Recognizing Plume Features on Enceladus

Each year, the Lunar and Planetary Science Conference condenses more and more people into the Waterway Marriott in The Woodlands, Texas. It’s reaching a critical mass. This year, around 2,046 abstracts were accepted for presentation in talk or poster form. There’s too much to sift through except in your specific field, but I wanted to branch out a bit. So I put out a call on Twitter for a random number between 1001 and 3046 (the numbering system starts at 1001). Lockwood chose 2601, and from that abstract (PDF link) we have the topic of this post: Plumes on Enceladus.

enceladus_cassini_PIA07800f
Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA, APOD

This well-written abstract covers a few interesting points on using satellite photos to infer geological processes. The Cassini spacecraft has a narrow angle camera with broadband filters. These filters allow specific wavelengths of light, ranging from ultraviolet to infrared, to pass through. The wavelengths of light, in turn, are affected by objects they bounce off of. Different minerals reflect or absorb specific wavelengths of light differently, a feature used in many remote observations of planetary bodies to infer (deduce?) surface composition.

 

Paul Schenk and the other authors of this abstract used surface images filtered using two infrared spectra and one ultraviolet spectrum. One of the main goals of their analysis was to use our observations of Enceladus to use as a comparison to what we are beginning to study on Europa. There are issues to be worked out, with, for example, how the orientation of the spacecraft relative to the surface of a world will affect the observed spectra. If light is uniformly scattered by an object, then we can observe it from any angle and it will look the same. But if there is directionality to the scattering, then the angle we observe will affect what we observe. That will be an issue if we have more limited observations (like at Europa).

The image above was featured in the Astronomy Picture of the Day

Examining Bubbles as a Novel Nuclear Forensics Tool

The project began with a simple question prompted by this thin section. Notice anything particular about it? Ruminate for a bit while I fill in some details.

ppl Trinitite
Image mosaic of a Trinitite sample viewed in cross section. Some salient features annotated. The sample is ~1 inch wide.

That there is a little slice of nuclear post-detonation material called Trinitite. I spent 2014 working the stuff, which is a glassy product of the first atomic bomb detonation. On a July morning in 1945, the Plutonium-powered nuclear device was detonated at the Trinity site of the White Sands Proving Grounds in Alamagordo, New Mexico. The test yielded the equivalent of ~20 kilotons, the fireball exceeded 8000ºC, and it created a ~1 km wide (shallow) crater.

trinity_explosion_sequence
Stills from the Trinity test. (Source: US Gov)

Typical atomic bomb stuff. The terrible fireball is eye-catching, sure, but as a geologist my gaze is usually drawn to ground level. Fewer people appreciate what’s going on down there (until there’s a house in frame to obliterate). There’s a supersonic shock wave, and the upper surface gets scoured by heat and wind. It is a chaotic mixture of bomb material and local geology — and an interesting, unique mixture at that. Nuclear materials have certain fingerprints – chemically and isotopically – that can be tied to a source or sources. During the explosive process, this signature gets imprinted in post-detonation material (typically acronym’d as PDMs). Trinitite is the first such human-made material. It is not the first ever because there are natural fission reactors in some parts of the world where uranium has built up over time and “gone nuclear”.

 

trinitite whole rock
Top down view of the sample sectioned above

So back to the thin section that started it all. Notice anything unusual about the bubbles? Our question was simple: why are there so many bubbles along the bottom? Is that a real feature, or are we seeing patterns in randomness?

In looking at that and a couple additional samples, it was apparent that something was different between the top and bottom vesicles. With a simple hypothesis to test, it was time to measure. So I ran statistical analysis of the size and shape of bubbles in vertically cut trinitite sections. What we found agreed with our initial impressions, and our findings were published in a PLOS ONE paper earlier this year (and free to read).

