Hubble Sees Intense Auroras on Uranus

This is a composite image of Uranus by Voyager 2 and two different observations made by Hubble — one for the ring and one for the auroras. These auroras occurred in the planet’s southern latitudes near the planet’s south magnetic pole. Like Jupiter and Saturn, hydrogen atoms excited by blasts of the solar wind are the cause for the glowing white patches seen in both photos. Credit: NASA/ESA

Earth doesn’t have a corner on auroras. Venus, Mars, Jupiter, Saturn, Uranus and Neptune have their own distinctive versions. Jupiter’s are massive and powerful; Martian auroras patchy and weak.

Auroras are caused by streams of charged particles like electrons that originate with solar winds and in the case of Jupiter, volcanic gases spewed by the moon Io. Whether solar particles or volcanic sulfur, the material gets caught in powerful magnetic fields surrounding a planet and channeled into the upper atmosphere. There, the particles interact with atmospheric gases such as oxygen or nitrogen and spectacular bursts of light result. With Jupiter, Saturn and Uranus excited hydrogen is responsible for the show.

These composite images show Uranian auroras, which scientists caught glimpses of through the Hubble in 2011. In the left image, you can clearly see how the aurora stands high above the planet’s denser atmosphere. These photos combine Hubble pictures made in UV and visible light by Hubble with photos of Uranus’ disk from the Voyager 2 and a third image of the rings from the Gemini Observatory in Hawaii and Chile. The auroras are located close to the planet’s north magnetic pole, making these northern lights.
Credit: NASA, ESA, and L. Lamy (Observatory of Paris, CNRS, CNES)

Auroras on Earth, Jupiter and Saturn have been well-studied but not so on the ice-giant planet Uranus. In 2011, the Hubble Space Telescope took the first-ever image of the auroras on Uranus. Then in 2012 and 2014 a team from the Paris Observatory took a second look at the auroras in ultraviolet light using the Space Telescope Imaging Spectrograph (STIS) installed on Hubble.

From left: Auroras on Earth (southern auroral oval is seen over Antarctica), Jupiter and Saturn. In each case, the rings of permanent aurora are centered on their planets’ magnetic poles which aren’t too far from the geographic poles, unlike topsy-turvy Uranus. Credit: NASA

Two powerful bursts of solar wind traveling from the sun to Uranus stoked the most intense auroras ever observed on the planet in those years. By watching the auroras over time, the team discovered that these powerful shimmering regions rotate with the planet. They also re-discovered Uranus’ long-lost magnetic poles, which were lost shortly after their discovery by Voyager 2 in 1986 due to uncertainties in measurements and the fact that the planet’s surface is practically featureless. Imagine trying to find the north and south poles of a cue ball. Yeah, something like that.

In both photos, the auroras look like glowing dots or patchy spots. Because Uranus’ magnetic field is inclined 59° to its spin axis (remember, this is the planet that rotates on its side!) , the auroral spots appear far from the planet’s north and south geographic poles. They almost look random but of course they’re not. In 2011, the spots lie close to the planet’s north magnetic pole, and in 2012 and 2014, near the south magnetic pole — just like auroras on Earth.

An auroral display can last for hours here on the home planet, but in the case of the 2011 Uranian lights, they pulsed for just minutes before fading away.

Want to know more? Read the team’s findings in detail here.

The Orbit of Uranus. How Long is a Year on Uranus?

Uranus is a most unusual planet. Aside from being the seventh planet of our Solar System and the third gas giant, it is also classified sometimes as an “ice giant” (along with Neptune). This is because of its peculiar chemical composition, where water and other volatiles (i.e. ammonia, methane, and other hydrocarbons) in its atmosphere are compressed to the point where they become solid.

In addition to that, it also has a very long orbital period. Basically, it takes Uranus a little over 84 Earth years to complete a single orbit of the Sun. What this means is that a single year on Uranus lasts almost as long as a century here on Earth. On top of that, because of it axial tilt, the planet also experiences extremes of night and day during the course of a year, and some pretty interesting seasonal changes.

Orbital Period:

Uranus orbits the Sun at an average distance (semi-major axis) of 2.875 billion km (1.786 billion mi), ranging from 2.742 billion km (1.7 mi) at perihelion to 3 billion km (1.86 billion mi) at aphelion. Another way to look at it would be to say that it orbits the Sun at an average distance of 19.2184 AU (over 19 times the distance between the Earth and the Sun), and ranges from 18.33 AU to 20.11 AU.

Images of Uranus taken over a four year period using the Hubble Space Telescope. Credit: NASA/ESA/HST

The difference between its minimum and maximum distance from the Sun is 269.3 million km (167.335 mi) or 1.8 AU, which is the most pronounced of any of the Solar Planets (with the possible exception of Pluto). And with an average orbital speed of 6.8 km/s (4.225 mi/s), Uranus has an orbital period equivalent to 84.0205 Earth years. This means that a single year on Uranus lasts as long as 30,688.5 Earth days.

However, since it takes 17 hours 14 minutes 24 seconds for Uranus to rotate once on its axis (a sidereal day). And because of its immense distance from the Sun, a single solar day on Uranus is about the same. This means that a single year on Uranus lasts 42,718 Uranian solar days. And like Venus, Uranus’ rotates in the direction opposite of its orbit around the Sun (a phenomena known as retrograde rotation).

Axial Tilt:

Another interesting thing about Uranus is the extreme inclination of its axis (97.7°). Whereas all of the Solar Planets are tilted on their axes to some degree, Uranus’s extreme tilt means that the planet’s axis of rotation is approximately parallel with the plane of the Solar System. The reason for this is unknown, but it has been theorized that during the formation of the Solar System, an Earth-sized protoplanet collided with Uranus and tilted it onto its side.

A consequence of this is that when Uranus is nearing its solstice, one pole faces the Sun continuously while the other faces away – leading to a very unusual day-night cycle across the planet. At the poles, one will experience 42 Earth years of day followed by 42 years of night.

This is similar to what is experienced in the Arctic Circle and Antarctica. During the winter season near the poles, a single night will last for more than 24 hours (aka. a “Polar Night”) while during the summer, a single day will last longer than 24 hours (a “Polar Day”, or “Midnight Sun”).

Meanwhile, near the time of the equinoxes, the Sun faces Uranus’ equator and gives it a period of day-night cycles that are similar to those seen on most of the other planets. Uranus reached its most recent equinox on December 7th, 2007. During the Voyager 2 probe’s historic flyby in 1986, Uranus’s south pole was pointed almost directly at the Sun.

Seasonal Change:

Uranus’ long orbital period and extreme axial tilt also lead to some extreme seasonal variations in terms of its weather. Determining the full extent of these changes is difficult because astronomers have yet to observe Uranus for a full Uranian year. However, data obtained from the mid-20th century onward has showed regular changes in terms of brightness, temperature and microwave radiation between the solstices and equinoxes.

These changes are believed to be related to visibility in the atmosphere, where the sunlit hemisphere is thought to experience a local thickening of methane clouds which produce strong hazes. Increases in cloud formation have also been observed, with very bright cloud features being spotted in 1999, 2004, and 2005. Changes in wind speed have also been noted that appeared to be related to seasonal increases in temperature.

Uranus Dark Spot
Close up of Uranus Dark Spot, taken by the Hubble Telescope. Credit: NASA/ESA/HST

Uranus’ “Great Dark Spot” and its smaller dark spot are also thought to be related to seasonal changes. Much like Jupiter’s Great Red Spot, this feature is a giant cloud vortex that is created by winds – which in this case are estimated to reach speeds of up to 900 km/h (560 mph). In 2006, researchers at the Space Science Institute and the University of Wisconsin observed a storm that measured 1,700 by 3,000 kilometers (1,100 miles by 1,900 miles).

