Mercury, fastest planet in the sky

--------- #1369 - Mercury - Planet Closest to the Sun

- Attachment: Mercury image

- Being closet to the Sun Mercury should be the hottest planet. Being closest causes it to feel 10 times the gravity that we feel on Earth. Mercury orbit’s the Sun in 88 days. But, it takes 58.6 days for Mercury to complete one rotation. The planet has a 3:2 resonance with 3 Mercury days = 58.6 * 3 = 176 days to 2 Mercury years = 88 * 2 = 176 days. Eventually Mercury will be locked in to a 1:1 resonance with one year equal to one day. Much as our Moon is locked in to 1:1 resonance as it orbit’s the Earth.

- Mercury’s orbit is very elliptical varying form 25.5 million miles to 43 million miles (Earth’s orbit is 93 million miles). This significant difference throughout the year varies its Solar Intensity from 6 times to 14 times what we experience on Earth.

- Watching Mercury form Earth its separation from the Sun varies from 18 degrees away to 27 degrees away. This is how much you can find Mercury above the western horizon when the Sun is setting.

- Mercury’s brightness magnitude varies from 5.1 Magnitude to -2.4 Magnitude. -2.4 Magnitude is twice as bright as the Dog Star Sirius, the brightest star in the night sky. Mercury is brightest when it is furthest away, because that is when we see its full face in the sunlight.

- The average temperature on Mercury is 163 C ( 325 F). It is much closer to the Sun than Venus yet Venus average temperature is 460 C ( 860 F). The difference is that Mercury has almost no atmosphere and Venus has a thick, greenhouse atmosphere of carbon dioxide. (See Review #1368 for more on Venus)

- If you ignore the effects of atmosphere the temperature of a body can be calculated by how close it is to the Sun and its reflectivity ( also called albedo).

--------- Mercury is 0.387 Astronomical Units from the Sun

-------- Mercury’s reflectivity is the same as the Moon, 12 %.

------- The formula for calculating the temperature is:

------------ T = 280K * [ ( 1 - reflectivity) / (distance)^2 ) ] ^ ¼

------------ T = 280K * [ ( 1 - 0.12) / (0.387)^2 ) ] ^ ¼

------------ T = 280K * [ ( 0.88) / (0.1497) ] ^ ¼

------------ T = 280K * [ ( 5.876) ] ^ ¼

------------ T = 280K * [ ( 0.88) / (0.1497) ) ] ^ ¼

------------ T = 280K * [ 1.557 ] ^ ¼

------------ T = 436 Kelvin ( 163 C )

- The average temperature on the surface of Mercury is 436 K - 273 C = 163 C.

- You can do the same calculation for Venus and you get -40 C for the average temperature. But, that is not what it is because of its greenhouse atmosphere which adds 500 C temperature rise to the surface. The average temperature on Venus is 460 C (860 F). (See Review # 1368 for more about Venus ).

- Earth, Mars, Saturn are planets with about 20 degree tilts in rotation compared to the plane of orbit. Mercury has no tilt. Its poles rotate straight up and down with the plane of orbit.

- Because Mercury’s poles never tilt toward the Sun there are places in the polar regions that never see the Sun. There could be frozen water ice patches in the depressions and craters near the poles on this planet closest to the Sun. Amazing!

- Like the Moon, Mercury has be geologically “ dead” for most of its history. Mercury has so little atmosphere there is no wind or weather of any kind. The surface pressure and reflectivity of sunlight are the same for both Mercury and the Moon.

- There is direct evidence that Mercury suffered a major asteroid impact at one point in its evolution. There is an enormous impact crater on one side of the planet ( Caloris Basin) and a shockwave basin on the opposite side of the planet ( Weird Terrain). The Caloris Basin spans more than half Mercury’s radius. It s multiple rings are evidence of an extremely violent impact. A direct hit. There are few craters inside the basin indicating the impact occurred after the Solar System’s heavies asteroid and comet bombardments.

- Mercury has a small , but strange Magnetic field. Its magnetic equator does not match up with its geographical equator. It lies 300 miles north. With Mercury’s small size and slow rotation, 59 days, we would not expect it to have a molten core or any metal convection to create a magnetic field. The explanation is that Mercury has an unusually large metal core that is still partly molten and convection is still occurring. An announcement will be made shortly, stay tuned.
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707-536-3272, Sunday, January 1, 2012