The opinions expressed here are well-reasoned and insightful -- needless to say they are not the opinions of my employers

20 December 2007

Pale Blue Dot

Phil Plait reminded me that Carl Sagan died 11 years ago today. I'm too old for this to be true, but Cosmos was what inspired me to a career in science. Better late than never.

In February 1990 the Voyager 1 spacecraft, by then 12.5 years and four billion miles from home, spun around to take a “family portrait” of our Solar System from about 32 degrees above the ecliptic. The image above is of Earth, a few pixels of blue against the blackness of space. The light streak is a ray from the sun, just off the image to the right.

Look again at that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every "superstar," every "supreme leader," every saint and sinner in the history of our species lived there — on a mote of dust suspended in a sunbeam.

The Earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by all those generals and emperors, so that, in glory and triumph, they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of this pixel on the scarcely distinguishable inhabitants of some other corner, how frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds.

Our posturings, our imagined self-importance, the delusion that we have some privileged position in the Universe, are challenged by this point of pale light. Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves.

The Earth is the only world known so far to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment the Earth is where we make our stand.

It has been said that astronomy is a humbling and character building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we've ever known.

-- Carl Sagan in a commencement address, May 11 1996

16 December 2007

The Reason for the Season

It's been a week since Phil Plait posted this link on his Bad Astronomy blog. It made me very angry, that I didn't think of it first. Nonetheless, it will from this year forward be my Christmas card...

Merry Christmas, y'all!

15 December 2007

Where On (Google) Earth? #80

Well I managed to win my first northern hemisphere WOGE challenge, though it looks like Ron may have had a better claim had he signed on a few minutes later, after Joe re-posted the image.

It only seems fair to stay with the recent trend and not invoke the Schott Rule this time.

However to add to the challenge this time I won't remind you to look at the north arrow.

13 December 2007

Accretionary Wedge #4: My Pet Rock

For show-and-tell today I brought this fulgurite specimen, which I unfortunately did not collect myself.

In 1998 I took a group of intro geology students on a trip to Capitol Reef National Park in southern Utah, and while we watched a storm play out over the Henry Mountains I gave an impromptu lecture on convective thunderstorms and lightning.

Lightning is the sudden release of huge static charges that develop in convective storms due to rapid vertical movement of air. No one really understands the details of the transfer of charge (the primary carrier of charge appears to be small fragments of ice crystals), but typically the cloud becomes a 5-10 km high capacitor with negative charge concentrated near the base. When the stress becomes too high the cloud discharges very rapidly. Globally there are roughly 5000-10000 lightning discharges per minute, mostly in the tropics, of which roughly one third strike the ground (the rest occur between clouds or within the cloud).

A cloud-to-ground strike transfers billions of joules of energy, most of which dissipates in the atmosphere (heating the air to tens of thousands of degrees and creating visible as well as infrared light that ranges down into the radio spectrum). In the end only a tiny amount of the original energy is delivered at the point of impact, but anyone who has observed or experienced the results will testify that it is sufficient.

Since the cloud is looking for the easiest path to the ground, it is most likely to strike objects above the local base level, as trees, houses, and people (especially those holding lightning rods golf clubs) are much better conductors than relatively dry air.

The subject of fulgurites, or fossil lightning, came up and I explained that quartz has a melting point of about 1600 degrees C, a temperature easily achieved in the near surface, though in very wet soils the charge can be dissipated pretty rapidly. The best examples therefore result when lightning strikes the ground directly in an area where the soil is dry and is thus not a good conductor.

Quartz sand works particularly well as it conducts heat as poorly as it conducts electricity. This prevents charge from dissipating rapidly, so the bolt can travel through the ground for several meters, fusing sediment to glass and vaporizing all of the volatile components. Some of the best examples of fulgurites are discovered in dune fields, where dry unconsolidated sands subsequently are eroded by winds, exposing the fused structure. Unfortunately fulgurites are quite fragile and tend to crumble when excavated.

One of the students on that trip was visiting relatives in Temecula later that year when she saw lightning strike the ground a few hundred feet from her back door. She got her husband to help her dig up the yard after the storm had passed, and she brought us a shopping bag full of this broken chunks of this fossilized lightning. The largest piece here is about 25 cm long and 8 cm in diameter and was found less than 40 cm beneath the surface.

The soil was very sandy in this location, and was fused to glass at temperatures exceeding 2000 degrees C. You can see what a poor conductor of heat this material is, as it transitions from glass to barely altered sand in less than a centimeter. There is a general relationship between current and diameter indicating that this sample was created by a fairly powerful lightning strike.

The moisture and organics flashed to vapor, leaving lots of vesicles. The explosive expansion of the volatiles can create hollow tubes down the center of the structure, pushing the molten (or melting) sand outward. There is a common misconception about these hollow tubes that they represent a zone where temps were high enough to vaporize quartz, but this is not true.

The second piece is from about a meter beneath the larger sample and is barely more than a glass tube. You can see the charge branching off from this piece in several directions simultaneously.

I've found that, in addition to being great samples to pass around when I’m talking about thunderstorms in class, fulgurites are great ringers to slip into an igneous rocks practical exam.

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