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Satellites of Uranus

Forming the satellites of Uranus

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Merry Christmas! Today we discuss how the satellites of Uranus were formed. It is usually thought the satellites of a giant planets formed from a protoplanetary nebula. Originally there was a cloud of gas and dust, mass accumulated in the center to form the planet, and protosatellites were created from the accretion of mass as well. This was well understood for the gas giants like Jupiter, but Uranus is much smaller (23 times lighter). The Swiss study I present today, In situ formation of icy moons of Uranus and Neptune, by Judit Szulágyi, Marco Cilibrasi and Lucio Mayer, solves this problem. This study has recently been published in The Astrophysical Journal Letters.

Outline

The satellites of Uranus
From the disk to the satellites
Hydrodynamic simulations
Yes, it is possible!
What about Neptune?
The study and its authors

The satellites of Uranus

Uranus has 27 known satellites, which can be classified into 3 groups:

  • the inner moons, which are small satellites embedded in the rings,
  • the main moons, which are mid-sized icy bodies. These are the ones we are interested in today,
  • and the irregular moons, which orbit very far from the planet, and on significantly eccentric and inclined orbits. These bodies are probably former asteroids, which were trapped by the gravitational field of Uranus.

As I said, we are interested in the main 5 satellites, which are listed below. Their semimajor axes are given with respect to the mean equatorial radius of Uranus, which is 25,559 km.

U-5 Miranda U-1 Ariel U-2 Umbriel U-3 Titania U-4 Oberon
Discovery 1948 1851 1851 1787 1787
Semimajor axis 5.062 RU 7.474 RU 10.408 RU 17.055 RU 21.070 RU
Eccentricity 0.0013 0.0012 0.0039 0.0011 0.0014
Inclination 4.232° 0.260° 0.205° 0.340° 0.058°
Orbital period 1.413 d 2.520 d 4.144 d 8.706 d 13.463 d
Diameter 471.6 ± 1.4 km 1157.8 ± 1.2 km 1169.4 ± 5.6 km 1576.8 ± 1.2 km 1522.8 ± 5.2 km
Density 1.20 g/cm3 1.66 g/cm3 1.40 g/cm3 1.72 g/cm3 1.63 g/cm3

As you can see, these 5 bodies are

  • A small one (Miranda), which is pretty close to the planet,
  • two larger ones, Ariel and Umbriel, which orbit further from the planet,
  • and two even larger ones, Titania and Oberon, which orbit even further from the planet.

Discoveries

Titania and Oberon have been discovered in 1787 by the German-British astronomer William Herschel, only 6 years after the same William Herschel discovered Uranus. Actually, Uranus was (and still is) visible to the naked eye, and had been observed many times before. But how to know it was a planet, and not a star? Well, a star does not move in the sky (actually, it does a very little…), while a planet moves. But since Uranus orbits very far from the Sun, its motion is pretty slow. Herschel detected such motion, but he thought at that time that Uranus was a comet. The computation of its motion showed a pretty circular orbit, proving it was a planet.
After that, Uranus has been observed many times, and Herschel noticed two dots following Uranus. Since they followed Uranus, it meant they were gravitationally bound to it, hence satellites. These two dots were the two largest of them, i.e. Titania and Oberon.

Seventy years after the discovery of Uranus, the British merchant and astronomer William Lassell, who by the way made his fortune as a beer brewer, built his own telescope. He polished himself the mirror, and pioneered the use of the equatorial mount, which facilitated the tracking of objects with respect to the rotation of the Earth. His telescope permitted him to discover the satellite of Neptune Triton, to co-discover the satellite of Saturn Hyperion, and to discover the satellites of Uranus Ariel and Umbriel.

For Miranda, we had to wait for the Dutch-American astronomer Gerard (Gerrit) Peter (Pieter) Kuiper. He discovered Miranda in 1948 and the satellite of Neptune Nereid in 1949, at McDonald Observatory (TX, USA). Kuiper is mostly known for having proposed the existence of the so-called Kuiper Belt, i.e. a belt of asteroids orbiting beyond the orbit of Neptune. He has also been the thesis advisor of Carl Sagan.

