Editing Miranda (moon) (section)

== Origin and formation ==
Several scenarios are proposed to explain its formation and geological evolution.<ref name="Plescia1987_442"/><ref name=Brahic2010_195/> One of them postulates that it would result from the [[Accretion (astrophysics)|accretion]] of a disk of gas and dust called a "subnebula".<ref name="Mousis2004">{{harvsp|O. Mousis|2004|p=373|id=Mousis2004}}</ref> This sub-nebula either existed around Uranus for some period of time after its formation, or was created following a [[Impact event|cosmic impact]] which would have given its great obliquity to the axis of rotation of Uranus.<ref name="Mousis2004"/> However, this relatively small moon has areas that are surprisingly young compared to the [[geological time scale]].<ref name="Peale1988_153">{{harvsp|S. J. Peale|1988|p=153|id=Peale1988}}</ref> It seems that the most recent geological formations only date back a few hundred million years.<ref name=Plescia1987_458/> However, thermal models applicable to moons the size of Miranda predict rapid cooling and the absence of geological evolution following its accretion from the subnebula.<ref name=Plescia1987_459/> Geological activity over such a long period cannot be justified either by the heat resulting from the initial accretion, nor by the heat generated by the [[nuclear fission|fission of radioactive materials]] involved in the formation.<ref name=Plescia1987_459 />

Miranda has the youngest surface among those of the satellites of the Uranian system, which indicates that its geography has undergone the most important evolutions.<ref name="Plescia1987_442"/> This geography would be explained by a complex geological history including a still unknown combination of different astronomical phenomena.<ref name=Brahic2010_195/> Among these phenomena would be [[tidal forces]], mechanisms of [[Orbital resonance|orbital resonances]], processes of [[Planetary differentiation|partial differentiation]], or even movements of [[convection]].<ref name=Brahic2010_195/>

The geological patchwork could be partly the result of a catastrophic collision with an [[Impact event|impactor]].<ref name=Brahic2010_195/> This event may have completely dislocated Miranda.<ref name=Plescia1987_442/> The different pieces would then have re-assembled, then gradually reorganized in the spherical form that the ''[[Voyager 2]]'' probe photographed.<ref name=Waldrop1986_916>{{harvsp| Waldrop|1986|p=916|id=Waldrop1986}}</ref> Some scientists even speak of several cycles of collision and re-accretion of the moon.{{sfn| Croft |Greenberg|1991|p=561}} This geological hypothesis was depreciated in 2011 in favor of hypotheses involving Uranian tidal forces. These would have pulled and turned the materials present under Inverness and Arden to create fault scarps. The stretching and distortion caused by Uranus's gravity, which alone could have provided the heat source necessary to power these uprisings.<ref name=Cowen1993>{{harvsp|R. Cowen|1993|p=300|id=Cowen1993}}</ref>

The oldest known regions on the surface of Miranda are cratered plains such as Sicilia Regio and Ephesus Regio.<ref name=Plescia1987_458>{{harvsp|J. B. Plescia|1987|p=458|id=Plescia1987}}</ref> The formation of these terrains follows the accretion of the moon<!--Previously said "star" which is obviously an error--> then its cooling.<ref name=Plescia1987_458/> The bottoms of the oldest craters are thus partially covered with material from the depths of the moon referred to as endogenous resurfacing, which was a surprising observation.<ref name=Plescia1987_458/> The geological youth of Miranda demonstrates that a heat source then took over from the initial heat provided by the accretion of the moon.<ref name=Plescia1987_458/> The most satisfactory explanation for the origin of the heat which animated the moon is the one which also explains the [[volcanism on Io]]: a situation of [[orbital resonance]] now on Miranda and the important phenomenon of [[tidal force]]s generated by Uranus.<ref name="Peale1988_153"/>

After this first geological epoch, Miranda experienced a period of cooling which generated an overall extension of its core and produced fragments and cracks of its mantle on the surface, in the form of [[graben]]s.<ref name=Plescia1987_458 /> It is indeed possible that Miranda, [[Ariel (moon)|Ariel]], and [[Umbriel]] participated in several important resonances involving the pairs Miranda/Ariel, Ariel/Umbriel, and Miranda/Umbriel.<ref name="Peale1988_154">{{harvsp|S. J. Peale|1988|p=154|id=Peale1988}}</ref> Unlike those observed on Jupiter's moon [[Io (moon)|Io]], these orbital resonance phenomena between Miranda and Ariel could not lead to a stable capture of the small moon.<ref name="Peale1988_154"/> Instead of being captured, Miranda's orbital resonance with Ariel and Umbriel may have led to the increase in its eccentricity and orbital inclination.<ref name="Peale1988_157">{{harvsp|S. J. Peale|1988|p=157|id=Peale1988}}</ref> By successively escaping several orbital resonances, Miranda alternated phases of heating and cooling.<ref name="Peale1988_169"/> Thus all the known grabens of Miranda were not formed during this second geological episode.<ref name=Plescia1987_458/>

