Thursday, February 12, 2009

Io Science with EJSM Part Deux

Now that the more detailed mission studies for both the Jupiter Europa Orbiter and the Jupiter Ganymede Orbiter are now online, we can delve a little deeper into the science plans for Io. For some background from the Joint Summary Reports posted last month, check out my first Io Science with EJSM post. Again, Van Kane will cover the Titan stuff from the TSSM in detail. Don't forget that the downselection meeting was today (no, I don't know who won), and an announcement should be made some time early next week.

The only comment that I have about the TSSM reports is that the ESA people should have checked with me to make sure that they had the right longitude system for that map of mine they were using. Though really they didn't have to, Kraken Mare was on that version of the ISS map [see the TSSM In-Situ Elements Assessment Report, page 71], and they still had their landing site off by 180 degrees.

The JEO Final Report is a fairly extensive document detailing the current of state of our knowledge of the Jovian system, the science objectives for the Jupiter Europa Orbiter and the Europa/Jupiter System Mission as a whole, the mission concept (including model payload, mission design, and spacecraft design using floor and baseline budgets), and planned science operations. The document itself weights in at almost 500 pages so it is a bit difficult to summarize the entire document in a single post. This document is more extensive than the EJSM Joint Summary Report discussed earlier. That's why I've chosen for this first post to focus on the Io-related science plans and mission concept.

The Final Report defines two primary science investigations for Io that would be performed by JEO: Investigate the nature and magnitude of tidal dissipation and heat loss on Io and Investigate Io's active volcanism for insight into its geological history and evolution (particularly of its silicate crust). These science objectives fit with in the mission goal of understanding Europa in the context of the Jupiter System.

To accomplish these goals, the JEO team would use most if not all of the instrument payload, which while not optimized for Io science, would go a long way to furthering our understanding of Io place in the Jupiter system and its evolution. For example, using the imaging systems, the JEO team plans to image 30% of Io with a resolution better than 1 km, which is comparable to coverage provided by Galileo and Voyager. The also plan on imaging 20% of Io at better than 200 meters/pixel (compared to 3% for Galileo), 5% at better than 50 m/pixel (compared to ~1% for Galileo) and a still to-be-determined amount at better than 10 m/pixel. 10 m/pixel imaging may be precluded by the slow reaction wheel turn time and fast speed of the two, planned close flybys of Io. In addition, the JEO team plans to acquire two Ice Penetrating RADAR swaths, 1000 km long, of Io during those two encounters. These would allow observations of sub-surface structures such as fault planes and shallow magma reservoirs. No information is provided on how deep IPR could sense in silicate rock. The Thermal Instrument will observe Io's night-side in order to better constrain Io's heat flow, which is important for understanding tidal heating on Io.

The above figure shows the ground track for the four planned Io flybys. The first, taking place shortly before JOI on December 21, 2025, would have an altitude of 1000 km and would be used to help setup the the Jupiter Orbit Insertion maneuver. As currently planned, no remote sensing observation would be acquired so as to reduce the risk to the all-important JOI, though further study if EJSM is selected would be conducted to see how much risk there actually is. The Io gravity assist would help save 200 m/s in delta-V, which amounts to a dry mass increase of 160 kg, over an earlier plan to use a Ganymede flyby before JOI to help slow the spacecraft into orbit around Jupiter. Even with the additional shielding that will be required, this still amounts to more than 100 kg in mass margin.

Between July and December 2026, three additional encounters will be performed [please note again, that details of these encounters would change, but we can still discuss them as they are described in the Final Report].

