Uranus

h2ouniverse · Aug 26, 2007

Andrew, for the supercritical ocean: Beyond 647K pure H2O can no longer be liquid in the strict sense. It becomes supercritical beyond 220 bars, or Ice VII beyond 100000 bars. As it is said to be blended with ammonia, that changes the picture. But it seems difficult to get an evidenced composition! Do you know the phase diagram of the suspected H2O+ammonia blend? Regards.

Aetius · Aug 26, 2007

When I think of aerobraking vehicles designed for use in the atmospheres of the gas giants, I imagine spacecraft that are shaped something like NASA's X-43A. If the thing's designed to fly at Mach 25-plus, plowing through the upper atmosphere of Uranus during each aerobraking encounter (maybe even loading its tanks with some hydrogen collected by an ISRU experiment as it goes along), a recessed payload bay mounted on the top of the main spacecraft bus could hold the meteorology microprobes. I'm curious as to what kind of potential issues might arise with that kind of design. The aerobraking phase might use less propellant [from Earth], and save precious weight for the science at Uranus. We are clearly talking about a mission decades in the future here. But when it comes to planet Uranus, any good science mission probably is decades in the future. <img src="/images/icons/frown.gif" /> P.S. Edited to change the phrase "...the aerobraking encounter...", to "...each aerobraking encounter..."

jaxtraw · Aug 26, 2007

"I would think of the microprobes as small items released from the mothership few weeks before the aerobraking (as posted by another poster on this thread), so that they enter the atm few days / hours before the aerobraking, giving time for a final boost on the mothership to adjust trajectory." It would be nice to arrange the approach trajectory so that a star is occulted by the atmosphere; that'd give a pretty good indicator of the atmosphere's gradient as well <img src="/images/icons/smile.gif" />

3488 · Aug 26, 2007

Hi Joel, I am afraid that I do not, my ignorance is showing there. I know about the role of Ammonia in cryovolcanism, lowering the melting point of water ice to minus 100 C / 173 Kelvin. This is almost certainly happening on Saturn's Titan & in the past on the Uranus moons Ariel, Titania & maybe Oberon & Pluto's Charon. However, I am not knowledgeable about the Ammonia / H20 mix in a supercritical ocean environment. Perhaps you could enlighten me? What I do know though, is that current ideas suggest that (liquid water), hot ice may exist in the 19 kilobar levels in both Uranus & Neptune. I wonder if above that they are surrounded by steam & the Methane, Ammonia, Hydrogen & Helium mix, a super compressed atmosphere? IMO it would not be pure. Surely carbon must be present also. If Uranus & Neptune did form from materials similar to Triton, Eris & Pluto type objects, surely carbon compounds must also reside here? Yes it looks as if the atmosphere of Uranus is far more active now than during the encounter with Voyager 2. We will see if the weather calms down again, as Uranus moves away from the equinox, to the Solstice in 2028 (more or less half a Uranus year after Voyager 2). Certainly wind speeds exceed 800 KPH / 500 MPH, & that spots & convectional storms are present at times. Even during Voyager 2's encounter, very powerful lightning was detected. Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

3488 · Aug 26, 2007

Hi jaxtraw. Yes a steller occultation would be perfect. The spectrum of the star occulted would be well known so it is a matter of the spectrometer onboard the approaching craft to relay the absorbtion lines within the atmosphere of Uranus to provide a more reliable compostion & how the light is refracted to get a more reliable density profile. Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

3488 · Aug 27, 2007

ANY mission to Uranus right now would be a good thing. A high resolution mutispectral imager package & communications is better than what we have now. Interesting Uranus weather around the Equinox. It is only because of the KECK & the Hubble Space telescope, that we are getting any decent Uranus science right now. Shame that New Horizons 2 was cancelled. Imagine in 2014, we could have had good coverage of all five of the main moons, particularly the hemispheres not seen by Voyager 2. Below Uranusshine on Ariel. I found the raw image on the NASA PDC site & enhanced it myself. Voyager 2. I will try & do likewise for the others, particulaly Miranda, but I think the viewing geometary may not be good for that. Clearly visible on the night Winter hemisphere on Ariel is at least one fresh impact crater, perhaps two small ones. I can also make out the continuation of at least two tectonic canyons, joining up to become one. With New Horizons 2 LORRI camera, perhaps this could have been seen sunlit?? NASA / JPL. Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

