[X] My Assistant

[Guest_BruceMoomaw_*]

OK, here are those crumbs. The JPL description is pretty good, but there are a few things missing from it:

(1) The 2002 Solar System Decadal Survey noted that the five main goals for the next Jupiter mission are: (A) Determine if Jupiter has a central core to constrain models of its formation; ( B ) determine the planetary water abundance; ( C ) determine if the winds persist into Jupiter's interior or are confined to the weather layer; (D) assess the structure of Jupiter's magnetic field to learn how the internal dynamo works; and (E) measure the polar magnetosphere to understand its rotation and relation to the aurora. Juno will do a nice job on all five - and while, the Survey's original desire for at least one and preferably 2 or 3 deep entry probes (down to 100 bars) would have further improved the data on ( B ) and ( C ), the added expense was so great that a deep Jovian Multiprobe Flyby mission by itself is now ranked pretty low on the list of desired New Horizons missions -- shallow Galileo-type entry probes of the other giant planets are higher-ranked. Moreover, the data from Juno will allow us to better plan the targeting of those deep Jupiter entry probes when we finally DO fly them. (Note also that -- if they absolutely have to descope Juno -- they could toss off every single instrument except the microwave radiometer and magnetometer, and lose only goal (E) in the process.)

(2) Currently we know Jupiter's gravity-field harmonics down to level 6 -- Juno will take it down to level 12 to 14. Not only can it nail down the size of any hevy-element core -- which is crucial to decide which of the two rival theories of giant-planet formation is true -- but it can measure that core's rotation rate, and even obtain profiles of the density of the planet's middle layers sensitive enough to determine how deep its convective wind cycles really run, all the way down 1/5 of the way to the core!

(3) Our current knowledge of Jupiter's magnetic-field harmonics is level 4. Juno will take it all the way down to level 20 -- much BETTER than we can ever obtain for Earth itself, where we're forever limited to level 14 due to interference from crustal fields! Thus Juno is likely to provide radical new information not only on the generative processes of Jupiter's magnetic field (including the dynamo radius and changes with time), but of Earth's field as well.

(4) Knowledge of the total oxygen content of Jupiter's atmosphere is crucial -- and the Galileo entry proe didn't get it because of its bad-luck fall (9-1 odds against) into a hot spot where a downdraft removed the local water vapor. The probe DID find not only that the concentration of the other heavier elements -- Ar, Kr, Xe, C, N and S -- was somewhat lower than expected, but that they were very consistent in being enriched about threefold relative to the Sun, whereas much bigger element-to-element differences had been expected in that ratio. This was a shock. The logical conclusion is that the icy planetesimals that formed Jupiter were actually made of much colder ice than that which existed at the planet's current distance from the Sun (150 K) -- those other elements were imprisoned either in regular ice at only 20-30 deg K or clathrates at >38 K, so either the planet itself formed much farther from the Sun and migrated a great distance inwards, or the planetesimals that formed it themselves came from much farther out and migrated inwards before accreting to form Jupiter at something like its present distance from the Sun. (The entry probe found further confirmation of this in the nitrogen isotopic ratios, which indicates that Jupiter's nitrogen was originally delivered as molecular N rather than as ammonia -- which in turn provides an odd clash that I've mentioned elsewhere with the indications from Huygens that Titan's nitrogen DID arrive as ammonia in relatively warm ice.)

Since water ice was the carrier of all these other heavier elements, we need to know the ratio of water ice to them -- for which we must know Jupiter's current oxygen content. If the planet's oxygen is enriched to only about the same degree relative to the Sun as all the other heavier elements measured by the Galileo entry probe, then they must have been carried into the planet in very cold water ice, from the Kuiper Belt or beyond -- and Jupiter itself may have originally accreted at that distance and then spiralled a great distance inwards. But if oxygen turns out to be enriched more relative to its solar abundance than those other elements -- say, about 9 times solar abundance -- then those other elements were trapped by water ice, and carried into the forming Jupiter, in a more diluted form as clathrate ices, which could have formed somewhat closer to the Sun.

