JWST/NIRSpec PRISM Transmission Spectra

• JWST/NIRSpec PRISM Transmission Spectra are low- to moderate-resolution observations spanning 0.6–5.3 μm that enable comprehensive exoplanet and galaxy studies.
• They leverage high photometric sensitivity and innovative noise mitigation techniques to support both faint-object detection and multiplexed surveys.
• Advanced Bayesian retrieval frameworks applied to these spectra yield precise molecular abundances and aerosol diagnostics, enhancing our understanding of exoplanet atmospheres.

JWST/NIRSpec PRISM Transmission Spectra are low- to moderate-resolution astronomical spectra collected using the Near-Infrared Spectrograph (NIRSpec) aboard the James Webb Space Telescope (JWST) in its PRISM disperser mode. This mode provides simultaneous coverage from 0.6 μm to 5.3 μm and is pivotal for exoplanet atmospheric transmission spectroscopy, high-redshift galaxy studies, and other astrophysical applications. The following sections summarize the key scientific, technical, and methodological aspects of JWST/NIRSpec PRISM transmission spectra as demonstrated by the principal literature.

1. Instrumentation and Observational Capabilities

The NIRSpec instrument’s PRISM mode covers 0.6–5.3 μm at resolving powers ranging from R ≈ 30 near the blue end to ≈330 at the red end (Jakobsen et al., 2022, Rustamkulov et al., 2022, Carter et al., 18 Jul 2024, Bunker et al., 2023). Its dispersion element is a double-pass CaF₂ prism, with non-uniform dispersion yielding minimal order overlap and spectra spanning ≈400 detector pixels. This broad spectral grasp is enabled by an all-reflective optical chain (except for the prism and some filters), providing high optical throughput, exceptional thermal stability at cryogenic temperatures, and minimized scattered light.

Key features of the PRISM mode:

• Broad spectral coverage: Enables detection of features from numerous atomic and molecular species (e.g., Na, K, H₂O, CH₄, CO₂, CO, SO₂), provides access to several strong and diagnostic bands for exoplanet atmospheres and galaxy redshift surveys (Rustamkulov et al., 2022, Shabram et al., 2010, Roy-Perez et al., 29 Jan 2025).
• Photometric sensitivity: Achieves faint-object detection limits (e.g., 132 nJy at 3 μm for R = 100 over 10,000 s) due to high photon conversion efficiency (>50%) and minimal reflection losses (Jakobsen et al., 2022, Bunker et al., 2023).
• Slit and MSA utilization: Up to four nonoverlapping PRISM spectra can be arranged on the detectors with the Micro-Shutter Array (MSA) or fixed slits, supporting deep, multiplexed spectroscopic surveys (Bunker et al., 2023).
• Noise and systematics: Features such as cosmic ray mitigation (up-the-ramp readout), low instrument noise, and correction for drift, jitter, and intrapixel sensitivity make high-precision time-series observations feasible (Rustamkulov et al., 2022).

2. Spectral Retrieval and Atmospheric Diagnostics

Comprehensive spectral retrievals leverage the PRISM’s wavelength range to infer atmospheric composition—quantifying molecular abundances, thermal profiles, and aerosol properties:

• Physical modeling: The transmission radius as a function of wavelength is given by

$\mathrm{d}R_{p}/\mathrm{d}\ln\lambda = \alpha(kT/\mu g) \label{eq:analytic-slope}$

where $\alpha$ characterizes the wavelength dependence of absorption, $H = kT/\mu g$ is the scale height, $T$ is temperature, $\mu$ the mean molecular weight, and $g$ gravity. This relation directly links observed slopes to atmospheric properties (Shabram et al., 2010).

• Vertical mixing and nonequilibrium effects: Models capture non-equilibrium products, notably HCN and C₂H₂, produced from methane photolysis and vertical transport, with adopted mixing ratios (e.g., 10⁻⁴ for HCN, 10⁻⁵ for C₂H₂) measurably affecting features at 1.5, 3.3, and 7 μm (Shabram et al., 2010). The widths of these features diagnose relative abundances and probe disequilibrium chemistry.
• Full Bayesian retrieval frameworks (e.g., nested sampling, Markov Chain Monte Carlo) model the forward problem as

$$L(\theta) \propto \exp\left[-\frac{1}{2}\sum_\lambda \frac{(d(\lambda)-F(\lambda, \theta))^2}{\sigma^2(\lambda)}\right]$$

with $\theta$ the vector of atmospheric parameters and $d(\lambda)$ the observed data (Chapman et al., 2017). High-performance retrievals, as implemented in tools such as BEAGLE, POSEIDON, and petitRADTRANS, are used for both exoplanet and galaxy spectra (Chevallard et al., 2017, Grübel et al., 12 Nov 2024).

The PRISM’s sensitivity to key bands gives order-of-magnitude improvements in abundance constraints for CO, CO₂, and CH₄ when the spectral window is extended beyond 2.5 μm, compared to shorter-wavelength instruments (Chapman et al., 2017).

