Geology of Pluto

The topic of this month's Accretionary Wedge is geology in space.

There is a lot of sexy stuff to talk about, but I thought, what about a space body that has been through some tough times recently. Pluto used to be a planet, the select club of only 8 other members in our solar system. But in 2006 it was downgraded to just a dwarf planet. It deserves some attention.

The hardest thing about studying the geology of Pluto is the distance. Pluto is at a minimum 4.28 billion km from earth. A little to far for a rock hammer. At that distance, even satellites have problems.



First, what is Pluto made of? Using spectroscopy, Pluto's surface has been determined to be mostly frozen N₂ with a small amounts of CH₄ and CO. Based on its density, Pluto is thought to have a solid rock core. The radioactive decay from this core could be enough to melt the bottom of Pluto's frozen surface, resulting in a layer of liquid between the core and surface.

Pluto's surface, however, is not flat and featureless, as Hubble images have reveled. Pluto has locations of varying albedo that are probably a result of not only surface frost, but also impact craters.



Despite Pluto's great distance from the sun, its atmosphere's composition is still controlled by the sun. Just like with ice caps on Earth, although the sun may not have enough energy to melt the frozen material, it can sublimate some of it. Because of this sublimation, Pluto's atmosphere has a similar composition to its surface. Pluto, like Saturn's moon, Titan, Neptune's moon, Triton, and the Earth, has a N₂ dominated atmosphere. However, this atmosphere is sparse, at only 3 microbars of pressure versus the Earth's average of 10130 microbars. Because of Pluto's thin atmosphere and great distance from the sun, the average surface temperature is only 40 Kelvin.

Pluto has not been forgotten. In 2006, the New Horizons mission was launched to study Pluto and the Kuiper belt. Pluto won't be reached 2015, but the satellite should great expand our knowledge of Pluto, the distant 9th planet dwarf plant of our solar system.

References
NASA's New Horizons Mission. What We Know About Pluto, Charon and the
Kuiper Belt. http://pluto.jhuapl.edu/science/whatWeKnow.html

Owen, T. C., Roush, T. L., Cruikshank, D. P., Elliot J. L., Young, L. A., De Bergh, C., Schmitt, B., Geballe, T. R., Brown, R. H., Bartholomew, M. J. 1993. Surface Ices and the Atmospheric Composition of Pluto. Science 261(5122): 745 - 748.

Stern, A., Buie, M. 1996. Hubble Reveals Surface of Pluto for First Time. Hubble Newscenter. http://hubblesite.org/newscenter/archive/releases/1996/09/

Williams, D. R. 2006. Pluto Fact Sheet. NASA Planetary Fact Sheets. http://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html

Ancient Permafrost

The current issue of Science has a short article about a 740,000 year old ice wedge found in a section of permafrost in Yukon, Canada [Froese et al., 2008].

An ice wedge is a vertically oriented section of ice the forms in periglacial environments. Water enters a crack in the soil and then freezes, forms an ice wedge, and expands the soil crack further, making the wedge larger (check out this cool animation). It is associated with permafrost.

As the climate warms, there are concerns that melting permafrost will release large amounts of CO₂ [Zimov et al., 2006]. However, we don't know exactly how permafrost and related periglacial features will react to warmer temperatures.

The researchers found an ice wedge under a volcanic ash layer, thereby predating the ash layer. The ash layer was dated to over 700,000 years ago. Not only does this make this ice wedge the oldest dated ice in North America, but it shows that ice has survived warmer interglacial periods in the past.

References
Froese, D. G., Westgate, J. A., Reyes, A. V., Enkin, R. J., Preece, S. J. 2008. Ancient Permafrost and a Future, Warmer Arctic. Science 321(5896): 1648.

Zimov, S. A., Schurr, E. A. G., Chapin III, F. S. 2006. Permafrost and the Global Carbon Budget. Science 312(5780): 1612-1613.

Image: Agriculture and Agri-Food Canada

Alexander von Humboldt

We have Darwin Day, why not Alexander von Humboldt day (September 14)? If you ask for the greatest natural scientist (a term not used until much later) in the 19th century, you would probably get Darwin. How about number two?

In these days of specialization, the range of topics people like von Humboldt examined is remarkable. Geography, volcanology, mineralogy, electromagnetism, biology, meteorology, and oceanography were all impacted by his work. On his famous South American travels, which later influenced Darwin on his journey on the Beagle, von Humboldt proved the link between the Amazon and Orinoco Rivers, found the earth's magnetic field decreased from the poles to the equator, and nearly reached the top of Chimborazo, what was then thought to be the tallest mountain the world.

The image below is a copy of his diagram of Chimborazo. Not only is it a beautiful image, but it also contains loads of information he gathered while climbing the peak. This includes data about altitude, appearance of electricity, vegetation, changes in weight, blueness of the sky, animals, location of the snowline, the boiling point of water, and atmospheric pressure, temperature, humidity, and composition. The site hosting it has even translated some of the German notes in English. So go take a look this von Humboldt day!