Astronomy college course/Miranda and Titan

Material taken from Miranda and Titan

Physical characteristics


Miranda's surface may be mostly water ice, with the low-density body also probably containing silicate rock and organic compounds in its interior. A flyby by Voyager 2 gave earthlings the first glimpse a geology like none other in the solar system (including planets and moons).

Early hypothesis of Miranda's geology
The "early hypothesis", proposed shortly after the 1985 Voyager 2 flyby, was that a previous incarnation of Miranda was shattered by a massive impact, with the fragments reassembling and denser ones subsequently sinking to produce the current strange pattern.

Subsequent hypotheses of Miranda's geology
Later hypotheses have suggested that the geology of Miranda could have resulted from conventional geological processes made more intense by tidal interactions with neighboring moons. The large 'racetrack'-like grooved structures could be a planetary corona associated with diapirs, or upwellings of warm ice. The ridges probably represent extensional tilt blocks.

The canyons probably represent graben formed by extensional faulting. Other features may be due to cryovolcanic eruptions of icy magma. The diapirs may have changed the density distribution within the moon, which could have caused Miranda to reorient itself, similar to a process believed to have occurred at Saturn's geologically active moon Enceladus.

Miranda's past geological activity is believed to have been driven by tidal heating at a time when its orbit was more eccentric than currently. Early in its history, Miranda was apparently captured into a 3:1 orbital resonance with Umbriel, from which it subsequently escaped. The resonance would have increased orbital eccentricity; resulting tidal friction due to time-varying tidal forces from Uranus would have caused warming of the moon's interior. In the Uranian system, due to the planet's lesser degree of oblateness, and the larger relative size of its satellites, escape from a mean motion resonance is much easier than for satellites of Jupiter or Saturn. Miranda's orbital inclination (4.34°) is unusually high for a body so close to the planet. Miranda probably escaped from its resonance with Umbriel via a secondary resonance, and the mechanism of this escape is believed to explain why its orbital inclination is more than 10 times those of the other large Uranian moons (see moons of Uranus)

Titan


Titan (or Saturn VI') is the largest moon of Saturn. It was discovered in 1655 by the Dutch astronomer Christiaan Huygens, and is the second-largest moon in the Solar System, after Jupiter's moon Ganymede, and is larger by volume than the smallest planet, Mercury, although only 40% as massive.

Titan is the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found. Titan could be described as the solar system's moon that most resembles Earth, except that it is much colder.

Titan is primarily composed of water ice and rocky material. With the arrival of the Cassini–Huygens mission in 2004, the shroud of mystery concerning the planets surface could be unveiled, and evidence emerged that liquid hydrocarbon lakes occupy Titan's polar regions. The surface is geologically young. It is smooth and few impact craters have been found, although mountains and several possible cryovolcanoes have been discovered.

The atmosphere of Titan is largely composed of nitrogen, which is the primary constituent of Earth's atmosphere. Minor components in the atmosphere of Titan lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas (probably of liquid methane and ethane), and deltas, and is dominated by seasonal weather patterns as on Earth. With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan's methane cycle is viewed as an analogy to Earth's water cycle, although at a much lower temperature.



Titan orbits Saturn once every 15 days and 22 hours. Like many of the other satellites of the gas giants and the Moon, its rotational period is identical to its orbital period; Titan is thus tidally locked in synchronous rotation with Saturn, and always shows one face to the planet.

The seasons last much longer, and approximately match the orbital period of Saturn around the Sun. One "year" on Titan lasts almost 30 years, so each of the four seasons lasts about seven and a half years.

Characteristics of Titan


Titan is likely differentiated into several layers with a 3400 km rocky center surrounded by several layers composed of different crystal forms of ice. Its interior may still be hot and there may be a liquid layer consisting of a "magma" composed of water and ammonia between the ice Ih crust and deeper ice layers made of high-pressure forms of ice. The presence of ammonia allows water to remain liquid even at temperatures as low as 176 K (for eutectic mixture with water). Evidence for such an ocean has recently been uncovered by the Cassini probe in the form of natural extremely-low-frequency radio waves in Titan's atmosphere. Titan's surface is thought to be a poor reflector of extremely-low-frequency radio waves, so they may instead be reflecting off the liquid–ice boundary of a subsurface ocean. Surface features were observed by the Cassini spacecraft to systematically shift by up to 30 km between October 2005 and May 2007, which suggests that the crust is decoupled from the interior, and provides additional evidence for an interior liquid layer. In other words, the crust seems to be "sloshing" back and forth as if it were not connected to the rest of the planet by solid matter.)