jaxtraw · Aug 27, 2007

I wonder if we're too ready to consider the attributes of our planetary neighbours as permanent, or stable. It may be that we just happen to be alive at one of those times when a giant planet is "on its side" and at other times Uranus has had a more conventional rotational axis, just as maybe we're lucky to be alive when one of the giants is surrounded by a young, glorious ring system (and maybe in the past that old, dark, inconspicuous ring around Uranus was a much larger, brighter, more spectacular affair...)

h2ouniverse · Aug 27, 2007

Jaxtraw, I fully agree that stationary behavior is just an illusion due to our short (for now) lifespan. For anything we think as "constant". This being said, as far as the rings are concerned, the 4 giants have some. So even if the saturnian ones are at their acme, may be Neptune or Jupiter will relay them. in the future. I think the statistics speak for themselves: rings should be more the norm than the exception. Regards.

h2ouniverse · Aug 27, 2007

Anvel, Moons are a stabilizing factor. However Uranian moons are not very big in proportion. They count just as 8% of the moment of inertia of Uranus system about its rotation axis. (whereas the Moon provides for 100 times more inertia than the Earth). The moons toggle Uranus up to about 25 km. The despinning of Uranus by its moons dissipates energy partly in heat in Uranus but essentially by accelerating the moons, i.e. increasing their orbit length. Hence their contribution to the moment of inertia. But only marginally. The spin is high however, which still provides for huge gyroscopic stiffness. That should be the major resistance again an axial tilt. Best regards.

3488 · Aug 27, 2007

Hi Joel, do you think that the five main moons of Uranus, have enough mass, to prevent Uranus, a planet some 13 times the mass of the Earth, to note wobble, betwwen its current 98 degree tilt to a more upright one over a long period?? Just a thought. Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

h2ouniverse · Aug 27, 2007

Hi Andrew, Actually I do not think they can, not because their mass is low, but because of their moment of inertia is low. Uranus moment of inertia is 2.27e40 kg.m2, and the total m.O.I of the subsystem is just 8% higher, at about 2.5e40kg.m2. Earth is just 1e38 kg.m2, while Earth+Moon is 1.1e40kg.m2. The half of the whole Uranian subsystem!!! I think the gyrscopic stiffness of Uranus alone is more significant.

3488 · Aug 27, 2007

Thanks Joel. Also Uranus is actually quite oblate, see my post in the Uranus Ring Plane Crossing thread. That oblateness caused by the rapid rotation would keep this axis stable long term IMO. Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

MeteorWayne · Aug 27, 2007

Indeed, a giant gaseous gyroscope! <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

3488 · Aug 27, 2007

It is indeed. A brilliant natural example. <img src="/images/icons/laugh.gif" /> Unusual to see Uranus like that, as Uranus needs to present the equator to us, only once every 42 years. I am surprised that on the Hubble Space Telescope / JPL sites, this has not been commented on. <img src="/images/icons/rolleyes.gif" /> Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

Aetius · Aug 27, 2007

Yeah, you're right. <img src="/images/icons/frown.gif" /> Galileo and Cassini have spoiled me. New Horizons 2 would have been incredible.

brellis · Aug 27, 2007

Maybe one day we'll decide to stabilize the entire solar system by sending RTG's into each planet - crazy idea of course! <div class="Discussion_UserSignature"> I'm a recovering optimist - things could be better. </div>

Aetius · Aug 27, 2007

How would radioisotope thermoelectric generators (RTGs) stabilize the solar system? I'm not saying it's a crazy idea, but I don't get it.

brellis · Aug 27, 2007

I was just making a joke about how some spacecraft use RTG's to stabilize their trajectories. Just a silly suggestion to lob some gigantic RTG's onto a highly radioactive giant planet like Jupiter, to make sure it doesn't flip on its side. <div class="Discussion_UserSignature"> I'm a recovering optimist - things could be better. </div>

