Identifying Water Ice in the Asteroid Belt with JWST

 

1.    Introduction

Asteroids are remnants which provide insight into the chemical and dynamical history of the early solar system. The origin and evolution of water in our solar system is of interest in planetary science, exoplanetary studies, astrobiology, and cosmochemistry, as well as to the general public, as water on Earth allowed for evolution of life. When observing in the near-infrared (NIR), hydration is indicated by a feature around 3.0 𝜇m [1]. It is difficult to identify hydration on most Main-Belt asteroids (MBAs) from ground-based telescopes, as telluric atmospheric water makes this wavelength region nearly opaque.

Two asteroid types, primitive carbonaceous C-types, and siliceous S-types, comprise ~90% of all asteroid taxonomies. C- and S-type asteroids formed in distinct regions of the solar system, where S-types formed closer to the Sun and experienced more heating, and C-types formed farther out [2]. Due to these formation locations, if C-types accreted water, their locations beyond the frost line would have allowed them to retain water ice [3,4,5]. In this work, we use JWST to observe MBAs at 3.0 𝜇m, giving us a glimpse into the current distribution of water in the Main Asteroid Belt.     

 

2.    Methods

2.1    Data Acquisition

The data for this work was obtained in JWST Cycle 1 pure parallel program #2211 (PI: Trilling) that was designed to obtain NIRCam (Near-Infrared Camera) data alongside pointings from various other programs within 15° of the ecliptic. These serendipitously detected asteroids were observed in three filters (1.5, 2.0, and 2.7-𝜇m) over approximately 40 hours of observing time. The JWST calibration pipeline flags MBAs as spurious with respect to the primary target, removing them from the final science image. However, because the data are collected in a read-up-the-ramp (RUTR) method, we can detect asteroids in locations where there is a jump in detected flux. Pulling the pixels with a jump flag, we created a binary image (Figure 1), where the streaks across the image are the asteroids captured. This allows us to measure the asteroid brightness at the correct time for each pixel.

 

Figure 1  A portion of a binary image created using the jump detection flags. The yellow represents all the pixels with detected jumps in flux. The green ellipses highlight the asteroids caught in the image, appearing as streaks as they move across the FOV.

 

2.2    Photometry and Colors

We tested our asteroid detection methods using MIRAGE¹ (Multi-Instrument RAmp GEnerator) simulated JWST data and implanted asteroids. We will also use MIRAGE data to determine how to best extract the asteroid photometric information. Several methods will be tested for estimating the asteroid fluxes, and the most accurate method(s) will be used on all MBAs captured. Determining the [F150W]–[F200W] color distinguishes between S-type and C-type asteroids, and the [F200W]–[F277W]  identifies if a 3.0 𝜇m hydration feature is present (Figure 2, 3).

Figure 2  A diagram of the color–color analysis. The x-axis is the [F150W]–[F200W] color, which identifies the asteroid type. The y-axis is the [F200W]–[F277W] color, which determines hydration. C-types are in blue and S-types in red. Small diamonds represent an asteroid with no 3.0 𝜇m feature and large diamonds represent an asteroid with a large 3.0 𝜇m feature. The yellow dot is the solar color.

 

Figure 3  The reflectance in the NIR for synthetic S-type (red) and C-type (blue) asteroids [6] extrapolated with meteorite spectra with (solid) and without (dashed) hydration features [7]. The shaded gray regions are the bandpasses for the 1.5, 2.0, and 2.7–𝜇m filters used. The cyan regions are the double-wide NIRCam filters, which are too wide to be used here. The orange curves show the thermal emission for objects at 250K (thick) and 200K (thin).  

 

3.    Expected Results

Our methods have been able to capture ~20 MBAs per pointing. We expect that >60% of the C-type asteroids will have hydration features [8], while a small percentage of S-type asteroids will have these features [9]. The spectral feature at 3.0 𝜇m is caused by a vibrational frequency of the hydrogen–oxygen bond, therefore it can be from water ice, hydrated minerals, OH from solar wind, or a combination. We make the assumption that hydration features identified on S-types are likely a result of solar wind implantation [9]. To determine the origin of hydration features on C-types, we will use detailed 3.0 𝜇m spectra of asteroids with known hydration features as a proxy for comparison. Performing a color–color analysis of these asteroids will allow us to determine which of our C-types contain water ice. 

 

4.    Scientific Implications 

JWST is successful at examining the 3.0 𝜇m region of MBAs. The limiting magnitude of JWST can observe MBAs of smaller sizes than ever before (d~100m), therefore many of the asteroids captured here will be new discoveries. This work will allow us to understand the distribution of water in the Main Belt, and the distribution of hydration among C-type and S-type asteroids. Since the Main Belt is the primary reservoir from which objects enter near-Earth space, the presence of water ice and hydrated minerals on MBAs supports that Earth’s water was delivered via an impact, subsequently giving rise to life.

 

 

References

[1] Baratta et al. 1991, Astronomy & Astrophysics, 252, 421

 [2] Gradie & Tedesco 1982, Science, 216, 1405

 [3] Takir & Emery 2012, Icarus, 219, 641

 [4] Rivkin et al. 2015, Asteroids IV

 [5] Rivkin et al. 2022, PSJ, 3, 153

 [6] DeMeo et al. 2009, Icarus, 202, 160

 [7] Takir et al. 2013, MPS, 48, 1618

 [8] Rivkin 2012, Icarus, 221, 744

 [9] McGraw et al. 2022, PSJ, 3, 243

 

  ¹  https://mirage-data-simulator.readthedocs.io/en/latest/index.html