Ep 57 — Fun Case Collection (Part 1): Infrared in Space - JWST and Mars Exploration
Series: Infrared Spectroscopy Encyclopedia: From Principles to Practice
Chapter: Part 6 - Practice and Expansion: From Laboratory to Infinite Possibilities
Target Audience: All series viewers, especially readers interested in astrochemistry, planetary science, and space exploration
Prerequisites: Ep 01–09 (Infrared Basics), Ep 28 (Atmospheric Gas Monitoring), Ep 56 (ftir.fun Tool)
Reading Time: Approximately 30 minutes
Introduction: When Infrared Spectroscopy Leaves Earth
In the previous 56 episodes, our infrared spectrometers have always stayed in laboratories on Earth—benchtop, portable, handheld—but ultimately on the 'ground'. However, the most magnificent application of infrared spectroscopy is actually far away in space, hundreds of millions of kilometers away.
On September 5, 2022, the James Webb Space Telescope (JWST) released the first near-infrared spectrum of Mars [1]. On this spectrum, the absorption features of CO₂, H₂O, and CO are clearly visible—molecules we learned about in Ep 28 on atmospheric monitoring, now 'seen' from over 100 million kilometers away.
'JWST's first observation of Mars yielded high-quality spectral data, allowing us to simultaneously detect key atmospheric components such as CO₂, H₂O, and CO in a single spectrum.' [1]
This is the romance of infrared spectroscopy—the same physical principle can analyze a plastic sample in your hand and 'sniff out' the chemical composition of the Martian atmosphere.
In this episode, we will leave the laboratory and go into space to see how infrared spectroscopy helps humanity explore the universe.
1. James Webb Space Telescope (JWST) and Infrared Spectroscopy
1.1 Why Infrared?
JWST is called an 'infrared telescope' for good reason. There are three core reasons for choosing the infrared band for cosmic observation:
First, cosmic expansion redshift. Light from distant galaxies is stretched in wavelength (redshifted) due to cosmic expansion, so ultraviolet/visible light becomes infrared by the time it reaches Earth. To see the early universe, we must observe in infrared.
'Due to cosmic expansion, ultraviolet light from galaxies formed about 200 million years after the Big Bang can only be observed in the infrared today.' [2]
Second, dust penetration. Interstellar dust is opaque in visible light, but infrared light can penetrate these dust clouds, allowing us to see star-forming regions behind the dust.
Third, molecular fingerprints. As we learned in Ep 05, vibrational-rotational transitions of molecules primarily fall in the infrared region. To detect molecules in the universe (water, CO₂, methane, organics), infrared is the most direct window.
Figure: JWST's golden primary mirror consists of 18 hexagonal segments, optimized for infrared observation. Source: NASA/ESA/CSA [2]
1.2 JWST's Infrared Instruments
JWST carries four core scientific instruments, two of which are directly related to infrared spectroscopy:
| Instrument | Full Name | Wavelength Range | Spectral Capabilities |
|---|---|---|---|
| NIRSpec | Near-Infrared Spectrograph | 0.6–5.3 μm | Multi-object spectroscopy, integral field spectroscopy |
| MIRI | Mid-Infrared Instrument | 5–28 μm | Medium-resolution spectroscopy |
| NIRCam | Near-Infrared Camera | 0.6–5 μm | Imaging (including slitless spectroscopy) |
| NIRISS | Near-Infrared Imager and Slitless Spectrograph | 0.8–5.0 μm | Slitless spectroscopy |
'NIRSpec can simultaneously obtain spectra of up to 100 targets in a single observation, making it JWST's core spectroscopic instrument. MIRI covers the mid-infrared band and is critical for detecting molecular vibrational bands.' [3]
Note the wavelength range: NIRSpec covers the near-infrared (0.6–5.3 μm, i.e., ~1900–16600 cm⁻¹) and MIRI covers the mid-infrared (5–28 μm, i.e., ~360–2000 cm⁻¹), together covering the familiar mid-infrared 'functional group region' and part of the far-infrared region.