Interestingly enough, while Uranus’ polar regions receive more energy on average over the course of a year than the equatorial regions, the equatorial regions have been found to be hotter than the poles. The exact cause of this remains unknown, but is certainly believed to be due to something endogenic.

Yep, Uranus is a pretty weird place! On this planet, a single year lasts almost a century, and the seasons are characterized by extreme versions of Polar Nights and Midnight Suns. And of course, an average year brings all kinds of seasonal changes, complete with extreme winds, massive storms, and thickening methane clouds.

We have written many articles about the length of a year on other planets here at Universe Today. Here’s How Long is a Year on the Other Planets?, How Long is a Year on Mercury?, How Long is a Year on Venus?, How Long is a Year on Earth?, How Long is a Year on Mars?, How Long is a Year on Jupiter?, How Long is a Year on Saturn?, How Long is a Year on Neptune? and How Long is a Year on Pluto?

If you’d like more info on Uranus, check out Hubblesite’s News Releases about Uranus. And here’s a link to the NASA’s Solar System Exploration Guide to Uranus.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.

Sources:

Sweet Sights for November Nights

Clear night ahead? Let’s see what’s up. We’ll start close to home with the Moon, zoom out to lonely Fomalhaut 25 light years away and then return to our own Solar System to track down the 7th planet. Even before the sky is dark, you can’t miss the 4-day-old crescent Moon reclining in the southwestern sky. Watch for it to wax to a half-moon by Thursday as it circles Earth at an average speed of 2,200 mph (3,600 km/hr). That fact that it orbits Earth means that the angle the Moon makes with the sun and our planet constantly varies, the reason for its ever-changing phase.

You'll see two and possibly three lunar "seas" tonight (Nov. 15). Only a portion of Mare Tranquilliitatis (Seas of Tranquility) is exposed. The large crater Janssen, 118 miles wide and 1.8 miles deep, is visible in binoculars. Credit: Virtual Moon Atlas / Legrande and Chevalley
You’ll see two and possibly three lunar “seas” tonight (Nov. 15). Only a portion of Mare Tranquilliitatis (Seas of Tranquility) is exposed. The large crater Janssen, 118 miles wide and 1.8 miles deep, is visible in binoculars. Credit: Virtual Moon Atlas / Legrande and Chevalley

With the naked eye you’ll be able to make two prominent dark patches within the crescent — Mare Crisium (Sea of Crises) and Mare Fecunditatis (Sea of Fecundity). Each is a vast, lava-flooded plain peppered with thousands of craters , most of which require a telescope to see. Not so Janssen. This large, 118-mile-wide (190-km) ring will be easy to pick out in a pair of seven to 10 power binoculars. Janssen is named for 19th century French astronomer Pierre Janssen, who was the first to see the bright yellow line of helium in the sun’s spectrum while observing August 1868 total solar eclipse.

Piscis Austrinus, the Southern Fish, has but one bright star, 1st magnitude Fomalhaut. It shines all by its lonesome in the south around 7 p.m. local time at mid-month. The star is located only 25 light years from Earth. Source: Stellarium
Piscis Austrinus, the Southern Fish, has but one bright star, 1st magnitude Fomalhaut. It shines all by its lonesome in the south around 7 p.m. local time at mid-month. The star is located only 25 light years from Earth. Source: Stellarium

English scientist Norman Lockyer also observed the line later in 1868 and concluded it represented a new solar element which he named “helium” after “helios”, the Greek word for sun. Helium on Earth wouldn’t be discovered for another 10 years, making this party-balloon gas the only element first discovered off-planet!

See the fish now? Greek mythology tells us that Piscis Austrinus is the "Great Fish", the parent of the two fish in the zodiacal constellation of Pisces the Fish. Source: Stellarium
See the fish now? Greek mythology tells us that Piscis Austrinus is the “Great Fish”, the parent of the two fish in the zodiacal constellation of Pisces the Fish. Source: Stellarium

Directing your gaze south around 7 o’clock, you’ll see a single bright star low in the southern sky. This is Fomalhaut in the dim constellation of Piscis Austrinus, the Southern Fish. The Arabic name means “mouth of the fish”. If live under a dark, light-pollution-free sky, you’ll be able to make out a loop of faint stars vaguely fish-like in form. Aside from being the only first magnitude star among the seasonal fall constellations, Fomalhaut stands out in another way — the star is ringed by a planet-forming disk of dust and rock much as our own Solar System was more than 4 billion years ago.

The planet Fomalhaut b orbits Fomalhaut inside a circumstellar disk of dust and rock, taking about 1,700 years to orbit. Brilliant Fomalhaut, represented by the small, white dot, has been masked from view, so astronomers could photograph the much fainter disk. Credit: NASA / ESA / Hubble Space Telescope
The planet Fomalhaut b orbits Fomalhaut inside a circumstellar disk of dust and rock, taking about 1,700 years to orbit. Brilliant Fomalhaut, represented by the small, white dot, has been masked from view, so astronomers could photograph the much fainter disk. Credit: NASA / ESA / Hubble Space Telescope

Within that disk is a new planet, Fomalhaut b, with less than twice Jupiter’s mass and enshrouded either by a cloud of dusty debris or a ring system like Saturn. Fomalhaut b has the distinction of being the first extrasolar planet ever photographed in visible light. The plodding planet takes an estimated 1,700 years to make one loop around Fomalhaut, with its distance from its parent star varying from about 50 times Earth’s distance from the sun at closest to 300 times that distance at farthest.

Shoot a diagonal across the Square of Pegasus to 4th magnitude Delta Piscium. Point your binoculars here and slide east to 4th magnitude Epsilon and 2° south to the planet Uranus shines at magnitude +5.7 and can be glimpsed with the naked eye from a dark sky site. Time shown is around 7 p.m. local time. See detailed map below. Source: Stellarium
Shoot a diagonal across the Square of Pegasus to 4th magnitude Delta Piscium. Point your binoculars here and slide east to 4th magnitude Epsilon and 2° south to the planet Uranus shines at magnitude +5.7 and can be glimpsed with the naked eye from a dark sky site. Time shown is around 7 p.m. local time. See detailed map below. Source: Stellarium

Next, we move on to one of the more remote planets in our own solar system, Uranus. The 7th planet from the sun, Uranus reached opposition — its closest to Earth and brightest appearance for the year — only a month ago. It’s well-placed for viewing in Pisces the Fish after nightfall high in the southeastern sky below the prominent sky asterism, the Great Square of Pegasus.

Wide-field binocular view of Uranus' travels now through next April. I've labeled two stars near the planet with their magnitudes - 5.5 and 6.0 - which are similar to Uranus in brightness, so you don't confuse them with the planet. The others are naked eye stars in Pisces. Source: Chris Mariott's SkyMap
Wide-field binocular view of Uranus’ travels now through next April. I’ve labeled several stars near the planet with their magnitudes, which are similar in brightness to Uranus, so you’ll know to tell them apart from the planet. The others are naked eye stars in Pisces. Source: Chris Mariott’s SkyMap

A telescope will tease out its tiny, greenish disk,  but almost any pair of binoculars will easily show the planet as a star-like point of light slowly marching westward against the starry backdrop in the coming weeks. Check in every few weeks to watch it move first west, in retrograde motion, and then turn back east around Christmas. For those with 8-inch and larger telescopes who love a challenge, use this Uranian Moon Finder to track the planet’s two brightest moons, Titania and Oberon, which glimmer weakly around 14th magnitude.

