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.