SOLAR SYSTEM DATA
Sun-SOHO/EIT THE SUN
The Sun is the largest and most massive body in the solar system. Located at the centre, all other objects in the solar system are, by definition, gravitationally bound to the Sun. The Sun is composed mostly of hydrogen and helium (92% H and 8% He by numbersof atoms) although almost any element you can name is present. Helium was first discovered on the Sun, by virtue of the spectral lines that appear in the Sun's light. Helium was later found on the Earth in natural gas deposits (mostly in the USA and Russia). Technetium, an element with atomic number of 43, does not occur naturally on the Earth (although it is now made in nuclear reactors), but is also present in the Sun. Although we refer to a solar surface (termed the photosphere), the Sun has no solid surface, and this so-called surface is simply the layer from which visible light and heat is radiated into space.

The Sun does not spin or rotate as a solid body. The equator rotates faster than do the polar regions. Different layers inside the Sun also rotate at different rates than the layers immediately above them. This differential rotation is of course only possible because the Sun is a gaseous body (a big ball of gas). Even with the very high estimated pressures at the solar core, the central temperature is high enough to keep the matter in a gaseous state.

Models and measurements from helioseismology lead us to believe that the Sun is divided internally into 3 different regions. The core (about 20% of the solar diameter) is where the nuclear fusion reactions occur that power the Sun. The radiative zone extends from the core out to a radius of about 70% of the Sun. This region is so dense that we believe it can take up to one million years for the particles of heat and light (photons) generated in the core to work their way through it. The outer 30% of the solar diameter is the convection zone where motions are very turbulent. The top of this zone is the photosphere.

The Sun has a constantly changing magnetic field, which is to believed to be generated at the base of the convection zone by the motions there. This magnetic field, twisted by the differential rotation of the Sun, is believed to produce sunspots on the solar surface together with solar activity such as flares and coronal mass ejections that can be observed with special filters.

The solar atmosphere is divided into a lower atmosphere (the chromosphere) and the outer atmosphere (the corona). The corona extends out through the solar system as what we term the solar wind. The coronal temperature near the Sun is about 2 million degrees (K).

APPROXIMATE SOLAR PARAMETERS

Diameter = 1,400,000 km ( = 109 times the Earth's diameter )
Mass = 2 x 1030 kg ( 330,000 Earth masses )
Volume = 1.4 x 1027 m3 ( 1.3 million times Earth's volume)
Average density = 1400 kg m-3 ( 1.4 times the density of water)
Surface gravity = 270 m s-2 ( 27 x Earth's gravity )
Surface temperature = 6,000 K
Surface pressure = 1 kPa ( 1/100 Earth's sea level pressure of 101 kPa)
Core temperature = 15 million K ( estimated from models )
Core pressure = 2.3 x 1013 kPa
Core density = 150,000 kg m-3
Average rotation rate = once every 27 days ( varies with latitude )
Luminosity = 3.8 x 1026 W
Fuel consumption = 5 millions tons of hydrogen per second
Fuel cycle - nuclear fusion ( 4 H atoms fuse to form a He atom )
Sunspot cycle period = 11 years
Age = 5,000 million years ( estimated from models )


Mercury-Mariner 10 MERCURY
Mercury is the closest planet to the Sun and is classified as a terrestrial or Earth-like planet. It is difficult to observe because it is always less than 28o from the Sun. Mariner 10 was the only spacecraft to visit Mercury (1974-5) until the recent pass of Messenger in 2008. The Mariner passes, supplemented by Earth-based radar have been the main sources of information about the planet.

The surface of Mercury is covered by craters, and one large impact feature, named Caloris, is spread over 1300 km and has 'wierd' terrain at its antipodal point. Of the four terrestrial planets (Mercury, Venus, Earth and Mars), only Mercury and the Earth have a global magnetic field.

ORBITAL PARAMETERS FOR MERCURY

Mean distance from the Sun = 57.9 million km (0.387 AU)
Orbital eccentricity = 0.206
Orbital inclination to the ecliptic = 7.01 degrees
Orbital period = 87.969 days
Mean orbital veloctiy = 47.9 km/sec
Advance in perihelion = 10 minutes of arc per century

PLANETARY PARAMETERS

Mass = 3.30 x 1023 kg
Mean radius = 2438 km
Mean density = 5430 kg/m3
Gravity = 3.7 m/sec2 = 0.38 g (g = Earth gravity)
Escape velocity = 4.25 km/sec
Synodic rotation period (mean) = 115.9 days
Sidereal rotation period = 58.6 days (= 2/3 of orbital period)
Surface temperature range = 100 - 700 K
Inclination of equator to orbit = 1o (approx)
Mean solar constant = 9940 W/m2