Atmosphere
See also: Atmosphere of Titan Titan is the only known moon with more than a trace of atmosphere. Its atmosphere is the only nitrogen-rich dense atmosphere in the Solar System aside from Earth's. Observations of its atmosphere made in 2004 by Cassini suggest that Titan is a "super rotator", like Venus, with an atmosphere that rotates much faster than its surface. Titan's atmosphere is denser than Earth's, with a surface pressure about 1.45 times that of Earth's. It supports opaque haze layers that block most visible light from the Sun and other sources and renders Titan's surface features obscure. Titan's lower gravity means that its atmosphere is far more extended than Earth's.

The atmosphere of Titan is opaque at many wavelengths and a complete reflectance spectrum of the surface is impossible to acquire from orbit. It was not until the arrival of the Cassini–Huygens mission in 2004 that the first direct images of Titan's surface were obtained.

Titan's atmospheric composition in the stratosphere is 98.4% nitrogen with the remaining 1.6% composed mostly of methane (1.4%) and hydrogen (0.1–0.2%), and trace amounts of various hydrocarbons.

Energy from the Sun should have converted all traces of methane in Titan's atmosphere into more complex hydrocarbons within 50 million years — a short time compared to the age of the Solar System. This suggests that methane must be somehow replenished by a reservoir on or within Titan itself. The origin of this methane in Titan's atmosphere may be its interior, released via eruptions from cryovolcanoes. On April 3, 2013, NASA reported that complex organic chemicals could arise on Titan. On June 6, 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons in the upper atmosphere of Titan. On September 30, 2013, propylene was detected in the atmosphere of Titan by NASA's Cassini–Huygens spacecraft, using its composite infrared spectrometer (CIRS). It is speculated that these hydrocarbons formed via the recombination of radicals formed by the ultraviolet photolysis of methane, the second-most common gas in Titan's atmosphere.

Climate
see also Climate of Titan

Titan receives about 1% of the amount of sunlight that Earth gets. Titan's surface temperature is about 94 Kelvins. At this temperature water ice has an extremely low vapor pressure, so the little water vapor (a greenhouse gas) present appears limited to the stratosphere. Haze in Titan's atmosphere contributes to an anti-greenhouse effect by reflecting sunlight back into space, cancelling a portion of the greenhouse effect warming and making its surface significantly colder than its upper atmosphere.

Titan's clouds, probably composed of methane, ethane or other simple organics, are scattered and variable, punctuating the overall haze. The findings of the Huygens probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto its surface. Clouds typically cover 1% of Titan's disk, though outburst events have been observed in which the cloud cover rapidly expands to as much as 8%.

Surface features
The surface of Titan is "complex, fluid-processed, [and] geologically young". Titan has existed since the Solar System's formation, but its surface is much younger, between 100 million and 1 billion years old. Geological processes may have reshaped Titan's surface. Titan's atmosphere is twice as thick as the Earth's, making it difficult for astronomical instruments to image its surface in the visible light spectrum. The Cassini spacecraft, using infrared instruments, radar altimetry and synthetic aperture radar (SAR) to map portions of Titan during its close fly-bys has revealed a diverse geology, with both rough and smooth areas. There are features that seem volcanic in origin, which probably disgorge water mixed with ammonia. There are also streaky features, some of them hundreds of kilometers in length, that appear to be caused by windblown particles. Examination has also shown the surface to be relatively smooth; the few objects that seem to be impact craters appeared to have been filled in, perhaps by raining hydrocarbons or volcanoes. Radar altimetry suggests height variation is low, typically no more than 150 meters. Occasional elevation changes of 500 meters have been discovered and Titan has mountains that sometimes reach several hundred meters to more than 1 kilometer in height.

Liquids
See: Lakes of Titan

The possibility of hydrocarbon seas on Titan was first suggested based on Voyager 1 and 2 data that showed Titan to have a thick atmosphere of approximately the correct temperature and composition to support them, but direct evidence was not obtained until 1995 when data from Hubble and other observations suggested the existence of liquid methane on Titan, either in disconnected pockets or on the scale of satellite-wide oceans, similar to water on Earth.

Overall, the Cassini radar observations have shown that lakes cover only a few percent of the surface, making Titan much drier than Earth. Although most of the lakes are concentrated near the poles (where the relative lack of sunlight prevents evaporation), a number of long-standing hydrocarbon lakes in the equatorial desert regions have also been discovered, including one near the Huygens landing site in the Shangri-La region, which is about half the size of Utah's Great Salt Lake. The equatorial lakes are probably "oases", i.e. the likely supplier is underground aquifers.

Specular reflections are indicative of a smooth, mirror-like surface, so the observation corroborated the inference of the presence of a large liquid body drawn from radar imaging. The observation was made soon after the north polar region emerged from 15 years of winter darkness. On July 8, 2009, Cassini's VIMS observed a specular reflection indicative of a smooth, mirror-like surface, off what today is called Jingpo Lacus, a lake in the north polar region shortly after the area emerged from 15 years of winter darkness.

