Many celestial bodies in the Kuiper Belt beyond Neptune's orbit appear reddish. The why has long occupied researchers. Now a team is presenting a possible explanation for the red coloring of the objects.
Astronomers have long puzzled over why many of the icy objects in the Kuiper belt beyond Neptune's orbit have a reddish-colored surface. Now three researchers from the USA believe they have found a solution: ice volcanism could be the cause. Methane also escapes from the interior of the celestial bodies, converts into heavier molecules under the influence of cosmic radiation and then deposits as a reddish film on the surface of the asteroid, the research team writes in the journal "Nature Communications".
Stephanie Menten, Michael Sori, and Ali Bramson of Purdue University came across this explanation while studying the dwarf planet Pluto's large moon Charon. This also shows a conspicuous red colouration, but only at its poles. Until now, planetary researchers have assumed that these red polar caps are caused by methane escaping into space from Pluto's atmosphere. Part of it migrates to the moon Charon and is deposited there - due to the rotation of the two celestial bodies - at the poles. Cosmic ray particles were thought to set off chemical reactions that, over long periods of time, lead to the formation of heavier organic molecules such as tholin - and this is what causes the reddish coloration.
"But there are also molecules formed from methane on objects of a similar size in the Kuiper belt that are not bound to a dwarf planet," explain Menten and her colleagues. That's why this explanation seemed too short-sighted - and they went in search of a cause that should get by without another celestial body.
The obvious idea: the methane comes from the interior of the reddish-colored objects themselves. In fact, there are traces of cryovolcanism on Charon - i.e. volcanism in which not hot rock magma but a mixture of frozen and liquid water from the interior penetrates to the surface. This ice magma also contains methane, which escapes quickly into space in the case of smaller celestial bodies. But a small proportion is deposited on the surface as methane ice and is then exposed to cosmic rays.
Menten and her colleagues have calculated that the cryovolcanism transported a total of more than a trillion tons of methane to the surface of Charon - and that this methane should have been deposited preferentially at the poles. This amount is enough to explain the reddish hue of the poles even without the influence of Pluto. And such a process is also possible on other objects in the Kuiper Belt, according to the research team: "The outflow of methane from the interior could therefore be a frequent and important process in the entire Kuiper Belt."