Mission to mysterious Uranus

Oct 12, 2011 by Michael Schirber
Voyager 2 snapped this photo of Uranus in 1986. Credit: NASA

Scientists want to send an orbiter and probe to the ice giant planet Uranus, but do the resources exist to support such an ambitious project?

Earlier this year, the Decadal Survey recommended that NASA consider sending a mission to the planet Uranus. With all the attention paid to Mars, Jupiter, and even poor little Pluto, what's the draw in going to Uranus?

Lots, says Mark Hofstadter of the Jet Propulsion Laboratory

"Uranus is a type of a planet that we know very little about," he says. "Thirty years ago we thought Uranus and Neptune were just smaller versions of Jupiter and Saturn."

We now know that the outermost planets in our solar neighborhood are not gas giants filled with hydrogen and , but rather "ice giants" containing a large mixture of water, methane, ammonia and carbon dioxide.

Current tallies of exoplanets suggest that ice giants are more common in our galaxy than the larger gas giants.

"We'd like to study our local examples of this common type of planet," Hofstadter says.

If you had to pick one, Uranus is probably the better destination than Neptune. It challenges scientific models with its unique rotation and puzzling . Moreover, planet number 7 is easier to get to than planet number 8.

Hofstadter is one of a group of scientists exploring the potential of sending an orbiter to Uranus. One particular proposal, called Uranus Pathfinder, was recently considered by the (ESA).

"Both the European and American sides are convinced that an orbiter is needed rather than a fly by," says Chris Arridge of the University College London and PI of Uranus Pathfinder. "But then costs rear their ugly head."

The Keck Telescope captured some severe weather on Uranus in 2004, demonstrating that the planet is not simply a boring blue ball. Credit: Lawrence Sromovsky, Space Science and Engineering Center

Uranus Pathfinder made it far in the selection process for ESA's next round of medium class missions, but ultimately it was passed up.

"The project is by no means dead," Arridge says. "Interest is building to go back to Uranus."

Unique Uranus

Go back, indeed.

In 1986, the Voyager 2 spacecraft flew by Uranus on its way to the edge of the solar system. The pictures that were beamed back showed a uniform blue ball that almost seemed boring in comparison to the rich, colored surfaces of Jupiter and Saturn. But Uranus's featureless façade hides the strangest interior in the solar system.

The first striking aspect of Uranus is the way that it's tipped over on its side, with its rotational axis lying in the disk of the solar system. Scientists speculate that some giant collision long ago knocked Uranus over.

The odd rotational orientation may have some effect on the internal dynamics. Uranus is the only planet we know of that can't be fit with a simple three-layer model comprised of a rocky core, a water-rich mantle, and a gas atmosphere. Inside Uranus, these different phases of matter must be mixed together in some complex way.

To sort out this internal structure, Hofstadter and Arridge and their colleagues want to send an orbiter to map the gravitational field around Uranus. In addition, remote measurements in microwave and infrared wavelengths could fill in details of the atmospheric composition, like the abundances of different ices.

[It should be noted that "ice" is used to describe molecules like water and methane that would be frozen at Uranus's . However, these compounds are likely to be in a dense liquid-like state at the lower depths of the planet where the pressure is extremely high.]

This illustration compares the interior ocean of Jupiter's moon Europa to a similar ocean suspected for Uranus's moon Titania. Credit: Chris Arridge/UCL/UP Consortium

A better understanding of the internal structure may help explain Uranus's bizarre magnetic field, which is tilted at 60 degrees with respect to the rotational axis. This would be like having Earth's magnetic pole going through New Orleans.

"It is not something we understand very well," Arridge says. "If we can study it more closely with magnetic field measurements, we might learn something about how planets produce magnetic fields in general."

Colder than cold

Because of Uranus's odd rotational orientation, winters and summers are literally night and day apart. During a quarter of the planet's 84-year orbit, one hemisphere monopolizes all the sunlight, while the other is completely dark. This was how Voyager 2 saw Uranus, with its southern hemisphere in full summer sun. And this might be why the planet looked so bland.

