The Juno spacecraft now orbiting Jupiter has a suite of science instruments which will investigate the existence of a possible solid planetary core, map Jupiter’s intense magnetic field, measure the amount of water and ammonia in the deep atmosphere, and observe the planet’s auroras.
Why do we care about this?
Jupiter is the largest planet in our solar system. If we can understand how giant planets like this form, it will help us understand how these huge planets played a role in forming the rest of the solar system.
Here is what NASA says about the origin and interior of Jupiter:
Theories about solar system formation all begin with the collapse of a giant cloud of gas and dust, or nebula, most of which formed the infant sun, our star. Like the sun, Jupiter is mostly hydrogen and helium, so it must have formed early, capturing most of the material left after our star came to be. How this happened, however, is unclear. Did a massive planetary core form first and capture all that gas gravitationally, or did an unstable region collapse inside the nebula, triggering the planet’s formation? Differences between these scenarios are profound.
Even more importantly, the composition and role of icy planetesimals, or small protoplanets, in planetary formation hangs in the balance — and with them, the origin of Earth and other terrestrial planets. Icy planetesimals likely were the carriers of materials like water and carbon compounds that are the fundamental building blocks of life.
Unlike Earth, Jupiter’s giant mass allowed it to hold onto its original composition, providing us with a way of tracing our solar system’s history. Juno will measure the amount of water and ammonia in Jupiter’s atmosphere and help determine if the planet has a core of heavy elements, constraining models on the origin of this giant planet and thereby the solar system. By mapping Jupiter’s gravitational and magnetic fields, Juno will reveal the planet’s interior structure and measure the mass of the core.
The Juno Spacecraft (Image courtesy of NASA/JPL)
Juno’s instruments (Image courtesy of NASA/JPL)
Gravity Science Experiment: Jupiter’s gravitational field
In order to understand Jupiter’s inner planet structure, Juno will precisely map the planet’s gravitational field in the Gravity Science Experiment. To do this scientists will measure changes in Juno’s velocity as it orbits Jupiter. They are looking for variations in Juno’s acceleration rate that tells scientists something about the changes in the gravitational field of Jupiter from variations in Jupiter’s inner planet structure. So they measure the spacecraft’s velocity as a function of time extremely accurately. This is actually done by measuring the Doppler shift of the radio signal.
The Doppler shift from Juno's radio signal will enable scientists to map variations in Jupiter's gravity field as the spacecraft orbits the planet. (Image courtesy of NASA/JPL)
The biggest part of the instrument system is a 34m diameter radio antenna at Goldstone California. See Figure 4.
The Goldstone 34m (110 ft.) diameter Beam Waveguide antenna on Earth. (Image courtesy of NASA/JPL)
On Juno there is a high gain antenna pointed at the Earth. See Figures 1 and 2 for the Gravity Science Experiment portion of the spacecraft where this antenna resides. The radio instrument measures the voltage coming in from the antenna from the Earth and then transmits a voltage that is fed out to the radio signal to send back to the Earth; then the gravity measurement tells us something about the density variations which tells us whether there are storms on the outside or whether they penetrate all the way through and whether there is some hint of a structure in the center other than just compressed Hydrogen gas.
Frequencies used for transmitting gravity data: X-band (wavelengths of 3 cm) and Ka-band which is for added accuracy (wavelengths of 1 cm).
Location: Saucer-shaped high-gain antenna on top of spacecraft and radio transponder within radiation vault. See Figures 1 and 2.
Another instrument called the Advanced Water Vapor Radiometer helps isolate the signal from any type of interference caused by Earth’s atmosphere.
JPL provided the Juno telecom system. The Italian Space Agency contributed the Ka-band translator system.
Watch for more Juno instruments in the coming week.