Properties

Let us go back to the table, and have a look at their properties. We can see that these bodies have small eccentricities and inclinations, i.e. they orbit in the equatorial plane of Uranus, on pretty circular orbits. There is anyway an exception to this rule, which is the significant inclination of Miranda (4.2°). This inclination has probably been excited by a past 3:1 mean motion resonance with Umbriel.

Another interesting point is the density of these bodies. 1g/cm3 means a composition close to water. Pure water ice would be a little less dense. Here we have densities between 1 and 2, which means that these bodies are mixtures of ice and silicates.

This property they share is a clue, which suggests a common formation process. Let us investigate the formation from the protoplanetary disk.

From the disk to the satellites

Let us figure out how a giant planet is formed. First you have a protoplanetary nebula, made of gas and dust. Matter accumulates and aggregates at its center, creating a star (if the nebula is massive enough). To compensate this accumulation at the center (conservation of the total angular momentum), the matter which is still outside the star accelerates, and the nebula becomes a disk, which orbits the star.
Then (may be a little meanwhile, actually), you have local accretions of matter, which create the planets. And sometimes, if you have enough matter, then you have a circumplanetary disk around some of the planets, in which matter aggregates… and creates the satellites! Well, this way, it seems to be easy.
One question is: how massive need the protoplanetary disk to be, to create the satellites. It was known that it works for Jupiter. This study wonders whether it works for Uranus.

Hydrodynamic simulations

To answer this question, the authors ran intensive numerical simulations, using the hydrocode JUPITER. By hydrocode I mean that it simulates a hydrodynamic system.

Actually, a disk is made of particles of gas and dust. It is highly challenging, even if it is sometimes tried, to consider all the particles constituting it, and model their motion and their interactions. Instead, you can consider that the whole disk acts as a gas, and model the collisions between the particles as a viscosity.

Simulating this motion requires to split the disk into cells, use the method of finite elements, i.e. the state of a given part of the disk depends on the state of its neighbors… This requires intensive computing facilities. In JUPITER, you can focus on a given region, for instance where a planet is created.

The authors ran 25,000 simulations, depending on the following parameters:

  • the disk dispersion timescale,
  • the dust-to-gas ratio,
  • the dust refilling timescale: when dust accumulates at the center to create the planet, the disk needs to reach a new equilibrium. This parameter controls the velocity of this process,
  • the distance from the planet where the first proto-satellites are created,
  • the initial temperature of the central planet.

It appeared that this temperature, set to 1000K, 500K and 100K, plays a critical role in the possibility to create the satellites. Consider this effect is possible in JUPITER since 2016 and the implementation of a module, which models the radiative transfer in the disk. As a consequence, it models the effects of the heating and cooling of the gas.

Yes, it is possible!

The simulations show that the circumplanetary gaseous disk was formed when the temperature dropped below 500K (227°C, or 441°F). In that case, icy moons were formed in most of the simulations, which strongly suggests that the present satellites of Uranus were formed that way.

What about Neptune?

Neptune is somehow like Uranus, by its size. This is why the authors ran similar simulations, which showed similar results, i.e. formation of icy, mid-sized satellites. But wait, this is not what we see.

When we observe the system of Neptune, we see a large satellite, Triton, which is highly inclined, on a retrograde orbit. As we discussed here, Triton behaved like a cuckoo.

The satellite of Neptune Triton seen by Voyager 2 in 1989. © NASA
The satellite of Neptune Triton seen by Voyager 2 in 1989. © NASA

Triton was an asteroid, which has been trapped in the gravity field of Neptune. Then it was so massive than it ejected the satellites, which were present… if they existed. What this study tells us is that they probably existed. Nereid was probably one of them. Where are the others now? In my opinion, they could be almost anywhere, since the Solar System is a mess.

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.

