Astrophile: Soggy bogs swallow craters on Titan

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Astrophile: Soggy bogs swallow craters on Titan

Post by bystander » Tue Dec 17, 2013 10:12 pm

Soggy bogs swallow craters on Titan
New Scientist | Astrophile | Lisa Grossman | 2013 Oct 18

Objects: Titanic wetlands
Likely source: Subsurface wells of hydrocarbons

The other meteors in the solar system were so lucky. They landed on nice, hard rock like on Mars or Mercury, exploding on the surface and excavating deep craters. If I had followed their lead, I could have left my mark! But no, I headed straight for this orange haze-ball Titan and plopped down in the northern marshes. I was a mere splash in an icy puddle, and all evidence of my existence was erased.

So was the fate of many a meteorite that struck Saturn's largest moon, according to a new analysis of Titan's topography. The work suggests that large, crater-free bands bordering the moon's equator are wetlands fed by subsurface springs, where any incoming meteors would have made more of a splash than a bang. Those same springs may be the source of Titan's polar lakes, feeding the world's unusual liquid cycle.

Among the many moons in our solar system, Titan is the most eerily Earth-like. It is the only other solid body to host a thick atmosphere and to have rainstorms, rivers and lakes. But unlike Earth, chilly Titan's liquids are methane and ethane. Titan also neatly divides its surface features into horizontal bands. The poles are covered with hydrocarbon lakes and boast few craters, while the equator is a vast dune field peppered with impact scars. The mid-latitudes are nearly featureless expanses, dubbed the Bland-lands.

"The complete lack of craters near the poles… People have noticed this for a while, but nobody's had any good explanations," says Catherine Neish of the Florida Institute of Technology in Melbourne. She wondered if craters there are simply melting away because the meteorites are landing in vast hydrocarbon swamps.

High and dry

On our home turf, craters that form in shallow seas or wetlands hardly make an imprint in the soggy ground and are quickly subsumed. "The wet sediment slumps back into the crater and basically fills it in, so you get a pretty flat crater even to start with," says Neish. The best way to identify submarine craters on Earth is with drilling or seismic surveys. "But if all you have is remote sensing data, it might be difficult to identify something as subtle as that."

If Titan's craters are being swallowed by soggy ground, then the ones we do see should appear more often at higher elevations, where the ground would be drier. To test this idea, Neish and her colleague Ralph Lorenz at the Johns Hopkins University Applied Physics Lab in Laurel, Maryland, examined a new map of Titan made from radar readings by NASA's Cassini orbiter, published earlier this year.

"It's the first global topographic map of Titan that's ever been made," says Neish. "This is the first chance to really explore this issue quantitatively, instead of just qualitatively." The team found that, statistically, more craters do exist at higher elevations all across the globe, while low zones have far fewer. The pair then went over alternative explanations for the distribution pattern, such as erosion by wind or burial under material flowing from ice volcanoes.

"In this paper I went through all the possibilities I could think of," says Neish. "None of them made sense, except that the mid-latitudes are or were recently extremely wet and that craters are there; we just can't see them."

Hidden sea

The presence of underground aquifers could explain another mystery. Titan's atmosphere is full of methane, which should react with ultraviolet radiation from the sun to produce liquid ethane. Over time, the moon should have built up an ethane ocean hundreds of metres deep. But when the Cassini probe arrived at Titan, it found only the polar lakes. "So where did all the ethane go?" says Neish. The presence of wetlands would suggest that it soaked into the ground, forming a subsurface liquid layer akin to groundwater on Earth.

"This is an important question," says Cassini team member Jonathan Lunine at Cornell University in Ithaca, New York, who was not involved in the new work. "We know that methane can't last over the age of the solar system. Unless we're looking at an unusual time in Titan's history, there must be a methane resupply mechanism. Anything we can do to figure out how much methane or ethane is in the crust is important."

He thinks the wetlands proposal is an interesting hypothesis. The next thing to do, he says, is get more radar data. "It's sobering to think we have just a little bit more than half of Titan mapped now with Cassini radar, which is the highest resolution data," he says. "Titan's dense atmosphere just makes everything more difficult. But Cassini is getting there."

Elevation distribution of Titan’s craters suggests extensive wetlands - C.D. Neish, R.D. Lorenz
Cassini Shapes First Global Topographic Map of Titan
NASA | JPL-Caltech | Cassini Solstice Mission | JHU-APL | 2013 May 15

A global topographic map of Titan - Ralph D. Lorenz et al
Cassini Gets New Views of Titan's Land of Lakes
NASA | JPL-Caltech | Cassini Solstice Mission | CICLOPS | 2013 Oct 23

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Astrophile: Titan lake has more liquid fuel than Earth

Post by bystander » Wed Dec 18, 2013 6:43 pm

Titan lake has more liquid fuel than Earth
New Scientist | Astrophile | Katia Moskvitch | 2013 Dec 13

Object: Mirror-smooth lake on Titan
Contents: Huge volume of pure liquid methane

Imagine standing on the lush shores of a lake. Now imagine that it isn't one on Earth. The vegetation disappears, the temperature drops to -179ºC, and the sky takes on a sickly orange hue. Despite a bone-chilling breeze, the lake's dark surface lacks even the tiniest ripple. Welcome to Ligeia Mare on Titan.

