Ikapati crater on Ceres: potential past cryogenic activities and brine layer evolution

Ceres is the only dwarf planet of the inner Solar System, locater in the main asteroid belt as a mean solar distance of 2.8 AU. The DAWN spacecraft explored Ceres between 2015 and 2018. Its onboard Framing Camera (FC) and the reflectance Spectrometer (VIR) mapped the entire illuminated surface between 0.4 and 4 µm. Ceres is very dark, with an average albedo below 0.1.

VIR data revealed the predominantly presence of (Mg-NH₄) phyllosilicates and (Mg-Ca) carbonates on the Cerean surface. In a number of craters, bright faculae indicate the presence of ammonium salts (NH₄Cl), halite (NaCl) and Na-carbonates as for example in the prominent Occator crater. These materials and salts are originating from endogenic processes, partly caused by cryovolcanism activity, indicating the presence of a brine layer underneath the surfac. Ahuna Mons, a prominent topographic elevation in the southern hemisphere, is a further identified cryovolcanic surface feature also showing Na-carbonates and salts.

By combining findings of geomorphology, surface composition analysis and surface age dating, we are able to identify traces of past cryovolcanism on the Cerean surface and expect to shed light into the evolution of the brine composition using the age of the flows. Among a few prominent crater candidates, we focused our study on Ikapati.

Ikapati has a diameter of 48 km, centered at 33°N 45°E. Its geomorphology, potential cryovolcanic features (pits, flows, fractures) and surfaces ages were studied by (Krohn et al., 2016; Schmedemann et al., 2016; Pasckert et al., 2018; Sizemore et al., 2019) using FC imagery. The type and absorption band parameters of the carbonates of this area was studied by (Carrozzo et al., 2018; Raponi et al., 2019).

In our present study we use the aforementioned methods to characterize the Ikapati crater in detail to establish when possible a chronology of the cryovolcanic features and an evolution in the brine layer composition.

We used the VIR infrared dataset, corrected following (Carrozzo et al., 2016; Ciarniello et al., 2017, 2020), and the clear filter FC LAMO mosaic processed by MPS. We calculated the absorption band depth of the main absorptions features in Ceres spectra, at 2.73, 3.06, 3.47 and 3.96 µm using the formula from (Viviano et al., 2014), as well as the slope between 2.42 and 1.7 µm (Figure 1). Figure 2 is a color composition of the crater Ikapati using VIR HAMO images.

Figure 1 : Exemple of VIR spectral profile (I/F as a function of the wavelength in µm) at the central peak of Ikapati. The blank spaces correspond to detectors limits. Orange lines are the continuum considered to calculate the absorption band depth between the shoulders of the absorption. The dashed lines show the wavelengths at which the band depths is calculated. The red, green and blue vertical lines correspond to figure 2 selected wavelengths.

We can observe different compositions between the smooth and pitted terrains south of the central peak, and south-west of the crater (light blue), the central peak and the west of the crater (light orange) due to carbonates (Carrozzo et al., 2018), the north-west ridge (brown), the south of the crater (olive), and the northern plateau of the crater (lilac). According to Pasckert et al., (2018), the carbonate deposits range from 20 Myr around the central peak to 29 Myr at the west of the crater using the ADM crater counting method. The olive patches are around 27 (left) and 43 (right) Myr. This give us confidence in being able to observe an evolution in the composition depending on the age of the surface.

Figure 2 : color composition with a resolution of 350m/pxl (R : 2.43,  V : 3.055, B : 3.97 µm ) of Ikapati, overlaid with the HAMO FC mosaic. The VIR cubes used from the HAMO mission phase are  VIR_IR_1B_1_494727886_ 1, VIR_IR_1B_1_498127790_1, VIR_IR_1B_1_494794304_1, VIR_IR_1B_1_495678838_ 1, VIR_IR_1B_1_495743991_ 1, VIR_IR_1B_1_511816236_1, VIR_IR_1B_1_495743991_1.

 

References

Carrozzo, F.G. et al. (2016) ‘Artifacts reduction in VIR/Dawn data’, Review of Scientific Instruments, 87(12), p. 124501. Available at: https://doi.org/10.1063/1.4972256.

Carrozzo, F.G. et al. (2018) ‘Nature, formation, and distribution of carbonates on Ceres’, Science Advances, 4(3), p. e1701645. Available at: https://doi.org/10.1126/sciadv.1701645.

Ciarniello, M. et al. (2017) ‘Spectrophotometric properties of dwarf planet Ceres from the VIR spectrometer on board the Dawn mission’, Astronomy & Astrophysics, 598, p. A130. Available at: https://doi.org/10.1051/0004-6361/201629490.

Ciarniello, M. et al. (2020) ‘Ceres observed at low phase angles by VIR-Dawn’, Astronomy & Astrophysics, 634, p. A39. Available at: https://doi.org/10.1051/0004-6361/201936492.

Krohn, K. et al. (2016) ‘Cryogenic flow features on Ceres: Implications for crater‐related cryovolcanism’, Geophysical Research Letters, 43(23). Available at: https://doi.org/10.1002/2016GL070370.

Pasckert, J.H. et al. (2018) ‘Geologic mapping of the Ac-2 Coniraya quadrangle of Ceres from NASA’s Dawn mission: Implications for a heterogeneously composed crust’, Icarus, 316, pp. 28–45. Available at: https://doi.org/10.1016/j.icarus.2017.06.015.

Raponi, A. et al. (2019) ‘Mineralogy of Occator crater on Ceres and insight into its evolution from the properties of carbonates, phyllosilicates, and chlorides’, Icarus, 320, pp. 83–96. Available at: https://doi.org/10.1016/j.icarus.2018.02.001.

Schmedemann, N. et al. (2016) ‘Timing of optical maturation of recently exposed material on Ceres’, Geophysical Research Letters, 43(23), p. 11,987-11,993. Available at: https://doi.org/10.1002/2016GL071143.

Sizemore, H.G. et al. (2019) ‘A Global Inventory of Ice-Related Morphological Features on Dwarf Planet Ceres: Implications for the Evolution and Current State of the Cryosphere’, Journal of Geophysical Research: Planets, 124(7), pp. 1650–1689. Available at: https://doi.org/10.1029/2018JE005699.

Viviano, C.E. et al. (2014) ‘Revised CRISM spectral parameters and summary products based on the currently detected mineral diversity on Mars’, Journal of Geophysical Research: Planets, 119(6), pp. 1403–1431. Available at: https://doi.org/10.1002/2014JE004627.