We’ve barely scratched the surface of the vacuum with these offerings; they’re just a few of the many highlights of mid-November nights that also include the annual Leonid meteor shower, which peaks Tuesday and Wednesday mornings (Nov. 17-18). So much to see!

The Planet Uranus

Uranus, which takes its name from the Greek God of the sky, is a gas giant and the seventh planet from our Sun. It is also the third largest planet in our Solar System, ranking behind Jupiter and Saturn. Like its fellow gas giants, it has many moons, a ring system, and is primarily composed of gases that are believed to surround a solid core.

Though it can be seen with the naked eye, the realization that Uranus is a planet was a relatively recent one. Though there are indications that it was spotted several times over the course of the past two thousands years, it was not until the 18th century that it was recognized for what it was. Since that time, the full-extent of the planet’s moons, ring system, and mysterious nature have come to be known.

Discovery and Naming:

Like the five classic planets – Mercury, Venus, Mars, Jupiter and Saturn – Uranus can be seen without the aid of a telescope. But due to its dimness and slow orbit, ancient astronomers believed it to be a star. The earliest known observation was performed by Hipparchos, who recorded it as a star in his star catalog in 128 BCE – observations which were later included in Ptolemy’s Almagest.

The earliest definite sighting of Uranus took place in 1690 when English astronomer John Flamsteed – the first Astronomer Royal – spotted it at least six times and cataloged it as a star (34 Tauri). The French astronomer Pierre Lemonnier also observed it at least twelve times between the years of 1750 and 1769.

A replica of the telescope which William Herschel used to observe Uranus. Credit:
A replica of the telescope which William Herschel used to observe Uranus. Credit: Wikipedia Commons

However, it was Sir William Herschel’s observation of Uranus on March 13th, 1781, that began the process of identifying it as a planet. At the time, he reported it as a comet sighting, but then engaged in a series of observations using a telescope of his own design to measure its position relative to the stars. When he reported on it to The Royal Society, he claimed it was a comet, but implicitly compared it to a planet.

Afterwards, several astronomers began to explore the possibility that Herschel’s “comet” was in fact a planet. These included Russian astronomer Anders Johan Lexell, who was the first to compute its nearly circular orbit, which led him to conclude it was a planet after all. Berlin astronomer Johann Elert Bode, a member of the “United Astronomical Society”, concurred with this after making similar observations of its orbit.

Soon, Uranus’ status as a planet became a scientific consensus, and by 1783, Herschel himself acknowledged this to the Royal Society. In recognition of his discovery, King George III of England gave Herschel an annual stipend of £200 on condition that he move to Windsor so that the Royal Family could look through his telescopes.

In honor of his new patron, William Herschel decided to name his discovery Georgium Sidus (“George’s Star” or “Georges Planet”). Outside of Britain, this name was not popular, and alternatives were soon proposed. These included French astronomer Jerome Lalande proposing to call it Hershel in honor of its discovery, and Swedish astronomer Erik Prosperin proposing the name Neptune.

Uranus. Image credit: Hubble
Images of Uranus captured by the Hubble Space Telescope. Image credit: NASA/ESA/Hubble

Johann Elert Bode proposed the name Uranus, the Latinized version of the Greek god of the sky, Ouranos. This name seemed appropriate, given that Saturn was named after the mythical father of Jupiter, so this new planet should be named after the mythical father of Saturn. Ultimately, Bode’s suggestion became the most widely used and became universal by 1850.

Uranus’ Size, Mass and Orbit:

With a mean radius of approximately 25,360 km, a volume of 6.833×1013 km3, and a mass of 8.68 × 1025 kg, Uranus is approximately 4 times the sizes of Earth and 63 times its volume. However, as a gas giant, its density (1.27 g/cm3) is significantly lower; hence, it is only 14.5 as massive as Earth. Its low density also means that while it is the third largest of the gas giants, it is the least massive (falling behind Neptune by 2.6 Earth masses).

The variation of Uranus’ distance from the Sun is also greater than that any other planet (not including dwarf planets or plutoids). Essentially, the gas giant’s distance from the Sun varies from 18.28 AU (2,735,118,100 km) at perihelion to 20.09 AU (3,006,224,700 km) at aphelion. At an average distance of 3 billion km from the Sun, it takes Uranus roughly 84 years (or 30,687 days) to complete a single orbit of the Sun.

The rotational period of the interior of Uranus is 17 hours, 14 minutes. As with all giant planets, its upper atmosphere experiences strong winds in the direction of rotation. At some latitudes, such as about 60 degrees south, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours.

Diameter comparison of Uranus and Earth. Approximate scale is 90 km/px. Credit: NASA
Diameter comparison of Uranus and Earth. Approximate scale is 90 km/px. Credit: NASA

One unique feature of Uranus is that it rotates on its side. Whereas all of the Solar System’s planets are tilted on their axes to some degree, Uranus has the most extreme axial tilt of 98°. This leads to the radical seasons that the planet experiences, not to mention an unusual day-night cycle at the poles. At the equator, Uranus experiences normal days and nights; but at the poles, each experience 42 Earth years of day followed by 42 years of night.

Uranus’ Composition:

The standard model of Uranus’s structure is that it consists of three layers: a rocky (silicate/iron–nickel) core in the center, an icy mantle in the middle and an outer envelope of gaseous hydrogen and helium. Much like Jupiter and Saturn, hydrogen and helium account for the majority of the atmosphere – approximately 83% and 15% – but only a small portion of the planet’s overall mass (0.5 to 1.5 Earth masses).

The third most abundant element is methane ice (CH4), which accounts for 2.3% of its composition and which accounts for the planet’s aquamarine or cyan coloring. Trace amounts of various hydrocarbons are also found in the stratosphere of Uranus, which are thought to be produced from methane and ultraviolent radiation-induced photolysis. They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), and diacetylene (C2HC2H).

In addition, spectroscopy has uncovered carbon monoxide and carbon dioxide in Uranus’ upper atmosphere, as well as the presence icy clouds of water vapor and other volatiles, such as ammonia and hydrogen sulfide. Because of this, Uranus and Neptune are considered a distinct class of giant planet – known as “Ice Giants” – since they are composed mainly of heavier volatile substances.

The ice mantle is not in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water, ammonia and other volatiles. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean.

Diagram of the interior of Uranus. Credit: Public Domain
Diagram of the interior of Uranus. Credit: Public Domain

The core of Uranus is relatively small, with a mass of only 0.55 Earth masses and a radius that is less than 20% of the planet’s overall size. The mantle comprises its bulk, with around 13.4 Earth masses, and the upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and extending for the last 20% of Uranus’s radius.

Uranus’s core density is estimated to be 9 g/cm3, with a pressure in the center of 8 million bars (800 GPa) and a temperature of about 5000 K (which is comparable to the surface of the Sun).

Uranus’ Atmosphere:

As with Earth, the atmosphere of Uranus is broken into layers, depending upon temperature and pressure. Like the other gas giants, the planet doesn’t have a firm surface, and scientists define the surface as the region where the atmospheric pressure exceeds one bar (the pressure found on Earth at sea level). Anything accessible to remote-sensing capability – which extends down to roughly 300 km below the 1 bar level – is also considered to be the atmosphere.

Using these references points, Uranus’  atmosphere can be divided into three layers. The first is the troposphere, between altitudes of -300 km below the surface and 50 km above it, where pressures range from 100 to 0.1 bar (10 MPa to 10 kPa). The second layer is the stratosphere, which reaches between 50 and 4000 km and experiences pressures between 0.1 and 10-10 bar (10 kPa to 10 µPa).

