As I write this, the New Horizons spacecraft is within a million kilometres of Pluto, which was until recently regarded as the outermost planet in the Solar System.
The images coming back are very intriguing, and in the next few weeks there should be a flow of new discoveries to puzzle over. To better appreciate these, it is worth looking at a planet at the opposite extreme: Mercury, the closest planet to the Sun.
Imagine a rocky, mountainous and cratered surface, rather like our Moon. The atmosphere is thin, so the sky is black. It is dominated by a Sun looking more than twice as large as it looks in our sky. The heat is intense. On Earth, if we had no atmosphere to reflect some of it back into space, a square metre of ground with the Sun overhead would receive just under 1,500 Watts of energy. A similar square metre on Mercury is getting around 9800 Watts. For comparison, an equivalent square metre of Pluto is getting less than 1 Watt of solar energy. Pluto is obviously a very cold place, whereas Mercury will be extremely hot.
Mercury is a rocky ball, rather like the Moon, with a diameter of about 4,900 km, compared with our Moon's diameter of almost 3,500. Both bodies are very cratered, with mountains and lava flows. As in the case of the Moon, the extensive cratering means Mercury has been geologically quiet for billions of years. That is not the case on Earth. The weather slowly erodes mountains and craters away, and our planet's surface is being continuously recycled by plate tectonics.Mercury rotates on its axis once every 30 or so of our days.
If the Sun is in the sky the surface gets very hot. When the Sun is overhead the temperature reaches about 500 Celsius, with an average daily temperature of around 200 degrees. With little atmosphere and long nights, temperatures drop rapidly after sunset, getting down to -150 C or so. The absence of a significant atmosphere in combination with the alternation of freezing and frying is very effective at removing water, so the planet's surface is very dry. It was therefore a surprise to find there is ice on Mercury.
It was nearly as big a surprise a few years ago to discover ice on the Moon's surface. Although the Moon's temperatures are not as extreme as those on Mercury, its surface has also become freeze-fry dried. However, a spacecraft surveying the Moon's Polar Regions found ice at the bottom of deep craters that the sunlight never reached. Spacecraft surveying Mercury have found the same thing. There are deep craters around the planet's poles with bottoms perpetually in shadow. Even that close to the Sun, with no solar heat ever getting to them, those crater bottoms are cold, around -170 C, which is cold enough for ice to accumulate. It sounds paradoxical that we can find some of the coldest places in the Solar System on the planet closest to the Sun. Unlike Venus, Jupiter and Saturn, which are currently prominent in our skies, it is actually in the sky now immediately before sunrise, but it's hard to spot because of the brightness of the sky. Since the planet orbits close to the Sun, it is never very far from the Sun in the sky, so we always see it against the sunset or sunrise sky glow.
In the 19th Century, French astronomer Urbain Leverrier proposed the existence of another planet, even closer to the Sun. It was even (appropriately) named, Vulcan, after the Roman god of fire. Always being buried in the Sun's glare it was not expected to be easy to find, so it was only after years of failed or mistaken observations and a lot of wishful thinking, that astronomers finally concluded that Vulcan does not exist. It would have been an amazing world, extremely hot and a very difficult spacecraft destination.
Venus and Jupiter still lie close together in the sunset glow. Saturn lies in the south. Mercury is very low in the sunrise glow. The Moon will reach First Quarter on the 23rd.
Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton.