Methane lakes on Titan
The complex surface of Saturn's moon Titan is partially covered with lakes of liquid methane and ethane . There is also water ice on the surface of the ice moon , but at the low temperatures that prevail there it has the consistency of silicate rock.
The large methane lakes (Mare, plural maria ) are named after mythological sea monsters; the smaller methane lakes (Lacus, plural lacūs ) after lakes on earth. Endorheic lakes (Lacuna, plural lacunae ), i.e. lakes without drainage, are named after endorheic lakes on earth.
surface
Of all bodies in the solar system, Saturn's moon Titan has the most Earth-like surface shapes, although the surface consists of completely different materials. The Cassini-Huygens space probe , which explored the Saturn system from July 2004 to September 2017, found rivers, lakes and small seas made of methane on the moon, which is surrounded by a dense nitrogen atmosphere . Methane plays the same role on Titan as water does on Earth and ensures the formation of very earth-like landscapes on the crust, which consists of hard-frozen water ice. It is liquid there because the surface temperature of titanium averages around minus 190 degrees Celsius. Such methane lakes were previously only known from the polar regions of Titan; in pictures by Cassini, methane lakes have now also been discovered in the “tropical” latitudes of the moon.
This was a surprise, because until now planetary researchers had assumed that permanent methane lakes could only last in the slightly cooler polar regions of the moon. In the lower latitudes, however, it should be too warm for such "waters". However, a team of researchers at the University of Arizona found methane lakes at least one meter deep in infrared images of Cassini in an area of 20 degrees north and south around the Titan equator. One of them can already be seen in Cassini's first pictures from 2004 and covers an area of 2,400 square kilometers, about half the area of the Great Salt Lake in the US state of Utah .
Liquid hydrocarbons
The liquids in the lake-like structures are relatively transparent, so that one could look into these "waters" like a clear earthly lake. According to NASA calculations, the supply of liquid hydrocarbons on Titan exceeds that on Earth by a hundred times. The atmospheric cycle, the raining down, collecting and flowing of hydrocarbons shaped the icy surface in a surprisingly similar way to how water forms the silicate rocks on earth. Even at first glance, entire river systems can be seen from a height of a few kilometers, liquid methane cuts erosively into the surface of the ice and forms a hilly, mountainous relief.
The three largest lakes Kraken Mare , Ligeia Mare and Punga Mare are referred to as “Mare” and with areas of over 100,000 square kilometers, they reach the dimensions of large terrestrial inland lakes and seas (for comparison: Upper Lake 82,100 square kilometers). At the beginning of the mission, the largest "lake" Ontario Lacus was discovered at the South Pole as the only methane lake in the southern hemisphere to date and named after Lake Ontario , which is about 20,000 square kilometers in size . DLR researchers announced on July 30, 2008 that ethane had been detected in it and that it probably also contained other alkanes . Evaluations of radar measurements in 2009 showed that the Ontario Lacus appears to be as smooth as a mirror. The variations in height were less than 3 mm at the time of measurement. This supports the suspicion that the Ontario Lacus is actually made of liquid hydrocarbons and not dried mud. It also showed that there was relatively no wind on the surface. The depth of the "Ligeia Mare", which is mostly filled with methane, was determined to be 170 m with the help of Cassini's radar. The smaller lakes include the Feia Lacus , the Kivu Lacus , the Koitere Lacus and the Neagh Lacus .
Lake types
There is a clear dichotomy in the types of lakes in the northern hemisphere of Saturn's moon. In the eastern half there are some extensive lake areas, almost seas, with flat shores and several islands. From the North Pole to the west, however, hundreds of smaller lakes dominate the surface. As early as 2018, radar data revealed that these small lakes have strikingly steep banks and are not located at sea level, but on table mountains hundreds of meters high and often extend several hundred meters into the subsurface, similar to earthly karst lakes . But this also means that these lakes cannot be fed by tributaries. Instead, these bodies of water have to get their liquid methane from the rain - similar to many isolated crater lakes on Earth. Benzene and acetylene in particular are as soluble in liquid ethane at minus 180 degrees as calcium-based minerals are in earthly water. This means that the rain on Titan could be sufficient to solve such holes more than a hundred meters deep in the underground over the course of ten to one hundred million years.
Most of the smaller titanium lakes are either empty or full, sharp-edged depressions with narrow, steep outer edges, a width of about 1 km and relatively flat bottoms with depths of up to 600 m. However, some lakes are surrounded by ring-shaped hills that extend tens of kilometers from the edges of some lakes. In contrast to the former forms, these ramparts completely enclose their host lake. For the first time, ESA scientists at the European Center for Space Astronomy (ESAC) combined spectral and radar data from Cassini to study five regions near the North Pole with filled lakes and raised ramparts, and three empty lakes from a nearby region. The lakes ranged in size from 30 to 670 square kilometers and were completely surrounded by 200 to 300 m high ramparts that extended up to 30 km out of the lake. Of the approximately 650 large and small lakes in the polar regions discovered by the Cassini space probe, only 300 were at least partially filled with a liquid mixture of methane and ethane.