During six flybys of Titan from 2006 to 2011, Cassini gathered radiometric tracking and optical navigation data from which investigators could roughly infer Titan's changing shape. The density of Titan is consistent with a body that is about 60% rock and 40% water. The team's analyses suggest that Titan's surface can rise and fall by up to 10 metres during each orbit. That degree of warping suggests that Titan's interior is relatively deformable, and that the most likely model of Titan is one in which an icy shell dozens of kilometres thick floats atop a global ocean. The team's findings, together with the results of previous studies, hint that Titan's ocean may lie no more than 100 km below its surface.

Impact craters


Data from Cassini have revealed few impact craters on Titan's surface. These impacts appear to be relatively young, compared to Titan's age. Many of Titan's craters or probable craters display evidence of extensive erosion, and all show some indication of modification. Most large craters have breached or incomplete rims, despite the fact that some craters on Titan have relatively more massive rims than those anywhere else in the Solar System. However, there is little evidence of formation of palimpsests through viscoelastic crustal relaxation, unlike on other large icy moons. Most craters lack central peaks and have smooth floors, possibly due to later eruption of cryovolcanic lava. Although infill from various geological processes is one reason for Titan's relative deficiency of craters, atmospheric shielding also plays a role; it is estimated that Titan's atmosphere reduces the number of craters on its surface by a factor of two.

Cryovolcanism and mountains
See also Cryovolcano

Scientists have long speculated that conditions on Titan resemble those of early Earth, though at a much lower temperature. The detection of argon-40 in the atmosphere in 2004 indicated that volcanoes had spawned plumes of "lava" composed of water and ammonia. Global maps of the lake distribution on Titan's surface revealed that there is not enough surface methane to account for its continued presence in its atmosphere, and thus that a significant portion must be added through volcanic processes.

Still there is a paucity of surface features that can be unambiguously interpreted as cryovolcanoes. One of the first of such features revealed by Cassini radar observations in 2004, called Ganesa Macula, resembles the geographic features called "pancake domes" found on Venus, and was thus initially thought to be cryovolcanic in origin, although the American Geophysical Union refuted this hypothesis in December 2008. The feature was found to be not a dome at all, but appeared to result from accidental combination of light and dark patches. In 2004 Cassini also detected an unusually bright feature (called Tortola Facula), which was interpreted as a cryovolcanic dome. But no similar features have been identified as of 2010. In December 2008, astronomers announced the discovery of two transient but unusually long-lived "bright spots" in Titan's atmosphere, which appear too persistent to be explained by mere weather patterns, suggesting they were the result of extended cryovolcanic episodes.

If volcanism on Titan really exists, the hypothesis is that it is driven by energy released from the decay of radioactive elements within the mantle, as it is on the Earth. Magma on Earth is made of liquid rock, which is less dense than the solid rocky crust through which it erupts. An alternate hypothesis has been proposed by which methane is not emitted by volcanoes but slowly diffuses out of Titan's cold and stiff interior.

Images of surface


In the first images of Titan's surface taken by Earth-based telescopes in the early 2000s, large regions of dark terrain were revealed straddling Titan's equator. Prior to the arrival of Cassini, these regions were thought to be seas of organic matter like tar or liquid hydrocarbons. Radar images captured by the Cassini spacecraft have instead revealed some of these regions to be extensive plains covered in longitudinal sand dunes, up to 330 ft high about a kilometer wide, and tens to hundreds of kilometers long. The longitudinal (or linear) dunes are presumed to be formed by moderately variable winds that either follow one mean direction or alternate between two different directions. Dunes of this type are always aligned with average wind direction. In the case of Titan, steady zonal (eastward) winds combine with variable tidal winds (approximately 0.5 meters per second). The tidal winds are the result of tidal forces from Saturn on Titan's atmosphere, which are 400 times stronger than the tidal forces of the Moon on Earth and tend to drive wind toward the equator. This wind pattern causes sand dunes to build up in long parallel lines aligned west-to-east. The dunes break up around mountains, where the wind direction shifts.

The sand on Titan is likely not made up of small grains of silicates like the sand on Earth, but rather might have formed when liquid methane rained and eroded the ice bedrock, possibly in the form of flash floods. Alternatively, the sand could also have come from organic solids produced by photochemical reactions in Titan's atmosphere. Studies of dunes' composition in May 2008 revealed that they possessed less water than the rest of Titan, and are most likely to derive from organic material clumping together after raining onto the surface.

On January 14, 2005, the Huygens probe landed on the surface of Titan, just off the easternmost tip of a bright region now called Adiri. The probe photographed pale hills with dark "rivers" running down to a dark plain.

Prebiotic conditions and search for life
See also: Life on Titan

Life on Titan
Although it is highly speculative, some scientists have proposed mechanisms by which there might be life on Titan. Arguments involve the possibility of life, possibly methane-based, existing as microbes in the thick atmosphere, in the lakes, or even under the surface where live might receive energy in the form of warmth from tidal heating of the planet.