The Origin of Water - and Other Pre-biotic Molecules - in Planetary Systems

Observations from ground and space-based telescopes have shown through infrared spectroscopy that in cold, dense regions of space key pre-biotic molecules, such as water (H₂O), carbon dioxide (CO₂), carbon monoxide (CO), methanol (CH₃OH), the nitrogen- bearing molecule ammonia (NH₃) plus various carbon-nitrogen stretch molecules (XCN), and the important sulfur-bearing molecule, carbonyl sulfide (OCS), are locked into ice in variable, but large amounts.  In dense interstellar clouds and in the cold midplanes of planet-forming [protoplanetary] disks, it is thought that 90-to-99% of these and similar molecules are found in icy mantles on the surface of interstellar dust grains. It is clear that ices play a major role in planetesimal formation within disks, and that ices are the source of water and organic molecules for newly forming planets. However, basic questions remain unanswered. For example, how do ices evolve from translucent to dense molecular clouds and, ultimately, protoplanetary disks? Are the ices within protoplanetary disks inherited from the interstellar medium, or are they instead altered significantly within disks? SPHEREx will address these and similar questions, by probing the abundances and composition of ices in all phases of star and planet formation.

SPHEREx will study interstellar and circumstellar ices through the use of absorption spectroscopy [Figure 2] at infrared wavelengths, where the ices have their unique spectral signatures.  Prior to the advent of JWST, discussed further below, there were only some 200 absorption spectra of ices available, obtained by the previous space missions ISO, Spitzer, and AKARI, severely limiting our ability to understand the formation and evolution of cosmic ices.    SPHEREx will increase the number of such spectra manifold, by surveying young stellar systems of all kinds (young stellar objects, protoplanetary disks, and molecular clouds; see Figure 1) throughout our Milky Way Galaxy. 

SPHEREx will observe the entire Milky Way plus the Large and Small Magellanic Clouds – neighboring dwarf galaxies - every six months. We estimate that SPHEREx will generate high quality spectra for a transformative 8 – 9 million sources.  This estimate is based on the number of point sources catalogued within ± 15 degree of the Galactic plane by NASA’s Wide-field Infrared Survey Explorer (WISE) mission that meet the selection criteria outlined by Matt Ashby and Joe Hora (CfA) in a recent publication which describes the SPHEREx target List of ICE Sources (SPLICES) for the community [see Ashby, M. et al, 2023, ApJ, 949, 105]. It is these select sources that collectively represent all phases of molecular cloud evolution, plus the central protostars in protoplanetary disks, that will serve as the ‘background candles’ against which SPHEREx’s absorption spectroscopy will be performed, as illustrated in Figure 2.

Figure 2. ISO spectrum toward the Galactic star forming region W33A obtained with a spectral resolving power (λ/∆λ) greater than 700 showing key ice absorption features. The SPHEREx spectral resolving powers, R, shown within each band, were selected to permit the depth of each ice feature to be accurately determined with no significant line blending.

The recently launched James Webb Space Telescope (JWST) is already making major strides in the study of ices in particular sources and regions.  JWST is much more sensitive than previous infrared missions and has spectrographic instruments covering ~1 to ~25 µm with much higher spectral acuity than has been available in the past.  These advances are illustrated in Figure 3, where the black line shows JWST’s spectrum of a star buried behind a large cloud of interstellar dust.  The absorptions due to water ice, CO, and CO₂ stand out in this spectrum, which covers a portion of the SPHEREx wavelength band, and weaker features due to other molecules are seen as well.    The red line is a simulated SPHEREx spectrum, including the SPHEREx spectral resolving powers, of the same line of sight assuming a background source brighter than the one observed by JWST. It illustrates that SPHEREx’s spectral resolution will permit us to readily identify and spectrally isolate the prominent ice features seen by JWST.  

We expect that JWST will obtain a few thousand spectra which show ice features such as those illustrated in Figure 3.  These detailed studies will provide new insights into the nature of interstellar ices and are already informing the approaches which will be taken to analyzing the SPHEREx data. SPHEREx’s survey will, in turn, trace out the distribution of interstellar and circumstellar ices throughout the Milky Way.  This will allow us to understand for the first time, in a statistically significant way, how ice content correlates with cloud density, internal temperature, presence or absence of embedded sources, external UV and X-ray radiation, elemental abundances (e.g., C/O ratio), gas-phase composition, and cosmic-ray ionization rate.  We can also see how the ice content evolves with time as material passes from one evolutionary stage to another, as illustrated in Figure 1. Thus, JWST and SPHEREx provide complementary capabilities for the study of interstellar ices.

Figure 3. The black line is the JWST spectrum of a source seen through a thick molecular cloud of interstellar dust, showing the strong features of the interstellar ice species H2O, CO2, and CO at wavelengths of 3.05, 4.27, and 4.67 microns (McClure et al. 2023, Nature Astronomy, 7, 431). Also shown are weaker bands due to less abundant ices and other diagnostic features. Overlaid in red is a simulated spectrum, taken with SPHEREx’s lower spectral resolving power, of a background source with 100x the JWST brightness in the SPHEREx range that shows the same absorption features as seen by JWST. Note that SPHERE reproduces almost all of the spectral structure apparent in the JWST spectrum.