Astronomy Picture of the Day
April 6, 2014

Uranus
Uranus

Credits: NASA/JPL-Caltech - Voyager 2 Mission; credits for the additional process. and color.: Dr Paolo C. Fienga/LXTT/IPF

Today's APOD is a view of Uranus that was recorded by the NASA - Voyager 2 Spacecraft in the AD 1986, as the Spacecraft was approaching the Gas-Giant Planet and its many moons behind, and then set forth on the cruise towards the last Gas-Giant Planet of our Solar System, Neptune.

Uranus possesses the third-largest Planetary Radius and the fourth-largest Planetary Mass in the Solar System. It was named after the ancient Greek Deity of the Sky, Uranus, the father of Cronus (such as Saturn) and grandfather of Zeus (such as Jupiter). Even though Uranus is (just theoretically) visible to the naked eye, like the other 5 (five) so-called "Classical Planets" (such as Mercury, Venus, Mars, Jupiter and Saturn, all clearly visible and easy to recognize in the Sky even to the unaided eye), it was never recognized as a Planet by ancient observers because of its dimness and slow orbit. As a matter of fact, it was Sir William Herschel who announced its discovery, on March 13, of the AD 1781, thus expanding the known boundaries of the Solar System for the first time in Modern history. Uranus was also the first Planet discovered by using a telescope.


Uranus is similar in composition to Neptune, and both Planets show different chemical composition as to the other two (larger) Gas-Giants Planets, Jupiter and Saturn. Astronomers sometimes place them (such as Uranus and Neptune) in a separate category called "Ice Giants" and that is because the Atmosphere of both Uranus and Neptune, while similar to the one of Jupiter and Saturn in its Primary Composition of Hydrogen and Helium, actually contains more "Ices", such as Water, Ammonia and Methane, along with traces of Hydrocarbons than the Atmospheres of Jupiter and Saturn. In addition to that, the Atmosphere of Uranus appears to be the coldest Planetary Atmosphere in the Solar System, with a minimum temperature of 49 K (such as − 224° Celsius). Uranus has also shown, sometimes in its Atmosphere, the presence of complex, layered Cloud Structures, with Water Ice Particles - which are thought to make up the lowest Clouds' Layers - and Methane, on the other hand, that is thought to form their uppermost Layer. In contrast, the interior of Uranus is thought to be mainly composed of Ices and Rock.


Like the other Gas-Giant Planets, Uranus possesses a Ring-System, a Magnetosphere, and a remarkable number of moons. The Uranian System has a unique configuration among the Planets because its Axis of Rotation is tilted sideways, nearly into the Plane of its Revolution about the Sun. This means that its Poles, therefore, lie where the other Planets of the Solar System have their Equators. In the AD 1986, images from NASA - Voyager 2 Spacecraft showed Uranus as a virtually featureless Planet in Visible Light (note that such a total lack of visible reliefs occurs all the times when its Upper Atmosphere does not show the presence of any Weather-related Phenomena, like the Cloud-Bands and huge Storms which are often associated with all the other three Gas-Giant Planets). Furthermore, throughout the analysis of several pictures taken by the NASA - Hubble Space Telescope, a few Astronomers have seen signs of seasonal change and increased Weather Activity in recent years as Uranus approached its Equinox. Last but not least (and just out of curiosity), the speed of the Uranian Winds (as it has been calculated by observing the movements of the Clouds located in its Upper Atmosphere along the Disc of the Planet) apparently can reach up to 250 meters per second (such as approx. 900 Km-per-hour, or either approx. 558,9 miles-per-hour).


In this specific picture, however, some very faint and small Round and Linear Clouds can be seen towards the Southern Latitudes of the Gas-Giant Planet.


This frame (which is an Original NASA - Voyager 2 Spacecraft Natural Color frame published on the NASA - Planetary Photojournal with the ID n. PIA 18182) has been additionally processed, contrast enhanced, magnified to aid visibility of the details and then re-colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - Voyager 2 Spacecraft and then looked outside, towards the Gas-Giant Planet Uranus), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among others, the existence of different Elements present in the Atmosphere of Uranus, each having a different Albedo (---> Reflectivity) and Chemical Composition.


Note: it is possible (but we, as IPF, have no way to be one-hundred-percent sure of such a circumstance), that the actual luminosity of Uranus - as it is in this frame - would appear, to an average human eye, way lower than it has been shown (or, better yet: interpreted) here.



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