NIRSpec/PRISM Transmission Spectrum Overview

• NIRSpec/PRISM transmission spectrum is a low-resolution measurement using JWST’s NIRSpec PRISM mode that spans 0.6–5.3 µm to capture diverse molecular and aerosol features in exoplanet atmospheres.
• Advanced calibration and retrieval methodologies mitigate challenges such as detector saturation and red noise, ensuring high-fidelity extraction of transit light curves.
• The broad spectral coverage enables robust detection of key species like H2O, CO2, CH4, and SO2, offering critical insights into atmospheric composition, metallicity, and photochemical processes.

A NIRSpec/PRISM transmission spectrum refers to the measurement of wavelength-dependent planetary transit depths obtained with the low-resolution PRISM mode of the Near Infrared Spectrograph (NIRSpec) instrument aboard NASA's James Webb Space Telescope (JWST). This observing mode provides continuous spectral coverage from the visible through the mid-infrared in a single integration, enabling simultaneous probing of multiple chemical and cloud signatures in planetary and extragalactic atmospheres. The following sections provide a comprehensive review of the instrumental characteristics, retrieval methodologies, major scientific results, and challenges associated with interpreting NIRSpec/PRISM transmission spectra.

1. Instrumental Characteristics and Observing Strategy

The NIRSpec PRISM mode delivers a resolving power that is strongly wavelength-dependent, starting with $R\sim30$ at 1.2 μm and increasing to $R>200$ by 4.3 μm, effectively spanning 0.6–5.3 μm in a single exposure (Shabram et al., 2010, Chapman et al., 2017, Rustamkulov et al., 2022, Sarkar et al., 10 May 2024). The design optimizes the photon-electron yield per spectral resolution element, with overall system throughput determined by the prism transmission (0.81), reflectivity across 14 surfaces (∼0.58), detector quantum efficiency (∼0.75), and telescope reflectivity (∼0.9) (Shabram et al., 2010, Rustamkulov et al., 2022). An efficient integration scheme (e.g., 1800 s, ~33% shorter than the full transit) is typical to maximize S/N while mitigating saturation for bright sources.

Mode selection is critically dependent on target brightness. For very bright hosts (e.g., GJ 436, WASP-39), detector saturation can limit PRISM usability, in which case higher-resolution (R~1000-2700) grating modes (e.g., G395H) may be preferable for the same wavelength region (Shabram et al., 2010, Sarkar et al., 10 May 2024, Carter et al., 18 Jul 2024).

2. Extraction and Calibration Methodologies

Data reduction for NIRSpec/PRISM transmission spectroscopy involves extraction of time-resolved spectra, removal of systematics (e.g., intrapixel sensitivity, PSF motion, and “blooming” from charge overflow), and transit light curve modeling. A combined model such as

$f(t) = f_0 \cdot T(t, \theta) \cdot S(\mathbf{x})$

is often implemented, where $T(t, \theta)$ models the transit, and $S(\mathbf{x})$ encodes instrumental systematics as low-order polynomials in detector position and possibly common-mode corrections (Rustamkulov et al., 2022, Sarkar et al., 10 May 2024). Residual correlated (red) noise is quantified—e.g., through the Allan deviation or scaled standard deviation versus bin size—to ensure photon-limited performance, with demonstrated detector noise floors below 14 ppm ($\lesssim10$ ppm at 1.7σ) after decorrelation (Rustamkulov et al., 2022).

Calibrations must correct for first-group effects, detector nonlinearity, and crosstalk in saturated data. Multiple pipelines (e.g., toteure, Eureka!, ExoTEP, tshirt, JexoPipe) have been benchmarked for spectral fidelity, particularly for the persistently saturated region (typically 0.63–2.06 μm for bright stars) (Sarkar et al., 10 May 2024, Carter et al., 18 Jul 2024). Uniform light curve fitting across datasets (including NIRCam and NIRISS) provides sub-percent-level consistency in system parameters and can reduce inter-instrument offsets to below 150 ppm (Carter et al., 18 Jul 2024).

