Pluto’s moon, Charon.

Pluto’s moon, Charon. NASA

The white heart of Pluto. Is there an ocean beneath?

The white heart of Pluto. Is there an ocean beneath? NASA

Migration of Spitnuk Planitia to the tidal axis of Pluto and Charon.

Migration of Spitnuk Planitia to the tidal axis of Pluto and Charon.
illustration by James Keane

by Kim Malville

In the evening, Venus will be getting brighter and higher in the southwestern skies at dusk this month. It will remain in the sky three hours after sunset at the start of the moon, increasing to three and a half hours at the end of the month. Not only does it get brighter, but also the height of Venus above the horizon at sunset increases from 24° to 36°.  Ruddy unblinking Mars is higher in the southwestern sky.

December 1: The slender crescent moon lies to the right of Venus. Look below the moon to find Mercury, which will be visible for the next three weeks.

December 4: Mars lies to the left of the moon.

December 12-13: The moon, one day away from full, will occult the bright red star Aldebaran. This is a BIG event. Aldebaran, the red eye of Taurus the Bull, will disappear behind the moon as seen in Crestone between 8:17 and 8:22pm. The moon will be one day away from full, but its leading edge will be dark, making it easy to view the disappearance of Aldebaran. The star will reappear around 9:30. This is a nicely dramatic evidence of the real movement of the moon eastward along the ecliptic. Aldebaran is below the Pleiades and above the constellation of Orion. Binoculars will enhance your enjoyment of the event.

December 13-14: Geminid meteor shower. It is typically a great shower, but the full moon on this night will mean that only the brightest meteors should be visible.

December 21: Winter solstice occurs at 3:44am.

Pluto continues to surprise us

In recent months there’s been growing evidence that Pluto is hiding a liquid water ocean beneath its frozen surface. Pluto’s ocean may be more than 60 miles deep, perhaps ten times deeper than any of the Earth’s oceans! That conclusion, which has recently been published in a number of scientific journals, is based upon new interpretations of Sputnik Planitia, the left-hand side of the enormous, heart-shaped crater that dominates the dwarf planet’s midsection. Sputnik Planitia sits close to the tidal axis linking Pluto to its largest moon, Charon. Mutual tides on the two objects tie them together. Mutual tides, which each produces on the other, bind them tightly, such that they rotate and revolve around each other at the same rate. Each keeps the same face to the other. The Earth’s moon is similarly tidally locked to the Earth; our moon always keeps the same face toward the Earth. We never see its backside. Pluto and its moon never see their companion’s backside. Sputnik Planitia may have played a role in choreographing this strange dance of Pluto and Charon around each other.

About 600 miles across, Sputnik Planitia is nearly free of craters, indicating that it is very young. It is rimmed by tall water-ice mountain ranges and with glaciers that flow downward. Sputnik Planitia appears to be an ancient crater, the result of an asteroid impact. That basin has been partly filled by frozen nitrogen, methane, and carbon monoxide, perhaps overlying a deep ocean of semi-liquid water.

These recent studies conclude that Sputnik Planitia lies almost directly opposite Charon not by accident—statistically, there’s only a 5% chance that it accidentally formed where it is now. Instead this deep ocean of water pulled the crust of Pluto around to its current location. It’s the tendency of spinning planets, dwarf planets, and moons to reorient themselves so that locations with mass excesses end up on the equator and mass deficiencies at the poles. Consider swinging a weight on a string around your head. The weight, always wanting to move forward in a straight line, pulls outward, producing tension in the string.  Swinging the weight too fast will, of course, produce enough tension to break the string. The result of this kind of tension in the crust of Pluto caused the basin to move outward and downward, deforming the crust as it went. This basin may actually have been formed much farther north, near latitude 60°, and over millions of years migrated equatorward.

The asteroid impact that produced Sputnik Planitia created a hole in Pluto’s ice crust. This region where the crust was thin allowed an underlying ocean, a high-pressure mixture of water and salt, to well upward to create a huge sub-surface bubble of water. Although Pluto is small, there may be enough residual heat in its core to keep a region of slush water ice under pressure, which could have been squeezed out like toothpaste, to flood the region beneath Sputnik Planitia.  Because liquid water is denser than its ice, the broad bulge created an initial mass excess. It is estimated that the salinity of the of this ocean is 30%. Besides acting as an antifreeze, salt increases the density of the water even more, enhancing the mass excess. Thirty percent salinity is about as salty as the Dead Sea. It’s worth noting that halophiles, salt-loving microbes, are found in the Dead Sea.

Pluto has now seemingly joined the ever-growing list of planets, moons, dwarf planets and exoplanets in our solar system that have oceans of liquid water and perhaps life. So far there’s evidence of water on the exoplanet Ceres, three of Jupiter’s moons (Europa, Ganymede and Callisto), three of Saturn’s moons (Titan, Enceladus, and Mimas), and one of Neptune’s moons (Triton). And there’s clear evidence Mars once had a giant ocean that covered nearly 20% of the planet.

It is very exciting that we have now entered a scientific era in which water seems to be more prevalent in our solar system than we had previously thought. Astrobiologists, in their search for life beyond the Earth, have argued that where there is water, there could be life. Could little Pluto, which suffered the humiliation of having been ejected from the family of planets, actually have life forms floating in its dark, deep, and slushy ocean?

Pluto’s moon, Charon.

Pluto’s moon, Charon. NASA