The microwave radiometer (whose viewfield is 1 degree at the equator and 4 degrees at the poles) should allow water abundance measurements down to about 100 bars -- plus better ammonia data (which is a bit fuzzier from the Galileo probe than we would like), thus nailing down both Jupiter's overall oxygen content, and further sharpen our data on its nitrogen content. It will also get more data on the temperature and cloud depth profiles in different parts of the planet, which in turn should help tell us more about just how deep the convective and wind patterns that create the belt-zone structures really run. But it can only do all this reliably because the Galileo entry probe measured the other trace components of Jupiter's atmosphere -- some of which, like PH3, have a significant effect on the planet's microwave spectrum.

(5) Juno's mission is scheduled to run 32 orbits of 11 days each -- and any extended mission will be only a month or so, because they want to make sure that they can crash it into Jupiter, and thus avoid any chance of contaminating Europa, before they lose control of it from radiation damage. In fact, they may end the mission ahead of time -- most of its science will come from its first 16 orbits, and its periapsis latitudes are designed to give it only 5% of its total radiation dosage during that period. (As the JPL paper says, 5 of its first 7 orbits are directed toward microwave radiometry, with all the rest of its orbits being devoted to precision tracking for gravity-field data.)

(6) Juno spins at 3 rpm. Its "JunoCam" -- the most dispensable of all its instruments, whose data will be processed by students at JPL -- should send back 5-10 images per orbit. Juno is focused entirely on the planet itself -- any data it does get on the moons will be pure gravy. For instance, it's very unlikely that they will be able to arrange for it to fly through Io's flux tube.

[nprev]

This topic will consist of discussion of Juno operations post-JOI until end of mission, currently anticipated in Feb 2018.

[Glenn Orton]

OK, here are those crumbs. The JPL description is pretty good, but there are a few things missing from it:

(1) The 2002 Solar System Decadal Survey noted that the five main goals for the next Jupiter mission are: (A) Determine if Jupiter has a central core to constrain models of its formation; ( B ) determine the planetary water abundance; ( C ) determine if the winds persist into Jupiter's interior or are confined to the weather layer; (D) assess the structure of Jupiter's magnetic field to learn how the internal dynamo works; and (E) measure the polar magnetosphere to understand its rotation and relation to the aurora. Juno will do a nice job on all five - and while, the Survey's original desire for at least one and preferably 2 or 3 deep entry probes (down to 100 bars) would have further improved the data on ( B ) and ( C ), the added expense was so great that a deep Jovian Multiprobe Flyby mission by itself is now ranked pretty low on the list of desired New Horizons missions -- shallow Galileo-type entry probes of the other giant planets are higher-ranked. Moreover, the data from Juno will allow us to better plan the targeting of those deep Jupiter entry probes when we finally DO fly them. (Note also that -- if they absolutely have to descope Juno -- they could toss off every single instrument except the microwave radiometer and magnetometer, and lose only goal (E) in the process.)

(2) Currently we know Jupiter's gravity-field harmonics down to level 6 -- Juno will take it down to level 12 to 14. Not only can it nail down the size of any hevy-element core -- which is crucial to decide which of the two rival theories of giant-planet formation is true -- but it can measure that core's rotation rate, and even obtain profiles of the density of the planet's middle layers sensitive enough to determine how deep its convective wind cycles really run, all the way down 1/5 of the way to the core!

(3) Our current knowledge of Jupiter's magnetic-field harmonics is level 4. Juno will take it all the way down to level 20 -- much BETTER than we can ever obtain for Earth itself, where we're forever limited to level 14 due to interference from crustal fields! Thus Juno is likely to provide radical new information not only on the generative processes of Jupiter's magnetic field (including the dynamo radius and changes with time), but of Earth's field as well.

(4) Knowledge of the total oxygen content of Jupiter's atmosphere is crucial -- and the Galileo entry proe didn't get it because of its bad-luck fall (9-1 odds against) into a hot spot where a downdraft removed the local water vapor. The probe DID find not only that the concentration of the other heavier elements -- Ar, Kr, Xe, C, N and S -- was somewhat lower than expected, but that they were very consistent in being enriched about threefold relative to the Sun, whereas much bigger element-to-element differences had been expected in that ratio. This was a shock. The logical conclusion is that the icy planetesimals that formed Jupiter were actually made of much colder ice than that which existed at the planet's current distance from the Sun (150 K) -- those other elements were imprisoned either in regular ice at only 20-30 deg K or clathrates at >38 K, so either the planet itself formed much farther from the Sun and migrated a great distance inwards, or the planetesimals that formed it themselves came from much farther out and migrated inwards before accreting to form Jupiter at something like its present distance from the Sun. (The entry probe found further confirmation of this in the nitrogen isotopic ratios, which indicates that Jupiter's nitrogen was originally delivered as molecular N rather than as ammonia -- which in turn provides an odd clash that I've mentioned elsewhere with the indications from Huygens that Titan's nitrogen DID arrive as ammonia in relatively warm ice.)