3. Retrieval of Molecular, Elemental, and Aerosol Properties

The NIRSpec/PRISM mode's broad spectral range is uniquely suited to constraining the abundance of key molecules—H$_2$O, CO, CO$_2$, CH$_4$, HCN, C$_2$H$_2$, SO$_2$—across a diversity of exoplanet atmospheres (Shabram et al., 2010, Chapman et al., 2017, Rustamkulov et al., 2022, Sarkar et al., 10 May 2024, Deka et al., 26 Apr 2025). For example:

• Water (H$_2$O): Strong $1.3$–$1.6\,\mu$m bands; detectable even with restricted coverage.
• Carbon chemistry: CO, CO$_2$, CH$_4$ feature most strongly beyond 2.5 μm (CO$_2$ at 4.3 μm, CO at 4.7–4.8 μm, CH$_4$ around 3.3 μm, HCN/C$_2$H$_2$ at 1.5 and 3.3 μm) (Shabram et al., 2010, Chapman et al., 2017, Rustamkulov et al., 2022).
• Sulfur chemistry: SO$_2$ absorption at 4.05 μm, a robust diagnostic for photochemistry (Rustamkulov et al., 2022, Deka et al., 26 Apr 2025).
• Sodium and Potassium: Na and K doublets (0.589/0.77 μm) in the optical, modulated by cloud opacity (Rustamkulov et al., 2022, Carter et al., 18 Jul 2024).

Retrieval frameworks typically employ either free or equilibrium chemistry. Forward models generate theoretical spectra incorporating molecular cross sections (pressure, temperature, wavelength dependent), and the likelihood is evaluated as

$$P(\theta|D) \propto \exp\left(-\chi^2/2\right) \cdot p(\theta),\quad \chi^2 = \sum_i \left[\frac{d_i-m_i(\theta)}{\sigma_i}\right]^2,$$

where $m_i(\theta)$ are the modeled transit depths and $\mathbf{\theta}$ the parameters (mixing ratios, cloud properties, T-P profile) (Chapman et al., 2017).

Aerosol/cloud properties are retrieved by parameterizing the wavelength dependence of extinction, e.g.,

$$\tau_\lambda = \tau_0 \left(\frac{\lambda}{\lambda_0}\right)^{-\alpha}$$

(Angström law), with additional physically motivated models using Mie theory or empirical log-normal size distributions (Roy-Perez et al., 29 Jan 2025). JWST/PRISM's spectral range allows direct determination of whether extinction sharply decreases (small particles), weakly increases (large particles), or is nearly flat, with substantial impact on inferred molecular abundances.

4. Scientific Results Across Planet Types

Hot Jupiters and Saturns

• WASP-39b: Detection of Na (19σ), H$_2$O (33σ), CO$_2$ (28σ), CO (7σ), and SO$_2$ (2.7σ) robustly established a super-solar metallicity, sub-solar C/O ($\sim$0.7), and active photochemistry leading to SO$_2$ production; C/O and elemental abundances were tightly constrained in hybrid retrievals, with clear disequilibrium between equilibrium and required SO$_2$ levels (Rustamkulov et al., 2022, Sarkar et al., 10 May 2024, Deka et al., 26 Apr 2025).
• TOI-5205b (Jupiter around M4): Pronounced stellar spot contamination muted H$_2$O features but enabled CH$_4$ and H$_2$S detection (statistically robust); atmospheric metallicity $[\mathrm{M/H}] \sim -2$, with super-solar C/O, was at odds with bulk metallicity in the deep interior, suggesting poor mixing (Cañas et al., 10 Feb 2025).

Warm Neptunes and Super-Earths

• GJ 436b (Neptune): Nonequilibrium products HCN, C$_2$H$_2$ produce diagnostic 1.5, 3.3, and 7 μm features; width and amplitude of these bands are direct tracers of vertical mixing ($K_{zz}$) and chemistry regime (Shabram et al., 2010).
• L 98–59c, L 168–9b (Super-Earths, R $<$1.5 R$_\oplus$): Spectra are featureless at $\sim$20–36 ppm precision; primordial H$_2$/He atmospheres and low mean molecular weight envelopes are ruled out at 3σ. Allowable scenarios require high metallicity ($>$300$\times$ solar) or no atmosphere, supporting a sharp cutoff in atmospheric retention as a function of planet size and mass (Scarsdale et al., 11 Sep 2024, Alam et al., 5 Nov 2024).
• Kepler-51d (Super-Puff): Featureless, sloped NIRSpec/PRISM spectrum consistent with low-metallicity, H/He-dominated atmospheres topped by very high altitude (∼1–100 μbar) submicron hazes; alternative ring interpretations are disfavored by the short ring lifetime relative to stellar age (Libby-Roberts et al., 27 May 2025).

