Jovian Seismology and JIVE

NASA's critical planetary science goal is to answer the question "How did the Sun's family of planets originate and evolve?" Jupiter played a major role in the formation of our Solar System. However, the manner in which Jupiter formed is still debated. This is due to the fact that its interior composition and structure is so poorly known that it could contain as little as none or as much as 40 Earth masses of elements other than hydrogen and helium. A similar uncertainty exists for Saturn. For both planets, the mass of their cores - the seeds of planetary formation in the early Solar System - are only crudely constrained by current observations. The large uncertainties in our understanding of these two massive planets that motivate this project are portrayed in Fig. 1.

The Jovian Interiors from Velocimetry Experiment in New Mexico (JIVE in NM) is a NASA EPSCoR project whose research activities are designed to address this question and determine for the first time the interior structure and composition of Jupiter and Saturn and to gain new insights into their dynamic atmospheres. This will be carried out by constructing an optimized instrument that is capable of detecting Jovian oscillations to allow for seismic measurements of the planetary interiors and direct inferences of atmospheric winds. It builds and improves upon the successful design of an instrument that provided the first ever confirmed oscillations on Jupiter. The seismic results obtained in this project will reduce the range in possible core mass and interior composition in Fig. 1 by a factor of 5-10, allowing us to finally discriminate between competing theories of planetary formation.

Seismology

The most promising way to infer the interior structure of giant planets is through seismology. While measurements of planets' gravity fields (gravimetry) can be used to discern the internal distribution of mass within, these data are most useful in the outer portions of the planet. Jovian seismology, on the other hand, provides a way to unambiguously determine interior structure using well-tested methods originally developed for the Sun and now being carried out on other pulsating stars. Seismology concerns the observations and interpretation of global resonant sound waves that, once excited, propagate throughout stars and planets. The observed frequencies of these oscillations are compared to those computed from theoretical interior models of the planet. The models are tuned so that they give frequencies that match as well as possible the observed frequencies. Very generally, the best resulting interior model describes the planet accurately.

Jupiter is expected to pulsate in a spectrum of acoustic modes that are driven by convective motions similar to the Sun (see left). It has been recognized for four decades that the detection of oscillations would provide a powerful probe of the interior structure, and numerous studies have been carried out to theoretically investigate the nature of Jupiter's oscillation modes. Because of the expected rapid variations and structural discontinuities in the interior, these are challenging computations. Nonetheless, agreement has been reached that the modes that are excited and probe the core pulsate at frequencies between about 1000 and 3000 muHz (periods of 5 to 15 min), where this range depends fundamentally on the properties of the sound speed in the metallic and molecular hydrogen layers of Jupiter.

Observationally, there have been several attempts to detect Jovian oscillations using infrared photometry, Doppler spectrometry, and careful searches for excitation of acoustic waves due to the impact of the Shoemaker-Levy 9 comet. In most of these campaigns, the signal-to-noise (SNR) ratio was too low or instrumental artifacts were present that inhibited any positive detection. The fast rotation of Jupiter also limits the precision these instruments were able to obtain.

Jovian seismology had to wait until 2011 to get the first strong evidence of the detection of oscillations using the SYMPA instrument, an imaging spectrometer upon which the JIVE design is based. This instrument was designed to overcome some of the earlier limitations by imaging the full planetary disk, similar to solar helioseismic instruments like GONG, MDI/SOHO, and HMI/SDO. As part of a 10-day observing run in 2005, the SYMPA instrument was able to produce a power spectrum of Jupiter's oscillations shown in Fig. 2. An excess of acoustic power is observed in the frequency range predicted by theory, as well as the comb-like structure of peaks that is also expected from interior models, thereby confirming Jupiter's global pulsations. Unfortunately, the level of noise in the data is too high to identify individual modes and decisively probe Jupiter's interior.

Jupiter's Atmosphere

By tracking the motion of visible clouds in the troposphere we know that the predominant weather pattern in the Jovian atmosphere consists of a series of alternating eastward and westward zonal jets that are remarkably steady over long time scales. We also know that, embedded between these alternating jets, other dynamical structures such as vortices and waves develop. Some of the most prominent of these features, like the Great Red Spot (GRS), are well characterized, and infrared observations indicate that Jupiter has a strong equatorial stratospheric jet.