2. JWST's First Infrared Spectrum of Mars
2.1 Observation Background
On September 5, 2022, JWST performed its first spectral observation of Mars using NIRSpec [1]. This was a technical challenge—Mars is 'too bright' for JWST and could damage sensitive detectors, so the team used short exposures and special occulting techniques.
Figure: Near-infrared spectrum of Mars obtained by JWST NIRSpec, with absorption bands of CO₂, H₂O, and CO labeled. Source: NASA/ESA/CSA [1]
2.2 Spectrum Analysis
On this spectrum of Mars, three main absorption features are clearly visible [1]:
| Absorption Band | Wavelength | Wavenumber (cm⁻¹) | Assignment | Description |
|---|---|---|---|---|
| CO₂ | ~2.7 μm | ~3700 | CO₂ combination band (NIR; not fundamental asymmetric stretch) | Mars atmosphere is ~95% CO₂; fundamental asymmetric stretch of gas phase is at 4.3 μm (~2349 cm⁻¹) |
| H₂O | ~2.6–3.5 μm | ~2860–3850 | H₂O stretching-related | Atmospheric water vapor + surface ice |
| CO | ~4.6 μm | ~2170 | CO stretching | Photochemical reaction product |
🔗 The infrared features of these molecules are exactly the same as on Earth. You can view the relevant functional group pages on ftir.fun:
- Water molecule: ftir.fun/ir/group/water
- For discussions on adsorbed/gaseous CO, refer to Adsorbed Carbon Monoxide (note that laboratory ATR conditions differ from planetary near-infrared observation); the organic carbonyl page should not be directly equated with gaseous CO.
Why these molecules? Martian atmospheric composition: CO₂ (95.3%), N₂ (2.7%), Ar (1.6%), O₂ (0.15%), CO (0.08%), H₂O (variable) [4]. CO₂ and H₂O have strong absorption bands in the near-infrared, and the CO band at 4.6 μm is also within NIRSpec's coverage.
2.3 Comparison with Earth's Atmosphere
Interestingly, the main component of Mars' atmosphere, CO₂, is exactly one of the most annoying interference sources in infrared spectroscopy experiments on Earth (mentioned in Ep 18 on spectral processing). In the lab, we subtract the absorption of atmospheric CO₂ and water vapor; when JWST observes Mars, these 'interferences' are precisely the target signals we want to detect.
'The beauty of infrared spectroscopy is that the same physical principle can be an 'interference' in the lab but a 'signal' in planetary observation. It all depends on which side you are looking from.' [5]
3. JWST's Planetary Science Observations
3.1 Martian Atmospheric Isotopes: HDO/H₂O
An important scientific goal of JWST is to measure isotopic ratios in the Martian atmosphere, especially the HDO (semi-heavy water)/H₂O ratio [6].
Why care about HDO?
- The D/H ratio is a key indicator for tracing the history of water escape on Mars
- Early Mars likely had abundant liquid water; the D/H ratio can infer the degree of water loss
- HDO has characteristic absorption bands in the infrared (O-D stretch ~2700 cm⁻¹, separate from O-H)
'The D/H ratio in the Martian atmosphere is about 5–6 times that of Earth, indicating that Mars has lost a large amount of water over billions of years. JWST's high-precision spectroscopy can further refine this estimate.' [6]
JWST's GTO (Guaranteed Time Observations) program 1415 is dedicated to Mars observations, led by principal investigator G. Villanueva [6].
Figure: JWST Mars GTO Program 1415 information page. Source: STScI [6]
3.2 Search for Organics
Infrared spectroscopy is a powerful tool for detecting organic matter. As we learned in Ep 05–06, C-H stretching (2800–3000 cm⁻¹), C=O stretching (~1700 cm⁻¹), etc., are the "fingerprints" of organics. JWST's NIRSpec and MIRI precisely cover these bands.
The search for organics on Mars has two implications:
- Prebiotic chemistry: If organics exist on Mars, it indicates that Mars has the chemical basis for the origin of life.
- Biomarkers: Certain organics (e.g., porphyrins, long-chain alkanes) may be products of biological activity.
