What's So Hot About Io?

To print images, click here

Grade Level: Activity 1, Middle School; Activity 2, High School

Description: Students will model the Near Infrared Mapping Spectrometer (NIMS) instrument by using a sheet of liquid crystal to detect areas of high infrared (IR) radiation (warm tiles or coins hidden beneath a photo of Io's surface). Advanced students then calculate the area and power output of the volcano Prometheus based on the IR data.

Materials for Activity 1: Two copies of image of Io's surface for each group (To print images click here); liquid crystal sheet (see Teacher Notes); scissors; three large coins or small ceramic tiles (tiles work best as they retain their heat longer); two small beakers; ice; Bunsen burner or hot plate; tongs or spoons for removing objects from beakers

Materials for Activity 2: NIMS Prometheus spectrum (graph); calculators

Vocabulary: Infrared, spectrometer, liquid crystal, wavelength, micron, power, Watt, Kelvin



Images of Jupiter's moon Io that were photographed in the visible part of the spectrum reveal a surface pockmarked with dark spots, many of which are active volcanoes. How do we know which areas are active and which are not? The Near-Infrared Mapping Spectrometer (NIMS) observes the infrared part of the spectrum, measuring the amount of heat that is produced by a moon or planet. The NIMS instrument is ideal for finding hot volcanoes on the surface of Jupiter's moon Io. It has already shown us that volcanoes on Io have the highest recorded surface temperatures of any planetary body in the solar system (one may be as hot as 3,100 degrees Fahrenheit!)


C. N. Hotspots: Infrared Detective!

In this activity, you will use a material similar to that on the NIMS detector to find a series of hidden hot spots. The color of a sheet of liquid crystals will change depending on the temperature of objects that are in close contact or nearby the sheet.

First, you will need to make two copies of the image of Io's surface. Cut out the pictures, and place one copy on a flat surface such as a table or desk.

Next, heat three coins or tiles to different temperatures: 0 Celsius (32 F), 37 Celsius (98.6 F), and 100 Celsius (212 F). To do this, use a beaker of ice water, the palm of your hand, and a beaker of boiling water. Be sure to follow all safety procedures outlined by your teacher.

When the coins or tiles have reached the proper temperature, have one member of your group (the "infrared detective") turn around and close their eyes. Quickly and carefully place the coins or tiles onto the image of Io on your desk, aligning them with randomly chosen features. Now place the second cut-out image of Io directly on top of the first, so that the coins or tiles in between are aligned with the same features as below. 

Finally, have the "detective" turn back around and try to find the active hot spots by placing the liquid crystal infrared detector on a portion of the image of Io's surface. Once they are located, observe the hot spots for several minutes, noting any changes in size or color.



  1. How many "active volcanoes" were you able to detect?
  2. Why doesn't your infrared detector "see" the coin or tile at 0 Celsius?
  3. How does the area of each hot spot relate to temperature? How might you explain this?
  4. Did the color of any hot spot change over time? If so, how does the color of a hot spot relate to its temperature?


The Power of Prometheus! [see Teacher Notes before attempting this activity]

In this exercise we are going to determine the area of the Prometheus volcano, one of many volcanoes on Io, and its total power output. The methods described here are used by scientists on the Galileo Project.

Figure 3 shows a spectrum of the hot spot Prometheus observed by NIMS. The x-axis of the graph shows wavelength (measured in microns: a micron is one-millionth of a meter). The y-axis shows the power (in Watts) as measured by NIMS. You can see that the hot spot emits different amounts of energy at different wavelengths. This distribution of energy output (the output spectrum) depends on the temperature of the hot spot. As different temperatures have different curve shapes, we can find the temperature whose curve gives the best fit. For Prometheus, the best fit is obtained with a temperature of 461 K.

Knowing the temperature of the hot spot, we can calculate the area. We determine the energy emitted per unit area, at a selected wavelength, by a body at the temperature of the hot spot. This is our measuring stick: it tells us that a known area at a known temperature at a known wavelength is emitting a certain amount of energy. We can then compare this calculated figure with the actual amount of energy that NIMS has measured from Prometheus and find the area.

First, select a wavelength (L) from the x-axis in figure 3. This will be used throughout the exercise. The amount of energy (Pm) emitted from 1 m2 of a body at the temperature of Prometheus (461 K) at our selected wavelength (L) is given by equation 1. The units of Pm are Watts per square meter per micron, or W/m2/mm. Calculate Pm for T = 461 K and wavelength L. By the way, e = 2.718.

equation 1:


Now, we need to find how much power (P) Prometheus is actually emitting at our selected wavelength. Using figure 2, at wavelength L, draw a line from the x axis up to the 461 K curve, and then across to the y-axis to find power P. This is how much energy Prometheus is releasing at wavelength L.

We also know that at wavelength L every square meter (m2) of the surface area of a body at 461 K emits power Pm . So how big does the volcano have to be to emit power P, at Pm per square meter? The area in square meters (Am) is given by equation 2.

equation 2:

What is the area of the volcano in square kilometers (Akm)? 1 km2 = 1,000,000 m2.

Total Power Output. We now know the average temperature and area of the Prometheus lava flows. The total power output, PT, in Watts, is given by equation 3. Find PT.

equation 3:

This is all from one volcano, and Io has many volcanoes! A light bulb uses 100 W: How many light bulbs does this hot spot represent?

The power generating capacity of the United States is 700,000 MegaWatts (700,000,000,000 Watts: Department of Energy, 1997 summer generation figures). Io emits 1014 W, much of it from volcanoes. How do these figures compare?

Teacher notes are available here.


Galileo/NIMS Home Page


The Electromagnetic Spectrum explained

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This module was written by Brian Exton (National Optical Astronomy Observatories, Tucson AZ).

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