Saturn's largest satellite is an eerie world about the size of Mercury. It is the only moon in the solar system with a thick, hazy atmosphere and the only place aside from Earth that has large bodies of liquid on its surface. But there's also one very big difference: on frigid Titan, the lakes and seas are filled with liquid hydrocarbons, such as methane. They also tend to be very smooth.

Earlier radar maps made by NASA's Cassini probe had revealed that the largest southern lake, Ontario Lacus, is super-smooth, varying in height by less than 3 millimetres. The latest Cassini results, presented yesterday at the American Geophysical Union meeting in San Francisco, show that Ligeia Mare near the moon's north pole is even flatter. Its surface height changes by no more than 1 millimetre.

"That's a very interesting result, because we know there are beautiful 100-metre-high dunes near Titan's equator, and in order to create dunes, you need wind," says Alexander Hayes at Cornell University in Ithaca, New York. His team is now developing models to figure out why the winds are not making waves on Titan.

Seasonally smooth

Flatness may be a seasonal trait, at least for Ligeia Mare, suggests Cassini project scientist Linda Spilker. A year on Titan corresponds to 30 Earth years, which means Cassini has been observing a long winter on the moon's northern half. But soon northern spring should begin.

"In the next few years we are entering the most exciting time for Titan weather," says Spilker. "What will these years bring as the north pole of Titan approaches summer and the sun is high in the sky? Will the lakes evaporate or fill with methane rain? Will the winds kick up, creating waves and little hurricanes on the lakes?"

Extreme flatness in the north wasn't the only surprise from Cassini's latest look at Titan. Previously, the liquid in Ligeia Mare was thought to be a chemical stew dominated by ethane. But the radar observations were able to peer all the way to the lake's bottom, which suggests that the liquid is unexpectedly clear and pure. The results also show that it is mostly filled with methane, the main component in natural gas.

"Measurements indicate that the lake is 160 metres deep, and it alone contains by volume about 40 times more hydrocarbon liquid than Earth's global oil reservoir," says Hayes. "Together, all of Titan's visible lakes and seas contain about 300 times the volume of Earth's proven oil reserves."

Could anything be alive inside these cold but carbon-rich seas?

"Titan has been an incredible mind-expander," says Jeff Kargel at the University of Arizona in Tucson. If the moon does host life, he says, our imagination is the only limit to what it could be.

Cassini Reveals Clues About Saturn Moon
NASA | JPL-Caltech | Cassini Solstice Mission | 2013 Dec 12

Wind driven capillary-gravity waves on Titan’s lakes: Hard to detect or non-existent? - A.G. Hayes et al << Previous Astrophile
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Re: Astrophile: Titan lake has more liquid fuel than Earth

Post by neufer » Wed Dec 18, 2013 8:24 pm wrote:
Wind driven capillary-gravity waves on Titan’s lakes: Hard to detect or non-existent? - A.G. Hayes et al Abstract: <<Saturn’s moon Titan has lakes and seas of liquid hydrocarbon and a dense atmosphere, an environment conducive to generating wind waves. Cassini observations thus far, however, show no indication of waves. We apply models for wind wave generation and detection to the Titan environment. Results suggest wind speed thresholds at a reference altitude of 10 m of 0.4–0.7 m/s for liquid compositions varying between pure methane and equilibrium mixtures with the atmosphere (ethane has a threshold of 0.6 m/s), varying primarily with liquid viscosity. This reduced threshold, as compared to Earth, results from Titan’s increased atmosphere-to-liquid density ratio, reduced gravity and lower surface tension. General Circulation Models (GCMs) predict wind speeds below derived thresholds near equinox, when available observations of lake surfaces have been acquired. Predicted increases in winds as Titan approaches summer solstice, however, will exceed expected thresholds and may provide constraints on lake composition and/or GCM accuracy through the presence or absence of waves during the Cassini Solstice Mission. A two-scale microwave backscatter model suggests that returns from wave-modified liquid hydrocarbon surfaces may be below the pixel-scale noise floor of Cassini radar images, but can be detectable using real-aperture scatterometry, pixel binning and/or observations obtained in a specular geometry.>> wrote:
<<In 1698, the Dutch astronomer Christian Huygens wrote these words: "Since 'tis certain that Earth and Jupiter have their Water and Clouds, there is no reason why the other Planets should be without them. I can't say that they are exactly of the same nature with our Water; but that they should be liquid their use requires, as their beauty does that they be clear. This Water of ours, in Jupiter or Saturn, would be frozen up instantly by reason of the vast distance of the Sun. Every Planet therefore must have its own Waters of such a temper not liable to Frost."