"JWST's Mid-Infrared Instrument (MIRI) can detect molecular features in the 5–28 μm band, including vibrational bands of organic molecules, providing a new window for the search for organics on Mars."[3]
However, note that JWST observes Mars at a "global scale" with limited spatial resolution. The actual search for organics on the Martian surface is carried out by rovers (see below).
3.3 Exoplanet Atmospheric Chemistry
One of the most exciting applications of JWST is exoplanet atmospheric detection. When an exoplanet transits in front of its host star, starlight passes through the planet's atmosphere, where molecules absorb specific wavelengths, forming a transmission spectrum.
JWST has conducted spectroscopic observations of several exoplanets, detecting:
- Water vapor (H₂O): Detected in the atmospheres of multiple hot Jupiters.
- Carbon dioxide (CO₂): First unambiguous detection on WASP-39b [7].
- Methane (CH₄): In some planetary atmospheres.
- Sulfur dioxide (SO₂): A photochemical product.
"JWST's observations of WASP-39b marked the first unambiguous detection of CO₂ in an exoplanet atmosphere, a milestone in understanding exoplanet atmospheric chemistry."[7]
The principle behind these detections is exactly the same as the infrared spectroscopy in our lab—molecular vibrations absorb infrared light. The only difference is that the light source changes from a silicon carbide rod in the instrument to a star tens of light-years away.
4. Infrared Spectroscopy and Vibrational Spectroscopy Payloads on Mars Rovers
JWST observes from afar, whereas rovers/orbiters observe up close. This section describes both actual infrared payloads and commonly discussed vibrational spectroscopy instruments, to avoid mistakenly referring to Raman as infrared.
4.1 Perseverance: SHERLOC (Raman) and Infrared-Related Payloads
NASA's Perseverance rover carries SHERLOC, a deep UV Raman + fluorescence instrument for in situ detection of organics and minerals [8]. It belongs to the same vibrational spectroscopy family as FTIR (Ep 09), but it is not an infrared spectrometer.
Other payloads like SuperCam provide spectroscopic capabilities closer to mineral/atmospheric observations (including infrared-related channels; details follow mission instrument documentation). Early Mars rovers also used true infrared instruments such as Mini-TES (thermal infrared emission spectroscopy).
Teaching tip: Do not assume "spectroscopy" automatically means mid-infrared FTIR.
4.2 Infrared Spectrometers on Mars Orbiters
CRISM: Onboard MRO, 0.4–4.0 μm, focused on mapping hydrous minerals [9].
TES: Onboard MGS, thermal infrared ~6–50 μm, used for mineral and atmospheric temperature profiles; covers the vibrational region of silicates and other minerals.
Figure: Refer to NASA's CRISM instrument page. Source: NASA [9]
5. Infrared Spectroscopy and the Search for Extraterrestrial Life
5.1 What is a Biosignature?
In astrobiology, a biosignature is a chemical or physical signal that indicates life activity. Potential biosignatures detectable by infrared spectroscopy include:
| Molecule | Infrared Feature | Biological Significance |
|---|---|---|
| O₂ | — | Product of photosynthesis (requires visible/UV, not IR) |
| CH₄ | ~3017 cm⁻¹ (C-H stretch) | Product of anaerobic metabolism |
| H₂O | ~3400 cm⁻¹ (O-H stretch) | Essential solvent for known life |
| N₂O | ~2220 cm⁻¹ (N-O stretch) | Product of denitrifying bacteria |
| Organics | C-H, C=O, etc. | Building blocks of biomolecules |
🔗 View relevant molecules at ftir.fun:
- Water: ftir.fun/ir/group/water
- Organics containing C=O: ftir.fun/ir/group/carbonyl
- Alkyl C-H: ftir.fun/ir/group/alkyl-c-h
5.2 Atmospheric Disequilibrium as a Sign of Life
Life activities create atmospheric chemical disequilibrium. For example, Earth's atmosphere contains both O₂ (produced by photosynthesis) and CH₄ (produced by anaerobic bacteria), which should not coexist stably in thermodynamic equilibrium—unless continuously replenished by life.
Telescopes like JWST use infrared spectroscopy to detect CH₄, H₂O, etc., in exoplanet atmospheres. Combined with visible-light detection of O₂, they can assess whether a planet exhibits "chemical disequilibrium"—an indirect sign of life.
