Juno’s other ‘cameras’

I was inspired to write this article by a comment on Emily's JunoCam post:

"The Juno spacecraft did not need a camera to accomplish its science goals."

So, what are the science goals for which a camera is not needed? Seems to me that for any such mission, the images are much more important and instructive than any other measurement.

Gerald: 06/11/2016 05:31 CDT

What are the science goals for which a camera is not needed? Gerald raises a good point. And if you’re generous in your interpretation of the word "camera", the answer to Gerald's question is: "none!" Cameras come in lots of shades, shapes, and sizes.

Juno’s science goals are to understand the origin and interior of Jupiter, focusing specifically on its atmosphere and magnetic field (or magnetosphere). Achieving these goals would answer questions we have about worlds far beyond Jupiter, affecting our current understanding of planetary formation, planetary fluid dynamics, and gas giant composition. Cameras can help answer some of these questions. 

Since our eyes see what we call visible light (with wavelengths of approximately 400-700 nanometers), we think of cameras as recording visible light. Juno has one camera that sees visible light, JunoCam. In my generous re-interpretation, however, a camera records electromagnetic waves of any flavor. This definition of the word "camera" stands in contrast to mass spectrometers, which must physically collect molecules to obtain data.

What other flavors of electromagnetic waves are there? Two familiar examples are infrared (with longer wavelengths than visible light) and ultraviolet (shorter wavelengths). Infrared cameras are often used in home security systems, or on the auto-focus on your DSLR. On Juno, the infrared camera is called JIRAM (Jovian Infrared Auroral Mapper). Juno also has a UV camera called the Ultraviolet Spectrograph, or UVS.

Ultraviolet and infrared cameras help perform a type of science called spectroscopy that can detect chemical signatures of gases without physically collecting those molecules. (More on this here.) When atmospheric gases are excited by radiation like sunlight, their molecules become excited and vibrate, releasing energy in unique and specific wavelengths that we can use to "fingerprint" atmospheric gases, just like our eyes use colors to help us identify objects in the visible range. On Juno, spectroscopic photographs taken by JIRAM and UVS will help determine the atmospheric composition both deep into the atmosphere and in Jupiter’s aurora (specifically, H₃⁺ molecules).

Radiometers are also kind of like a one-dimensional, single-pixel camera with a flash (bear with me here). When you photograph an apple with a flash, you send a pulse of white light to the apple. The apple absorbs some of the light and reflects some of it back to you. The color of the light that gets reflected gives you useful information about the apple: is it green, or red, or maybe rotting and not safe to eat? A microwave radiometer does something similar: it sends out a pulse of radiation in the microwave band of the electromagnetic spectrum (with wavelengths of 1 to 1000 millimeters), and records the return information. The radiometer records the delay and intensity of the return radiation. Microwaves are useful at Jupiter because they can penetrate clouds. On Juno, the Microwave Radiometer (MWR) tells us about the deep structure of Jupiter by measuring the return time and intensity of microwave pulses in six specific frequencies. This tells us the quantity of water, ammonia and temperature  at different levels in the atmosphere,  improving our understanding of Jovian dynamics and, more generally, the planetary dynamics of gas giants. 

At the end of the day, scientists are relying on images and cameras of all sorts to do the critical science of Juno, seeing everything from the internal structure of storms and flow within Jupiter’s upper atmosphere to the composition and mass of the deep interior. 

More reading/watching:

NASA's Juno Mission Information Page

Wikipedia: Microwave Radiometer

Dr. Fran Bagenal explains the current state of knowledge on Jupiter as well as  Juno’s science goals (12:05 for the science objectives specifically)

Great explainers on Juno's instruments