Miranda (satellite)

Miranda was discovered on February 16, 1948 by the Dutch (since 1933 living in the USA ) astronomer J. Kuiper at the MacDonald Observatory in Texas 97 years after the discovery of Titania and Oberon . Kuiper's goal was to measure the relative magnitudes of the four known satellites of Uranus: Ariel , Umbriel , Titania and Oberon ^[1] .

In accordance with the proposal of John Herschel - the son of the discoverer of Titania and Oberon - all the satellites of Uranus are named after the characters of the works of William Shakespeare and Alexander Pope . Miranda was named after William Shakespeare's character The Tempest ( Prospero’s Daughters) ^[1] . All the details of the relief of this satellite are named after the places where the works of William Shakespeare take place ^[5] .

Miranda is the closest to Uranus from its large satellites : it is at a distance of about 129,900 km from the planet. The eccentricity at its orbit is small (0.0013), and the inclination to the equatorial plane of Uranus is much greater than that of all the other regular satellites: 4.232 ° ^{[6] [7]} . In other words, Miranda’s orbit is almost circular, and its plane (like the equatorial plane of Uranus) is almost perpendicular to the plane of the planet’s orbit. The large inclination of the orbit to the equator of Uranus is probably due to the fact that Miranda could be in orbital resonance with other satellites - for example, at 3: 1 resonance with Umbriel and, probably, at 5: 3 resonance with Ariel ^[8] . The orbital resonance with Umbriel could increase the eccentricity of the Miranda orbit by slightly changing Umbriel's orbit. The large eccentricity of the orbit leads to a regular change in the magnitude of the tidal forces and, as a consequence, to friction in the depths of the satellite and their heating. This could be a source of energy for geological activity ^[8] . Due to the low flatness and small size of Uranus, its satellites are much easier to leave orbital resonance than Saturn or Jupiter satellites. An example of this is Miranda, who has gone out of resonance (through a mechanism, which, probably, gave her an orbit an abnormally large inclination) ^{[9] [10]} .

The orbital period is 1.41347925 Earth days and coincides with the rotation period ^[11] . Miranda is always turned to Uranus by one side, her orbit is completely in his magnetosphere ^[12] , but she has no atmosphere. Therefore her the slave hemisphere is constantly bombarded by magnetospheric plasma particles that move in orbit much faster than Miranda (with a period equal to the period of the axial rotation of Uranus) ^[13] . Perhaps this leads to the darkening of the slave hemisphere, which is observed in all the satellites of Uranus, except Oberon ^[12] . Voyager-2 registered a clear decrease in the concentration of the ions of the Uranus magnetosphere near the satellite ^[14] .

Since Uranus revolves around the Sun "on its side", and its equatorial plane roughly coincides with the equatorial plane (and orbits) of its large satellites, the change of seasons on them is very distinctive. Each pole of Miranda is 42 years old in total darkness and 42 years is continuously lit, and at the time of the summer solstice the Sun at the pole almost reaches its zenith ^[12] . The span of the Voyager-2 in January 1986 coincided with the summer solstice in the southern hemisphere, whereas almost all of the north was in complete darkness.

Once in 42 years - at the equinox on Uranus - the Sun (and with it the Earth) passes through its equatorial plane, and then the mutual covers of its satellites can be observed. Several such events were observed in 2006–2007, including the covering of Ariel Miranda on July 15, 2006 at 00:08 UT and the covering of Umbriel Miranda on July 6, 2007 at 01:43 UT ^{[15] [16]} .

The shape of satellites is closely related to their size: objects with a diameter of more than 400 km usually have a spherical shape ^[5] . The diameter of Miranda is about 470 km and, thus, it is located on the border between small and large satellites ^[17] . Its density is the lowest among the main satellites of Uranus: 1.15 ± 0.15 g / cm ³ , which is fairly close to the density of ice ^[18] . Observations of the surface in the infrared range allowed us to detect water ice mixed with silicates and carbonates ^[18] , as well as ammonia (NH ₃ ) in an amount of 3% ^[18] . Based on the data obtained by Voyager-2, it was concluded that the stones constitute 20-40% of the mass of the satellite ^[18] .

