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We present a JWST/NIRCam transmission spectrum from $3.9-5.0$ $\mu$m of therecently-validated sub-Earth GJ 341b ($\mathrm{R_P} = 0.92$$\mathrm{R_{\oplus}}$, $\mathrm{T_{eq}} = 540$ K) orbiting a nearby bright M1star ($\mathrm{d} = 10.4$ pc, $\mathrm{K_{mag}}=5.6$). We use three independentpipelines to reduce the data from the three JWST visits and perform severaltests to check for the significance of an atmosphere. Overall, our analysisdoes not uncover evidence of an atmosphere. Our null hypothesis tests find thatnone of our pipelines' transmission spectra can rule out a flat line, althoughthere is weak evidence for a Gaussian feature in two spectra from differentpipelines (at 2.3 and $2.9\sigma$). However, the candidate features are seen atdifferent wavelengths (4.3 $\mu$m vs 4.7 $\mu$m), and our retrieval analysisfinds that different gas species can explain these features in the tworeductions (CO$_2$ at $3.1\sigma$ compared to O$_3$ at $2.9\sigma$), suggestingthat they are not real astrophysical signals. Our forward model analysis rulesout a low mean molecular weight atmosphere ($< 350\times$ solar metallicity) toat least $3\sigma$, and disfavors CH$_4$-dominated atmospheres at $1-3\sigma$,depending on the reduction. Instead, the forward models find our transmissionspectra are consistent with no atmosphere, a hazy atmosphere, or an atmospherecontaining a species that does not have prominent molecular bands across theNIRCam/F444W bandpass, such as a water-dominated atmosphere. Our resultsdemonstrate the unequivocal need for two or more transit observations analyzedwith multiple reduction pipelines, alongside rigorous statistical tests, todetermine the robustness of molecular detections for small exoplanetatmospheres.

JWST/NIRCam Transmission Spectroscopy of the Nearby Sub-Earth GJ 341b