Temperature profile of the Uranian troposphere and lower stratosphere. Cloud and haze layers are also indicated. Credit: Wikipedia/Ruslik0
Temperature profile of the Uranian troposphere and lower stratosphere. Cloud and haze layers are also indicated. Credit: Wikipedia/Ruslik0

The troposphere is the densest layer in Uranus’ atmosphere. Here, the temperature ranges from 320 K (46.85 °C/116 °F) at the base (-300 km) to 53 K (-220 °C/-364 °F) at 50 km, with the upper region being the coldest in the solar system. The tropopause region is responsible for the vast majority of Uranus’s thermal infrared emissions, thus determining its effective temperature of 59.1 ± 0.3 K.

Within the troposphere are layers of clouds – water clouds at the lowest pressures, with ammonium hydrosulfide clouds above them. Ammonia and hydrogen sulfide clouds come next. Finally, thin methane clouds lay on the top.

In the stratosphere, temperatures range from 53 K (-220 °C/-364 °F) at the upper level to between 800 and 850 K (527 – 577 °C/980 – 1070 °F) at the base of the thermosphere, thanks largely to heating caused by solar radiation. The stratosphere contains ethane smog, which may contribute to the planet’s dull appearance. Acetylene and methane are also present, and these hazes help warm the stratosphere.

The outermost layer, the thermosphere and corona, extend from 4,000 km to as high as 50,000 km from the surface. This region has a uniform temperature of 800-850 (577 °C/1,070 °F), although scientists are unsure as to the reason. Because the distance to Uranus from the Sun is so great, the amount of heat coming from it is insufficient to generate such high temperatures.

Like Jupiter and Saturn, Uranus’s weather follows a similar pattern where systems are broken up into bands that rotate around the planet, which are driven by internal heat rising to the upper atmosphere. As a result, winds on Uranus can reach up to 900 km/h (560 mph), creating massive storms like the one spotted by the Hubble Space Telescope in 2012. Similar to Jupiter’s Great Red Spot, this “Dark Spot” was a giant cloud vortex that measured 1,700 kilometers by 3,000 kilometers (1,100 miles by 1,900 miles).

Uranus’ Moons:

Uranus has 27 known satellites, which are divided into the categories of larger moons, inner moons, and irregular moons (similar to other gas giants). The largest moons of Uranus are, in order of size, Miranda, Ariel, Umbriel, Oberon and Titania. These moons range in diameter and mass from 472 km and 6.7 × 1019 kg for Miranda to 1578 km and 3.5 × 1021 kg for Titania. Each of these moons is particularly dark, with low bond and geometric albedos. Ariel is the brightest while Umbriel is the darkest.

A montage of Uranus's moons. Image credit: NASA
A montage of Uranus’s moons. Image credit: NASA

All of the large moons of Uranus are believed to have formed in the accretion disc, which existed around Uranus for some time after its formation, or resulted from the large impact suffered by Uranus early in its history. Each one is comprised of roughly equal amounts of rock and ice, except for Miranda which is made primarily of ice.

The ice component may include ammonia and carbon dioxide, while the rocky material is believed to be composed of carbonaceous material, including organic compounds (similar to asteroids and comets). Their compositions are believed to be differentiated, with an icy mantle surrounding a rocky core.

In the case of Titania and Oberon, it is believed that liquid water oceans may exist at the core/mantle boundary. Their surfaces are also heavily cratered; but in each case, endogenic resurfacing has led to a degree of renewal of their features. Ariel appears to have the youngest surface with the fewest impact craters while Umbriel appears to be the the oldest and most cratered.

The major moons of Uranus have no discernible atmosphere. Also, because of their orbit around Uranus, they experience extreme seasonal cycles. Because Uranus orbits the Sun almost on its side, and the large moons all orbit around Uranus’ equatorial plane, the northern and southern hemispheres experience prolonged periods of daytime and nighttime (42 years at a time).

As of 2008, Uranus is known to possess 13 inner moons whose orbits lie inside that of Miranda. They are, in order of distance from the planet: Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Cupid, Belinda, Perdita, Puck and Mab. Consistent with the naming of the Uranus’ larger moons, all are named after characters from Shakespearean plays.

Uranus and Moons
Uranus and its system of Moons. Credit: NASA/JPL

All inner moons are intimately connected to Uranus’ ring system, which probably resulted from the fragmentation of one or several small inner moons. Puck, at 162 km, is the largest of the inner moons of Uranus – and the only one imaged by Voyager 2 in any detail – while Puck and Mab are the two outermost inner satellites of Uranus.

All inner moons are dark objects. They are made of water ice contaminated with a dark material, which is probably organic materials processed by Uranus’ radiation. The system is also chaotic and apparently unstable. Computer simulations estimate that collisions may occur, particularly between Desdemona and Cressida or Juliet within the next 100 million years.

As of 2005, Uranus is also known to have nine irregular moons, which orbit it at a distance much greater than that of Oberon. All the irregular moons are probably captured objects that were trapped by Uranus soon after its formation. They are, in order of distance from Uranus: Francisco, Caliban, Stephano, Trincutio, Sycorax, Margaret, Prospero, Setebos, and Ferdinard (once again, named for characters in Shakespearean plays).

Uranus’s irregular moons range in size from about 150 km (Sycorax) to 18 km (Trinculo). With the exception of Margaret, all circle Uranus in retrograde orbits (meaning they orbit the planet in the opposite direction of its spin).

Uranus’ Ring System:

Like Saturn and Jupiter, Uranus has a ring system. However, these rings are composed of extremely dark particles which vary in size from micrometers to a fraction of a meter – hence why they are not nearly as discernible as Saturn’s. Thirteen distinct rings are presently known, the brightest being the epsilon ring. And with the exception of two very narrow ones, these rings usually measure a few kilometers in width.

A Sharper View Of Uranus
Uranus viewed in the infrared spectrum, revealing internal heating and its ring system. Credit: Lawrence Sromovsky (Univ. Wisconsin-Madison)/Keck Observatory

The rings are probably quite young, and are not believed to have formed with Uranus. The matter in the rings may once have been part of a moon (or moons) that was shattered by high-speed impacts. From numerous pieces of debris that formed as a result of those impacts, only a few particles survived, in stable zones corresponding to the locations of the present rings.

The earliest known observations of the ring system took place on March 10th, 1977, by James L. Elliot, Edward W. Dunham, and Jessica Mink using the Kuiper Airborne Observatory. During an occultation of the star SAO 158687 (also known as HD 128598), they discerned five rings existing within a system around the planet, and observed four more later.

The rings were directly imaged when Voyager 2 passed Uranus in 1986, and the probe was able to detect two additional faint rings – bringing the number of observed rings to 11. In December 2005, the Hubble Space Telescope detected a pair of previously unknown rings, bringing the total to 13. The largest is located twice as far from Uranus as the previously known rings, hence why they are called the “outer” ring system.

In April 2006, images of the new rings from the Keck Observatory yielded the colors of the outer rings: the outermost is blue and the other one red. In contrast, Uranus’s inner rings appear grey. One hypothesis concerning the outer ring’s blue color is that it is composed of minute particles of water ice from the surface of Mab that are small enough to scatter blue light.

Exploration:

Uranus has only been visited once by any spacecraft: NASA’s Voyager 2 space probe, which flew past the planet in 1986. On January 24th, 1986, Voyager 2 passed within 81,500 km of the surface of the planet, sending back the only close up pictures ever taken of Uranus. Voyager 2 then continued on to make a close encounter with Neptune in 1989.