The first will take place on July 9, 2026 and will have a close approach altitude of 300 km. On approach, JEO will be able to view portions of the anti-jovian and trailing hemispheres, between 150° and 260° West longitude, at high resolution. The more northerly approach also provides an opportunity to view the north polar region at high emission angles. The Amirani plume maybe visible along the bright limb. That will be useful as the close approach point for Io-1 is directly above the Amirani plume source. If the structure of the Amirani plume is similar to Thor's (as seen by Galileo in 2001), then this encounter would allow JEO to sample the high-altitude gas plume at Amirani. The Ion and Neutral Gas Spectrometer should be able to measure the abundances of various gas components in the plume, which could provide another method for measuring the volatile abundances on Io's surface. The Ice Penetrating Radar and Laser Altimeter should be used during this encounter. The ground track for JEO during Io-1 passes over the mountains Euxine Mons and Monan Mons with an altitude of less than 1000 km, so LA should be able to provide height and structure information about the mountains, while IPR should provide quite a bit of information about near-surface faults and flexure, which can provide a lot of details about Io's crust. IPR could also help our understand of the shallow magma reservoirs and conduits for the Amirani volcano. Because the spacecraft will use reaction wheels to control pointing near close approach, the spacecrafts speed relative to Io at C/A (9 km/s), will require a kind of skeet shoot to keep the LA and IPR pointed at Io nadir and INMS pointed in the direction of motion. So these observations maybe limited to the few minutes surrounding close approach.

The second encounter (Io-2) would take place on September 3, 2026, shortly before perijove on the next orbit. This encounter is a slower, relatively high-altitude (3125 km) flyby over the south polar region of Io. This should allow high resolution (less than 50 m/pixel) NAC imaging of the south polar region. The high altitude, however, precludes Ice Penetrating RADAR observations. Inbound, JEO will observe, like the previous encounter, the terrain between 150° and 260° West longitude at low phase angles. The lower latitude of the Io-2 encounter should allow for stereo analysis of this region, which would greatly expand topographic information from the limited swaths acquired by LA and IPR. Outbound, JEO would observe the sub-jovian hemisphere in Jupiter-shine, providing an opportunity for the Thermal Instrument and the VIS-IR Spectrometer to observe Io's volcanic activity and thermal emission.

The third encounter (Io-4) would take place on December 27, 2026, two orbits after Io-2 and would include the closest approach altitude of the four encounters, only 75 km. At such a low altitude, the high priority is to sample a plume using the INMS. However, the close approach point is located between Malik and Altjirra Patarae, where no previous plume has been observed. If the IPR is pointed at nadir when JEO flies over Malik and Altjirra, the spacecraft could go a long way toward constraining the formation models of Ionian paterae by looking at the sub-surface structure of these two (fault planes and perhaps shallow magma reservoirs). Once again, inbound JEO should be able to image the portions of the anti-jovian and trailing hemispheres between 150° and 260° West longitude in sunlight. The Final Report includes a detailed observation scenario for I4, ±30 minutes of closest approach, which is graphically shown at right. IPR would observe Io ±1 minute surrounding closest approach. The most of the optical remote sensing instruments would operate prior to close approach, except TI whould would be run continuously after closest approach. The laser altimeter would acquire a ground track ±6 minutes surrounding closest approach.

In addition to these flybys, sixteen non-targeted encounters during the Jupiter tour with close approaches distances between 56,300 km and 479,000 km have been identified. Most of these encounters would provide opportunities to monitor activity on Io's anti-jovian hemisphere with resolutions ranging from 560 m/pixel to 4.8 km/pixel. One interesting opportunity comes on October 29, 2026 when JEO approaches the trailing hemisphere from a distance of 185,428 km. This should allow imaging of the Pele-Loki hemisphere at resolutions approach 1.85 km/pixel. While not as good as the 500 m/pixel imaging planned (and lost due to budget constraints) for orbit A34 in November 2002, this non-targeted flyby should provide a nice change to image Pele at reasonable emission angles, Loki, and Ra as well as terrain last best seen by Voyager 1 near the south pole. During these encounters and other opportunities during the Jupiter tour and during the fourth Europa Campaign, JEO will conduct monitoring observation, searching for surface changes, plumes (both longitudinal coverage to search for them and to create plume movies), and monitoring the evolution of Io's volcanic activity over a 2.5- to 3-year time scale.

I think I'm pretty well exhausted from typing this up. I will try to writeup more tomorrow and over the weekend. In the mean time, you all can check out the two EJSM final reports for the Jupiter Europa Orbiter and the Jupiter Ganymede Orbiter. Really, more info than my brain can handle at the moment.

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