Miranda is possibly partially differentiated into a silicate core covered with an ice mantle ^[19] . If so, the thickness of the mantle is about 135 km, and the core radius is about 100 km ^[19] . In this case, heat is removed from the subsoil by heat conduction ^[19] . However, the presence of crowns on the satellite may indicate convection . According to one hypothesis, ice on Miranda forms a clathrate with methane ^[20] . In addition to methane, water clathrates can capture carbon monoxide and other molecules, forming a substance with good thermal insulation properties - the thermal conductivity of clathrates will be only from 2 to 10% of the thermal conductivity of ordinary ice ^[21] . Thus, they can prevent the outflow of heat from the depths of the satellite, which is released there during the decay of radioactive elements. In this case, it would take about 100 million years to heat the ice to 100 ° C ^[21] . Thermal expansion of the core could reach 1%, which would lead to cracking of the surface ^{[20] [21]} . Its heterogeneity may be due to the heterogeneity of the flow of thermal energy from the depths ^[22] .

Miranda has a unique surface ^[5] with a wide variety of landforms. These are cracks , faults , valleys , craters , ridges , depressions, rocks and terraces ^{[17] [23]} . The surface of this satellite, the size of Enceladus, is an amazing mosaic from very diverse zones. Some regions are old and inexpressive. They are dotted with numerous impact craters, as one would expect from a small inert body ^[5] . Other regions are intersected by complex interweaving of ridges and ledges and covered with rectangular or ovoid systems of light and dark stripes, which indicates the unusual composition of Miranda ^[11] . Most likely, the satellite's surface consists of water ice, and the deeper layers of silicate rocks and organic compounds ^[11] .

This led to the assumption that the surface of this satellite was rebuilt up to 5 times throughout its history. On the images of Miranda, a structure in the form of the Latin letter “V” is visible, alongside there are mountain ranges and valleys, old craterous and young smooth areas, shaded canyons up to 20 km deep. Just below the center is the large crater of Alonso, 24 km deep.

Several hypotheses have been put forward to explain the strong inhomogeneity of the Miranda surface. According to one of them, Miranda was split as a result of a collision with a large celestial body, but then the pieces were reunited again. However, it remains unclear why the impact craters remained on the remaining parts of the satellite surface. Another hypothesis suggests that there was an uneven heating of the Miranda bowels.

Areas

Large areas of the surface, which differ from the neighboring ones in color or brightness, are called regions in the planetary nomenclature ( lat. Regio , plurality regiones ). The Miranda areas, which can be seen in the pictures of Voyager-2, were named “Mantua Region”, “Efes Region”, “Sicily Region” and “Dunsinan Region” ^[24] . These are more or less highly craterized hilly plains ^[11] . In some places there are faults and ledges, some of which are as old as the regions themselves, while others are supposed to have appeared quite recently, with the formation of rims ^[11] . These faults are accompanied by grabens , which indicates the presence of tectonic activity in the past ^[11] . The surface of the regions is almost uniformly dark, but brighter rocks are visible on the slopes of the craters ^[11] .

Crowns

Miranda - one of the few satellites in the solar system , with the crowns ( lat. Corona , pl. Coronae ) - a kind of annular or oval surface details. Modeling showed that they could occur due to convection in the depths. It is assumed that in the past, Miranda had a more elongated orbit and was deformed on each revolution due to the change in the magnitude of the tidal forces from Uranus. This caused heating of its subsoil, and warm plastic ice rose in several streams to the surface. Interacting with it, these flows formed crown rings ^{[25] [26]} .

Three crowns found by Voyager-2 are now known: the Arden crown (located on the leading hemisphere), the Elsinore crown (on the slave hemisphere) and the Inverness crown (located at the south pole). Contrasts of albedo on the surface of Miranda are most pronounced on the crowns of Arden and Inverness ^[11] .

Crown Inverness

The Inverness Circlet is a trapezoid area of ​​about 200 km² located near the south pole. Its outer boundary, like the inner ridges and stripes, forms a polygon ^[11] . It is bounded on three sides (south, east, and north) by a complex system of faults. The nature of the western edge is less clear, but it can also be the result of tectonic activity. Most of the area of ​​the crown is occupied by parallel grooves, separated by gaps of several kilometers ^[27] . A small number of impact craters indicates a smaller Inverness crown than the other two crowns ^[27] .

