Europa, in all its frozen glory


Researchers create first ever complete thermal map of Jupiter’s moon. Brian W. Pulling reports.


A thermal image of Europa, superimposed on a Hubble telescope image of Jupiter.

A thermal image of Europa, superimposed on a Hubble telescope image of Jupiter.

ALMA (ESO/NAOJ/NRAO), S. Trumbo et al.; NRAO/AUI NSF, S. Dagnello; NASA/Hubble

For the first time, researchers have taken photographs which create a complete thermal map of the surface of Jupiter’s moon, Europa.

Using the Atacama Large Millimetre/submillimetre Array (ALMA) in Chile, researchers at the California Institute of Technology in the US, led by Samantha Trumbo and Bryan Butler, have compiled four photographs showing temperature gradients across the moon’s icy surface.

The images are published on the website of the US National Radio Astronomy Observatory.

The data to create the thermal map was collected during November, 2015.

ALMA “uses many radio dishes spaced far apart to achieve the spatial resolution of a telescope that is much larger than any of the individual dishes,” explains Trumbo.

“It senses wavelengths in the submillimetre to millimetre range, so the light it receives from Europa is thermal emission from Europa’s surface, rather than reflected sunlight.”

To interpret the photographs, the researchers extrapolated from previous data about the geometry of the moon, collected by the Voyager and Galileo missions, now collected into a single resource called the Europa Global Mosaic.

Trumbo describes the mosaic as “a greyscale global map of Europa’s surface made up of images from the two spacecraft missions”.

In addition to these photographs, the researchers created a thermal model using data on Europa’s absorption and re-radiation of sunlight.

“This tool allowed us to understand our thermal data, and what it might mean in terms of Europa’s surface thermal properties and potential for current geologic activity,” says Trumbo.

The images revealed a cold spot in the northern hemisphere of the moon which has not yet been explained by the thermal modelling, any satellite, or by ALMA.

According to a paper written by Trumbo and colleagues, and published in The Astronomical Journal, this cold spot is also the iciest region on the surface and “does not coincide with any unique geologic or morphological features”.

Trumbo is planning several more observations using ALMA, which she says will allow her to “better untangle the thermal properties and the explanations for discrepancies with the modelling”.

The fresh information is expected to be useful for the upcoming NASA Europa Clipper mission, set to launch as early as 2022.

The mission includes the Europa Thermal Emission Imaging System (E-THEMIS), an early version of which was used on NASA’s Mars Odyssey orbiter.

“It’s really incredible that we can get thermal data from the ground that are of comparable spatial resolution to those achieved by spacecraft,” Trumbo says. “That’s how powerful ALMA is.”

  1. https://public.nrao.edu/news/2018-alma-image-europa/
  2. https://astrogeology.usgs.gov/maps/europa-voyager-galileo-global-mosaics
  3. http://iopscience.iop.org/article/10.3847/1538-3881/aada87
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