A recent multi-chord occultation measurement of the dwarf planet ( 136108 ) Haumea ( ) revealed an elongated shape with the longest axis comparable to Pluto ’ s mean diameter .
The chords also indicate a ring around Haumea ’ s equatorial plane , where its largest moon , Hi ’ iaka , is also located .
The Haumea occultation size estimate ( size of an equal-volume sphere D _ { equ } = 2 \cdot ( a \cdot b \cdot c ) ^ { 1 / 3 } . D _ { equ } = 1595 km ) is larger than previous radiometric solutions ( equivalent sizes in the range between 1150 and 1350 km ) , which lowers the object ’ s density to about 1.8 g/cm ^ { 3 } , a value closer to the densities of other large TNOs .
We present unpublished and also reprocessed Herschel and Spitzer mid- and far-infrared measurements .
We compare 100- and 160- \mu m thermal lightcurve amplitudes - originating from Haumea itself - with models of the total measured system fluxes ( ring , satellite , Haumea ) from 24 – 350 \mu m. The combination with results derived from the occultation measurements allows us to reinterpret the object ’ s thermal emission .
Our radiometric studies show that Haumea ’ s crystalline water ice surface must have a thermal inertia of about 5 J K ^ { -1 } m ^ { -2 } s ^ { -1 / 2 } ( combined with a root mean square of the surface slopes of 0.2 ) .
We also have indications that the satellites ( at least Hi ’ iaka ) must have high geometric albedos \gtrsim 0.5 , otherwise the derived thermal amplitude would be inconsistent with the total measured system fluxes at 24 , 70 , 100 , 160 , 250 , and 350 \mu m. The high albedos imply sizes of about 300 and 150 km for Hi ’ iaka and Namaka , respectively , indicating unexpectedly high densities > 1.0 g cm ^ { -3 } for TNOs this small , and the assumed collisional formation from Haumea ’ s icy crust .
We also estimated the thermal emission of the ring for the time period 1980-2030 , showing that the contribution during the Spitzer and Herschel epochs was small , but not negligible .
Due to the progressive opening of the ring plane , the ring emission will be increasing in the next decade when JWST is operational .
In the MIRI 25.5 \mu m band it will also be possible to obtain a very high-quality thermal lightcurve to test the derived Haumea properties .