Satellites of Uranus

Uranus’s satellites are red

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Hi there! Today, we speak about the satellites of Uranus. They have been visited only once by a spacecraft, i.e. Voyager 2 in 1986, but we dispose of Earth-based facilities, which are able to give us some clues. The study I present today, Red material on the large moons of Uranus: Dust from irregular satellites? discusses the fact that the main moons appear to be redder than the large moons of Jupiter and Saturn.

Let me define redder first: the surface of these bodies reflects the Solar incident light. A spectral decomposition of the reflected light tells us something on the material coating the surface. And the relative response of the surface in the different wavelengths is higher in the infrared for the large moons of Uranus, than it is for the moons of Jupiter and Saturn.

This study, by Richard Cartwright et al., has recently been published in Icarus.

Outline

The satellites of Uranus
Voyager 2 at Uranus
Observations at IRTF
Geometrical constraints
A red leading side
Pollution by the irregular moons
The study and its authors

The satellites of Uranus

First: Uranus. This is the seventh planet of our Solar System, which orbits in 84 years, and which seems to roll on its orbit. Actually, its rotation axis is tilted by nearly 90° (actually 97.8°), and its main satellites and rings orbit close to its equatorial plane. Their orbits are tilted as well.

The satellites of Uranus, all named after Shakespeare’s characters, can be categorized into 3 groups:

  1. The 13 small, inner satellites, which are embedded into the rings,
  2. the 5 main ones,
  3. and the 9 irregular satellites, which orbit much further from Uranus, and which orbits may be tilted. Contrary to the other two groups, they have probably not been formed in the proto-Uranus nebula, but were former asteroids, which have been trapped by Uranus.

You can find below some properties and orbital characteristics of the main satellites. All of these bodies have been discovered from the Earth. These are the targets of the study I present.

Discovery Semimajor axis Eccentricity Inclination Orbital period Mean diameter
Miranda 1948 129,900 km 0.0013 4.338° 1.413 d 471.6 km
Ariel 1851 190,900 km 0.0012 0.041° 2.520 d 1,157.8 km
Umbriel 1851 266,000 km 0.0039 0.128° 4.144 d 1,169.4 km
Titania 1787 436,300 km 0.0011 0.079° 8.706 d 1,577.8 km
Oberon 1787 583,500 km 0.0014 0.068° 13.46 d 1,522.8 km

You can see that they have limited eccentricities and inclinations, except for the inclination of Miranda, which probably results from a past resonant forcing by Umbriel. In the past, the orbital period of Umbriel was exactly thrice the one of Miranda, and this has forced its inclination, which was thus initially very small. Anyway, it remains close to the equatorial plane for Uranus.

You can see below that things are different for the small satellites.

Discovery Semimajor axis Eccentricity Orbital period Mean diameter
Francisco 2001 4,282,900 km 0.13 267 d ≈22 km
Caliban 1997 7,231,100 km 0.18 580 d ≈72 km
Stephano 1999 8,007,400 km 0.22 677 d ≈32 km
Trinculo 2001 8,505,200 km 0.22 749 d ≈18 km
Sycorax 1997 12,179,400 km 0.52 1,288 d ≈150 km
Margaret 2003 14,146,700 km 0.68 1,661 d ≈20 km
Prospero 1999 16,276,800 km 0.44 1,978 d ≈50 km
Setebos 1999 17,420,400 km 0.59 2,225 d ≈48 km
Ferdinand 2001 20,430,000 km 0.40 2,790 d ≈20 km

These are very small bodies, which orbit very far from Uranus, on eccentric orbits. Besides, their orbital planes have just nothing to do with the equatorial plane of Uranus. This is why we believe they are former asteroids. Beside Margaret, they all orbit on retrograde orbits, while all the regular moons are prograde. Discovering them required to use telescopes of a least 5 m, the satellites discovered in 2003 having been discovered during a systematic survey with the Subaru telescope at Mauna Kea, Hawaii, over a field of 3.5 degrees. They all have apparent magnitudes larger than 20.

