Trojans, Moons and Dwarf Planets Quiz

Answer: Phobos

Phobos and the second Martian moon, Deimos, are named after sons of Ares, the Greek version of Roman god Mars. Phobos is the larger with an average diameter of 22 km (13.7 mi). With an orbit at just under 6,000 km (3,700 mi) from Mars on average, it is the closest a moon orbits a planet in the solar system and, due to its equatorial orbit, cannot be seen at latitudes of just over 70 degrees on the planet. It has a decreasing orbit and is predicted to survive another 43 million years, according to one study.

Material ejected from its largest crater, Stickney (at 9 km or 5.6 mi in diameter), was at one time thought to be the cause of prominent grooves on the surface. However, material ejected from impacts on Mars itself is now considered to be the likely source.

Answer: Ganymede

Ganymede is one of Jupiter's four Galilean moons. It is the only moon with a substantial magnetic field, although it lacks much of an atmosphere. It is of a modest density and has an internal ocean which may exceed the volume of all of Earth's oceans combined.

Tidally locked, a revolution of the moon (its 'day') takes seven days and three hours, the same as its orbit. It is in orbital resonance with the inner two moons Europa and Io, which complete two and four orbits respectively for every one Ganymede orbit.

Answer: Selene

There are a few names for this particular satellite: Moon, Luna, even Cynthia. Selene, also known as Mene, is the Greek goddess and personification of the Moon. 'Moon' itself is derived from a time-related Proto-Indo-European word - in other words from 'month'.

The Earth and Moon (Selene) rotate about their shared centre of mass which lies beneath the Earth's surface. There is some eccentricity in the moon's orbit. Being tidally locked, the lunar month is the same as a lunar day. The gravitational interplay between the Earth, Moon and Sun creates tidal forces on the Moon and, rarely, mild moonquakes which can last as long as an hour.

Answer: Triton

Triton, Greek god of the sea, was the son of Poseidon, the Greek equivalent of Neptune. It is unusual for a large moon to have a retrograde orbit, going in the opposite direction to Neptune's direction of rotation. A current theory is that it must have come from elsewhere in the solar system, such as the Kuiper Belt where Pluto can be found.

A large number of Kuiper Belt objects (including Pluto) have orbital resonance with Neptune. Triton is tidally locked to Neptune, meaning the same face is kept pointing to the planet, and its orbit is also a nearly perfect circle.

Answer: Pluto

Pluto's largest moon, Charon, is half its diameter and the two act as a binary system, according to some. They are also tidally locked. Usually the smaller satellite is locked to the larger (as with Earth's moon), however Pluto and Charon are locked to each other. Rotating around their combined centre of mass, it is considered a binary system when the centre of mass lies outside both bodies. Although one-third of the volume of Earth's moon, Pluto is only one-sixth of its mass. In mythology, Pluto was the Roman god of the underworld.

Answer: Titan

Titan is unusual in having an atmosphere denser than Earth's and is rich in nitrogen. Apart from Earth, it is the only known object to have stable surface liquid bodies, such as liquid hydrocarbon lakes. Its climate creates rivers and dunes and its atmosphere rotates faster than the moon itself does. Venus, Jupiter and Saturn are other examples where this occurs. Titan is rich in organic compounds but is believed too cold on its surface at -179 deg C (-290 deg F) for life.

It is speculated that any ocean within the moon itself might be suitable for microbial life.

Answer: Eris

Eris, named after the goddess of strife and discord, is slightly smaller than Pluto but, being denser, is more massive. At one time it was thought to be bigger than Pluto, perhaps contributing to Pluto's demotion to dwarf status. Eris is tidally locked to its smaller companion Dysnomia. Dysnomia's density suggests that it is mostly made of ice and yet it has a very dark surface, contrasting strongly with the highly-reflective surface of Eris.

The pirouetting pair take nearly 560 years to orbit the Sun and lie further out than Neptune.

Answer: Titania

Taking its name from Shakespeare's fairy queen, Titania was one of the first two moons discovered around Uranus. It is the planet's largest moon and eighth largest in the Solar system and possibly arose from leftover material from the planet's formation.

The surface shows canyons, scarps and craters. The craters appear to have been partially smoothed by tectonic activity in the past. Titania orbits within the planet's magnetosphere, which appears to cause the observed darkening of the trailing hemisphere of Titania and the other moons lying inside the planet's orbit.

Answer: Helene

Trojan moons are only known in the Saturnian system. They sit in or around two of the Lagrange points of larger moons. The Lagrange points are places where the gravitation influence of two large orbiting bodies and the centrifugal force balance each other out. Minimal energy is required to sit in these places which makes them good for artificial satellites around Earth. Dione has two such Trojans with Helene being one of them.

The name is a reference to Helen of Troy (the Trojan connection).

Answer: Ceres

Found in 1801 some 45 years before Neptune and 129 years before Pluto, it was at that time considered the eighth planet of the Solar system. Eventually classified as a dwarf planet, it is the only one not beyond Neptune. It is named after the Roman goddess of agriculture.

Being in the middle of an asteroid belt, it has amassed at least 50 known Trojans. Ceres accounts for around 40% of the mass of the asteroid belt and takes 4.6 years to orbit the Sun. It appears to have originated outside the belt, perhaps between Jupiter and Saturn. Cryovolcanically active, water and brines drive the geology with a theory that asteroid impacts on one side can create a cryovolcano on the other. Viscous relaxation seems to smooth out impact craters over time.