It turned out there was more to see than just “more vesicles at greater depth”. In each of the samples we studied, the upper 2-3 mm was relatively devoid of bubbles. Below this region, there was an increase in the size, number, and elongation of vesicles. All this points to a different formation mode between the upper and lower zones. Our current sequence of events starts with the blast heat melting the desert surface to a few mm deep. There was enough moisture to cause extreme bubbling as the melt degassed (released water vapor and other volatiles to the atmosphere). As the fireball grew into the classic mushroom shape, it drew air inward. This cooler air quenched the upper surface of the glass, but there was enough heat and/or water in the glass to cause further bubbling while maintaining at least a semi-molten state. However, the bubbles were trapped and couldn’t degas at the surface. Thus the lower bubbles had time to grow together and flattened out below the quench zone, in the 2-3 mm deep region. There was also some “late” (a few seconds to minutes) contribution from fallback, where particles in the fireball settled out in a molten (or partially molten) rain. Our sequence matches well with previous interpretations of Trinitite formation. We hope it shows this type of textural analyses is a useful complement to other techniques in nuclear forensic analysis.

In the end, it was a fun little article to write, and reinforced the idea that it just takes an off-the-cuff observation to start an interesting research project.

Full Article: P. H. Donohue and A. Simonetti (2016) Vesicle Size Distribution as a Novel Nuclear Forensics Tool. PLOS ONE 11(9) e0163516.

Lunar geochemical datasets on MoonDB

The Apollo missions returned 2200 samples comprising “382 kilograms (842 pounds) of lunar rocks, core samples, pebbles, sand and dust from the lunar surface” (quoted from ref). Since then, we’ve sliced, diced, dissolved, vaporized, irradiated, and applied just about every other analytical tool to investigate these samples. An incomplete list of geochemical investigative techniques used includes: electron microprobe, instrumental neutron activation analysis, various mass spectrometry techniques (multi-collector, laser ablation, thermal ionization, time-of-flight, secondary ion), X-ray fluorescence, and cathodoluminescence. The results of these studies are scattered across the past 46 years of published literature, in various formats and accessibility.

Most lunatics probably have a ragtag collection of spreadsheets incorporating personally vetted data from the literature. But it is easy to miss good papers, and time consuming to re-type poorly scanned copies. I have spent a fair bit of time hunting down literature data on lunar basalts, and these important datasets are not always easy to find or re-use in a useful way. To make this process easier, MoonDB aims to serve as an open access data repository for lunar geochemical and petrological data. It is led by Kerstin Lehnert, PI of the EarthChem/PetDB project.

MoonDB_screenshot

MoonDB (http://www.moondb.org/) is launching a program to restore and preserve lunar geochemical analytical data. The framework will be nearly identical to the Petrological Database (PetDB), where geochemical data from terrestrial studies are freely available. The framework is still under development, but it appears they have data from 500+ papers archived and awaiting release.

In addition to incorporating peer reviewed article data, MoonDB plans to serve as a “publishing” venue for otherwise unpublished datasets. Researchers occasionally collect more data then they eventually need to include in a manuscript. Or they might perform foray analyses on samples for projects that never go anywhere. MoonDB hopes to rescue these from eventual hard drive failure and publish them in a citable format. It would also be a way for instrument techs and lab managers, who might not be publishing regularly, to get some additional credit via citable works. For researchers with NASA funding, MoonDB may suffice to meet the newly implemented Data Management Plan requirements.

The topic of unpublished datasets drew the most discussion at a MoonDB lunch/seminar during the Lunar and Planetary Science Conference in March 2016. Quality control was probably the biggest concern, especially when discussing the possibility of future versions of MoonDB incorporating geochronology datasets. Without context, you can calculate just about any date you want from a sample set (but really, this applies to published values as well). To combat this and other potential issues, unpublished datasets will be subject to internal review, and/or may be sent to external reviewers. In some cases, these may end up being more closely scrutinized than if they had been published online as supplementary material to a paper. On the subject of context, there is a potentially powerful (if it gets used) ability to link analyses with specific locations to sample photos (e.g., on thin sections). 

MoonDB is still seeking feedback from lunar researchers on community needs. Those who have previously submitted data to PetDB or earthchem could be especially helpful in spotting customizations necessary for lunar datasets.

