Pluto, discovered in 1930, has long been considered the ninth planet of the solar system. After the discovery of the first large objects of the Kuiper belt in 1992 and the more massive (but smaller) small planet of Eris, the astronomical community revised the definition of Pluto, and in 2006 it was “reduced” in status to a dwarf planet. But, albeit with the change of status, Pluto is still considered one of the largest trans-Neptune objects.
In early 2006, NASA sent the New Horizons station into space, which was to study Pluto, its satellite Charon, and other Kuiper belt space bodies. Information collected by the mission on July 15, 2015, when it passed alongside Pluto, turned out to be a real gift for astrophysicists, and the results of research based on information obtained from New Horizons have already begun to be published in scientific journals. Today we will talk about two such articles (1 and 2), published in the journal Nature.
On the surface of Pluto there is a vast drop-shaped depression, the so-called Sputnik Plain, measuring 1300 x 900 km and about 3.5 km deep relative to its surrounding elevations. Most likely, it was formed due to the powerful impact of a comet or an asteroid. The surface of the plain is very smooth and practically has no craters. In fact, it is a mixture of ices from volatile gases: nitrogen, methane and carbon monoxide. In the ice crust, there is a constant convection – “mixing” – due to the difference in temperature and gravity.
The plain of Sputnik is suspiciously close to the axis of the tides that Charon creates. Usually, the orientation of such geological objects in the direction of the main axes of rotation indicates the movement of the poles, and previous works by Pluto researchers indicate that this has happened. More precisely, because of the impact, the asteroid crust rose, and the plain area became denser, which led to the so-called positive gravitational anomaly: a local increase in gravity with respect to the average value for the object.
However, in order to explain such a strong positive gravitational anomaly in view of the new information from the New Horizons, the “shock compaction” alone is not enough. You need something more massive, for example, water. The presence of water on the surface of Pluto, in general, is not surprising, since the objects from the Kuiper belt mostly consist of a frozen chemical “cocktail” which includes carbon monoxide, methane, nitrogen and water. It is curious that during the time elapsed since the formation of the Satellite Plain, the water did not freeze, despite the processes of convection in the ice layers. Most likely, this was due to the fact that the nitrogen ice does not conduct heat well.
It is worth noting that one of the alternative scenarios suggested the formation of ice II under layers of nitrogen and methane. So called one of the phase varieties of ice, which is formed from ordinary ice at a temperature of from -83 ° C to -63 ° C (190–210 K) and a pressure of 300 MPa. Ice II has a trigonal crystal lattice, unlike the hexagonal lattice of ordinary ice, and, therefore, its density is significantly higher.
The presence of ice II could explain the formation of a vast area of positively gravitational anomaly, but in this case, certain compression tectonic effects, which, however, are not present in the depressions, should also have happened here. At the moment, researchers are developing alternative mechanisms for how positive gravity could arise in the Sphere of Satellite Plain area – a more detailed study of Pluto’s tectonics will help determine which theory is correct.
If there is a liquid ocean under the ice of Pluto, then due to “re-freezing” it must be under increased pressure, which in turn should lead to cryo-volcanic activity and fractures of the lithosphere, similar to those observed on the surface of the planet. And, most likely, many other Kuiper belt objects, similar to Pluto in their characteristics, can also “hide” liquid oceans under their surface.