Despite the wealth of atmospheric information derived from ground-based observational campaigns and from space missions like Voyager and Galileo, many questions still remain unanswered. The mechanisms maintaing the dominant alternating jets, vortices and waves in the troposphere as well as their structure below the visible cloud level is largely unconstrained by the existing observations. The relationship between small-scale variability in the jets and the observed atmospheric morphology variability is also poorly understood. Finally, it remains unclear the precise role that eddies (large and small) and waves (large and small) play in governing Jupiter's weather pattern and its variability (both in the troposphere and in the stratosphere). Overall, the Jovian weather pattern is a complex system involving many different phenomena at different spatial and temporal scales, and we lack the continuous high-resolution observations of Jupiter's weather system as a whole needed to understand such dynamics.

JIVE will extend our knowledge by providing two-dimensional maps of wind velocities in Jupiter's atmosphere with a precision of 10-20 m/s for each Jovian rotation, and an averaged latitudinal profile at the level of 1-2 m/s. Unlike the winds derived from cloud tracking techniques that assume cloud motions represent wind motions and can only be carried out where cloud features actually exist (Fig. 3), JIVE will measure wind motions directly. Such data will allow new calculations to be performed with unprecedented accuracy, and will show us what powers Jupiter's Great Red Spot, how it and similar structures evolve, and how the zonal jet streams and cloud bands are connected to the interior. Detailed studies of Jovian climatology will not only provide new insights about the solar system's largest planets, but will also enable the framework for the characterization of exoplanets.

Details of the Instrument

Theoretical models and observations suggest that that our giant gas planets oscillate with periods of about 5 to 15 minutes, and amplitudes in the 10-100 cm/s range. The average amplitudes of the detected modes from SYMPA are approximately 40 cm/s using about 50 hr of data with a duty cycle of 22%. The SNR of these measurements was only about 3. One of JIVE's objectives is to improve the performance over SYMPA by approximately an order of magnitude, so that we can perform a seismic analysis using as many detected oscillations as possible. Thus, to carry out this groundbreaking science we require JIVE to provide:

• the sensitivity to detect modes with amplitudes as low as 1-10 cm/s;
• a SNR to exceed 10;
• a target frequency range of [500 - 4000 muHz];
• a frequency resolution of better than 1 muHz;
• accuracy of averaged zonal wind measurements in the Jovian atmosphere of approximately 1-2 m/s;

No such Jovian ground-based instrument with these capabilities exists, nor does one for current or planned space missions to the giant planets. JIVE is an imaging spectrometer specifically designed to help achieve the scientific goals of this project and to meet these technical specifications. It will measure the Doppler shift in solar absorption lines from light that is reflected by clouds in Jupiter's upper troposphere, providing spatially resolved line-of-sight velocity images of the whole planet at that altitude. More precisely, JIVE is a Fourier transform tachometer that will simultaneously produce a visible image and a Doppler-velocity image of the planet at a regular temporal interval.

Jovian oscillation periods and wavelengths dictate the rate and resolution that JIVE samples the surface velocity. Thus, JIVE is designed to provide Doppler-velocity images every 1 minute with a spatial resolution of about 1 arcsec (dependent on seeing), corresponding on average to about 3000 km on Jupiter (diameter of 140,000 km), or about 6000 km on Saturn (diameter of 120,000 km). The velocity images will be used to compute seismic observables like power spectra for mode identification, and can be averaged to obtain a latitudinal profile of winds sensitive to 1-2 m/s fluctuations.

Telescope

JIVE will be operated at the Dunn Solar Telescope at Sacramento Peak in Sunspot, NM.

