Seismology Research at NMSU

The Jovian Interiors Velocimetry Experiment in New Mexico (JIVE in NM), a NASA-funded project, will determine the interior structure and composition of Jupiter using seismology. A sensitive imaging spectrometer will be built by close collaborators in Nice, France that can measure the confirmed oscillations of these planets to a precision high enough to enable detailed studies of the planetary interiors. Since the gas-giant planets played such a critical role in the formation of the Solar System, and since so little about their core and compositional properties is constrained by observation, the seismic discoveries we will make with JIVE will finally allow us to discriminate between competing theories of planetary formation. Furthermore, precise measurements of the atmospheric winds will uncover new details into the physical processes that drive the zonal jets, and provide the data necessary to carry out monitoring of Jovian climatology to understand its complex dynamics.

JIVE in NM strongly aligns with several current and planned space missions carried out in the Planetary Division of NASA's Science Mission Directorate, such as Juno and Cassini. It will establish and foster important partnerships between New Mexico State University and the New Mexico Institute of Mining and Technology. It will strengthen ties to another major statewide facility, Los Alamos National Laboratory, through its interior modeling component. It expands the collaboration to scientists with critical expertise located at three NASA centers: Ames, the Jet Propulsion Laboratory, and Goddard Space Flight Center. Finally, it leverages the team's connections to a group of scientists and engineers in Nice, France who paved the way for this groundbreaking research by designing the initial instruments that demonstrated that Jupiter pulsates, and whose latest design we will adapt for JIVE.

This is a project of collaboration among undergraduate students, graduate students, faculty, and professional scientists and engineers geared towards ultimately solving fundamental questions in planetary science.

Fig. 1. The range of Jupiter and Saturn core masses allowed by current theory. The core and heavy element masses permitted by the gravitational harmonics and planetary rotation rates are shown. There is over a factor of two uncertainty in Saturn's core mass and only an upper limit (15 Earth masses) on Jupiter's core mass. (Click for a larger image)

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.

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.

Fig. 2. Evidence of the first detection of Jupiter's global modes from the SYMPA instrument, similar to the one to be built in this project. It shows the power spectrum of the mean velocity time series obtained in 2005. Excess oscillation power is detected between 800 and 3400 muHz, as well as a comb-like structure of regularly spaced peaks. The thick lines are smoothed data. (Click for a larger image)

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.

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.

Fig. 3. Example intensity image (top panel) whose clouds are tracked to give an estimated velocity map (bottom panel) within a zonal band on Jupiter. JIVE will be able to distinguish wave motion and zonal flows, critical for understanding what powers the jets. (Click for a larger image)

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.