Visit MoonDB.org

SOEST Open House 2015

Started in 1991 (I think) and held every two years, SOEST Open House is a massive science outreach event. Over the course of two days, we showcase science to more than 4,000 people, most of them grade-school students.

This is my first Open House, and I helped out at the Colors of Space exhibit. Our puny human eyes only view a narrow band of light (~380 nm to 700 nm) compared to possible spectra. We’ve developed instruments to artificially extend our vision beyond human shortcomings.

NOAA_visible_spectrum

At the Colors of Space exhibit, we had two thermal infrared cameras, a near infrared spectrometer, and a couple of microscopes with lunar samples. One thermal camera was pointed down the hall to grab the attention of visitors.

Our thermal cameras monitored the 10 micron wavelength. One use of thermal cameras is to locate rocky regions on other planets. Large rocks have higher thermal inertia, so rocky regions remain warmer longer than smooth surfaces. We also use near infrared spectrometer to discern between similar-appearing things. Our demo used common cooking materials, like flour vs cornstarch, and sugar vs salt.

And of course, visible light is still an important part of research. We had Apollo samples on display under a binocular microscope, and a thin section of olivine basalt 12008 (from the Apollo 12 mission) in a petrographic microscope. This is where I posted up. I would have them look at the thin section in normal light, and then put in the cross-polarization filter to show how we use light properties to identify minerals.

The change caused a lot of eyes to widen in amazement. “It looks like a church”, one kid said. Other exclamations included “kaleidoscope!” and “a bunch of dead butterflies”. Kids are weird.

To cap it off, we had an infrared photobooth with some sweet plastic props. The cold plastic props, transparent to our eyes, were opaque at the 10 micron wavelength the infrared camera monitored. Note that you can’t really see the props in the below video until it pans over to the IR screen.

Visitors of all ages were able to take home a printout of their thermal image with some of the science behind it shoehorned in. Everyone loves a keepsake!

Leading up to this year’s open house, I had tried finding some details from past years. There were surprisingly few photos or details posted, and I realized there wasn’t much documentation of events happening. I had originally planned to set up a twitter account to tweet photos from our photobooth. That idea began to grow. Why not use it to share all aspects of open house? But how? I was going to be rooted to the Colors of Space exhibit all day. How could I find out who would be tweeting within the department? Instead of me searching for them, why not let them come to me? I sent out an email with the password to the account to the department mailing list. Unfortunately, the only people that took me up on it were also at my table, so it didn’t expand our feed as much as I had hoped. But I also teamed up with volcanology prof/chair Ken Rubin, which gave us a little more variety.

All in all, it was worth a shot, and we had a bit of engagement from the UH Manoa twitter accounts and some visitors. I think next open house, it would help to get the word out earlier, and have an option for people to send photos without having to be familiar with twitter.

Mt. St. Helens, Part III: Epilogue

Morgen and I spent a few hours hiking in the blast zone of Mt. St. Helens. Around us were signs of recovery from that singular event. But in reality, it really wasn’t a single event, isolated in time. Especially for Washingtonians. The dramatic and deadly initial blast rightfully receives significant coverage when talking about May 18th. But for ten hours (hours!) afterward, Mt St. Helens continued to erupt rock fragments (tephra) that spread across eastern Washington.

Our base of operations was in central Washington near Yakima, 120 miles east of Mt. St Helens. After our hike, our host told us her story about that day in 1980 Yakima: The weather forecast was for sunny skies; Yakima gets 300 days of sun a year, you know. But as she readied for church, the skies began to darken. It wasn’t long before she recognized the event would be rather unique. She placed a small bowl outside the door to catch some of the falling pyroclastic material. With my geology background, she knew she had my attention when she said “I think I still have it around here tucked away in a closet somewhere…Would you want to take some with you?”

Mt_St_Helens_Yakima_Ash
Mt St. Helens ash that fell on Yakima, WA in May, 1980

Photo by my friend Ben (the Beekeeper). Visit the USGS site for a high-mag view of volcanic ash.

