EJSM Jupiter Science

Two previous blog entries (see Overview and Satellite Flyby Science) have looked at the science that the Europa Jupiter System Mission (EJSM) could perform prior to the two spacecraft entering orbit around Europa and Ganymede.  This blog entry looks at the science that could be performed by observing Jupiter itself.  The material for this discussion is drawn from the Jupiter Science and Capabilities on the Europa Jupiter Science Mission presentation made at the last OPAG meeting in February.  The presentation focuses on science that could be performed on the visible outer layers of Jupiter, including the weather layers with their swirling cloud decks.

To provide new science, the EJSM mission must build on results provided by past and planned missions.  Four spacecraft with instruments suitable for observing this world's weather have conducted studies: the two Voyager spacecraft, Cassini, and New Horizons.  All were limited by short observation times and except for Cassini possessed either 1970s technology instruments or instruments not optimized for gas giant observations.  The Galileo craft orbited Jupiter for a number of years, but it suffered from both 1970s vintage instruments and a malfunctioning antenna that severely crippled planned observations of Jupiter.  The Galileo probe did successfully enter Jupiter's atmosphere and provided irreplaceable composition measurements that, unfortunately, were not fully typical of the planet since the the probe entered a hot spot, which is the equivalent of a Jovian desert.  Key measurements of water and clouds could not be made.  In addition to these past missions, by the time EJSM arrives in the mid 2020s, the Juno orbiter will have completed its studies of the bulk composition of Jupiter and structure of the planet below the cloud tops.

The EJSM mission would bring several new capabilities to the study of Jupiter:

• Modern instruments that span the spectral range from 70 nanometers (ultraviolet) to 2 micron (mid-infrared)
• Multi-year observations with large data storage systems and high bandwidth communications systems
• Two spacecraft that can study the clouds decks at different times and at other times observe the same features to produce 3D images
• Two spacecraft that can probe the atmospheric structure by tracking each other's radio signals as they pass behind Jupiter as viewed from each other

A science working group identified three key science objectives: atmospheric dynamics and circulation, atmospheric composition and chemistry, and atmospheric vertical structure.  Some of the more detailed questions that struck me as particularly interesting (and you can read the entire list on pages 19-21 of the presentation) included:

• Investigating the dynamical of the weather layer to understand the redistribution of energy and momentum (particularly interesting on a world where a solid surface doesn't steal energy from wind systems)
• Coupling of the weather layer patterns with the deep interior
• Study localized variations in atmospheric composition
• Determine the 3D structure of Jupiter's atmosphere from deep within the clouds to the upper troposphere

The strawman payload of the NASA Jupiter Europa orbiter (JEO) would provide a range of resolutions (quoted resolutions are from 9.5 Jupiter radii):

• 7km/pixel from the narrow-angle camera
• 70km/pixel from the medium-angle camera
• 170km/pixel from the near-IR spectrometer
• 700km/pixel from the wide angle camera and UV spectrometer
• 1700km/pixel from the thermal

The planned orbits for the two craft would place perijove on the sunlit side of the planet.  These means that for the majority of each orbit, the instruments would view the night side of planet.  As a result, most of the atmospheric observations would take place in the three days centered on each perijove when the sunlit Jupiter would be visible (which represents approximately 10% of the time).  During this time, both contextual images and hundreds of narrow angle camera images would be taken to produce nearly 25GB of data per orbit.  A key strategy would be to repeatedly image the same locations on the cloud deck in multiple wavelengths to track evolution of cloud structure across the daytime and between days.

The current strawman instrument list and mission orbits are optimized for studying the Galilean moons, not the atmosphere of Jupiter.  The science working group identified a number of areas in which the mission could be enhanced:

• Optimize the spectral coverage of the instruments to observed key wavelengths to probe structure and composition
• Improve spatial resolution of the near-IR and UV spectrometers
• Add instruments specifically designed for probing Jupiter's atmosphere including a mid-to-far IR (5-500 microns) spectrometer and a sub-mm (500-1000 micron) spectrometer on the NASA spacecraft and a thermal imager on the European spacecraft
• Consider lengthening the orbital tour so that the apoapses of later orbits could lie on the daylight side of Jupiter for more observing time

Even without these enhancements, the EJSM mission would provide a major advance in our understanding of Jupiter and by extension the gas giants that have been found orbiting other stars.  Where Juno will focus on the interior of Jupiter, EJSM will focus on the top-most levels of the atmosphere.  In the words of the presentation, "Together, EJSM and Juno would provide the tools to significantly enhance our understanding of the fundamental physical processes in gas giant atmospheres... A long baseline of high resolution observations would allow us to construct a Jupiter 'climate database' to inform detailed physiochemical models of the atmosphere and the coupling processes between layers."