Seasonal fluctuations
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Since it is actually too warm for methane rain in these low latitudes of Saturn's moon, the researchers suspect that the methane in the tropical lakes comes from underground inflows from the ice crust. In the area of the lakes it emerges in springs and constantly fills the lakes, although they are constantly losing liquid to the atmosphere through evaporation.
In the previous circulation models, the planetary researchers assumed that precipitation is controlled by the distinct seasons on Titan. Titan orbits Saturn in its equatorial plane, whereby the ring planet is inclined around 27 degrees to its orbit plane. In the course of the Saturn year, which is around 30 earth years long, there are seasons on both the ring planet and on Titan that each last around eight earth years. When Cassini arrived in 2004, the northern hemispheres of Saturn and Titan were in winter and the northern polar regions were in the dark of the polar night. Using radar observations, the Cassini probe at the North Pole of Titan encountered large methane lakes, the largest of which reached roughly the area of the Caspian Sea .
After the onset of spring in the northern hemisphere, when the sun returns to the polar region, large amounts of methane evaporate and rise into the atmosphere. Clouds of condensed methane form there, move to more southerly latitudes and ultimately cause heavy rainfall in the now colder southern polar region. In the lower latitudes there is no longer any major precipitation. So far, precipitation has only been observed once in the last eight years at low latitudes on Titan, but this is not enough to fill the lakes that have been sighted there.
The newly discovered lakes are located in the Shangri-La region , not far from the landing site of the European Huygens atmospheric probe , which landed there in January 2005. There, Huygens' operating heat released methane from the ice floor below the probe, an indication that the soil was moist. The researchers now want to investigate the weather on Titan in more detail.
Bubbling lakes
The methane lakes on Titan can bubble with the changing of the seasons. Theoretical model calculations together with experimental data show that nitrogen dissolved in the liquid methane can bubble out under certain conditions. The resulting carpets of centimeter-sized bubbles look like short-lived bright islands on radar images .
Daniel Cordier and his colleagues have now been able to show that the most likely explanation for the appearance of bright surfaces that briefly appeared and then disappeared again, especially in the northern lake called Ligeia Mare , consist of nitrogen bubbles. Just as carbon dioxide is dissolved in ocean water on earth, liquid methane on titanium absorbs nitrogen. And this mixture can become unstable under certain conditions, as the researchers' theoretical models show.
According to the calculations, ethane plays an important role in the equilibrium in a methane lake. This organic substance is formed by photochemical processes in the atmosphere, rains down with methane and thus becomes another component of the lakes. A stratification then forms with an ethane-rich liquid at the bottom of the lake and a methane-rich layer near the surface. Precipitation causes currents that locally swirl this stratification and thus also disturb the equilibrium - the nitrogen segregates and bubbles up to the surface in centimeter-sized bubbles.
Floating hydrocarbon ice
The triple point of methane and ethane is close to the temperature and pressure values that are reached on the surface of titanium (maximum 90–94 Kelvin, minimum 91–93 Kelvin), which is why Jason Hofgartner and Jonathan Lunine made the assumption in a scientific paper explained that solid ice from hydrocarbons could form on the surface of some methane lakes. The prerequisite for this is that the density of the solid hydrocarbons is lower than the density of the liquid hydrocarbons. In methane-rich lakes made from methane and ethane, ice will float on the surface of the lake at all temperatures below the melting point of pure methane (−182 ° C or 90.7 Kelvin). In ethane-rich lakes, however, ice can only float on the surface if the ice contains more than 5 percent nitrogen by volume . In such lakes, the effect could even be that when the ambient temperature cools down, the ice sinks to the lake floor, with some ice lying on the lake floor as well as floating on the surface of the lake on certain days. The two authors predict that this effect could be seen in radar images.
List of named lakes
Web links
- USGS : Saturn / Titan / Mare, maria. Retrieved August 6, 2010 .
- USGS : Saturn / Titan / Lacus, lacūs. Retrieved August 6, 2010 .
Individual evidence
- ↑ a b c d Tropical methane lakes on Titan. June 14, 2012, accessed March 15, 2019 .
- ↑ Saturn's moon Titan - streams and lakes of liquid hydrocarbons. German Aerospace Center (DLR), July 30, 2008, accessed on March 15, 2019 .
- ↑ Ontario Lacus, a lake filled with liquid hydrocarbons with a "beach" on Titan. German Aerospace Center (DLR), accessed on March 15, 2019 .
- ↑ Lisa Grossman: Saturn moon's mirror-smooth lake 'good for skipping rocks'. New Scientist, August 21, 2009, accessed March 15, 2019 .
- ↑ NASA's Cassini Spacecraft Reveals Clues About Saturn Moon . In: jpl.nasa.gov. December 12, 2013, accessed March 15, 2019.
- ↑ scinexx.de: Saturn moon surprises with lakes in exotic karst and disappearing pools , from April 17th, 2019
- ↑ phys.org: Cassini explores ring-like formations around Titan's lakes , July 18, 2019
- ↑ Bubble streams in Titan's seas as a product of liquid N2 + CH4 + C2H6 cryogenic mixture. In: nature.com. April 18, 2017, accessed March 15, 2019 .
- ↑ Does ice float in Titan's lakes and seas? In: astro.princeton.edu. November 30, 2012, accessed July 24, 2019 .