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Uranus and its moons

Eclipses and occultations provide data to improve our understanding of the structure of planets


Uranus is rarely talked about. The seventh planet from the Sun, this celestial body named after a Greek god who represented the heavens, was only discovered in 1781 and doesn’t get the same coverage in the media as Mars, which might lead many people to imagine that it is a minor planet. However, not in the view of Roberto Vieira Martins. He and his collaborators from Brazil’s National Observatory and from the Valongo Observatory, in Rio de Janeiro, have recently witnessed a rare sequence of occultations and eclipses involving the main moons of Uranus. The measurements that they took, the most precise ever of these moons, should help us to learn more about the path of these satellites and even about the internal structure of this blue-green planet shrouded in thick layers of cloud.

Since he returned from his doctorate in France, in 1982, the Brazilian astronomer has been continuously registering the celestial revolutions of Uranus and its five largest moons: Miranda, Ariel, Umbriel, Titania and Oberon. From August to November 2007, Martins and the astronomers Marcelo Assafin, Felipe Braga-Ribas, Dario da Silva Neto and Alexandre Andrei took turns at the telescope of the Pico dos Dias observatory – the largest in Brazil, in the Brasópolis, state of Minas Gerais – to monitor a series of occultations and eclipses involving 5 of the 27 moons of Uranus, the sole planet in the Solar System whose rotation axis is inclined by slightly more than 90 degrees relative to that of the Earth.

The Rio de Janeiro team observed 5 occultations, when a satellite covers another one entirely or in part, plus two eclipses, when the shadow of one moon totally or partially covers the other. It was a rare opportunity, because Uranus is only in a favorable position for the observation of eclipses and occultations twice in the 84 years it takes to completely orbit the Sun once. Besides the seven events, described in April of 2009 in the Astronomical Journal, the group from Rio monitored something even rarer: a simultaneous occultation and eclipse, involving the same pair of satellites: Ariel, which has a diameter of 1,150 km, and Miranda, almost 2.5 times smaller.

Brightness and orbit
The passage of a satellite or of its shadow in front of another blocks part or even all of the light reflected by the body that is farthest from the Earth – and, in this case, near Uranus. Being aware of the reduction in brightness, astronomers are able to calculate the distance between the objects. In general, one needs to take measurements with various telescopes to establish accurately the position of the satellites in a planet’s orbit. However, everything becomes much simpler when at the same time there is an eclipse and an occultation of one moon over another, as was the case with Ariel and Miranda. “This information, which is to be published shortly, allows one to establish more precisely the geometry of the orbit of these satellites, with a margin of error of 30 kilometers”, states Assafin, from Valongo’s team, linked to the Federal University of Rio de Janeiro.

And 30 kilometers are virtually nothing when it comes to the most distant planet that one can see with the naked eye, at some 2.9 billion kilometers from the Sun, a distance 20 times greater than that which separates the Earth from its star. Not even the space probe Voyager 2, which visited the neighborhood of the blue-green planet in 1986, generated such precise data on the orbit of these satellites. “Our observations allowed us to collect data that is at least 10 times more precise than what was available before”, tells us Martins, who is also an associate researcher at the Paris Observatory.

NASAIn rising order: the moons Miranda, Ariel, Umbriel, Titania and OberonNASA

Combining these data with that of international groups, Martins and his team hope to establish the orbits of the moons around Uranus more exactly, as well as the forces that influence them. “The factors that determine the orbit are not always obvious”, Martins tells us. One factor that astronomers believe can interfere in the orbit of the moons is the so-called tidal force, a secondary consequence of the gravitational attraction between two bodies. Like the tidal force, which causes a slow variation in the satellites’ orbits, other forces, which result from the irregular distribution of matter inside the planet, also have an influence on the orbits.

Based on this information, astronomers can infer the composition of the planet’s innermost layers. Under clouds that are thousands of kilometers thick, for instance, there may be oceans of water diluted in methane and perhaps even a solid surface. However, all one can see from the Earth is the atmosphere. “We don’t know what’s down there”, Martins tells us.

Learning about the structure of the Solar System’s outermost planets, such as Jupiter, Saturn, Uranus and Neptune, should reveal details of the environment in which they and the Sun were formed 4.5 billion years ago. Nevertheless, even before one obtains this type of information, Assafin believes that Uranus will become better known, because in the quest for planets that are rocky like the Earth (see Pesquisa FAPESP issues 104 and 164), the smallest ones found are the same size as Uranus. “To understand why they are there”, says Assafin, “one must find out why they also exist around here”.

Scientific article
ASSAFIN, M. et al. Observations and analysis of mutual events between the Uranus main satellitesThe Astrophysical Journal. V. 137, p. 4046-53. Apr. 2009.