OBSERVATION OF THE BLUE COMA OF 174P/ECHECLUS
174P/Echeclus is a centaur; these objects currently orbit the Sun mostly between the orbits of Jupiter and Neptune (the orbit diagram below was created by NASA JPL’s Small Body Browser; Echeclus’ orbit is white, the orbits of the giant planets from Jupiter out to Neptune are plotted in colour). Centaurs are thought to be TNOs that have been pulled in from their original orbits beyond Neptune by repeated tugs from Neptune’s gravity1. Centaur orbits are unstable, and many of these objects are predicted to be ejected from the Solar System by the planets; some, however, survive and eventually move into the inner Solar System, becoming Jupiter Family Comets (JFCs; for example 67P/Churyumov-Gerasimenko)1. As the bridge between the TNOs and the JFCs, centaurs can be studied to better understand how icy minor planets evolve as they move from the frigid outer Solar System to its warmer inner regions.
Echeclus is one of around 13% of centaurs that have been observed to present cometary activity2, and it’s thought that this activity may drive a change in colour (a decrease in redness) that many icy minor planets appear to undergo as they get closer to the Sun3. In 2014 we observed a reflectance spectrum of Echeclus when it was inactive. In 2016 Echeclus unexpectedly had a short burst of cometary activity (see image below), so we repeated our 2014 observation soon after the outburst to see if Echeclus’ surface properties, such as colour, had changed. Unfortunately, we didn’t observe any change in the colour or reflectance properties of Echeclus itself.
Surprisingly, however, we did observe that the remnant of Echeclus’ transient coma was blue in colour; Echeclus is slightly red so it was interesting that the dust coming off it was coloured so differently. This observation goes against the normal trend observed for active centaurs and JFCs, where the colour difference between cometary comae and their parent nuclei is usually much smaller2,4. The dust may be blue because it is rich in amorphous carbon produced as the Solar wind strips hydrogen atoms from hydrocarbons in the coma’s dust grains5. Unfortunately from our observations alone, this can’t be confirmed. It’s possible, as predicted previously3, that if enough blue dust of this kind falls back to the red surface of Echeclus following cometary outbursts, it may act to reduce the redness of Echeclus’ colour.