Size, shape, and composition The size of a Solar System object can be deduced from its
optical magnitude, its distance, and its
albedo. Objects appear bright to Earth observers either because they are large or because they are highly reflective. If their reflectivity (albedo) can be ascertained, then a rough estimate can be made of their size. For most distant objects, the albedo is unknown, but Haumea is large and bright enough for its
thermal emission to be measured, which has given an approximate value for its albedo and thus its size. However, the calculation of its dimensions is complicated by its rapid rotation. The
rotational physics of
deformable bodies predicts that over as little as a hundred days, a body rotating as rapidly as Haumea will have been distorted into the
equilibrium form of a
triaxial ellipsoid. It is thought that most of the fluctuation in Haumea's brightness is caused not by local differences in albedo but by the alternation of the side view and ends view as seen from Earth. The rotation and amplitude of Haumea's
light curve were argued to place strong constraints on its composition. If Haumea were in
hydrostatic equilibrium and had a low
density like Pluto, with a thick mantle of
ice over a small
rocky core, its rapid rotation would have elongated it to a greater extent than the fluctuations in its brightness allow. Such considerations constrained its density to a range of 2.6–3.3 g/cm3. By comparison, the Moon, which is rocky, has a density of 3.3 g/cm3, whereas Pluto, which is typical of icy objects in the Kuiper belt, has a density of 1.86 g/cm3. Haumea's possible high density covered the values for
silicate minerals such as
olivine and
pyroxene, which make up many of the
rocky objects in the Solar System. This also suggested that the bulk of Haumea was rock covered with a relatively thin layer of ice. A thick ice mantle more typical of Kuiper belt objects may have been blasted off during the impact that formed the Haumean collisional family. Because Haumea has moons, the mass of the system can be calculated from their orbits using
Kepler's third law. The result is , 28% the mass of the Plutonian system and 6% that of the
Moon. Nearly all of this mass is in Haumea. Several ellipsoid-model calculations of Haumea's dimensions have been made. The first model produced after Haumea's discovery was calculated from
ground-based observations of Haumea's
light curve at
optical wavelengths: it provided a total length of 1,960 to 2,500 km and a
visual albedo (pv) greater than 0.6. The most likely shape is a triaxial ellipsoid with approximate dimensions of 2,000 × 1,500 × 1,000 km, with an albedo of 0.71. Observations by the
Spitzer Space Telescope gave a diameter of and an albedo of , from
photometry at
infrared wavelengths of 70 μm. Subsequent light-curve analyses have suggested an equivalent circular diameter of 1,450 km. In 2010 an analysis of measurements taken by
Herschel Space Telescope together with the older Spitzer Telescope measurements yielded a new estimate of the equivalent diameter of Haumea—about 1300 km. These independent size estimates overlap at an average
geometric mean diameter of roughly 1,400 km. In 2013 the Herschel Space Telescope measured Haumea's equivalent circular diameter to be roughly . However the observations of a
stellar occultation in January 2017 cast a doubt on all those conclusions. The measured shape of Haumea, while elongated as presumed before, appeared to have significantly larger dimensions according to the data obtained from the occultation Haumea is approximately the diameter of Pluto along its longest axis and about half that at its poles. The resulting density calculated from the observed shape of Haumea was about more in line with densities of other large TNOs. This resulting shape appeared to be inconsistent with a homogenous body in hydrostatic equilibrium, though Haumea appears to be one of the largest trans-Neptunian objects discovered nonetheless, smaller than , , similar to , and possibly , and larger than , , and . A 2019 study attempted to resolve the conflicting measurements of Haumea's shape and density using
numerical modeling of Haumea as a differentiated body. It found that dimensions of ≈ 2,100 × 1,680 × 1,074 km (modeling the long axis at intervals of 25 km) were a best-fit match to the observed shape of Haumea during the 2017 occultation, while also being consistent with both surface and core scalene ellipsoid shapes in hydrostatic equilibrium. The revised solution for Haumea's shape implies that it has a core of approximately 1,626 × 1,446 × 940 km, with a relatively high density of ≈ , indicative of a composition largely of hydrated silicates such as
kaolinite. The core is surrounded by an icy mantle that ranges in thickness from about 70 km at the poles to 170 km along its longest axis, comprising up to 17% of Haumea's mass. Haumea's mean density is estimated at ≈ , with an albedo of ≈ 0.66.
Surface In 2005, the
Gemini and
Keck telescopes obtained
spectra of Haumea which showed strong crystalline
water ice features similar to the surface of Pluto's moon
Charon. This is peculiar, because crystalline ice forms at temperatures above 110 K, whereas Haumea's surface temperature is below 50 K, a temperature at which
amorphous ice is formed. In addition, the structure of crystalline ice is unstable under the constant rain of
cosmic rays and energetic particles from the Sun that strike trans-Neptunian objects. The timescale for the crystalline ice to revert to amorphous ice under this bombardment is on the order of ten million years, yet trans-Neptunian objects have been in their present cold-temperature locations for timescales of billions of years. Radiation damage should also redden and darken the surface of trans-Neptunian objects where the common surface materials of
organic ices and
tholin-like compounds are present, as is the case with Pluto. Therefore, the spectra and
colour suggest Haumea and its family members have undergone recent resurfacing that produced fresh ice. However, no plausible resurfacing mechanism has been suggested. Haumea is as bright as snow, with an albedo in the range of 0.6–0.8, consistent with crystalline ice. Other large TNOs such as appear to have albedos as high or higher. Best-fit modeling of the surface spectra suggested that 66% to 80% of the Haumean surface appears to be pure crystalline water ice, with one contributor to the high albedo possibly
hydrogen cyanide or
phyllosilicate clays. Inorganic cyanide salts such as copper potassium cyanide may also be present. However, further studies of the visible and near infrared spectra suggest a homogeneous surface covered by an intimate 1:1 mixture of amorphous and crystalline ice, together with no more than 8% organics. The absence of ammonia hydrate excludes
cryovolcanism and the observations confirm that the collisional event must have happened more than 100 million years ago, in agreement with the dynamic studies. The absence of measurable
methane in the spectra of Haumea is consistent with a warm
collisional history that would have removed such
volatiles, in contrast to . In addition to the large fluctuations in Haumea's light curve due to the body's shape, which affect all
colours equally, smaller independent colour variations seen in both visible and near-infrared wavelengths show a region on the surface that differs both in colour and in albedo. More specifically, a large dark red area on Haumea's bright white surface was seen in September 2009, possibly an impact feature, which indicates an area rich in minerals and organic (carbon-rich) compounds, or possibly a higher proportion of crystalline ice. Thus Haumea may have a mottled surface reminiscent of Pluto, if not as extreme. == Ring ==