More recent images taken with ground telescopes during the southern autumn season have discovered clouds and huge wind storms traveling as fast as 900 km/h (500 mph).

"Uranus is more active than the Voyager images implied," Hofstadter says.

The climate activity on Uranus may have a very different origin than the well-known weather systems on Jupiter and Saturn. The impressive storms and clouds that decorate the facades of the are powered mostly by internal heat rising to the surface, but this heat supply is largely missing from Uranus.

Scientists are curious to study this in more detail. Could it be that Uranus's strange internal structure is blocking the flow of heat to the surface on a continual or seasonal basis? Or perhaps the heat was "knocked out" during the supposed collision that toppled the planet over? An orbiter mission could get to the bottom of this by measuring temperature variations on the surface that would relate to heat transport.

The lack of heat from below makes Uranus the coldest planet in the solar system, with an average temperature of –224 degrees Celsius. (Pluto may be colder, but it's not a planet -- if you haven't heard.)

A variety of science targets were considered for the proposed Uranus Pathfinder mission. Credit: Chris Arridge/UCL/UP Consortium

Ironically, the thermodynamics on super-cold Uranus might tell us something about "hot Jupiters," which are exoplanets in extremely tight orbits around their host stars. The outer temperature of a hot Jupiter is assumed to be controlled by the incoming stellar radiation.

"Likewise, Uranus might be getting all of its energy from sunlight," Hofstadter explains.

What's in a name? That which we call a moon

The planet is not the only thing to visit. Scientists are also interested in the satellite and ring systems around Uranus.

The 27 known moons are named – not for the gods and heroes of ancient mythology -- but after characters from the plays of Shakespeare and the poems of Alexander Pope.

Voyager 2 only saw one side of these moons, since they were all turned like Uranus with their poles toward the sun. An orbiter mission would give a much more complete picture of the Uranian satellites.

"The Cassini mission has revealed a lot about Saturn's moons, and I think just as rich a return awaits us at Uranus," Arridge says.

One of the more intriguing moons is Miranda. Its surface is carved with canyons that suggest intense geologic activity. "It looks like pieces of different puzzles stuck together," Hofstadter says. The moon also boasts the highest cliff in the solar system, Verona Rupes, which has a drop-off of over 5 kilometers.

Scientist have suggested that the two largest moons, Titania and Oberon, might have liquid oceans below their crusts. Only closer observations would be able to say for sure.

Intermixed with the moons are 13 narrow rings, which are so faint they were only detected by accident in 1977. The Uranian rings are distinct from the broad, bright rings of Saturn, so they offer a nice counter point in trying to understand the general physics of ring formation, Hofstadter says.

"Nice model" for astrobiology

Although the chances of life finding a niche somewhere around Uranus are unlikely, astrobiologists are interested in the role that Uranus and Neptune might have played in the formation of the terrestrial planets.

The so-called "Nice model" (after the city in France where it was first discussed) says that the late heavy bombardment may have been initiated around 4 billion years ago by a migration of Uranus and Neptune away from the Sun. Such a planetary shift could have scattered comets in the outer solar system, putting some of them on collision trajectories with Earth and the other terrestrial planets.

If the late heavy bombardment did originate this way, then it could explain where much of our water and atmosphere may have come from.

"Did the arrival of these icy comets help to make Earth habitable?" Hofstadter wonders.

A Uranus mission could test the Nice model by carrying a probe, which could be dropped into Uranus's atmosphere. The probe could measure the abundances of certain noble gases, which can tell scientists at what distance from the Sun Uranus formed.

If this data were to say that Uranus was born at a position closer to the Sun than it currently resides, that would support an earlier migration. It would also help to explain why Uranus and Neptune are larger than certain planetary models predict.

Where from here

The best time to launch a mission to Uranus would be sometime in the early 2020s, when the planet alignment would be prime for such a long journey. Depending on the chosen propulsion technology, the trip would take roughly 10 to 15 years.

Although most of the technology is at hand, Arridge says there are questions about how to supply electric power to the orbiter when it is almost 3 billion kilometers from the Sun. Solar panels would need to be an unwieldy 400 square meters is size. Radioactive power is the alternative, but plutonium is becoming harder to come by.