Huygens discovered Titan in 1655, which is why the probe is named after him. In those days, Titan was just a pinprick of light in a telescope. Huygens could not see Titan's clouds, pregnant with rain, or Titan's hillsides, sculpted by rushing liquids, but he had a fine imagination.

Titan's "water" is liquid methane, CH4, better known on Earth as natural gas. Regular Earth-water, H2O, would be frozen solid on Titan where the surface temperature is 290º F below zero. Methane, on the other hand, is a flowing liquid, of "a temper not liable to Frost."

Jonathan Lunine, a professor at the University of Arizona, is a member of the Huygens mission science team. He and his colleagues believe that Huygens landed in the Titan-equivalent of Arizona, a mostly-dry area with brief but intense wet seasons. "The river channels near the Huygens probe look empty now," says Lunine, but liquids have been there recently, he believes. Little rocks strewn around the landing site are compelling: they're smooth and round like river rocks on Earth, and "they sit in little depressions dug, apparently, by rushing fluids."

The source of all this wetness might be rain. Titan's atmosphere is "humid," meaning rich in methane. No one knows how often it rains, "but when it does," says Lunine, "the amount of vapor in the atmosphere is many times that in Earth's atmosphere, so you could get very intense showers." And maybe rainbows, too. "The ingredients you need for a rainbow are sunlight and raindrops. Titan has both," says atmospheric optics expert Les Cowley.

On Earth, rainbows form when sunlight bounces in and out of transparent water droplets. Each droplet acts like a prism, spreading light into the familiar spectrum of colors. On Titan, rainbows would form when sunlight bounces in and out of methane droplets, which, like water droplets, are transparent.

"Their beauty [requires] that they be clear...."

"A methane rainbow would be larger than a water rainbow," notes Cowley, "with a primary radius of at least 49º for methane vs 42.5º for water. This is because the index of refraction of liquid methane (1.29) differs from that of water (1.33)." The order of colors, however, would be the same: blue on the inside and red on the outside, with an overall hint of orange caused by Titan's orange sky.

One problem: Rainbows need direct sunlight, but Titan's skies are very hazy. "Visible rainbows on Titan might be rare," says Cowley. On the other hand, infrared rainbows might be common. Atmospheric scientist Bob West of NASA's Jet Propulsion Laboratory explains: "Titan's atmosphere is mostly clear at infrared wavelengths. That's why the Cassini spacecraft uses an infrared camera to photograph Titan." Infrared sunbeams would have little trouble penetrating the murky air and making rainbows. The best way to see them: infrared "night vision" goggles.
[img3="An infrared rainbow on Earth, photographed by Prof. Robert Greenler. Reference: Science 173,1231 (1971). "A rainbow on Titan might look like this," notes Les Cowley. "It would be larger than the visible methane 'bow'
with a radius slightly more than 49-52 degrees."
"] ... _strip.jpg[/img3]
All this talk of rain and rainbows and mud makes liquid methane sound a lot like ordinary water. It's not. Consider the following: The density of liquid methane is only about half the density of water. This is something, say, a boat builder on Titan would need to take into account. Boats float when they're less dense than the liquid beneath them. A Titan-boat would need to be extra lightweight to float in a liquid methane sea. (It's not as crazy as it sounds. Future explorers will want to visit Titan and boats could be a good way to get around.)

Code: Select all

Attribute         liquid water  liquid methane
density	            1 g/cc      0.45 g/cc
surface tension	   70 dyne/cm  17 dyne/cm
viscosity	          1.54 cP     0.184 cP
index of refraction	1.33        1.286
Liquid methane also has low viscosity (or "gooiness") and low surface tension. Surface tension is what gives water its rubbery skin and, on Earth, lets water bugs skitter across ponds. A water bug on Titan would promptly sink into a pond of flimsy methane. On the bright side, Titan's low gravity, only one-seventh Earth gravity, might allow the creature climb back out again.

Propellers turning in methane would need to be extra-wide to "grab" enough of the thin fluid for propulsion. They'd also have to be made of special materials resistant to cracking at cryogenic temperatures. And watch out for those waves! European scientists John Zarnecki and Nadeem Ghafoor have calculated what methane waves on Titan might be like: seven times taller than typical Earth-waves (mainly because of Titan's low gravity) and three times slower, "giving surfers a wild ride," says Ghafoor.

Last but not least, liquid methane is flammable. Titan doesn't catch fire because the atmosphere contains so little oxygen--a key ingredient for combustion. If explorers visit Titan one day they'll have to be careful with their oxygen tanks and resist the urge to douse fires with "water."

Infrared rainbows, towering waves, seas beckoning to sailors. Huygens saw none of these things before it plopped down in the mud. Do they really exist? "...there is no reason why the other Planets should be without them.">>
Art Neuendorffer