These two pictures of Uranus -- one in true color (left) and the other in false color -- were compiled from images returned Jan. 17, 1986, by the narrow-angle camera of Voyager 2. Image credit: NASA/JPL
These two pictures of Uranus — one in true color (left) and the other in false color — were compiled from images returned Jan. 17, 1986, by the narrow-angle camera of Voyager 2. Credit: NASA/JPL

The possibility of sending the Cassini spacecraft from Saturn to Uranus was evaluated during a mission extension planning phase in 2009. However, this never came to fruition, as it would have taken about twenty years for Cassini to get to the Uranian system after departing Saturn.

In terms of future missions, multiple proposals have been made. For instance, a Uranus orbiter and probe was recommended by the 2013–2022 Planetary Science Decadal Survey published in 2011. This proposal envisaged a launch taking place between 2020–2023 and a 13-year cruise to Uranus. A New Frontiers Uranus Orbiter has been evaluated and was recommended in the study, The Case for a Uranus Orbiter. However, this mission is considered to be lower-priority than future missions to Mars and the Jovian System.

Scientists from the Mullard Space Science Laboratory in the United Kingdom have proposed a joint NASA-ESA mission to Uranus known as Uranus Pathfinder. This mission would involve launching a medium-class mission by 2022, and estimates place its cost at €470 million (~$525 million USD).

Another mission to Uranus, called Herschel Orbital Reconnaissance of the Uranian System (HORUS), was designed by the Applied Physics Laboratory of Johns Hopkins University. The proposal is for a nuclear-powered orbiter carrying a set of instruments, including an imaging camera, spectrometers and a magnetometer. The mission would launch in April 2021 and arrive at Uranus 17 years later.

Uranus. Image credit: Hubble
Uranus, as imaged by the Hubble Space Telescope. Image credit: NASA/Hubble

In 2009, a team of planetary scientists from NASA’s Jet Propulsion Laboratory advanced possible designs for a solar-powered Uranus orbiter. The most favorable launch window for such a probe would be in August 2018, with arrival at Uranus in September 2030. The science package may include magnetometers, particle detectors and, possibly, an imaging camera.

Suffice it to say, Uranus is a hard target when it comes to exploration, and its distance has made the process of observing it recognizing it for what it was problematic in the past. And in the future, with most of our mission focused on exploring Mars, Europa, and Near-Earth Asteroids, the prospect of a mission to this region of the Solar System doesn’t seem very likely.

But budget environments change, as do scientific priorities. And with interest in the Kuiper Belt exploding thanks to the discovery of many Trans-Neptunian Objects in recent years, it is entirely possible that scientists will demand that a mission to the out solar system be mounted. If and when one occurs, it may be possible to have the probe swing by Uranus on its way out, gathering information and pictures to help advance our understanding of this “Ice Giant”.

We have many interesting articles about Uranus here at Universe Today. We hope you find what you are looking for in the list below:

Ice Giants at Opposition

It seems as if the planets are fleeing the evening sky, just as the Fall school star party season is getting underway. Venus and Mars have entered the morning sky, and Jupiter reaches solar conjunction this week. Even glorious Saturn has passed eastern quadrature, and will soon depart evening skies.

Enter the ice giants, Uranus and Neptune. Both reach opposition for 2015 over the next two months, and the time to cross these two out solar system planets off your life list is now.

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Looking east at dusk in late August, as Uranus and Neptune rise. Image credit: Stellarium

First up, the planet Neptune reaches opposition next week in the constellation Aquarius on the night of August 31st/September 1st. Shining at magnitude +7.8, Neptune spends the remainder of 2015 about three degrees southwest of the +3.7 magnitude star Lambda Aquarii.  It’s possible to spot Neptune using binoculars, and about x100 magnification in a telescope eyepiece will just resolve the blue-grey 2.3 arc second disc of the planet. Though Neptune has 14 known moons, just one, Triton, is within reach of a backyard telescope. Triton shines at magnitude +13.5 (comparable to Pluto), and orbits Neptune in a retrograde path once every 6 days, getting a maximum of 15” from the disk of the planet.

Nep Aug-Nov Triton aug 31
The path of Neptune from late August through early November 2015. Inset: the position of Neptune’s moon Triton on the evening of August 31st: Image credit: Starry Night Education software

Uranus reaches opposition on October 11th in the adjacent constellation Pisces.  Keep an eye on Uranus, as it nears the bright +5.2 magnitude star Zeta Piscium towards the end on 2015. Shining at magnitude +5.7 with a 3.6 arc second disk, Uranus hovers just on the edge of naked eye visibility from a dark sky site.

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Uranus, left of the eclipsed Moon last October. Image credit and copyright: A Nartist

It’ll be worth hunting for Uranus on the night of September 27th/28th, when it sits 15 degrees east of the eclipsed Moon. Uranus turned up in many images of last Fall’s total lunar eclipse.  This will be the final total lunar eclipse of the current tetrad, and the Moon will occult Uranus the evening after for the South Atlantic. This is part of a series of 19 ongoing occultations of Uranus by the Moon worldwide, which started in August 2014, and end on December 20th, 2015. After that, the Moon will move on and begin occulting Neptune next year in June through the end of 2017.

Occultation
The visibility footprint of the September 29th occultation of Uranus by the Moon. Image credit: Occult 4.0.

Uranus has 27 known moons, four of which (Oberon, Ariel, Umbriel and Titania) are visible in a large backyard telescope. See our extensive article on hunting the moons of the solar system for more info, and the JPL/PDS rings node for corkscrew finder charts.

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The path of Uranus, from late August through early December 2015. Inset: the position of the moons of Uranus on the evening of October 12th. Image credit: Starry Night Education software

The two outermost worlds have a fascinating entwined history. William Herschel discovered Uranus on the night of March 13th, 1781. We can be thankful that the proposed name ‘George’ after William’s benefactor King George the III didn’t stick. Herschel initially thought he’d discovered a comet, until he followed the slow motion of Uranus over several nights and realized that it had to be something large orbiting at a great distance from the Sun. Keep in mind, Uranus and Neptune both crept onto star charts unnoticed pre-1781. Galileo even famously sketched Neptune near Jupiter in 1612!  Early astronomers simply considered the classical solar system out to Saturn as complete, end of story.

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A classic 7″ Merz refractor at the Quito observatory, nearly identical to the instrument that first spied Neptune. Image Credit: Dave Dickinson

And the hunt was on. Astronomers soon realized that Uranus wasn’t staying put: something farther still from the Sun was tugging at its orbit. Mathematician Urbain Le Verrier predicted the position of the unseen planet, and on and on the night of September 23rd, 1846, astronomers at the Berlin observatory spied Neptune.

In a way, those early 19th century astronomers were lucky. Neptune and Uranus had just passed each other during a close encounter in 1821. Otherwise, Neptune might’ve remained hidden for several more decades. The synodic period of the two planets—that is, the time it takes the planets to return to opposition—differ by about 2-3 days. The very first documented conjunction of Neptune and Uranus occurred back in 1993, and won’t occur again until 2164. Heck, In 2010, Neptune completed its first orbit since discovery!

To date, only one mission, Voyager 2, has given us a close-up look at Uranus and Neptune during brief flybys. The final planetary encounter for Voyager 2 occurred in late August in 1989, when the spacecraft passed 4,800 kilometres (3,000 miles) above the north pole of Neptune.

All thoughts to ponder as you hunt for the outer ice giants. Sure, they’re tiny dots, but as with many nighttime treats, the ‘wow’ factor comes with just what you’re seeing, and the amazing story behind it.