A bigger issue is the cost. The ballpark estimate for a Uranus mission is between 1.5 and 2.7 billion dollars, according to Hofstadter who was lead author on a recent mission study. This price tag puts it in the territory of a Flagship Mission, the largest and most expensive missions in the NASA rubric.

In March 2011, the National Research Council released its vision for Planetary Science in the coming decade 2013-2022. This Decadal Survey ranked the priorities for Flagship Missions and gave the two top spots to a Mars sample return mission (MAX-C) and a Jupiter Europa Orbiter (JEO). For the third highest priority, the Survey chose the "Uranus Orbiter with Probe".

At the end of this past July, NASA responded to the Decadal Survey's recommendations. There was agreement over the prioritizing of Flagship missions, but under the current fiscal challenges, "it is unlikely that NASA can execute the Survey's recommendation to initiate the Uranus Orbiter with Probe mission along with MAX-C and JEO in this decade," quoting the official response. NASA will continue studying a Uranus mission for the future.

There is some talk of perhaps a collaboration between and ESA to pool their resources together, Hofstadter says. If not this decade, then maybe next. Uranus will wait, even if Earth-bound scientists have trouble doing so.

"I'm 48 years old, so the clock is ticking," Hofstadter says.

Explore further: Image: Comet 67P/Churyumov–Gerasimenko

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User comments : 22

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Nanobanano
1.3 / 5 (13) Oct 12, 2011
Inside Uranus, these different phases of matter must be mixed together in some complex way.


Two hypothesis:

Perhaps a rocky object is in "orbit" about the CoG beneath the cloud layer, or perhaps resting "under" the oceans, but not centered on the CoG. So essentially a planetesmal sized gravel pile not centered on the center of gravity of the planet. This would be akin to having a giant boulder resting on top of a glacier, after having slid down a nearby hill side.

This could explain the magnetic field not being aligned with the axis of rotation.

Alternatively, the core of the planet may rotate around a different axis than the oceans and atmosphere. this would create a lot of friction and should have eventually moderated itself, but if you can imagine how fast this thing is spinning, and you have dense core spinning very fast, it may exhibit some sort of exotic ultra-low friction effect, allowing the masses to spin past each other. superconducting magnetic levitation.
Nanobanano
1 / 5 (11) Oct 12, 2011
Consider, that one.

We know of hydroplaning on Earth, where your tire can skid across a water puddle, and even "catch air", so this is an example of an ultra-low friction, but it only happens at a high relative velocity.

Also, ice can slide past something at ultra-low friction, even "against the grain" so to speak.

So I'm offering some hypothesis of how the core of the planet might be rotating on a different axis from the mantle, ocean, and atmosphere, in order to explain the highly inclined magnetic field.

Now 60 degrees tilt from the axis is 38 degrees from normal relative to orbital plane, which isn't so far from the magnetic field of the other planets in the solar system.

So I vote for some sort of superconducting magnetic levitation effect. The cryogenic temperatures are there, though not as idealistic as laboratory conditions.

Imagine a hot ball of metal or rock, spinning inside an ice cube, with a thin layer of water vapor between the core and the ice...
Nanobanano
1.1 / 5 (13) Oct 12, 2011
Just to give an example of how you could hide an "orbiting" solid object inside Uranus asymetrically...

The uncertainty in Uranus' mass is more than 1/5th of the mass of Earth's moon, or more than 16 times the mass of Ceres.

So you could clearly have entire planetesmals inside the cloud layer or mantle "asymetrically" and not know the difference.
El_Nose
1 / 5 (5) Oct 12, 2011
- there is no real evidence for superconducting magnetic levitation -- granted i think a study should be done but that is not even a viable theory on earth let alone a planet far far away

-- where is your evidence of ultra low friction ???

sorry let me back up
-- where is your evidence of a rocky core ??

wouldn't the density of the planet stabilize to spread density close to evenly - how would a large rocky mass avoid this??

