James Webb's Observations on Red Planet

On September 5, 2022, the first images of Mars, captured by James Webb’s Near-Infrared Camera (NIRCam), revealed reflective and thermal properties of the planet with sufficient sensitivity and resolution to explore localized occurrences. 

Webb’s unique observation post nearly a million miles away at the Sun-Earth Lagrange point 2 which is known as L2, provides a view of Mars’ observable disk (the portion of the sunlit side that is facing the telescope). As a result of that, James Webb can capture clearest 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 such as 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 and infrared light that James Webb is designed to detect. This causes special challenges for 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.

Image Credits: NASA, ESA, CSA, STScI, Heidi Hammel (AURA), Mars JWST/GTO Team

A reference map of Mars from NASA and the Mars Orbiter Laser Altimeter(MOLA) shows the view from James Webb, with the orientation and lighting of the planet at the day and time of the observation. On September 5, 2022, during summer in Mars’ southern hemisphere, the observation was made. The central longitude is approximately 80 degrees east. The axis is tilted 25 degrees from perpendicular to the orbital plane. The eastern section of the disc is in the evening shadow. The map shows geographic features and surface coloring typically visible in reflected sunlight. Three features are labeled: Syrtis Major, a dark-colored volcanic region; the Huygens Crater, a complex impact crater; and the Hellas Basin, the largest preserved impact structure on Mars. 

Image Credits: NASA, ESA, CSA, STScI, Heidi Hammel (AURA), Mars JWST/GTO Team

A Near-Infrared Camera image shows 2.1-micron (F212 filter) reflected sunlight, revealing surface features similar to those seen in the base map, including the dark volcanic rocks of Syrtis Major, light rings of the Huygens Crater, and a section of the Hellas Basin, which may be coated in a layer of light-colored dust. The field of view is outlined in blue on the base map. 

Image Credits: NASA, ESA, CSA, STScI, Heidi Hammel (AURA), Mars JWST/GTO Team

Another NIRCam image captured simultaneously, shows ~4.3-micron (F430M filter) discharged light that reveals temperature differences with latitude and time of day, as well as darkening caused by atmospheric effects. The measured brightness is highest near the subsolar region. It is shown in the above image with yellow color. It decreases noticeably toward the northern hemisphere which is experiencing winter and toward the western hemisphere which is in night-time darkness. However, the measured brightness of the Hellas Basin is less than its surroundings not because it has a lower temperature, but rather because it is deeper and experiences higher air pressure. It is more than 4 miles, or 7 kilometers, below the surrounding terrain. This dampens the thermal emission at this wavelength: Some of the 4.3-micron light emitted from the basin is absorbed by molecules like carbon dioxide in Mars’ atmosphere.

Mars Atmosphere Composition
Image Credits: NASA, ESA, CSA, STScI, Heidi Hammel (AURA), Mars JWST/GTO Team

The spectrum shows a combination of sunlight reflected from Mars’ surface and atmosphere, and light emitted by the planet as it gives off heat. Wavelengths between 1 and 3 microns are dominated by reflected light. Wavelengths between 3 and 5 microns are dominated by emitted light. Both lights pass through Mars’ atmosphere, affecting the brightness of various wavelengths and the shape of the spectrum in various ways.

The deep valleys are absorption features caused when specific wavelengths are blocked by gases such as carbon dioxide, water, and carbon monoxide. Other details, like the broad shape of the spectrum and the slope of the curve at different points, reveal information about dust, clouds, and surface features. 

The data were collected using six different high-resolution grating modes (spectroscopy modes), each of which covers a different wavelength range. The white line is not continuous because there are small gaps in coverage. The best-fit model takes into account the data as well as other known properties of Mars. Constructing a best-fit model using a tool such as the Planetary Spectrum Generator makes it possible to estimate the abundance of given molecules in the atmosphere.