Question

what characteristic is unique to mercury?
mercury has no moons.
mercurys day lasts for two - thirds of a year.
mercury has a system of scarps
mercury has an atmosphere

Explanation:

• Mercury has no moons: Venus also has no moons, so this is not unique to Mercury.
• Mercury's day lasts for two - thirds of a year: Mercury's orbital period (a year) is about 88 Earth days, and its rotation period (a day) is about 59 Earth days. \(\frac{59}{88}\approx\frac{2}{3}\). This is a unique characteristic.
• Mercury has a system of scarps: Other celestial bodies may also have similar geological features.
• Mercury has an atmosphere: It has a very thin exosphere, but it is not unique in having some form of atmospheric - like layer (e.g., Mars has an atmosphere).

Answer:

Mercury’s day lasts for two - thirds of a year.