TMEubanks
not rated yet Oct 12, 2011
The odd rotational orientation may have some effect on the internal dynamics. Uranus is the only planet we know of that can't be fit with a simple three-layer model comprised of a rocky core, a water-rich mantle, and a gas atmosphere. Inside Uranus, these different phases of matter must be mixed together in some complex way.

This really should be the only _gas-giant_ planet with these attributes. The Earth, for example, does not have a rocky core, and Mars and Mercury almost certainly don't either.
Nanobanano
1.4 / 5 (8) Oct 12, 2011
- there is no real evidence for superconducting magnetic levitation -- granted i think a study should be done but that is not even a viable theory on earth let alone a planet far far away


I know I've seen demonstrations of levitation involving super conductors or super magnets.

http://en.wikiped...vity.JPG

-- where is your evidence of ultra low friction ???


Like I said, a Hypothesis based on circumstancial evidence in the form of "similar" known natural phenomena; i.e. hydroplaning, and Leidenfrost effect.

sorry let me back up
-- where is your evidence of a rocky core ??wouldn't the density of the planet stabilize to spread density close to evenly - how would a large rocky mass avoid this??


Large rocky masses avoid this on Earth all the time: moutain ranges, continents, geodetic and monolith inclusions in magmas..

Uranus is very large, and not very dense...cont...
El_Nose
1 / 5 (2) Oct 12, 2011


-- the uncertainty of the mass is on the order of a planetesmals but that does not mean there is a planetesmal in the planet itself.

if i were to say my uncertainty of the mass of the moon was give or take 5% -- that does not imply that that 5% could be pure water. it just means i cannot give an accurate measurement - it gives a constraint that the mass is no more or less than 5% of this number -- to say that its on the upper end and not only that but is made of water is well a very very bad statement to put it very nicely.
Nanobanano
1.1 / 5 (9) Oct 12, 2011
The scale is quite different, but I see no reason "something" solid couldn't exist in there.

It's not as hot as saturn or jupiter, where you'd expect metallic materials to be vaporized or in a super-critical condition.

The mean density of Uranus is 1.27, only a bit more than water.

Water flows over the Earth's sub-oceanic mountains all the time. Earth isn't symetrical, we just think of it as such because we like to over simplify things.

But when you're dealing with the sheer scale of this planet, being 4 times the radius of earth, or about 64 times the volume, things like turbulence and eddies in the sub-surface could be totally concealed at the tops of the clouds. Sort of like how a whale or a sub-marine passes by through the water, and you don't even know it above on the surface.

The pressure is very, very high, ergo hypothetical exotic state of matter in the core.
ACW
4.3 / 5 (3) Oct 12, 2011
If Uranus was "knocked over" by a giant collision, why are all of the moons knocked over as well.
Nanobanano
1 / 5 (4) Oct 12, 2011
el_nose:

I merely cited the uncertainty as evidence that there MIGHT be something unknown and assymetric of a planetesmal scale.

I was not saying that "extra" mass was definitely there.

I was saying an assymetry of that scale could exist and be hard to detect directly.
Nanobanano
1 / 5 (8) Oct 12, 2011
If Uranus was "knocked over" by a giant collision, why are all of the moons knocked over as well.


yeah, I already discussed that in another thread, and the collision cannot explain that.

Ghost gave me hell about it though, but oh well.

The planetary formation models are over simplified, and typically only examine one orbital plane, when it's obvious there are infinitely many orbital planes, although in present day, most of the mass happens to be on about the same plane, but there are still comets and planetesmals that orbit tens or even scores of degrees from the 8 planets.

Uranus axial tilt AND moons ARE explained by my hypothesis of objects on different orbital planes converging over eons due to gravitational acceleration gradually "flattening out" the planes.

What computer models should do is assume a condition of scores of proto-planetesmals on different planes which "flatten out" gravitationally over cosmic time scales.
Blakut
not rated yet Oct 12, 2011
"Ice giant planet Uranus". And here i thought Uranus was a gas giant, not an ice giant...
El_Nose
5 / 5 (1) Oct 12, 2011
i take back that statement about superconducting magnetic levitation --- i whole heartedly agree there is superconducting magnetic levitation -- for some reason i thought you were referring to rotating superconductors lowering the mass of objects placed above them
Jonseer
1.3 / 5 (8) Oct 13, 2011
Titan the big moon of Neptune MUST be considered when deciding which planet Uranus or Neptune is more worthy of a visit.

