Using the Near-Infrared Camera (NIRCam) and the Near-Infrared Spectrograph (NIRSpec) aboard the NASA/ESA/CSA James Webb Space Telescope, astronomers have captured the infrared images of a region in the eastern hemisphere of Mars and the near-infrared spectrum of the planet.
Mars is one of the brightest objects in the night sky in terms of both visible light and the infrared light that Webb is designed to detect.
This poses special challenges to the observatory, which was built to detect the extremely faint light of the most distant galaxies in the Universe.
Webb’s instruments are so sensitive that without special observing techniques, the bright infrared light from Mars is blinding, causing a phenomenon known as detector saturation.
The astronomers adjusted for Mars’ extreme brightness by using very short exposures, measuring only some of the light that hit the detectors, and applying special data analysis techniques.
Captured by the NIRCam instrument, Webb’s first images of Mars show a region of the planet’s eastern hemisphere at two different infrared wavelengths.
The NIRCam shorter-wavelength (2.1 microns) image is dominated by reflected sunlight, and thus reveals surface details similar to those apparent in visible-light images.
The rings of Huygens crater, the dark volcanic rock of Syrtis Major, and brightening in the Hellas Basin are all apparent in this image.
The NIRCam longer-wavelength (4.3 microns) image shows thermal emission — light given off by the planet as it loses heat. The brightness of 4.3-micron light is related to the temperature of the surface and the atmosphere.
The brightest region on the planet is where the Sun is nearly overhead, because it is generally warmest.
The brightness decreases toward the polar regions, which receive less sunlight, and less light is emitted from the cooler northern hemisphere, which is experiencing winter at this time of year.
However, temperature is not the only factor affecting the amount of 4.3-micron light reaching Webb with this filter.
As light emitted by the planet passes through Mars’ atmosphere, some gets absorbed by carbon dioxide molecules.
The Hellas Basin — which is the largest well-preserved impact structure on Mars, spanning more than 2,000 km (1,200 miles) — appears darker than the surroundings because of this effect.
“This is actually not a thermal effect at Hellas,” said Dr. Geronimo Villanueva, a researcher at NASA’s Goddard Space Flight Center.
“The Hellas Basin is a lower altitude, and thus experiences higher air pressure. That higher pressure leads to a suppression of the thermal emission at this particular wavelength range (4.1-4.4 microns) due to an effect called pressure broadening. It will be very interesting to tease apart these competing effects in these data.”
Whereas the new Webb images show differences in brightness integrated over a large number of wavelengths from place to place across Mars at a particular day and time, the near-infrared spectrum of the planet shows the subtle variations in brightness between hundreds of different wavelengths representative of the planet as a whole.
The preliminary analysis of the spectrum shows a rich set of spectral features that contain information about dust, icy clouds, what kind of rocks are on the planet’s surface, and the composition of the atmosphere.
The spectral signatures of water, carbon dioxide, and carbon monoxide are easily detected with Webb.
In the future, the team will be using these imaging and spectroscopic data to explore regional differences across the planet, and to search for trace gases in the atmosphere, including methane and hydrogen chloride.
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