can explore this thin section for yourself at The Open University-NASA Virtual Microscope.
Image Credit: NASA/Open Univ.
So, the questions now is, where did this water originally come from? If the Moon formed when a Mars-sized object crashed into the early Earth, the ejected debris that formed the Moon really would have lost all its volatiles to space. One theory is that comets that impacted into the Moon very early in its history could have delivered enough water to seed the interior with the needed volatiles. The problem, however, is that analyses of lunar water show that it is more similar to asteroid and terrestrial water than water from comets.  So, the water we see could not have come from comets.

We know that currently water on the surface is being replenished by impacts and solar wind. Even small hypervelocity (~2-20 meters per second) impacts can crush atomic molecules, breaking them up and leaving "dangling bonds" of oxygen (oxygen is a major component of all rocks, and so is very plentiful). Hydrogen from the solar wind then bonds to these available oxygen atoms, creating water. However, this process works only on the surface, and can't account for water deep in the lunar interior.

At the moment, we have no idea how water from the Earth and asteroids got into the Moon's interior. A lot of work still needs to be done to understand this aspect. But, the field is hopping, with lots of renewed interest in a topic that was, until recently, thought to be impossible.  Stay tuned....

Taylor L., 2013, Where and in what physical form do volatiles exist: Perspective from sample analysis? NLSI Workshop without Walls - Lunar Volatiles, May 2012. AbstractTalk.