I have been asked by one reader what I plan to do now that we have looked at the habitability of the Earth compared it with Venus and Mars. I plan to check out the other planets in our solar system one by one in terms of the details of their favourability to life. Then I will return you to Earth safe and sound and we will investigate this “small blue dot” once more to reveal some more astounding facts about its positioning.
As the person who asked said, “Why would you do that Ian, when you have amply demonstrated that even our two nearest neighbours are not suitable for life; it is for sure there will be no life on any planet closer to the sun than Venus or further away than Mars. Simply because I like to be thorough. I like to look into all the details and compare them. You never know what you will turn up.
Mercury is the least understood of the terrestrial planets. Most of what we know of Mercury has been learned within the last 50 years due to the fly past by Mariner 10 in 1974 and then again in the 1975. Following that MESSENGER orbited Mercury 4,000 times over a 4 year period from 2004. These “visits” have provided us with a wealth of information. Unfortunately none of which will encourage Elon Musk to plan a visit.
Our observation of Mercury is complicated by its proximity to the Sun. It is the closest planet to the sun in our solar system. We would not expect life to be present on Mercury given what I wrote last week. It is a mere 57.91 million kilometres from the sun at its closest point. (Earth is 149.6 million kms away from the sun). Mercury is the smallest and innermost planet in the Solar System From our vantage point on Earth, Mercury is lost in the Sun’s glare for much of the time. Mercury can be observed from some parts of the earth for only a brief period during early morning or evening twilight at the best point of its orbit. It is always low on the horizon from a vantage point on Earth.
The sky on Mercury is always black due to the fact it has no atmosphere. A person would be blinded instantly in the bright sunlight and harsh glare. The intensity of sunlight on Mercury’s surface ranges between 5 to 10 times the solar constant. Mercury is too small and hot for its gravity to retain any significant atmosphere over long periods of time. It does have a tenuous surface bounded exosphere containing hydrogen, helium, oxygen, sodium calcium and potassium in larger percentages. However the surface pressure is so minimal the exosphere is unstable. Atoms are continuously lost and replenished from a variety of external sources. Nothing is bound to the planet by virtue of its own atmosphere, which is why it is called an exosphere.
Mercury’s orbital period around the Sun of 87.97 days, is the shortest of all the planets in the Solar System. Furthermore it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two Mercurian years. Having no atmosphere to retain heat, it has surface temperatures that vary diurnally more than on any other planet in the Solar System. Mercurian temperatures range from 100 Kelvin (−173 °C or −280 °F) at night to 700 Kelvin (427 °C or 800 °F) during the day across the equatorial regions. The polar regions are constantly below 180 K (−93 °C or −136 °F).
Mercury’s axial tilt is virtually zero with the best measured value as low as 0.027 degrees. This is significantly smaller than that of Jupiter which has the second smallest axial tilt of all planets at 3.1 degrees. This means that to an observer at Mercury’s poles, the centre of the Sun never rises more than 2.1 arc minutes above the horizon. At certain points on Mercury’s surface, an observer would be able to see the Sun peek up about halfway over the horizon, then reverse and set before rising again, all within the same Mercurian day. Remember there is only one such day every two years. There are two points on Mercury’s equator, 180 degrees apart in longitude, at either of which, around perihelion in alternate Mercurian years (once a Mercurian day), the Sun passes overhead, then reverses its apparent motion and passes overhead again, then reverses a second time and passes overhead a third time, taking a total of about 16 Earth-days for this entire process. Then it is gone for another two years.
The varying distance to the Sun leads to Mercury’s surface being flexed by tidal bulges caused by the sun’s gravitational pull which are 17 times stronger than the Moon’s gravitational on Earth. Combined with a 3:2 spin-orbit ratio of the planet’s rotation around its axis, it also results in complex variations of the surface temperature. The combination makes a single solar day on Mercury last exactly two Mercurian years, or about 176 Earth days.
Mercury’s surface appears heavily cratered and is similar in appearance to the Moon’s, indicating that it has been geologically inactive for a long time. Mercury has experienced intense crater formation, receiving impacts over its entire surface ranging in diameter from small bowl-shaped cavities to multi ringed impact craters hundreds of kilometres in diameter. They appear in all states of degradation, from relatively fresh craters to highly degraded crater remnants. Mercurian craters differ from lunar craters in that the area blanketed by their ejecta is much smaller, a consequence of Mercury’s stronger surface gravity not allowing the detritus to fly very far from the point of impact. Warning take your hard hat with you.
That is the end of my dossier on your new home on Mercury. I think you will agree: scratch Mercury off the list of habitable planets. I think what surprised me the most in my investigation was the variation in temperature diurnally despite the fact that the sun is that much closer. It doesn’t bode well for our future investigative expeditions does it?
Next Nugget we will move on to Jupiter.
(Nudge, nudge, wink wink; just looking at the photo of the planets in the real estate agent’s window above I know which one I would choose.)