This image comes from the innermost planet of the solar
system, and shows a map of illumination around the South Pole of Mercury.
NASA’s description of the image states that it is coloured “on the basis of the
percentage of time that a given area is sunlit. Areas appearing black in the
map are regions of permanent shadow.”
This is an interesting image because darkness and light are
very important to our understanding of Mercury. This is because the cycle of day and night there isn’t
like that on the Earth. We are used to a year being much longer than a day, as
our planet rotates on its axis hundreds of times for each revolution around the
sun. However this is not the case on all solar system bodies.
For a long time it was thought that the day on mercury was
the same length as its year. This would mean that, much like the relationship
between the Moon and the Earth, one side would always face the sun, while the
other would be in perpetual darkness. This process is called tidal locking, as
the tidal forces exerted by the two objects gradually shift the orbital and
rotational speeds until they form a resonance. In the case of the Earth-moon
system this resonance is 1:1. The planet rotates once for every trip around the
sun, and thus the same face of the moon is always visible from the Earth.
This seemed to be the case on Mercury. Numerous telescopic
observations confirmed that the same face of the planet always seemed to be
pointed away from the sun. It appeared to be tidally locked. If this were also true
on mercury then it would make it a very unusual environment. Since mercury has
very little atmosphere to speak of, the redistribution of heat from the sun
facing day side to the colder night side would be expected to be quite slow. Temperatures
vary massively ranging from around -170 oC at night, to over 400 oc
in the day time. If water exists on mercury, it would thus be expected to only
be found on the night side, as any that made its way to the day side would
quickly evaporate and be lost to space.
This model for day and night on Mercury informed our
understanding of the planet’s environment right up until the 20th
century. However in the mid 1960s it became apparent that, like many early models,
it didn’t quite describe the reality. Radar observations were conducted to
measure the rotation of the planet, and concluded that mercury was rotating on
its axis substantially faster than previously thought. A year didn’t seem to be
one day long, but rather three. This was a very big change, and initially it
seemed to contradict vast amounts of observational data. Mercury had been known about since antiquity,
and so has been studied extensively for as long as telescopic observations have
been possible. The same face had always seemed to point towards the sun. Astronomers
had examined this wealth of data, and seen a clear pattern, which fit well with
a certain model of how mercury’s orbit could work, namely a 1:1 resonance. But
how could this be the case if it was rotating at the speed these new
observations suggested?
As is often the case in science it turned out that a “confounding
variable” was influencing our previous observations. The incorrect model
assumed that the reason we always saw the same face of mercury was because of
its orbital resonance. In fact there was another reason which hadn’t been
considered. Mercury and Earth are both in orbit around the sun, but it takes
them different lengths of time to complete a trip around the sun. The result of
this is that they are not always close together. There are times when Earth and
Mercury pass close to one another, and other periods when they are much further
apart. Close approaches make detailed observations more practical, while at
other times of year it is much harder to see one planet from the other. This meant
that the times at which all of the historical observations had been made
conformed to a certain pattern, relative to the orbital properties of the two
planets.
Mercury’s orbital resonance meant that, like clockwork,
whenever it passed close to the Earth, the same face was always pointed towards
the sun. It wasn’t until new observational techniques could be used, that it
became clear that this property of Mercury’s orbit was confounding the previous
observations, and had led to an inaccurate model of the system.
We now know that Mercury is actually in a 3:2 resonance with
the sun. This means that it rotates around its axis three times, for every two orbits around the sun.
This doesn’t seem very good for our chances of finding water on Mercury. All of the planet’s
surface will become staggeringly hot during the day when it is directly
illuminated by the sun, before dropping massively at night. Not only can there
not be an entire night side covered with glaciers and ice, but will there be
any water at all?
Fortunately for anyone who might want to visit Mercury in
the future, the image we started with shows that there are places which are always in the dark. Because of the angle
of the sun, the polar craters shown in the image we started with are always in
shadow. Chao Meng-Fu, a 180 km impact crater near the South Pole is considered
very likely to contain water ice, and so would be a prime site for a future
mission to the closest planet to the sun.
Image
Credit: NASA/Johns
Hopkins University Applied Physics Laboratory/Carnegie Institution of
Washington https://photojournal.jpl.nasa.gov/catalog/PIA19416
No comments:
Post a Comment