Since water ice was the carrier of all these other heavier elements, we need to know the ratio of water ice to them -- for which we must know Jupiter's current oxygen content. If the planet's oxygen is enriched to only about the same degree relative to the Sun as all the other heavier elements measured by the Galileo entry probe, then they must have been carried into the planet in very cold water ice, from the Kuiper Belt or beyond -- and Jupiter itself may have originally accreted at that distance and then spiralled a great distance inwards. But if oxygen turns out to be enriched more relative to its solar abundance than those other elements -- say, about 9 times solar abundance -- then those other elements were trapped by water ice, and carried into the forming Jupiter, in a more diluted form as clathrate ices, which could have formed somewhat closer to the Sun.

The microwave radiometer (whose viewfield is 1 degree at the equator and 4 degrees at the poles) should allow water abundance measurements down to about 100 bars -- plus better ammonia data (which is a bit fuzzier from the Galileo probe than we would like), thus nailing down both Jupiter's overall oxygen content, and further sharpen our data on its nitrogen content. It will also get more data on the temperature and cloud depth profiles in different parts of the planet, which in turn should help tell us more about just how deep the convective and wind patterns that create the belt-zone structures really run. But it can only do all this reliably because the Galileo entry probe measured the other trace components of Jupiter's atmosphere -- some of which, like PH3, have a significant effect on the planet's microwave spectrum.

(5) Juno's mission is scheduled to run 32 orbits of 11 days each -- and any extended mission will be only a month or so, because they want to make sure that they can crash it into Jupiter, and thus avoid any chance of contaminating Europa, before they lose control of it from radiation damage. In fact, they may end the mission ahead of time -- most of its science will come from its first 16 orbits, and its periapsis latitudes are designed to give it only 5% of its total radiation dosage during that period. (As the JPL paper says, 5 of its first 7 orbits are directed toward microwave radiometry, with all the rest of its orbits being devoted to precision tracking for gravity-field data.)

(6) Juno spins at 3 rpm. Its "JunoCam" -- the most dispensable of all its instruments, whose data will be processed by students at JPL -- should send back 5-10 images per orbit. Juno is focused entirely on the planet itself -- any data it does get on the moons will be pure gravy. For instance, it's very unlikely that they will be able to arrange for it to fly through Io's flux tube.

This is a nice summary, but I'm going to correct and update a little.

(5) PH3 will have a minor, although non-zero, effect on the microwave spectrum. The biggest effect by far is that of absorption by gaseous ammonia (NH3), and we need to sort it out carefully before we can find the holy grail of water vapor abundance at depth, from which we will derive the presumptive O/H ratio at depth.

We thought a couple of years ago that the 11-day orbit would be too short if there were a spacecraft 'safing' event, as these have taken typically 3-5 days to recover from. So we requested NASA to move to 14-day orbit, but retain the 32-orbit mission. They agreed, despite the increased cost. It still seems like a good idea.

(6) JunoCam's operation accepts images from all amateur astronomers focusing on Jupiter: they are our extended Co-Investigator team! We at JPL create cylindrical maps every 2 weeks and post these, maintaining a thread of discussion on each feature and noting when a feature no longer exists. About two weeks before the time of perijove 4 (PJ4), we will solicit and gather votes on which features to observe that will be in our expected field of view for that orbit, gather the votes in priority order, then upload commands to the spacecraft on what to observe. These images will then be transmitted back to earth and we'll post them in the same Mission Juno / JunoCam web site. Students at Caltech may well work on them, but so can the entire rest of the world. Processed images can be uploaded to the site. The only orbit where we don't intend to post immediate images is the current orbit (Orbit 0), when we're engaging in a lot of testing of procedures that may or may not work. We will also take science-driven images of the polar regions on each orbit before the features voted on by the public.

-Glenn Orton, JPL
Juno Science Team member

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