"If O₂ and CH₄ are simultaneously detected in a terrestrial planet's atmosphere with concentrations unexplained by abiotic processes, that would be one of the strongest signs of life."[10]
5.3 The Martian Methane Mystery
Methane (CH₄) in the Martian atmosphere is a long-standing puzzle. Ground-based telescopes and orbital infrared spectrometers have reported detecting trace amounts of methane (~10 ppb), but concentrations vary with time and location [4].
Two possible sources of methane:
- Biological: Produced by subsurface microbial metabolism (e.g., methanogens on Earth).
- Abiological: Serpentinization reactions (water-olivine reaction), UV decomposition of carbonaceous meteorites.
JWST's high-precision infrared spectroscopy may provide new data to solve the methane mystery [6].
6. Infrared Spectroscopy in Astrochemistry
6.1 Interstellar Molecular Clouds
In the interstellar space of our galaxy, there are enormous molecular clouds (e.g., the Orion Nebula). These clouds are cradles of star and planet formation and are among the richest molecular regions in the universe.
Infrared spectroscopy is crucial in studying interstellar molecular clouds:
- Water ice (H₂O ice): 3.05 μm (~3280 cm⁻¹) O-H stretch—detected on dust grains enveloped in a thick layer of water ice in cold clouds.
- CO ice: 4.67 μm (~2140 cm⁻¹)—carbon monoxide ice layer.
- CO₂ ice: 4.27 μm (~2340 cm⁻¹)—the same absorption band as atmospheric CO₂ on Mars.
- Polycyclic aromatic hydrocarbons (PAHs): 3.3, 6.2, 7.7, 8.6, 11.3 μm—aromatic C-H and C=C vibrations.
"More than 200 types of organic molecules have been detected in interstellar molecular clouds, many via infrared spectroscopy. These molecules may be chemical precursors to life."[11]
🔗 The infrared features of these interstellar molecules are identical to those on Earth. Visit ftir.fun/ir/group/water to see the vibrational frequency of water molecules—whether in an Earth lab or in the Orion Nebula, the O-H stretch frequency is ~3400 cm⁻¹.
6.2 Cometary Composition
Comets are "time capsules" from the early solar system. Infrared spectroscopy is a core method for analyzing cometary composition:
- Water (H₂O): Main component of cometary outgassing, strong absorption at 2.7 μm.
- CO₂: 4.27 μm, second most abundant after water.
- CO: 4.67 μm.
- Organics: C-H stretching band (3.3–3.5 μm), including methanol, formaldehyde, etc.
"JWST observations of comet 81P/Wild revealed the relative abundances of H₂O, CO₂, and CO in cometary outgassing, providing key data for understanding solar system formation."[11]
7. From Earth to Space: The Unity of Infrared Spectroscopy
7.1 Same Principle, Different Scales
Reviewing the content of this collection, we find a profound fact: The physical principle of infrared spectroscopy is universal.
| Scenario | Light Source | Sample | Detector | Principle |
|---|---|---|---|---|
| Laboratory ATR | Silicon carbide rod | Plastic powder | DTGS | Molecular vibration absorption |
| Mars orbiter | Reflected sunlight | Martian atmosphere/surface | MCT array | Molecular vibration absorption |
| JWST exoplanet | Host star | Planetary atmosphere | H2RG array | Molecular vibration absorption |
| Interstellar molecular cloud | Background star | Interstellar dust ice layer | Infrared array | Molecular vibration absorption |
Whether the sample is plastic in a lab, Martian surface soil, the atmosphere of a planet tens of light-years away, or interstellar ice grains thousands of light-years away—as long as molecules vibrate, infrared spectroscopy can "see" them.
"The greatness of infrared spectroscopy lies in its universality: the same O-H stretching frequency defines both a glass of water on Earth and a grain of ice dust in the Orion Nebula." [5]
7.2 ftir.fun and Astrochemistry
Interestingly, the functional group frequencies recorded on ftir.fun also apply to molecules in the universe. When you visit ftir.fun/ir/group/water to view the infrared features of water, these data can also help you understand the water vapor absorption bands observed by JWST on Mars—because the laws of physics are consistent throughout the universe.