Crown Arden

Crown Arden is located on the leading hemisphere of Miranda, and extends for 300 km from east to west. Its size from north to south is unknown, because while photographing it with Voyager-2, the northern hemisphere was located behind the terminator (it was immersed in darkness). This crown is formed by a bright, beveled rectangle at least 100 km wide, which is surrounded by darker parallel stripes. In general, a kind of “egg-shaped” figure is obtained ^[11] . The inner and outer parts of the Arden crown are very different. The inner zone has a smooth relief and a “marble” pattern of large light areas scattered over a dark surface. The stratigraphic relationship between the dark and the light surface cannot be determined due to the low resolution of Voyager-2 images. The outer part of the Arden crown is formed by light and dark stripes that extend from the western part of the crown, where they cross the crater surface (about 40 ° longitude), to the eastern part, where they go to the night side (about 110 ° longitude) ^[27] . These bands are formed by cliffs, which gradually fade away on the border between the Arden crown and the craterized region of Mantua ^[27] . Arden was formed earlier than Inverness, and at the same time with the crown Elsinore ^[27] .

Crown Elsinore

The crown Elsinore is located on the slave hemisphere of Miranda and in the Voyager photographs is located near the terminator. In size and structure, it is similar to the Arden crown. Both crowns have an outer belt about 100 km wide that girds the inside ^[11] . The relief of this part is a complex complex of depressions and hills that break off at the border of the outer belt, formed by almost parallel linear ridges. In the depressions there are small segments of hilly and craterized terrain ^[11] . There are also potholes within the crown of Elsinore - systems of roughly parallel depressions and ridges comparable to those on Ganymede , the satellite of Jupiter ^[11] .

Offsets

On the surface of Miranda there are ledges . Some are older than crowns, while others are younger. The most colorful - the ledge of Verona - is observed on the edge of the deep depression extending beyond the terminator.

This depression starts from the northwestern side of the crown of Inverness ^[11] , where the ledge Algeria is located, and stretches towards the convergence of the bands of this crown, after which it is sent to the terminator ^[11] . There it has a width of about 20 km, and its edge forms a huge bright precipice - the ledge of Verona. The height of this ledge is 10-15 km ^[11] , which is much higher than the walls of the Grand Canyon on Earth. The height of this rock is especially surprising compared to the small size of Miranda: 2-3% of the diameter of the satellite. All these conclusions were made from the pictures from Voyager-2, where Verona’s ledge leaves behind the terminator. It is likely that this step continues to the night side, and its full length is even longer ^[27] .

Impact Craters

By the number of impact craters, you can determine the age of the surface of a solid celestial body devoid of the atmosphere - the more craters, the older the surface ^{[27] [5]} .

During the overflight of the Voyager-2 space station, only craters on the south side of the satellite were studied. Their diameters vary from 500 m to 50 km ^[27] . Craters are very diverse in shape. Some very well visible edges, and often they are surrounded by a substance thrown on impact. Others are so destroyed that they can be seen with difficulty ^[27] .

No complex craters with central slides or craters girdled with multiple rings were found on Miranda. The detected craters are simple (with a cup-shaped bottom) or transitional (with a flat bottom), and the dependence of the shape of craters on their size is not observed ^[27] . Also known are simple craters with a diameter of about 15 km, and transition craters with a diameter of only 2.5 km ^[27] . Miranda craters are rarely surrounded by emissions, and for craters with a diameter of more than 15 km, emissions are completely unknown ^[27] . With a crater diameter of less than 3 km, its emissions are usually lighter than the surrounding surface, and with a diameter of 3 to 15 km, it is darker. But among craters of any size, there are those whose emissions have the same albedo as the surrounding surface ^[27] .

On the example of this satellite one can observe interesting geological phenomena ^[27] . To explain its formation and geological evolution, the scientific community proposed several theories ^[5] . One of them is that Miranda was formed from a gas-dust nebula or an accretion disk around Uranus. This disk either existed since the formation of the planet, or was formed by a huge collision , which most likely gave Uranus a large tilt of the axis of rotation ^[28] . Meanwhile, there are parts on this relatively small satellite, whose age is surprisingly short compared with the age of Miranda itself ^[29] . Apparently, the age of the youngest geological formations of Miranda is only a few hundred million years ^[27] . The simulation of the thermal history of small satellites (Miranda size) predicts rapid cooling and the complete absence of geological evolution after satellite accretion from a nebula ^[27] .Geological activity for such a long time can not be explained either by the energy from the initial accretion, or by the fission energy of radioactive elements ^[27] .

Миранда по сравнению с остальными спутниками Урана имеет самую молодую поверхность. Это указывает на то, что поверхность Миранды недавно претерпела значительные изменения ^[27] . Нынешнее её состояние объясняется её сложной геологической историей, в которой имели место редкие сочетания различных астрономических явлений ^[5] . Среди этих явлений могут быть и приливные силы , и явления орбитальных резонансов , и процессы конвекции и частичной дифференциации ^[5] .