Read Part I (Rainier) and Part II (Mt. St. Helens)

Advances in Nuclear Forensics: GSA 2014 Technical Session

The lunar basalts in my doctoral research were almost four billion years old, plus or minus a couple hundred million years. The rocks I study now were created on July 16th, 1945, at 05:29:45 AM (Mountain War Time). It’s a strange thing to know so precisely. But how can I pinpoint the exact second of creation? Because these rocks are trinitite, the glassy result of a sandy New Mexico desert experiencing the first atomic bomb blast.

trinitite_top_and_bottom
Two views of a common green glass variety of trinitite. Image from the Simonetti Lab at Notre Dame.

The first nuclear bomb test, codenamed Trinity, was performed at the White Sands Proving Grounds (near Alamogordo, New Mexico). The device, Gadget, was an implosion-type design with a plutonium (Pu-239) core. The heat resulting from the 18 kiloton explosion melted the desert sand surface out to distances of 400 meters from ground zero. trinitite_thin_section The surface sand melted to form a glassy layer (1-2 cm) on top of incipiently melted desert sand together, these form trinitite (alternatively, Alamogordo glass). This post-detonation material is a valuable tool in nuclear forensics research. Trinitite incorporated pieces of Gadget and the blast tower, and one of our goals is to identify and characterize the distribution and composition of individual components through geochemical and radionuclide analysis. At right, a vertical cross-section of trinitite is shown in thin section.

The analysis of postdetonation material (like trinitite) is one arm of the nuclear forensics field. An effective nuclear forensic analysis requires technical information and relevant databases, and specialized skills and expertise to generate, analyze, and interpret the data. This analysis combined with law enforcement and intelligence data can provide valuable information on the provenance of such materials, and processing history so as to improve source attribution. Identifying the source(s) of stolen or illicitly trafficked nuclear materials will therefore prevent, or make more difficult, terrorist acts that would use material from these same sources. Moreover, effective forensic analysis of postdetonation materials in the unlikely event of a nuclear terrorist attack is also expected to deter individuals or groups involved, and provides incentives to countries to enhance their security and safeguards relative to their nuclear materials and facilities.

The microscopic and macroscopic appearance, as well as the elemental and isotopic composition of nuclear materials, i.e. its ‘signature’ reflects its entire history. The term ‘signature’ is used to describe material characteristics that may be used to link nuclear samples to people, places, and processes, much as a written signature can be used to link a document to a particular person. Forensic methods employed to establish signatures in nuclear materials typically combine physical and chemical (e.g. X-ray fluorescence, scanning electron microscopy, electron microprobe analysis, secondary ion mass spectrometry) characterization and radiometric measurements (e.g. alpha, beta and gamma spectroscopy). The methodologies and interpretation of forensic analyses are constantly being advanced and perfected.

gsa-logo_14CAt this year’s annual meeting of the Geological Society of America, the Notre Dame crew (Drs. Tony Simonetti, Sara Mana, and myself) are chairing a session to update the geoscience community on the latest developments of nuclear forensics. The cleverly-titled session, “Advances in Nuclear Forensics”, will emphasize analytical techniques, database development, and implications for our ability to identify and possibly prevent nuclear attacks and trafficking of illicit nuclear materials.

UPDATE (Aug 9, 2014): The session has been designated a poster session.

Note: A significant portion of this post was reused from our session proposal, which isn’t published by GSA.

Further Resources:

Conversation with a Microbiologist (audio)

Do you know how to complement a bacterium? What about the difference between flagellum and Type 4 pili (and why it matters)? Listen and learn! Headphones recommended.

This was an in-person chat with Morgen Anyan, PhD candidate at the University of Notre Dame (research page). Morgen is researching environmental and morphological effects on the behavior of the bacteria Pseudomonas aeruginosa.

For those few who listened to my previous interview with Ben the Beekeeper, you’ll be pleasantly surprised to find that the audio is much better thanks to a Zoom H2n recorder. I’ve also edited it a bit heavier to keep it clipping along (mostly cutting out myself as much as possible to let Morgen tell her story). Avery made the cut, though.

AveryDance