Only one space mission visited them: Voyager 2, in January 1986.

Voyager 2 at Uranus

The spacecraft Voyager 2 was launched from Cape Canaveral, Florida, in August 1977. It benefited from a favorable geometrical configuration of the 4 giant planets to visit all of them. Unfortunately, this required the spacecraft to travel too fast to permit an orbital insertion. So, contrary to Cassini which toured around Saturn during 13 years, Voyager 2 just passed by.

Its closest approach to Uranus was on January 24, 1986, at a distance of 81,500 km from the planet’s cloud tops. It permitted the discovery of 11 inner satellites, and partly imaged the large ones. It revealed in particular geological features on Miranda, and analyzed the light reflected by the surface of these bodies. The study we discuss today supplements these measurements.

Miranda seen by Voyager 2. © NASA/JPL/USGS
Miranda seen by Voyager 2. © NASA/JPL/USGS

Observations at IRTF

The authors used NASA’s InfraRed Telescope Facility (IRTF). This is a 3-meter telescope, optimized for infrared astronomy. It is located at the Mauna Kea Observatory (altitude: 4,200 m) in Hawaii (USA), and 50% of the observation time is devoted to planetary observation.
Several instruments are available, the authors used the spectrograph-imager SpeX, which decomposes the incident light between 0.8 and 5.4 µm. In that study, the authors limited to 4.2 µm.

NASA's InfraRed Telescope Facility at Maune Kea, Hawaii. © Afshin Darian
NASA’s InfraRed Telescope Facility at Maune Kea, Hawaii. © Afshin Darian

The outcome of such observations is a plot amplitude vs. wavelength of a given surface element of a satellite. It is interesting to keep in mind that the regular moons rotate synchronously, permanently showing the same face to Uranus. The consequence is that they have a leading and a trailing hemisphere. During their orbital motion, the same hemisphere always leads. And this has implications for the surface composition, because the leading hemisphere can be polluted by the dusty environment. In other words: when you observe something on the leading hemisphere, which is not present on the trailing one, this is probably pollution.

When you observe, you actually observe the surface element which faces you. And this depends on the dynamics of the planet.

Geometrical constraints

As you know, Uranus rolls on its orbit, while the satellites have an equatorial orbit. As a consequence, during a 84-y orbit of Uranus around the Sun, the Earth crosses twice the orbital plane, and two periods are favorable for the observation of the poles of Uranus and the satellites. The northern hemispheres of these bodies face us during half the orbit (42 years), while the southern ones face us during the other half.

The last transition happened in 2007. Since then, the northern hemispheres of the satellites face us. And part of the visible face belongs to the leading hemisphere.

A red leading side

The results show that for Ariel and Umbriel, and even more for Titania and Oberon, the leading hemisphere is significantly redder than the trailing one, while it is not the case for the major satellites of Jupiter and Saturn. Titania and Oberon are the outermost of the satellites of Uranus, and the largest ones as well.

To understand the chemical nature of this reddening, previous studies have conducted lab experiments, consisting in reproducing the spectrum of mixtures of chemical elements, which could be found on the natural satellites of the outer planets. Of course, the conditions of temperature and pressure are considered. Then the spectrums are compared to the actually observed ones. And it appears that the reddening agents should be tholins and pyroxene.

Titania seems to have a red spot on its surface, which makes it the redder of the main Uranian satellites. Contrariwise, Miranda does not present this reddening. Latitudinal variations of color are not obvious, while they are in longitude, since they depend on the leading / trailing effect.

Now, the question is: how did these agents reach the satellites? They are probably not endogenous, since similar satellites around Jupiter and Saturn do not have them.

Pollution by the irregular moons

The smoking gun is the irregular moons: they are pretty red. And numerical simulations of the motion of dust expelled from these satellites by impacts show how they are likely to coat the leading sides of Oberon, Titania, Ariel and Umbriel.
And this is what we observe!

Of course, a space mission to Uranus would be very helpful… but this is another story.

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.