Aetius · Aug 27, 2007

Oh! Now I get it. <img src="/images/icons/wink.gif" />

3488 · Aug 28, 2007

Flipping heck brellis!!!! (crap pun intended) <img src="/images/icons/laugh.gif" /> Below is a Hubble Space Telescope image of Uranus, taken in May this year, flipped over (no I've got the flipping disease <img src="/images/icons/shocked.gif" />), showing how Uranus would look, if the planet has a more normal axial tilt. The oblateness, caused by the rapid 17 hours & 54 minute rotation is probably even easier to see. NASA. Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

h2ouniverse · Aug 28, 2007

Anvel, I think that if an impact tilts the primary, the moons will not prevent it and should follow with time (but not immediately) because the tidal despinning gradually transfers velocity to the moons normally to the new rotation axis. When the moon is in the equatorial plane, this means an acceleration increasing the semi-major axis. Otherwise this means an out-of-moon-plane acceleration, i.e. the equivalent of an inclination correction maneuver. On the opposite, if the source of the tilt is a slow process either internal to the primary or a gradual external force, then the global moment of inertia will be at play and resist the source of the tilt. In the case of Uranus, with a minor effect of the moons, and less resistance, the tilt will occur globally with the moons. Regards.

h2ouniverse · Aug 28, 2007

In both cases you end up with moons in the equatorial plane.

brellis · Aug 28, 2007

I just watched a Science channel program quoting a scientist who has models theorizing that Uranus and Neptune formed between the orbits of Jupiter and Saturn, and were then bumped out by the big bully, Jupiter. That would certainly help explain Uranus' tilt. Apologies if this was discussed already. Here's a space.com article from 1999 raising the question. Uranus and Neptune couldn't have formed as quickly as they did, unless they spent their formative period elsewhere. Martin Duncan and his colleagues propose this theory. "We realized that perhaps Uranus and Neptune actually formed closer to the sun, in the same region as Jupiter and Saturn," said Duncan, of Queen's University in Kingston, Canada. Duncan paints a protoplanetary picture with four somewhat huddled embryonic cores, Jupiter being the closest to the sun and perhaps already slightly larger than the others. Presently, Jupiter is 483 million miles (777 million kilometers), while Saturn is 886 million miles (1425 million kilometers), from the sun. Being close, Jupiter had the most material to work with, Duncan said. When it reached a certain size, about five to 15 times the mass of Earth, Jupiter's gravity began quickly pulling in surrounding gas and soon became the mostly gaseous giant that it is today. In this scenario, it is likely that something similar happened to Saturn. It was the sudden presence of another giant or two that threw the whole system into chaos, Duncan said. Uranus and Neptune were flung outward, as though from a slingshot, by the sudden introduction of nearby mass. Their orbits became erratic, swinging into an egg-shaped pattern and slipping above and below the imaginary plane along which most planets move around the sun. Disorder reigned f <div class="Discussion_UserSignature"> I'm a recovering optimist - things could be better. </div>

3488 · Aug 29, 2007

Hi all, Having technical troubles again, so this quick reply from work. I had heard a theory that Neptune was the next one out from Saturn & Uranus was once the furthest planet from the Sun. There are ideas that Jupiter & Saturn have migrated slightly outward, & Uranus & Neptune swapped places. Galileo atmospheric probe told a differnt story, that Jupiter MAY have formed as far out as Uranus at the very least & migrated inward (we see evidence of this sort of thing with the 'hot Jupiters' orbiting other sunlike stars & Jupiter stopped @ abput 5.2 AU), due to the abundance of gasses like Argon & Xenon (which were believed to be almost non existent in the solar nebular at Jupiter & Saturn's distances from the proto Sun). If this is true, than the outer planets were either all much further from the Sun at one time, or were more closely bunched together. Myself I dunno, but this was one topic I had very interesting discussions with Dr Eielene Theilig, when she was Galileo mission manager & she thought that Jupiter DID migrate closer in, during its youth. A most fascinating subject. I will return later. Now theres a threat. <img src="/images/icons/crazy.gif" /> Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

h2ouniverse · Aug 29, 2007

Hi Andrew, The theory of outward migration of Neptune expressed by Morbidelli seems to be well correlated with the composition of the Kuiper Belt, its remaining mass, and the eccentricity-inclination distribution in the Scattered Disk.

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