Tricks to Remember the Planets

Need an easy way to remember the order of the planets in our Solar System? The technique used most often to remember such a list is a mnemonic device. This uses the first letter of each planet as the first letter of each word in a sentence. Supposedly, experts say, the sillier the sentence, the easier it is to remember.

So by using the first letters of the planets, (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), create a silly but memorable sentence.

Here are a few examples:

  • My Very Excellent Mother Just Served Us Noodles (or Nachos)
  • Mercury’s Volcanoes Erupt Mulberry Jam Sandwiches Until Noon
  • Very Elderly Men Just Snooze Under Newspapers
  • My Very Efficient Memory Just Summed Up Nine
  • My Very Easy Method Just Speeds Up Names
  • My Very Expensive Malamute Jumped Ship Up North

    Sun and Planets
    The Sun and planets to scale. Credit: Illustration by Judy Schmidt, texture maps by Björn Jónsson

    If you want to remember the planets in order of size, (Jupiter, Saturn, Uranus, Neptune, Earth, Venus Mars, Mercury) you can create a different sentence:

  • Just Sit Up Now Each Monday Morning
  • Jack Sailed Under Neath Every Metal Mooring 

    Rhymes are also a popular technique, albeit they require memorizing more words. But if you’re a poet (and don’t know it) try this:

    Amazing Mercury is closest to the Sun,
    Hot, hot Venus is the second one,
    Earth comes third: it’s not too hot,
    Freezing Mars awaits an astronaut,
    Jupiter is bigger than all the rest,
    Sixth comes Saturn, its rings look best,
    Uranus sideways falls and along with Neptune, they are big gas balls.

    Or songs can work too. Here are a couple of videos that use songs to remember the planets:

    If sentences, rhymes or songs don’t work for you, perhaps you are more of a visual learner, as some people remember visual cues better than words. Try drawing a picture of the planets in order. You don’t have to be an accomplished artist to do this; you can simply draw different circles for each planet and label each one. Sometimes color-coding can help aid your memory. For example, use red for Mars and blue for Neptune. Whatever you decide, try to pick colors that are radically different to avoid confusing them.

    Or try using Solar System flash cards or just pictures of the planets printed on a page (here are some great pictures of the planets). This works well because not only are you recalling the names of the planets but also what they look like. Memory experts say the more senses you involve in learning or storing something, the better you will be at recalling it.

    Planets made from paper lanterns. Credit: TheSweetestOccasion.com
    Planets made from paper lanterns. Credit: TheSweetestOccasion.com

    Maybe you are a hands-on learner. If so, try building a three-dimensional model of the Solar System. Kids, ask your parents or guardians to help you with this, or parents/guardians, this is a fun project to do with your children. You can buy inexpensive Styrofoam balls at your local craft store to create your model, or use paper lanterns and decorate them. Here are several ideas from Pinterest on building a 3-D Solar System Model.

    If you are looking for a group project to help a class of children learn the planets, have a contest to see who comes up with the silliest sentence to remember the planets. Additionally, you can have eight children act as the planets while the rest of the class tries to line them up in order. You can find more ideas on NASA’s resources for Educators. You can use these tricks as a starting point and find more ways of remembering the planets that work for you.

    If you are looking for more information on the planets check out Universe Today’s Guide to the Planets section, or our article about the Order of the Planets, or this information from NASA on the planets and a tour of the planets.

    Universe Today has numerous articles on the planets including the planets and list of the planets.

    Astronomy Cast has an entire series of episodes on the planets. You can get started with Mercury.

Moonspotting-A Guide to Observing the Moons of the Solar System

Like splitting double stars, hunting for the faint lesser known moons of the solar system offers a supreme challenge for the visual observer.

Sure, you’ve seen the Jovian moons do their dance, and Titan is old friend for many a star party patron as they check out the rings of Saturn… but have you ever spotted Triton or Amalthea?

Welcome to the challenging world of moon-spotting. Discovering these moons for yourself can be an unforgettable thrill.

One of the key challenges in spotting many of the fainter moons is the fact that they lie so close inside the glare of their respective host planet. For example, +11th magnitude Phobos wouldn’t be all that tough on its own, were it not for the fact that it always lies close to dazzling Mars. 10 magnitudes equals a 10,000-fold change in brightness, and the fact that most of these moons are swapped out is what makes them so tough to see. This is also why many of them weren’t discovered until later on.

But don’t despair. One thing you can use that’s relatively easy to construct is an occulting bar eyepiece.   This will allow you to hide the dazzle of the planet behind the bar while scanning the suspect area to the side for the faint moon. Large aperture, steady skies, and well collimated optics are a must as well, and don’t be afraid to crank up the magnification in your quest. We mentioned using such a technique previously as a method to tease out the white dwarf star Sirius b in the years to come.

Image credit
A homemade occulting bar eyepiece with the barrel removed. One bar is a strip of foil, and the other is a E-string from a guitar. Image credit: Dave Dickinson

What follows is a comprehensive list of the well known ‘easy ones,’ along with some challenges.

We included a handy drill down of magnitudes, orbital periods and maximum separations for the moons of each planet right around opposition. For the more difficult moons, we also noted the circumstances of their discovery, just to give the reader some idea what it takes to see these fleeting worlds.  Remember though, many of those old scopes used speculum metal mirrors which were vastly inferior to commercial optics available today. You may have a large Dobsonian scope available that rivals these scopes of yore!

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The orbits of the Martian moons. Image credit: Starry Night Education Software

Mars- The two tiny moons of Mars are a challenge, as it’s only possible to nab them visually near opposition, which occurs about once every 26 months.   Mars next reaches opposition on May 22nd, 2016.

Phobos:

Magnitude:  +11.3

Orbital period:  7 hours 39 minutes

Maximum separation: 16”

Deimos:

Magnitude:  +12.3

Orbital period: 1 day 6 hours and 20 minutes

Maximum separation: 54”

The moons of Mars were discovered by American astronomer Asaph Hall during the favorable 1877 opposition of Mars using the 26-inch refracting telescope at the U.S. Naval Observatory.

Jupiter- Though the largest planet in our solar system also has the largest number of moons at 67, only the four bright Galilean moons are easily observable, although owners of large light buckets might just be able to tease out another two.  Jupiter next reaches opposition March 8th, 2016.

Ganymede:

Magnitude: +4.6

Orbital period: 7.2 days

Maximum separation: 5’

Callisto

Magnitude: +5.7

Orbital period: 16.7 days

Maximum separation: 9’

Io

Magnitude: +5.0

Orbital period: 1.8 days

Maximum separation: 1’ 50”

Europa

Magnitude: +5.3

Orbital period: 3.6 days

Maximum separation: 3’

Amalthea

Magnitude:  +14.3

Orbital period: 11 hours 57 minutes

Maximum separation: 33”

Himalia

Magnitude: +15

Orbital period: 250.2 days

Maximum separation: 52’

Note that Amalthea was the first of Jupiter’s moons discovered after the four Galilean moons. Amalthea was first spotted in 1892 by E. E. Barnard using the 36” refractor at the Lick Observatory. Himalia was also discovered at Lick by Charles Dillon Perrine in 1904.

Titan and Rhea imaged via Iphone and a Celestron NexStar 8SE telescope. Image credit: Andrew Symes (@failedprotostar)
Titan and Rhea imaged via Iphone and a Celestron NexStar 8SE telescope. Image credit: Andrew Symes (@failedprotostar)

Saturn- With a total number of moons at 62, six moons of Saturn are easily observable with a backyard telescope, though keen-eyed observers might just be able to tease out another two:

(Note: the listed separation from the moons of Saturn is from the limb of the disk, not the rings).