JIVE and Juno

JIVE strongly aligns with NASA's Juno mission, whose primary scientific goal is to significantly improve our understanding of the formation, evolution and structure of Jupiter, and will arrive at Jupiter in July 2016. Juno will make key contributions with precise measurements of Jupiter's gravity and magnetic fields and will radiometrically sound the deep atmosphere. While these gravitational measurements are primarily sensitive to the outer envelope of the planet, the acoustic waves that JIVE will measure propagate all the way to the core and are thus sensitive throughout the interior. Indeed, as Fig. 4 shows, JIVE is a perfect complement to Juno, and its observational campaigns will overlap with Juno's mission in Jupiter's orbit. Thus it extends Juno's capability, adds to its scientific return, and allows the possibility of critical cross-comparisons of results from two distinct types of measurements.

As part of a ``deep-interior seismology program'' using JIVE, we will carry out a detailed analysis of a time series for all modes of amplitudes above 5 cm/s and l<=10. Once these modes are identified and frequencies measured, our analysis will proceed through well-known forward and inversion techniques. With the modes measured in the deep-interior program, the structure of the core, its size, and whether it is well defined or diluted will be determined. With good seeing at the telescope, we anticipate reaching sufficient spatial resolution to measure up to l<=25, which will allow us to probe the near-surface layers and the molecular/metallic hydrogen transition. We denote this the ``envelope seismology program.'' Fig. 4 illustrates the distinct interior regions we will be able to study using JIVE. In particular, note the overlap between the JIVE and Juno experiments near the expected metallic-to-molecular hydrogen transition in Jupiter. Juno is designed to measure Jupiter's gravity and magnetic fields, and JIVE will therefore allow for a crucial inter-comparison between gravimetry and seismology.

Saturn

Regarding seismology of Saturn, recent analysis by Hedman+ of occultation observations using the NASA Cassini spacecraft at Saturn shows exciting evidence of planetary modes that manifest themselves in its rings. This possibility was first proposed by team member Mark Marley and Carolyn Porco. The basic idea is that wave features in Saturn's C rings could be created by resonant interactions with internal oscillation modes, since these modes perturb the internal density profile and, therefore, the external gravity field. The observations of Hedman+ are the indirect evidence of these wave forcings. JIVE will be able to confirm this with direct observations of the global resonant oscillations on the planet. For a description of how the detailed features of these modes might be interpreted, see the excellent work by Jim Fuller.

Impacts of This Project

There are several important goals for JIVE that span instrumentation, science, education, and collaboration.

Instrumentation:

• Build an imaging spectrograph capable of measuring Jovian oscillations within the three years;
• Adapt an instrument design that has an expected order of magnitude more sensitivity than previous instruments;
• Mount the instrument on a suitable telescope to carry out monitoring of giant planets;
• Develop the software needed to control the instrument and perform data acquisition and reduction;
• Assemble a team of experts who regularly meet and review construction progress.

Science:

• Measure Jupiter and Saturn's core mass to within several Earth masses;
• Measure the total mass of heavy elements to within several Earth masses;
• Identify structural discontinuities of the interior density and sound-speed profiles;
• Validate and compare JIVE sub-surface inferences with those from the NASA Juno mission.
• Determine wind speeds directly and compare to cloud-tracking results;
• Measure the momentum cycle driving zonal jets by calculating eddy momentum fluxes;
• Directly characterize the planetary-scale waves in the wind signatures in the Jovian atmosphere;
• Indirectly probe the deep convective region of the planet to advance our understanding of tropospheric-stratospheric coupling.

Education:

• Hire three graduate students in engineering and astronomy whose work in JIVE will form the bulk of their graduate degrees;
• Involve up to six undergraduate students in all aspects of the project;
• Provide effective mentoring and advising practices to help form pathways for future student participation in JIVE.

Collaboration:

• Engage researchers in New Mexico's universities and national laboratories whose interests overlap with JIVE;
• Utilize existing collaborations with key NASA partners to strengthen the relevance of the project to NASA's scientific priorities;
• Leverage existing collaborations with the Observatoire de Cote D'Azur in Nice, France who has critical expertise in this area, and build the case for a future global network of similar instruments.

Further Information

If you are interested in further information about Jovian seismology, here is a collection of some important technical references.