The Origin of Water in Planetary Systems

Water Ice and Biogenic Molecules

SPHEREx will be a game changer in resolving long-standing questions about the amount and evolution of key biogenic molecules (H2O, CO, CO2, and CH3OH) throughout all phases of star and planetary formation.

Early stages of planet formation

Fig. 1: Early stages of star and planet formation.

From Molecular Clouds To Protoplanetary Disks

In the early stages of star and planet formation (figure at left), the vast majority (99%) of molecules like water, carbon monoxide, and other volatile species are thought to exist in the solid phase, as ices.  So in order to measure the amounts of these molecules in young systems, and to answer basic questions about whereand when they formed, they need to be detected via absorption spectroscopy.  In this approach, nascent systems are seen in projection against bright background stars, and the ices generate absorptions (dips) in their spectra (figure below) from which their relative abundances can be measured.

Ices Spectra

Presently, there are only some 200 absorption spectra of ices in existence, hampering our ability to understand their formation and evolution.  Fortunately, SPHEREx will increase the number of such spectra by orders of magnitude, by surveying young stellar systems of all kinds (young stellar objects, protoplanetary disks, and molecular clouds) throughout the Milky Way. Thus, we will 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.

SPHEREx MIDEX Ice Recovery

Fig. 2: Simulated SPHEREx observations accurately reproduce synthetic input ice spectra. Left panel: A synthetic SPHEREx spectrum (dashed-orange line) of a K5 star seen through high extinction (Av=14) including simulated ice absorption features. Red bars indicate the corresponding end-of-ission Nyquist-sampled SPHEREx spectra generated by the simulator, including noise (measurement residuals in middle-left panel). We fitted the simulated SPHEREx ice features to estimate optical depths and column densities, creating the recovered spectrum (green), which agrees with the input spectrum to within 10%. Middle and Right panels: each input column density for both H2O and CO2 ice was simulated 100 times accounting for variation due to noise. The recovered column densities (purple, with 1-sigma uncertainties) are compared to the input column densities for sources seen by SPHEREx with SNRs of 100 on the continua. Fractional differences between the recovered and input column densities are shown in the bottom panels.