Some Moon stuff

By which I don’t just mean our Moon, but various moons through the solar system, some of which have been in the spotlight.

Starting out at both Saturn and Earth – and the two are united in having Moons that are slowly shrinking as they cool, the effect being, as on Earth and more dramatically on Mercury, that the outer layer develops wrinkles, seen as mountain chains. Cassini made the discovery for Titan when mapping the icy world’s topography.  Ripples of mountains, all orientated the same and clustered around the equator fitted a computer model of a small body, made of ice and rock with a subsurface ocean, slowly cooling over time as radioactive isotopes decay away. This causes the body to shrink, but the outer parts of the subsurface ocean to freeze and expand. Losing seven kilometres in radius and one percent of volume over the four billion or so years since formation, Titan has achieved two kilometre high mountains in its equatorial ranges. The features seen on our own Moon, by comparison, are much shorter – nine metres or so – but run on for kilometres and cover the whole surface. Equatorial ‘lobate scarps’ were first seen by the Apollo program, but only at the equator. More recent observations with the Lunar Reconnaissance Orbiter have identified further scarps dotted around the Moon. estimates of the shrinking of the Moon from the features suggests around 100m has been lost in around 800 million years, putting the Moon at a lower rate of radius loss than Titan. There is also the suggestion that instabilities in the contraction process are responsible for some of the Moon-quakes observed with Apollo hardware.

Titan wasn’t the only moon under the microscope of Cassini. During one flyby, the probe took images of three moons – Enceladus, Tethys and Dione. Images like this enable scientists to look at changing shadows on the surface of different moons that may help to eke out a little extra detail, or may even reveal a new area of the surface in unprecedented resolution at a given wavelength. They also help others to create computer models of moon surfaces for virtual flyby videos like these.

Of course, they’re also good just for being nice pictures. Stuart Atkinson has put up a post with one of the Enceladus pictures taken by Cassini, and another picture taken from the Mercury bound Messenger probe of a planet and a Moon. The planet happens to be Earth. Messenger wasn’t idly glancing into the void, however, the image was taken during a sweep for Vulcanoids – asteroids lying between the Earth and the Sun, unseen by terrestrial observatories due to the glare of our nearest star. These particular asteroids, should they exist, would be trapped in orbits that never take them as far out as ourselves, rather than actual asteroids or spent comets that have drifted inward. They shouldn’t exist, according to prevailing ideas on the formation of the solar system, so if just one is seen and later confirmed, it would be interesting. It would also mean that alongside the Oort Cloud, the Kuiper-Edgeworth Belt and the Asteroid Belt, we’d also have a Vulcanoid Belt, with new objects to study and slot into the history of the solar system.

For now, we’ll just have to study the solar system through the most ancient rocks available to present day geologists. The most recent block of ancient rock to be found has been dated at around 4.45 billion years old. The Earth is estimated to be 4.54 billion years old and so should the result be confirmed, the arctic rocks would date to a period before the crust of the Earth had formed, but after the core was created. The scientists measured the age looking at well known radioactive decay signatures, though others have suggested a better way would be to look for signatures from istopes believed to be around at the time, but which quickly decayed away in the earliest parts of terrestrial history.

The past and future evolution of planetary systems can have an impact on whether or not there are moons to be found. Hot Jupiter planets – gas giants that have migrated closer to their parent stars – are unlikely to have held onto their moons as they rode the gravitational turbulence further in and dealt with the gravitational forces from the closer proximity to their host star according to new research. Exomoon hunters (like David Kipping, quoted in the article) aren’t detered as the planets do still provide a testbed for observational techniques until our methods and instruments become capable of tackling extra solar planets with a higher likelihood of companion bodies.

Finally, the troubled evolution of Jupiter has been in the news (along with the evolution of its clouds, visible even in relatively small telescopes). The largest of our solar system’s planets has a small problem – its core is a little depleted compared to that of second largest planet Saturn. Current theory suggests that gas giants start out as a giant rocky/ice body, perhaps ten times the mass of the Earth, which then gravitationally hoovers up the gas surrounding it. The trouble is while Saturn shows evidence for the right size of core, Jupiter is a little lacking given its incredible overall mass. A new suggestion has been made that Jupiter’s own core has been in collision with four or five super-Earth’s, each skimming a bit off the top. It is hoped that the forthcoming Juno mission will be able to add observational evidence to the idea.


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