A third major geological epoch occurs with the orbital reorientation of Miranda and the formation of Elsinore and Arden coronae.<ref name=Plescia1987_458/> A singular volcanic event, made of flows of solid materials, could then to have taken place, within the coronae in formation.<ref name=Jankowski1988_1325>{{harvsp| Jankowski |Squyres|1988|p=1325|id=Jankowski1988}}</ref> Another explanation proposed for the formation of these two coronae would be the product of a [[diapir]] which would have formed in the heart of the moon.{{sfn|Pappalardo|Greeley|1993}}{{sfn|Pappalardo|Reynolds|Greeley|1997}} On this occasion Miranda would have at least partially differentiated.{{sfn|Pappalardo|Greeley|1993}} Considering the size and position of these coronae, it is possible that their formation contributed to changing the [[moment of inertia]] of the moon.<ref name=Plescia1987_455 /> This could have caused a 90° reorientation of Miranda.<ref name=Plescia1987_455/> Doubt remains as to the concomitant existence of these two formations.<ref name=Plescia1987_455/> It is possible that at this time, the moon was distorted to the point that its asphericity and eccentricity temporarily caused it to undergo a chaotic rotational movement, such as that observed on [[Hyperion (moon)|Hyperion]].<ref name="Peale1988_169"/> If Miranda's orbital reorientation occurred before the two coronae formed on the surface, then Elsinore would be older than Arden.<ref name=Plescia1987_458/> Chaotic movement phenomena generated by the entry into 3:1 resonance between the orbit of Miranda and that of Umbriel could have contributed to an increase in Miranda's orbital inclination greater than 3°.<ref name="Peale1988_157"/>

A final geological episode consists of the formation of Inverness which seems to have induced surface tensions which gave rise to the creation of additional grabens including Verona Rupes and Argier Rupes.<ref name=Plescia1987_458/> Following this new cooling of Miranda, its total volume could have increased by 4%.<ref name="Croft1992_416">{{harvsp|Croft|1992|p=416|id=Croft1992}}</ref> It is probable that these different geological episodes followed one another without interruption.<ref name=Plescia1987_458/>

Ultimately, Miranda's geological history may have spanned a period of more than 3 billion years. It would have started 3.5 billion years ago with the appearance of heavily cratered regions and ended a few hundred million years ago, with the formation of the coronae.<ref name=Plescia1987_459>{{harvsp|J. B. Plescia|1987|p=459|id=Plescia1987}}</ref>

The phenomena of orbital resonances, and mainly that associated with [[Umbriel]], but also, to a lesser extent, that of [[Ariel (moon)|Ariel]], would have had a significant impact on the orbital eccentricity of Miranda,{{sfn|Tittemore|Wisdom|1990}} and would also have contributed to the internal heating and geological activity of the moon. The whole would have induced convection movements in its substrate and allowed the start of planetary differentiation.{{sfn|Tittemore|Wisdom|1990}} At the same time, these phenomena would have only slightly disturbed the orbits of the other moons involved, which are more massive than Miranda.{{sfn|Tittemore|Wisdom|1990}} However, Miranda's surface may appear too tortured to be the sole product of orbital resonance phenomena.<ref name="Peale1988_169">{{harvsp|S. J. Peale|1988|p=169|id=Peale1988}}</ref>

After Miranda escaped from this resonance with Umbriel, through a mechanism that likely moved the moon into its current, abnormally high orbital tilt, the eccentricity would have been reduced.{{sfn|Tittemore|Wisdom|1990}} The tidal forces would then have erased the eccentricity and the temperature at the heart of the moon. This would have allowed it to regain a spherical shape, without allowing it to erase the impressive geological artifacts such as Verona Rupes.<ref name="Peale1988_169"/> This eccentricity being the source of the [[Tidal force|tidal forces]], its reduction would have deactivated the heat source which fueled the ancient geological activity of Miranda, making it a cold and inert moon.{{sfn|Tittemore|Wisdom|1990}}