3. Empirical Results: Exoplanet Atmospheres

NIRSpec PRISM spectra have yielded decisive insights into a variety of exoplanet atmospheres:

• Hot Neptunes and Jupiters: For GJ 436b, nonequilibrium hydrocarbon products generate broad absorption features used to constrain vertical mixing rates and atmospheric metallicity (Shabram et al., 2010). For WASP-39b, robust detections include Na (19σ), H₂O (33σ), CO₂ (28σ), CO (70–75σ), with a notable SO₂ feature likely tracing photochemistry (Rustamkulov et al., 2022). The absence of CH₄ and strong CO₂ point to super-solar metallicity and C/O ratios near 0.7.
• Low-metallicity and super-puff planets: In Kepler-51d, the spectrum is dominated by a sloped continuum with no strong molecular features, implying a high-altitude haze layer of submicron particles in a low-metallicity H/He-dominated atmosphere (Libby-Roberts et al., 27 May 2025).
• Clouds and aerosols: Broad-band PRISM spectra enable discrimination between different aerosol extinction laws. For WASP-39b, retrievals favor an extinction law weakly increasing or sharply decreasing with wavelength—non-grey cloud models are decisively supported, influencing gas abundance retrievals (Roy-Perez et al., 29 Jan 2025). The Angström exponent $\alpha$ serves as a quantitative proxy for aerosol particle size distribution and wavelength dependence.
• Stellar contamination in M dwarfs: Transmission spectra of planets such as TOI-5205b and TRAPPIST-1e are strongly contaminated by unocculted starspots and other photospheric heterogeneity, causing elevated transit depths at blue wavelengths and introducing degeneracies with atmospheric retrievals (Cañas et al., 10 Feb 2025, Espinoza et al., 5 Sep 2025). For TRAPPIST-1e, Gaussian Process marginalization on the logarithm of transit depths allows robust ruling out of cloudy H₂-dominated atmospheres at >3σ significance (Espinoza et al., 5 Sep 2025).

4. Galaxy Surveys, Redshifts, and Physical Properties

The PRISM mode is extensively utilized for deep galaxy surveys and high-redshift studies:

• Multi-object spectroscopy: Deep integrations (e.g., 28 hr in JADES) allow emission line detection down to ≈10⁻¹⁹ erg cm⁻² s⁻¹ in galaxies as faint as AB=29, yielding robust spectroscopic redshifts (confirmed up to z=13.2) and line diagnostics (Hβ, [O III], Lyα, etc.) (Bunker et al., 2023).
• Physical parameter recovery: Full-spectrum fitting, as performed with BEAGLE, recovers star formation rates, gas-phase metallicity, ionization parameter, and dust attenuation, generally within a factor of 1.5 for SFRs and metallicity, 2 for mass-to-light ratios, and 3 for galaxy ages (Chevallard et al., 2017).
• Contribution to cosmic reionization: Detection and quantification of physical conditions in faint, high-redshift galaxies enable assessment of their role in cosmic reionization, specifically through measurements of the production efficiency of ionizing photons.

5. Noise Properties, Systematics, and Data Analysis

PRISM spectra are characterized by low instrumental noise floors (≤14 ppm at 3σ; consistent with <10 ppm at 1.7σ), as verified in bright object time-series laboratory analogs (Rustamkulov et al., 2022).

• Systematic noise sources: Intrapixel sensitivity variations, small PSF shifts, 1/f detector noise, and correlated noise in saturated regions are identified as principal sources. Polynomial detrending and simultaneous systematics–transit model fitting are highly effective at suppressing these contributions.
• Partial detector saturation: For bright targets (WASP-39b), partial saturation in the blue and near-IR (0.63–2.06 μm) induces a “diluted” transit depth, requiring corrections (e.g., −177 ppm dilution) and urging caution or preference for higher spectral resolution modes in severely affected regions (Carter et al., 18 Jul 2024, Sarkar et al., 10 May 2024).
• Data processing frameworks: Advanced pipelines (e.g., JexoPipe) address saturation handling, bad pixel correction, “flipping noise” mitigation due to group read count variations, and optimal extraction schemes to ensure robust transmission spectra (Sarkar et al., 10 May 2024).

6. Applications, Limitations, and Future Directions

NIRSpec PRISM’s wide spectral range and sensitivity are leveraged in diverse science cases:

• Chemical and elemental abundance retrievals: Broad coverage captures key absorbers (CO, CO₂, CH₄, H₂O, SO₂, HCN, PAHs), supports precision abundance constraints, and enables comparisons to planetary formation models (e.g., super-solar metallicity, C/O, S/H).
• Cloud/aerosol diagnostics: JWST/PRISM data now resolve non-grey, physically motivated aerosol extinction laws, allowing quantification of cloud impacts on gas retrievals and assessment of haze and cloud stratification in super-puff and hot gas giant atmospheres (Roy-Perez et al., 29 Jan 2025, Libby-Roberts et al., 27 May 2025).
• Brown dwarf weather: High S/N time-resolved PRISM spectra combined with clustering algorithms elucidate vertical structure (pressure levels), variability morphology changes, and the presence of small grain silicate clouds in brown dwarfs (Biller et al., 12 Jul 2024).
• Detection thresholds for biosignatures and photochemical tracers: Simulated and real data demonstrate the potential and challenges of detecting biomarkers (CH₄, O₃, N₂O) and photochemical species (SO₂) in terrestrial and giant exoplanets—even in challenging, contamination-limited regimes (Lin et al., 2021, Espinoza et al., 5 Sep 2025).
• Degeneracies and contamination: Stellar heterogeneity in M dwarfs, incomplete aerosol modeling, or insufficient spectral resolution can induce biases or degenerate solutions—necessitating advanced statistical approaches (e.g., Gaussian Process marginalization) and physically realistic extinction models to decouple stellar, planetary, and cloud signals.

Future developments will refine contamination modeling, extend to longer wavelengths (e.g., MIRI synergy), and expand to larger statistical samples with multi-epoch monitoring and deeper integrations. JWST/NIRSpec PRISM thus provides the backbone for detailed inventory and structural analysis of cold and hot exoplanet atmospheres, brown dwarf weather, and the high-redshift galaxy population, substantially advancing the state of observational astrophysics.