Titan is a perfect stand in for Pluto and all the Plutinos. In fact it is considered to have been one prior to being captured by Neptune.

So a visit to Neptune would not only tell us more about both Neptune and Uranus, but also give us endless information on the dwarf planets that orbit beyond Neptune.

Uranus has no such moon to compliment the worth of exploring it further.

Thus despite the greater difficulty, Neptune/Triton is many times more worthy of a return orbital visit than Uranus.
PinkElephant
5 / 5 (4) Oct 13, 2011
If Uranus was "knocked over" by a giant collision, why are all of the moons knocked over as well.
It would imply that the moons were most likely formed by subsequent high-velocity (e.g. cometary) impacts on the already-tilted Uranus: the impact debris would be ejected into orbit, form rings, and eventually coalesce into moons (just like what happened with Earth's moon).

The plane of the ejecta, rings, and eventual moons would tend to align roughly with Uranus' equatorial plane because Uranus' rotation would naturally give the ejecta a "kick" in that direction (just as Earth space launches into anterograde orbits are most efficient from the equator, because the planet's rotation adds the largest "free" velocity boost there.)

Also, tidal interactions between Uranus' upper layers and the orbiting debris rings and moons would tend to further gradually pull the orbiting satellites into the planet's equatorial plane over time.
Robert_Wells
not rated yet Oct 13, 2011
Titan the big moon of Neptune MUST be considered when deciding which planet Uranus or Neptune is more worthy of a visit.


and here i thought Titan was Saturn's moon...or have i lost it?

i think there is a Triton orbiting Neptune, and another similarly name moon orbiting Uranus, Titania or something along those lines, its been years since I learned the moons of the outer planets.

oh well, maybe you meant to say Triton and mis-typed and this is a waste of a comment. maybe i am just continuing my bad day after midnight and i'm completely wrong here. I dunno maybe i mixed up Triton and Titania(?).

too lazy to google, eyes slowly closing, laptop falls to floor...zzzzz

edit: heh, so tired i had to edit this whole thing, quoted, forgot to close quote when i modified original.
El_Nose
5 / 5 (1) Oct 14, 2011
I am so proud no one took a jab at the article title
PinkElephant
3 / 5 (1) Oct 14, 2011
I am so proud no one took a jab at the article title
You have no idea how tempted I was, particularly with all the subsequent references to variously mixed solids and fluids, moons, "nice models", and talk of penetrating probes... I mean, this sentence alone is worth its bits in gold:

"A Uranus mission could test the Nice model by carrying a probe, which could be dropped into Uranus's atmosphere"

*sigh*

I guess I've gone and done it now. Damn you, El_Nose!
Yelmurc
not rated yet Oct 15, 2011
Maybe one day we will have orbiters around all the planets
Pirouette
1 / 5 (6) Oct 16, 2011
Shame on you. boys.
tee hee
Hev
Oct 16, 2011
This comment has been removed by a moderator.
antialias_physorg
not rated yet Oct 17, 2011
Just an idea for the strange matter asymmetry:

With so much of Uranus being deprived of sunlight (even at that distance) for so long there might be significant buildup of frozen material on the night side which melts/sublimates as the region comes into sunlight. This sublimation process might also account for the storms observed. If so then the storms should be concentrated around the terminator.

Perhaps a rocky object is in "orbit" about the CoG beneath the cloud layer

Unlikely. Atmospheric resistance would have caused something like this to crash long ago.

But if an impact with a massive object created the strange axis alignment then remnant material from that crash might also account for an imbalance.
Sinister1811
1 / 5 (6) Oct 17, 2011
I'm still curious as to whether Uranus and Neptune are both gas giants, or consist mostly of ices. I guess more orbiters and missions can give us more information about the exact composition of these planets.