Mars Is Mighty in First Webb Observations of Red Planet

The James Webb Space Telescope captured its first images and spectra of Mars on 5 September 2022. The telescope, an international collaboration between NASA, ESA and the Canadian Space Agency, provides a unique perspective with its infrared sensitivity on our neighbouring planet, complementing data being collected by orbiters, rovers, and other telescopes.

Webb’s unique observation post nearly 1.5 million kilometres away at the Sun-Earth Lagrange point 2 (L2) provides a view of Mars’ observable disk (the portion of the sunlit side that is facing the telescope). As a result, Webb can capture images and spectra with the spectral resolution needed to study short-term phenomena like dust storms, weather patterns, seasonal changes, and, in a single observation, processes that occur at different times (daytime, sunset, and nighttime) of a Martian day.

Because it is so close, the Red Planet is one of the brightest objects in the night sky in terms of both visible light (which human eyes can see) 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.” 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.

Webb’s first images of Mars, captured by the Near-Infrared Camera (NIRCam), show a region of the planet’s eastern hemisphere at two different wavelengths, or colours of infrared light. This image shows a surface reference map from NASA and the Mars Orbiter Laser Altimeter (MOLA) on the left, with the two Webb NIRCam instrument field of views overlaid. The near-infrared images from Webb are shown on the right.

The NIRCam shorter-wavelength (2.1 microns) image [top right] is dominated by reflected sunlight, and thus reveals surface details similar to those apparent in visible-light images [left]. The rings of the 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 [lower right] 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 (CO2) molecules. The Hellas Basin – which is the largest well-preserved impact structure on Mars, spanning more than 2,000 kilometres – appears darker than the surroundings because of this effect.

In the future, Webb will be using this imaging and spectroscopic data to explore regional differences across the planet, and to search for trace species in the atmosphere, including methane and hydrogen chloride.

These observations of Mars were conducted as part of Webb’s Cycle 1 Guaranteed Time Observation (GTO) Solar System program led by Heidi Hammel of AURA.

ESA operates two Mars orbiters, Mars Express and the ExoMars Trace Gas Orbiter, that have brought a treasury of insight into the Red Planet’s atmosphere and surface. Furthermore, ESA collaborates with the Japanese Aerospace Exploration Agency (JAXA) on the Martian Moons eXploration (MMX) mission, soon to launch for Mars’ moon Phobos.

Note: This post highlights images from Webb science in progress, which has not yet been through the peer-review process.

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