EDITOR'S NOTE 16 Oct 2016: Because of the delay of Juno's Periapsis Raise Maneuver, Juno's orbital tour has changed. As a result, while the general discussion about Juno's capabilities to observe Jupiter's moons remains true, the dates of specific observations are now incorrect. An updated version of this post will eventually be published when new encounter dates and distances are known. --ESL
It is not easy to observe Jupiter’s moons as more than points of light with Juno, because Juno will never get very close to any of the moons. The spacecraft is in a polar, elliptical orbit, but the moons orbit in the plane of Jupiter's equator. Swinging high above and below the equatorial plane, the spacecraft doesn't currently approach the equatorial plane except near perijove (the closest approach to Jupiter) and apojove (its farthest distance). Perijove occurs between Jupiter and its innermost ring, far inside the orbits of the major moons. Apojove is beyond the orbit of Callisto.
Gradually, Juno's orbit will evolve, from apojove and perijove being close to the equator, to apojove being south of Jupiter and perijove occurring at northern mid-latitudes. Eventually the northern (inbound) leg of the orbit will begin to cut through the equatorial plane at distances within the orbits of the Galilean satellites. We call this the “rotation of the line of apsides”.
Although satellite science is not a driver for the Juno mission there will be opportunities to collect satellite data, occurring more frequently as the orbit shifts. We will use Juno’s remote sensing instruments to observe the moons:
- JunoCam (JNC) for visible images
- Jovian InfraRed Auroral Mapper (JIRAM) for near infrared spectra and images
- Ultraviolet Spectrograph (UVS) for ultraviolet spectra
- Advanced Stellar Camera (ASC) for visible images at a spacecraft orientation offset from the other 3 instruments.
We have the additional constraint that these instruments have limited capability to point off-nadir. That limits the observations to the following range of moon-Juno-earth angles: 87 – 93o for JIRAM; 61 – 119o for JunoCam; 60 – 120o for UVS; and 158 – 174o for ASC.
Jupiter's moons fall into three major categories: 1) the Galilean satellites, 2) the ring moons, and 3) the outer irregular moons.
1. The Galilean satellites: Io, Europa, Ganymede and Callisto
Io is the most volcanically active body in the solar system. Juno observations will extend the current time record by looking for changes in some of the larger deposits and looking for new large eruptions. What volcanoes are currently erupting? JunoCam and ASC can image large plumes. JIRAM will return spectra in the 2 – 5 micron range, and view polar regions better than previous missions. This range captures sulfur signatures in several compounds.
To observe Io’s hotspots and plumes, we can image at a maximum range of 400,000 kilometers for JunoCam and UVS, and 800,000 kilometers for JIRAM and ASC. At 800,000 kilometers there is an opportunity almost every orbit to observe Io, although not necessarily always with the best spacecraft orientation.
Throughout the prime mission, there are 24 opportunities to observe Io with all instruments at ranges less than 400,000 kilometers. There are 3 Io flybys at distances less than 200,000 kilometers: 2017-10-03 155,454 km 2017-10-17 182,346 km 2018-02-20 105,015 km
The most significant opportunity for Juno to do Europa science would be to follow up on the plumes possibly detected by Hubble Space Telescope. Confirming Hubble's detection would be very scientifically valuable. Any information on the source location would be valuable. This science goal just may not be possible with the large distances from Juno to Europa, but we will look.
JunoCam or ASC can only detect plumes if they contain fine particles. The Hubble discovery (if real) only shows the presence of water vapor. We can predict by analogy to Enceladus that water vapor plumes will also contain particles. However, it is important to remember that the Hubble discovery was of gas, not particles. If the putative Europa plumes are Enceladus-like and do contain particles, they would not be as tall as Enceladus', because of Europa's higher gravity. Scaling for Europa’s gravity gives a maximum plume height of under 140 kilometers. To detect plumes, we need at least two pixels, so the image spatial scale would need to be better than 70 kilometers, at a relatively high phase angle where the particles would forward-scatter light to JunoCam and ASC.
To achieve resolutions better than 70 kilometers per pixel, UVS needs to be within 40,000 kilometers of Europa; JunoCam, 100,000 kilometers; and ASC, 170,000 kilometers. For the cameras, given the low expected height of the plumes, there is not much flexibility.
There are just four orbits that have Europa flybys that are closer than 300,000 km. Juno reaches the best available geometry in September 2017 as the rotation of the line of apsides brings Juno’s orbit close to Europa’s orbit: 2017-03-08 253,118 km 2017-09-19 264,043 km 2017-10-03 92,267 km 2017-10-17 204,654 km
Ganymede and Callisto
Science objectives for Ganymede and Callisto will mostly be focused on UVS and JIRAM spectra: the state of surface ices at the poles compared to equatorial regions, and leading vs. trailing hemispheres. In analogy to Saturn’s moons, the uppermost skin of the surface sensed in the UV is very responsive to the radiation environment, while values at longer wavelengths may be different. For these objectives, ranges of 300,000 kilometers or closer will be best.
There are four Ganymede flybys with ranges under 300,000 kilometers: 2017-02-21 235,981 km 2017-11-27 271,412 km 2017-12-12 108,457 km 2017-12-26 253,294 km
Juno will have no Callisto flybys closer than 400,000 kilometers, so spectral comparisons will be limited to hemispheres.
2. Ring moons: Metis, Adrastea, Amalthea, Thebe
Although there are relatively close flybys of the ring moons they are still too tiny to resolve by JunoCam and are just at the few-pixel level for JIRAM and ASC. These moons will be imaged opportunistically.
3. Outer Irregular Satellites
The largest outer irregular satellites of Jupiter are: Pasiphae, Themisto, Taygete, Lysithea, Himalia, Isonoe, Sinope, Mneme, Ananke, and Leda. They are too tiny and too distant to be detected by JunoCam, JIRAM or UVS. Only ASC has a chance of observing the largest of these moons (Lysithea, Leda, and Himalia).
The science objectives for these distant moons are to: a) determine the rotation period, and b) estimate the shape model and pole orientation.