Titan

Magnitude: +8.5

Orbital period: 16 days

Maximum separation: 3’

Rhea

Magnitude: +10.0

Orbital period: 4.5 days

Maximum separation: 1’ 12”

Iapetus

Magnitude: (variable) +10.2 to +11.9

Orbital period: 79 days

Maximum separation: 9’

Enceladus

Magnitude: +12

Orbital period: 1.4 days

Maximum separation: 27″

Dione

Magnitude: +10.4

Orbital period: 2.7 days

Maximum separation: 46”

Tethys

Magnitude: +10.2

Orbital period: 1.9 days

Maximum separation: 35”

Mimas

Magnitude: +12.9

Orbital period: 0.9 days

Maximum separation: 18”

Hyperion

Magnitude: +14.1

Orbital period: 21.3 days

Maximum separation: 3’ 30”

Phoebe

Magnitude: +16.6

Orbital period: 541 days

Maximum separation: 27’

Hyperion was discovered by William Bond using the Harvard observatory’s 15” refractor in 1848, and Phoebe was the first moon discovered photographically by William Pickering in 1899.

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The orbits of the moons of Uranus. Image credit: Starry Night Education software

Uranus- All of the moons of the ice giants are tough. Though Uranus has a total of 27 moons, only five of them might be spied using a backyard scope. Uranus next reaches opposition on October 12th, 2015.

Titania

Magnitude: +13.9

Orbital period:

Maximum separation: 28”

Oberon

Magnitude: +14.1

Orbital period: 8.7 days

Maximum separation: 40”

Umbriel

Magnitude: +15

Orbital period: 4.1 days

Maximum separation: 15”

Ariel

Magnitude: +14.3

Orbital period: 2.5 days

Maximum separation: 13”

Miranda

Magnitude: +16.5

Orbital period: 1.4 days

Maximum separation: 9”

The first two moons of Uranus, Titania and Oberon, were discovered by William Herschel in 1787 using his 49.5” telescope, the largest of its day.

Triton in orbit around Neptune near opposition in 2011. Image credit: Efrain Morales
Triton in orbit around Neptune near opposition in 2011. Image credit: Efrain Morales

Neptune- With a total number of moons numbering 14, two are within reach of the skilled amateur observer. Opposition for Neptune is coming right up on September 1st, 2015.

Triton

Magnitude: +13.5

Orbital period: 5.9 days

Maximum separation: 15”

Nereid

Magnitude: +18.7

Orbital period: 0.3 days

Maximum separation: 6’40”

Triton was discovered by William Lassell using a 24” reflector in 1846, just 17 days after the discovery of Neptune itself. Nereid wasn’t found until 1949 by Gerard Kuiper.

Pluto-Yes… it is possible to spy Charon from Earth… as amateur astronomers proved in 2008.

Charon

Magnitude: +16

Orbital period: 6.4 days

Maximum separation: 0.8”

Image credit
Pluto! Click here for a (possible) capture of Charon as well. Image credit: Wendy Clark

In order to cross off some of the more difficult targets on the list, you’ll need to know exactly when these moons are at their greatest elongation. Sky and Telescope has some great apps in the case of Jupiter and Saturn… the PDS Rings node can also generate corkscrew charts of lesser known moons, and Starry Night has ‘em as well. In addition, we tend to publish cork screw charts for moons right around respective oppositions, and our ephemeris for Charon elongations though July 2015 is still active.

Good luck in crossing off some of these faint moons from your astronomical life list!

How Many Moons Does Uranus Have?

Uranus and Moons

In the outer Solar System, there are many worlds that are so large and impressive to behold that they will probably take your breath away. Not only are these gas/ice giants magnificent to look at, they are also staggering in size, have their own system a rings, and many, many moons. Typically, when one speaks of gas (and/or ice) giants and their moons, one tends to think about Jupiter (which has the most, at 67 and counting!).

But have you ever wondered how many moons Uranus has? Like all of the giant planets, it’s got rather a lot! In fact, astronomers can now account for 27 moons that are described as “Uranian”. Just like the other gas and ice giants, these moons are motley bunch that tell us much about the history of the Solar System. And, just like Jupiter and Saturn, the process of discovering these moons has been long and involved on multiple astronomers.

Continue reading “How Many Moons Does Uranus Have?”

Uranus’s Moon Oberon

In 1610, Galileo’s observed four satellites orbiting the distant gas giant of Jupiter. This discovery would ignite a revolution in astronomy, and encouraged further examinations of the outer Solar System to see what other mysteries it held. In the centuries that followed, astronomers not only discovered that other gas giants had similar systems of moons, but that these systems were rather extensive.

For example, Uranus has a system of 27 confirmed satellites. Of these, Oberon is the outermost satellite, as well as the second largest and second most-massive. Named in honor of a mythical king of fairies, it is also the ninth most massive moon in the Solar System.

Discovery and Naming:

Discovered in 1787 by Sir William Herschel, Oberon was one of two major satellites discovered in a single day (the other being Uranus’ moon of Titania). At the time, he reported observing four other moons; however, the Royal Astronomical Society would later determine that these were spurious. It would be almost five decades after the moons were discovered that an astronomer other than Herschel observed them.

Initially, Oberon was referred to as “the second satellite of Uranus”, and in 1848, was given the designation Uranus II by William Lassell. In 1851, Lassell discovered Uranus’ other two moons – later named Ariel and Miranda – and began numbering them based on their distance from the planet . Oberon was thus given the designation of Uranus IV.

Size comparison between the Earth, the Moon, and Saturn's moon of Oberon. Credit: Tom.Reding/Public Domain
Size comparison between the Earth, the Moon, and Uranus’ moon of Oberon. Credit: Tom.Reding/Public Domain

By 1852, Herschel’s son John suggested naming the moon’s his father observed Oberon and Titania, at the request of Lassell himself. All of these names were taken from the works of William Shakespeare and Alexander Pope, with the name Oberon being derived from the King of the Fairies in A Midsummer Night’s Dream.

Size, Mass and Orbit:

With a diameter of approx. 1,523 kilometers, a surface area of 7,285,000 km², and a mass of 3.014 ± 0.075 x 10²¹ kilograms, Oberon is the second largest, and second most massive of Uranus’ moons. It is also the ninth most massive moon in the solar system.

At a distance of 584,000 km from Uranus, it is the farthest of the five major moons from Uranus. However, this distance is subject to change, as Oberon has a small orbital eccentricity and inclination relative to Uranus’ equator. It has an orbital period of about 13.5 days, coincident with its rotational period. This means that Oberon is a tidally-locked, synchronous satellite with one face always pointing toward the planet.

Since (like all of Uranus’ moons) Oberon orbits the planet around its equatorial plane, and Uranus orbits the Sun almost on its side, the moon experiences a rather extreme seasonal cycle. Essentially, both the northern and southern poles spend a period of 42 years in complete darkness or complete sunlight – with the sun rising close to the zenith over one of the poles at each solstice.

Voyager 2:

So far, the only close-up images of Oberon have been provided by the Voyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986.  The images cover about 40% of the surface, but only 25% of the surface was imaged with a resolution that allows geological mapping.

In addition, the time of the flyby coincided with the southern hemisphere’s summer solstice, when nearly the entire northern hemisphere was in darkness. This prevented the northern hemisphere from being studied in any detail. No other spacecraft has visited the Uranian system before or since, and no missions to the planet are currently being planned.

Composition:

Oberon’s density is higher than the typical density of Uranus’ satellites, at 1.63 g/cm³. This would indicate that the moon consists of roughly equal proportions of water ice and a dense non-ice component. The latter could be made of rock and carbonaceous material including heavy organic compounds.

