Mercury (the closest planet to the sun) revolves around the sun in a highly eccentric (e ≈ 0.2056) and inclined (7.00° to the ecliptic) orbit, with a semi-major axis of 0.3871 AU, and a sidereal orbital period of 87.96d. The greatest elongation viewed from the Earth, ranges from 18° (at perihelion) to 28° (at aphelion) with an average of 23°. Its rotation period is ≈ 59d direct and the planet’s equator is essentially parallel to its orbital plane.

Mercury is in a spin-orbit resonance with the Sun, with its sidereal rotation period, two-thirds of its sidereal orbital period.

A radar pulse (λ ≈ 1 to 10cm) is usually wider in angle than a planet it is aimed at. The return pulse therefore has time delayed and Doppler shifted components. The shortest time delay, tells us the planet’s distance. The longest time delay defining the limbs and so its radius: As the planet rotates, the reflected radar is Doppler shifted which reveals the rotation speed.

For a spherical planet, the maximum linear rotation rate occurs at the equator, veq = 2πR/P and because each limb contributes to the total shift, Δλ /Δλ0 = 4πR/Pc.

If T is the period of revolution of Mercury about the Sun (88d), P the sidereal period of rotation (58.7d), and S the synodic period of rotation (the Solar Day); after one Earth day, Mercury will have rotated 360°/P with respect to the stars, but only 360°/S with respect to the Sun. The difference between these two angles is equal to the angular distance mercury moved in its orbit, 360°/T

i.e. 360°/P – 360°/S = 360°/T

S ≈ 176 days

Mercury’s solar day is twice the length of its sidereal year. Mercury’s rotation is locked into a 2:3 commensurability by the strong tidal forces it experiences at perihelion, due to its eccentric orbit.

Mercury’s radius is 2,440km. Its mass (deduced from perturbations upon spacecraft) is 0.055M (3.3 x 1023kg). The average density is 5,420kgm-3, typical of terrestrial planets. It is likely that Mercury’s interior has a rocky mantle and a large metallic core (about 75% of its radius).

Using the equation of hydrostatic equilibrium, Mercury’s central pressure is,

Pc = (2/3)πG<ρ>2R2 ≈ 2.3 x1010Pa

About 0.1 that of the Earth’s.

The surface Albedo is very low (0.056 at visual wavelengths), indicative of a rocky material darker than the Moon’s.

Surface temperatures vary from ~ 700K at the perihelion subsolar point, to about 100K on the dark side. The long hot solar day, and low escape velocity (4.3kms-1), suggests very little atmosphere will be retained. The mariner 10 spacecraft detected a thin H and He atmosphere of 10-12 atm. No significant atmosphere means no insulation and hence the huge noon to midnight temperature range.

The surface of Mercury is similar to the Moon, although it has fewer mid-sized craters, no mountain ranges, shallow scalloped cliffs (scarps), and fewer basins and large lave flows.

The characteristics of the scarps suggest that Mercury’s radius has shrunk (by about 1 to 3km) from cooling of its core, crust, or both: Thereby forming the thrust faults that created the scarps.

Mercury has a much higher percentage of metals than is predicted, and too small a mantle. It is possible that a giant impact early in Mercury’s history could have stripped off a rocky mantle, leaving mostly metallic core behind.

Mariner 10 deteced a weak magnetic field (300nT). Although small, this is sufficient to create a magnetosphere in the solar wind.

Because Mercury does not have a substantial atmosphere, weathering does not erode the surface.

It is likely that the intense sculpting of Mercury’s surface took place 4 x 109 years ago, not long after it was formed. The earliest phase of catering was wiped out by volcanism. Basin formation would have come at the end of the heavy bombardment. Since then, a few impacts have punched out the rayed craters.


~ by jamesdow2013 on April 15, 2013.

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