5. Diagnostics, Information Content, and Degeneracy Breaking

NIRSpec/PRISM’s continuous 0.6–5.3 μm coverage is essential for breaking degeneracies:

• Molecular Degeneracy: Shorter wavelengths constrain H$_2$O, while the 2.5–5 μm range is required for CO, CO$_2$, CH$_4$ (with up to three orders of magnitude reduction in uncertainty for CO in low-$\mu$ atmospheres) (Chapman et al., 2017, Guzmán-Mesa et al., 2020).
• Cloud/Haze versus Gas: The long baseline enables separation of scattering slopes from genuine molecular bands (β in $$\sigma(\lambda) = \sigma_0 (\lambda/\lambda_0)^{-\beta}$$), quantifying clouds/hazes and their effects on continuum and absorption feature contrast (Chapman et al., 2017, Roy-Perez et al., 29 Jan 2025).
• Photochemical Disequilibrium: Ratios such as $$\frac{X_\mathrm{CO}X_\mathrm{H_2O}}{X_\mathrm{CO_2}}$$ serve as pressure-independent equilibrium diagnostics (Guzmán-Mesa et al., 2020).
• PAH Detection Feasibility: Simulations for WASP-6b show that only NIRSpec/PRISM’s broad coverage can robustly detect PAH absorption features at $\gtrsim 0.1\%$ of ISM abundance, otherwise continuum degeneracies from cloud/haze slopes dominate (Grübel et al., 12 Nov 2024).

Information content analyses, increasingly using machine learning approaches (e.g., random forest regression, feature importance), reveal that mid-IR ($>$2.5 μm) data dominate constraints for key molecular abundance and cloud parameters in 800–1200 K Neptunes and more (Guzmán-Mesa et al., 2020).

6. Challenges and Limitations

Despite high precision, several challenges persist:

• Detector Saturation and Systematics: Bright hosts induce persistent saturation, especially at short wavelengths, with systematic offsets of ≥100 ppm requiring custom correction (e.g., offset of –177 ppm applied to PRISM for WASP-39b) (Carter et al., 18 Jul 2024, Sarkar et al., 10 May 2024). Saturated regions present flattish, biased transit depths; caution is required in their interpretation.
• Correlated Noise: Unmodeled time-correlated (red) noise results in residuals that bin more slowly than $1/\sqrt{N}$, especially in saturated wavelength regions (Sarkar et al., 10 May 2024).
• Cloud Model Degeneracies: Large uncertainties and covariances in retrieved gas abundances arise if the cloud extinction law is inadequately or incorrectly parameterized; up to orders-of-magnitude changes in VMRs result from different cloud assumptions (Roy-Perez et al., 29 Jan 2025).
• Stellar Contamination: Unocculted starspots (especially for planets transiting active, cool hosts) can mimic or mask short-wavelength features, necessitating joint modeling of stellar photosphere/spot spectra (Cañas et al., 10 Feb 2025).
• Interpretation at Low S/N: For super-Earths with featureless spectra, even 20–36 ppm precision may be insufficient to distinguish a high-$\mu$ secondary atmosphere from no atmosphere at all without complementary constraints or further observations (Scarsdale et al., 11 Sep 2024, Alam et al., 5 Nov 2024).

7. Scientific and Methodological Impact

The NIRSpec/PRISM transmission spectrum constitutes a transformative advance in comparative atmospheric and cloud studies:

• Distinctive Molecular, Elemental, and Photochemical Diagnostics: It enables simultaneous measurement of water, carbon, sulfur, and alkali carriers crucial for determining atmospheric metallicity, C/O ratio, and the activity of photochemistry (e.g., SO$_2$), inaccessible in previous space-based datasets (Rustamkulov et al., 2022, Deka et al., 26 Apr 2025).
• Aerosol/Cloud Microphysics: JWST’s spectral reach enables, for the first time, robust inference of cloud extinction wavelength dependencies, particle size distributions, and composition—essential for reliable molecular retrievals (Roy-Perez et al., 29 Jan 2025).
• Data-Model Synergy: Integrated retrieval frameworks (e.g., NEXOTRANS) blending Bayesian and machine learning methods, with flexible chemistry and cloud models, are necessary to fully exploit the JWST PRISM data’s potential while rigorously quantifying uncertainties and degeneracies (Deka et al., 26 Apr 2025).
• Exoplanet Population Insights: The wide applicability across planet masses (from super-Earths to hot Jupiters) and the ability to test hypotheses about atmospheric retention, interior–envelope mixing, and haze/ring physics establish the NIRSpec/PRISM transmission spectrum as a benchmark for exoplanet atmospheric studies (Scarsdale et al., 11 Sep 2024, Libby-Roberts et al., 27 May 2025).

In summary, the NIRSpec/PRISM transmission spectrum is the foundational observational data product for JWST exoplanet spectroscopy, providing direct, multi-wavelength constraints on composition, aerosols, and disequilibrium processes. Its full scientific utility is realized only through careful data calibration, advanced retrieval methodologies, and physically motivated interpretations of both gaseous and solid-state atmospheric constituents.