Spectroscopic observations have confirmed the presence of crystalline water ice in the surface of the moon. It is believed that Oberon, much like the other Uranian moons, consists of an icy mantle surrounding a rocky core. If this is true, then the radius of the core (480 km) would be equal to approx. 63% of the radius of the moon, and its mass would be around 54% of the moon’s mass.

A computer-projected false-color image of Oberon. The white region has not yet been photographed by a spacecraft. The large crater with the dark floor (right of center) is Hamlet; the crater Othello is to its lower left, and the 'canyon' Mommur Chasma is at upper left. Credit: USGS Astrogeology Research Program
False-color image of Oberon, showing the Hamlet and Othello craters (right of center and lower left) and the Mommur Chasma (upper left). Credit: USGS Astrogeology Research Program

Currently, the full composition of the icy mantle is unknown. However, it it were to contain enough ammonia or other antifreeze compounds, the moon may possess a liquid ocean layer at the core–mantle boundary. The thickness of this ocean, if it exists, would be up to 40 km and its temperature would be around 180 K.

It is unlikely that at these temperatures, such an ocean could support life. But assuming that hydrothermal vents exist in the interior, it is possible life could exist in small patches near the core. However, the internal structure of Oberon depends heavily on its thermal history, which is poorly known at present.

Interesting Facts:

Oberon is the second-darkest large moon of Uranus (after Umbriel), with a surface that appears to be generally red in color – except where fresh impact deposits have left neutral or slightly blue colors. In fact, Oberon is the reddest moon amongst its peers, with a trailing hemisphere that is significantly redder than its leading hemisphere.

The reddening of the surfaces is often a result of space weathering caused by bombardment of the surface by charged particles and micrometeorites over many millions of years. However, the color asymmetry of Oberon is more likely caused by accretion of a reddish material spiraling in from outer parts of the Uranian system.

Oberon’s surface is the most heavily cratered of all the Uranian moons, which would indicate that Oberon has the most ancient surface among them. Consistent with the planet’s name, these surface features are named after characters in Shakespearean plays. The largest known crater, Hamlet, measures 206 kilometers in diameter, while the Macbeth, Romeo, and Othello craters measure 203, 159, and 114 km respectively.

Uranus and its five major moons
Uranus and its five major moons. Credit: space.com

Other prominent surface features are what is known as chasmata – steep-sided depressions that are comparable to rift valleys or escarpments here on Earth. The largest known chasmata on Oberon is the Mommur Chasma, which measures 537 km in diameter and takes its name from the enchanted forest in French folklore that was ruled by Oberon.

As you can plainly see, there is much that remains unknown about this satellite. Much like its peers, how they came to be, and what secrets may lurk beneath their surfaces, is still open to speculation. One can only hope that future generations will choose to mount another Voyager-like expedition to the Outer Solar System for the sake of studying the Uranian satellites.

We have written many interesting articles on the moons of Uranus here at Universe Today. Here’s How Many Moons Does Uranus Have? and Interesting Facts About Uranus.

For more information, check out NASA’s Solar System Exploration page on Oberon and Nine Planet’s page on Oberon.

Astronomy Cast also has a good episode on the subject. Here’s Episode 62: Uranus.

Sources:

Slender Moonspotting, Occultations, Daytime Planets and More

One of nature’s grandest ‘occultations’ of all is coming right up this Friday, as the Moon passes in front of the Sun for viewers in the high Arctic for a total solar eclipse. And although 99.999+% percent of humanity will miss totality, everyone can trace the fascinating path of the Moon as it moves back into the evening sky this weekend.

As of this writing, it looks like the fickle March weather is going to keep us guessing right up to eclipse day. Fear not, as the good folks over at the Virtual Telescope Project promise to bring us views of the eclipse live.  Not only does this eclipse fall on the same day as the start of astronomical spring in the northern hemisphere known as the vernal (northward) equinox, but it also marks the start of lunation 1141.

Ever try hunting for the slender crescent Moon in the dawn or dusk sky? The sport of thin Moon-spotting on the days surrounding the New Moon can push visual skills to the very limit. Binoculars are your friend in this endeavor, as you sweep back and forth attempting to see the slim fingernail of a Moon against the low contrast background sky.  Thursday morning March 19th provides a great chance for North American observers to spy an extremely thin Moon about 24 hours prior to Friday’s eclipse.

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Projected locales for the first sightings of the slim crescent Moon on the evening of March 20th. Credit: Created by author.

Unfortunately, most of North America misses the eclipse, though folks on the extreme east coast of Newfoundland might see a partially eclipsed sunrise if the day dawns clear.

The Moon will first be picked up in the evening sky post-eclipse this weekend. On Friday evening, folks in the southern United States might just be able to spy a 15 hour old Moon with optical assistance if skies are clear.

As the Moon fattens, expect to see it at its most photogenic as Ashen light or Earthshine illuminates its nighttime side. What you’re seeing is sunlight from the Earth being reflected back in a reverse (waning gibbous) phase as seen from the earthward side of the Moon. The prominence of Earthshine can vary depending on the amount of cloud and snow cover currently turned moonward, though of course, if it’s cloudy from your location, you won’t see a thing…

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The universe smiles back: A skewed emoticon grouping of Venus, Mars and the Moon plus Earthshine on February 20th. Photo by author.

Watch that Moon over the coming weeks, as it has a date with destiny.

The Moon occults (passes in front of) two planets and one bright star in the coming week. First up is an occultation of Uranus on March 21st at around 11:00 UT/7:00 AM EDT. Sure, this one is for the most part purely academic and unobservable, as it occurs over central Africa in the daytime and is only 15 degrees east of the Sun. Still, if you can pick up the Moon on the evenings of March 20th or March 21st, you might just be able to spy nearby Uranus shining at +6th magnitude nearby before it heads towards solar conjunction on April 6th.

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The visibility footprint of the March 21st occultation of Mars by the Moon. Credit: Occult 4.1.

Next, the Moon occults Mars on March 21st at 22:00 UT/6:00 PM EDT for the southern Pacific coast of South America. North America will see an extremely close photogenic pairing of Luna and the Red Planet. This is one of seven occultations of a naked eye planet by the Moon for 2015, and the first of two for Mars for the year, the next falling on December 6th.

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The Moon pairs with Venus on the evening of March 22nd. Credit: Stellarium.

Next up, the Moon has a tryst with brilliant Venus, passing 2.8 degrees from the Cytherean world on March 22nd. Can you spy -4th magnitude Venus near the two day old Moon before sunset? This is the stuff that has inspired astronomically-themed flags and skewed emoticon ‘smiley face conjunctions’ of yore, including the close pairing of Mars, Venus and the Moon seen worldwide last month.

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The occultation of Aldebaran by the Moon on March 25th. Credit: Occult 4.1.

Next up, the 30% illuminated Moon occults the bright star Aldebaran for Alaskan viewers at dusk on March 25th. This is the third occultation of the star by the Moon in the ongoing cycle, and to date, no one has, to our knowledge, successfully caught an occultation of Aldebaran in 2015… could this streak be broken next week?

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The daytime Moon paired with Jupiter on March 30th. Credit: Starry Night Education software.

And speaking of daytime planet-spotting, Jupiter will sit only five degrees south of the waxing gibbous Moon on the evening of March 30th. Can you spy the giant planet near the daytime Moon in the afternoon sky using binocs? And finally, watch that Moon, as it heads for the third total lunar eclipse of the last 12 months visible from the Americas and the Pacific region on the morning of April 4th

More to come!