Episode Summary
| Key Point | Content |
|---|---|
| Why JWST chose infrared | Cosmic redshift, penetrating dust, molecular fingerprints |
| JWST infrared instruments | NIRSpec (0.6–5.3 μm), MIRI (5–28 μm) |
| First infrared spectrum of Mars | CO₂ (2.7 μm), H₂O (2.6–3.5 μm), CO (4.6 μm) |
| Martian isotope study | HDO/H₂O ratio to infer water escape history (GTO Program 1415) |
| Exoplanet atmospheres | JWST first definitively detected CO₂ in WASP-39b |
| Mars rover spectroscopy | Perseverance SHERLOC (UV Raman), orbiter CRISM/TES |
| Biosignatures | Infrared features of CH₄, H₂O, N₂O, organics |
| Interstellar molecular clouds | Infrared detection of water ice, CO ice, CO₂ ice, PAHs |
| Core insight | The principles of infrared spectroscopy are universal, from lab to cosmos |
Discussion Questions
When JWST observes Mars, are the absorption bands of CO₂ and H₂O exactly the same in wavenumber as those you see on a laboratory ATR for the same molecules? Why? What factors need to be considered?
Mars' atmosphere is 95% CO₂, while Earth's atmosphere is only 0.04% CO₂. Yet Mars is much colder than Earth. Explain this "paradox" from the perspective of infrared spectroscopy and the greenhouse effect. (Hint: atmospheric pressure, other greenhouse gases)
If you were an astrobiologist searching for signs of life on exoplanets, which three molecules would you prioritize detecting with infrared spectroscopy? Why is the combination of these three more convincing than a single molecule?
The O-H stretching frequency of water molecules recorded on ftir.fun (~3400 cm⁻¹) and the water vapor absorption frequency observed by JWST on Mars are theoretically the same physical quantity. But in practice, the observation conditions differ vastly. List at least three physical factors that affect the observed frequency.
References
[1] NASA/ESA/CSA. "Mars (Near-Infrared Spectrum)." JWST First Images, 2022.
https://webbtelescope.org/con…
[2] NASA. "The James Webb Space Telescope Mission." JWST Overview.
https://webbtelescope.org/con…
[3] Gardner, J. P. et al. "The James Webb Space Telescope." Space Science Reviews, 2006, 123:485–606.
https://www.springer.com/jour…
[4] NASA. "Mars Fact Sheet." Planetary Sciences.
https://nssdc.gsfc.nasa.gov/p…
[5] ftir.fun project. Infrared Spectroscopy Functional Group Database.
https://ftir.fun
[6] Villanueva, G. et al. "JWST Mars GTO Program 1415." STScI Program Information.
https://www.stsci.edu/jwst/sc…
[7] JWST Transiting Exoplanet Community Early Release Science Team. "Identification of carbon dioxide in an exoplanet atmosphere." Nature, 2023, 614:649–652.
https://www.nature.com/articl…
[8] NASA. "SHERLOC - Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals." Mars 2020 Mission.
https://mars.nasa.gov/mars202…
[9] NASA. "CRISM Instrument." Mars Reconnaissance Orbiter.
https://science.nasa.gov/miss…
[10] Kaltenegger, L. "How to Characterize the Atmosphere of a Transiting Exoplanet." AIP Conference Proceedings, 2017.
https://doi.org/10.1063/1.497…
[11] van Dishoeck, E. F. "Astrochemistry of dust, ice and gas: introduction and overview." Faraday Discussions, 2014, 168:9–47.
https://doi.org/10.1039/C4FD0…
Next Episode Preview: Ep 58 — Fun Case Studies (Part 2): Art Authentication, Archaeological Discoveries, Forensic Stories
Leaving space and returning to Earth, infrared spectroscopy also shines in the humanities and social sciences. From authenticating Renaissance frescoes to unraveling the secrets of ancient Egyptian mummies, to matching paint chips from crime scenes in court—infrared spectroscopy is a bridge between science and humanities. Next time, we'll share some "story-rich" cases.