Mercury, Venus, MarsPart I

Lecture 20

Observing Mercury and Venus from Earth

Elongations of Mercury Mercury moves around the Sun every 88 days in a rather eccentric orbit. As seen

from the Earth, the angle between Mercury and the Sun at greatest eastern or western elongation can be as large as 28° (when Mercury is near aphelion) or as small as 18° (near perihelion). By contrast, Venus follows a larger, nearly circular orbit with a 224.7-day period. The angle between Venus and the Sun at eastern or western elongation is 47°.

Maximum separation from the Sun Mercury’s greatest elongation : 18° to 28° Venus’s greatest elongation : 47°

Favorable and unfavorable elongationsNot only the separation, but also the combined effect of the tilt of Earth’s axis, the inclination and eccentricityof Mercury’s orbit play a role

Favorable and unfavorable oppositions of MarsMars can be0.37AU awayat the most favorable oppositioncan be 3.5times brighterthan Sirius! At least

favorableopposition:0.67AU

Mercury’s Rotation

Synchronous rotation?

Due to the elliptical orbit of the Mercury (can never force the elongation always pointing to the Sun), Mercury’s long axis point to the Sun only when it is nearest to the Sun (i.e., at perihelion).

Mercury spins 1.5 times while it make one orbit (1.5 to 1 spin-orbit coupling or 3:2).

3 to 2 spin-orbit coupling

Venus is always covered with thick clouds and have to see through this thick cloud radio and IR radar technique.

Venus’s rotation period (solar day) is 116.8 Earth days.

And, Venus rotates in the opposite direction (clock-wise) retrograde rotation. Sun rises from the West on Venus!

Venus’s Rotation

Mercury’s surface

As expected for a small planet, Mercury’s surface has a lot ofcraters…

A scarp : 2km high and 600km longCooling Mercury contracted and made a scarp…

Mercury’s interior

Too large a core : 42% volume One possibility

Mercury has a weak magnetic field (1% strength of that of the Earth). unexpected for a small planet covered with lots of

craters. small size and slow rotation (58.6 days) make the

existing magnetic field a puzzle.

Unexpected magnetic field in Mercury

Speculations on Venus in the pre-spacecraft era

Tropical Paradise?With no high concentration of CO2

in the Venusian atmosphere, it would have achieved a hot summer tropical weather (45°C).

But, Mariner 2 found out that the Venus should be at least 400°C hot and later Venera 7 (descended to the ground of Venus) measured a temperature of 460°C!

Speculations on Mars in the pre-spacecraft era

William Herschel (late 1700s)• discoverer of Uranus• discovered polar ice caps of Mars• discovered the length of the Martian day

(24h 37 min)• noticed permanent marks on the surface“… inhabitants probably enjoy a situation in

many respects similar to our own!”

Ground-based image of Mars

Giovanni Schiaparelli (late 1800s)• discovered 79 linear features

(“canali”)•meaning “channels”, but

incorrectly translated as “canals”

Then, Percival Lowell…

Mirage of the Martian Canals

Percival Lowell(1855-1916)

He knew that not all astronomers see Schiaparelli’s canals…

He thought that it is due to poor “seeing”

On top of Mauna Kea mountain

Built an observatory in 1894 at Flagstaff, AZ to observe Mars…

Due to the astronomical seeing effect, long exposed pictures get blurry.

Lowell’s interpretation of Martian surface

Mapped 200 canals. He knew that Mars is arid and its polar ice caps… Canals transportation of water from pole to equator cities… One global network a single nation.

But, many scientists did not believe his claim (w/o confirmation). Excepting a couple of large geometrical features, his canals were purely imaginary! All scientists abandoned Martian canals (by 1910), the idea persisted among public for a

long long time!

In truth, Mars is a windblown, cratered world

In summary…

Important ConceptsFavorable and unfavorable

configurations of Mercury, Venus, and Mars

Mercury’s 3:2 spin-orbit couplingVenus’s retrograde rotationSurface features of MercuryLarge iron core of MercurySurprising magnetic field in

MercuryFantasies on Venus/Mars during

pre-space era

Important TermsElongationSpin-orbit couplingRetrograde rotation

Chapter/sections covered in this lecture : sections 11-1 through 11-4