Удивительная геологическая структура поверхности, состоящей из резко отличающихся областей, может быть результатом того, что Миранда была разбита на части при катастрофическом столкновении с другим небесным телом ^{[5] [27]} , а затем заново собралась из кусков под действием силы гравитации ^[30] . Некоторые учёные предполагают даже несколько этапов столкновений и повторной аккреции спутника ^[31] . Эта версия стала менее привлекательной в 2011 году из-за появления данных в пользу гипотезы, объясняющей особенности рельефа Миранды действием приливных сил Урана. Видимо, эти силы могли создать крутые разломы, наблюдаемые в венцах Инвернесс и Арден. Источником энергии для таких преобразований могла быть только сила притяжения Урана ^[32] .

В конечном счёте, формирование поверхности Миранды могло длиться более 3 млрд лет. Оно началось примерно 3,5 млрд лет назад с появления сильно кратерированных районов и закончилось сотни миллионов лет назад образованием венцов ^[27] .

Явления орбитальных резонансов (в большей степени с Умбриэлем , чем с Ариэлем ) оказали значительное влияние на эксцентриситет орбиты Миранды ^[8] , что могло привести к разогреву недр и геологической активности спутника ^[8] . Нагрев способствовал конвекции внутри Миранды, которая положила начало дифференциации её вещества ^[8] . В то же время орбитальный резонанс слабо изменил бы орбиты других, более массивных, спутников ^[8] . Но, вероятно, поверхность Миранды искорёжена слишком сильно, чтобы это можно было объяснить только этим механизмом ^[29] .

Миранда ушла от резонанса с Умбриэлем в ходе процесса, который придал её орбите аномально большое наклонение к экватору Урана ^[8] . Большой ранее эксцентриситет уменьшился из-за действия приливных сил: изменения их величины на каждом витке орбиты приводят к подвижкам и трению в недрах. Это стало причиной нагрева спутника и позволило ему вернуть шарообразную форму, но при этом Миранда сохранила впечатляющие геологические образования, такие как уступ Верона ^[29] . Поскольку первопричиной геологической активности был эксцентриситет орбиты, его уменьшение привело к затуханию этой активности. В результате Миранда стала холодным инертным спутником ^[8] .

«Вояджер-2», изучавший систему Урана в январе 1986 года, сблизился с Мирандой намного теснее, чем с любым другим спутником Урана (на 29 000 км), и поэтому заснял её намного детальнее ^[33] . Наилучшие фотографии Миранды имеют разрешение 500 м. Заснято около 40 % поверхности, но только 35 % — с качеством, пригодным для геологического картирования и подсчёта кратеров . При пролёте «Вояджера» вблизи Миранды Солнце освещало только южное её полушарие, и поэтому северное осталось неизученным ^[11] . Никакой другой космический корабль никогда не посещал Миранду (и вообще систему Урана). В 2020-х годах, возможно, будет запущен исследовательский аппарат НАСА « Uranus orbiter and probe ». В его состав будет входить орбитальный модуль и атмосферный зонд. Кроме того, группа из 168 учёных представила Европейскому космическому агентству программу миссии « Uranus Pathfinder » для путешествия к внешней части Солнечной системы, в котором конечной целью будет Уран ^[34] . Цель вышеназванных программ — уточнение данных об Уране и его спутниках (в том числе и о Миранде).

Дэвид Нордли посвятил Миранде фантастический рассказ «В пещерах Миранды», где рассказывается о путешествии по спутнику.

• Силкин Б. И. В мире множества лун / Под ред. Е. Л. Рускол. — Москва: «Наука». Главная редакция физико-математической литературы, 1982. — 208 с. - 150 000 copies
• Gerard P. Kuiper . The Fifth Satellite of Uranus (Eng.) // Publications of the Astronomical Society of the Pacific . - 1949. - Vol. 61, no. 360 . - P. 129. - DOI : 10.1086 / 126146 . - .

• Miranda: Overview (eng.) . Nasa The appeal date is September 15, 2011. Archived January 24, 2012.
• Miranda, A Moon of Uranus (English) . Views of the Solar System . The appeal date is September 15, 2011. Archived January 24, 2012.
• Miranda (eng.) . The Nine Planets Solar System Tour (15 December 2004). The appeal date is September 15, 2011. Archived January 24, 2012.