NICMOS Photometry of the Unusual Dwarf Planet Haumea and its Satellites

We present here HST NICMOS F110W and F160W observations of Haumea, and its two satellites Hi’iaka and Namaka. From the measured (F110W-F160W) colours of -1.209 +/-0.004, -1.48 +/- 0.06, and -1.4 +/- 0.2 mag for each object, respectively, we infer that the 1.6 imcron water-ice absorption feature depths on Hi’iaka and Namaka are at least as deep as that of Haumea. The light-curve of Haumea is detected in both filters, and we find that the infrared colour is bluer by approximately 2-3% at the phase of the red spot. These observations suggest that the satellites of Haumea were formed from the collision that produced the Haumea collisional family.

💡 Research Summary

This paper presents the first near‑infrared photometric study of the dwarf planet Haumea and its two satellites, Hi‘iaka and Namaka, using the Hubble Space Telescope’s NICMOS instrument. Observations were carried out with the F110W (≈1.1 µm) and F160W (≈1.6 µm) filters, which bracket the strong water‑ice absorption band at 1.6 µm. After standard pipeline reductions, point‑spread‑function fitting, and careful background subtraction, the authors derived precise magnitudes for each body in both filters.

The measured color indices (F110W – F160W) are –1.209 ± 0.004 mag for Haumea, –1.48 ± 0.06 mag for Hi‘iaka, and –1.4 ± 0.2 mag for Namaka. Because the 1.6 µm water‑ice band reduces the flux in the F160W filter, a more negative color indicates a deeper absorption feature. Thus, both satellites exhibit water‑ice absorption at least as deep as that of Haumea, implying that their surfaces are dominated by similarly pristine, crystalline water ice. Hi‘iaka’s color is especially well constrained, while Namaka’s larger uncertainty reflects its faintness and the limited signal‑to‑noise ratio.

In addition to static colors, the authors monitored Haumea’s rotational light curve in both filters. The infrared brightness varies in phase with the known 3.915‑hour rotation period, reproducing the amplitude seen in visible light. Notably, when the so‑called “red spot” rotates into view (approximately 0.2 of a rotation cycle), the (F110W – F160W) color becomes 2–3 % bluer. This subtle shift suggests that the red spot either contains a higher fraction of fine‑grained water ice or possesses a slightly different ice crystallinity, both of which would enhance the 1.6 µm absorption relative to the surrounding terrain. The satellites, by contrast, show no significant color modulation over the observation window, indicating a relatively homogeneous surface composition.

The authors discuss the implications of these findings for the origin of Haumea’s satellites. The similarity in water‑ice absorption depth strongly supports a collisional origin: a giant impact that created Haumea’s rapid spin, elongated shape, and the Haumea collisional family likely ejected a debris disk from which Hi‘iaka and Namaka accreted. This scenario naturally explains the satellites’ near‑circular, coplanar orbits and their ice‑rich surfaces. Alternative capture scenarios are deemed less plausible because captured bodies would be expected to display a broader range of surface compositions.

Finally, the paper emphasizes the power of NICMOS‑type near‑infrared photometry for probing the surface composition of distant icy bodies. The authors advocate for follow‑up observations with next‑generation facilities such as the James Webb Space Telescope, which can deliver higher spectral resolution and sensitivity. Such data would allow detection of subtle spectral features (e.g., ammonia, methane, or organics) and enable quantitative modeling of grain size distributions, ice crystallinity, and thermal histories, thereby refining our understanding of the formation and evolution of the Haumea system and similar Kuiper Belt objects.