Let us first consider the air we breathe on Earth.
It is a mixture of gases, as well as water vapour (from 0% to 4%) and dust.
The main gases are:
– Nitrogen (N₂): 78.08%, a molecule made of two nitrogen atoms
– Oxygen (O₂): 20.95%, a molecule made of two oxygen atoms
– Argon (Ar): 0.93%, a noble gas—so called because it does not bind into molecules (its outer electron shell is complete, with the 8 electrons allowed)
– Carbon dioxide (CO₂): about 0.04%, made of one carbon atom and two oxygen atoms.
With rounding, the total comes to 100%, but there are also traces of other gases, such as neon (Ne), helium (He), methane (CH₄), ozone (O₃). From everyday experience we know its elastic properties, which make it compressible and give concrete meaning to the concept of pressure (force applied on a surface), but we rarely think about its weight.
One litre, i.e., one cubic decimetre, of dry air at sea level and at the standard temperature of 15°C weighs about 1.2 grams. Stated like that it sounds like little, but it means that 1 cubic metre weighs 1.2 kg. That is why the kilometres of air above our heads ‘weigh’: as you go up the gases become increasingly rarefied and the air weighs less, but we are talking about tens of kilometres, if not more (to give a more precise estimate one would have to set a limit in pressure or in specific weight). This weight, obviously determined by gravitational attraction, is substantial: the standard pressure of 1 atmosphere corresponds to about 1 kg of weight per square centimetre. In other words, for every square centimetre of our body (a little less than an average fingernail) the acting force is equivalent to 1 kg of weight.
We are not crushed by it and we scarcely notice it because from the inside our body exerts an equal and opposite pressure: we are essentially like balloons that do not burst because the pressures balance. In addition, pressure acts in all directions, not only vertically, so we are not ‘pushed’ one way, we receive the push from every side.
The instrument for measuring atmospheric pressure was invented in 1643 by Evangelista Torricelli (1608–1647), a student of Galileo Galilei (1564–1642). He observed that a transparent tube closed at one end and filled with mercury, when inverted into a basin also containing mercury, maintained a height of about 76 cm, as in Figure 1. Therefore, the weight of the mercury column in the tube had to be equivalent to the weight of the air. The standard unit of measurement for atmospheric pressure is the Pascal (or the hectoPascal, hPa, equal to 100 Pascals, called millibar in meteorology as a submultiple of the bar). One atmosphere, defined as the average pressure at sea level, corresponds to 1,013 hPa, and is the simplest measure for quick explanations. For example, as you gain altitude the trend is not linear because, as mentioned, the air becomes lighter; but at first the pressure halves roughly every 5,500 metres. To give easily testable figures, at 1,500 metres above sea level the pressure is about 84%, at 3,000 metres it is about 69%. That is to say: at 1,500 metres altitude the pressure difference (negative) is comparable to that of a couple of metres of water depth—this is why, even when going into the mountains, it is often necessary to equalize the pressure on our eardrums.
Figure 1 – Torricelli’s barometer (source: ecoage.it, under CC Creative Commons licence)
In this post I will not go into meteorological questions, but it is clear that depending on humidity content and temperature this weight changes, determining differences between locations that generate weather phenomena: wind, clouds, rain, air-mass fronts, etc.
Returning to weight, by analogy with the measure of one atmosphere, note that it corresponds to a column of water about 10 metres high (because of the large difference in density between the two fluids: mercury is almost 14 times as dense as water). Thus, the pressure of the kilometres of air above us is equivalent to 10 metres of water; that is, when we dive, at 10 metres depth we experience a pressure about double what it was before we entered the water. Okay, this is not something people normally do, but anyone who has dived even just a few metres has clearly felt the external pressure (and the need to equalize the internal pressure in the ears). With simple multiplication, this means that at 50 metres depth the “weight” is 5 kg per cm², and at 1,000 metres it is 100 kg per cm².
In the Mariana Trench, where the bathyscaphe Trieste (built in Italy, Figure 2) first arrived with the exploration pioneers Jacques Piccard and Don Walsh in 1960, at nearly 11,000 metres depth, the pressure was over one tonne per cm². To withstand it, the steel hull was about 13 cm thick!
Figure 2 – Bathyscaphe Trieste (source: repubblica.it)
And how much does the air weigh on Mars?
The Martian atmosphere is composed of about 95% carbon dioxide (CO₂) (oxygen is only about 0.13%). The pressure is very low, about 1% of Earth’s, around 6 hPa compared with our 1013 hPa. It might seem like a paradise for plants that ‘breathe’ CO₂, but aside from the fact that at night plants also breathe oxygen (so they would suffocate), the very low pressure and extremely low temperature (−60°C) would not allow water to be liquid, so the plant’s biochemistry would come to a halt as well.
A significant problem with very low pressure for the human body is the so-called Armstrong limit (about 63 hPa), below which water and blood (at body temperature) would begin to ‘boil’ spontaneously because the external pressure is too low to keep them in the liquid state. A walk on Mars without a pressurized suit is strongly discouraged, regardless of the temperature and the lack of oxygen! On Earth this limit is reached at around 18 km of altitude.
On Jupiter things ‘get worse’ considerably: first of all there is no solid surface; the planet is entirely gaseous, made of about 90% hydrogen (H₂) and 10% helium (He), plus traces of other gases (which are what determine the coloured bands and spots we observe, see Figure 3). Moving inward from the outer layers, because of pressure these gases become liquids and temperatures become extremely high, perhaps 30,000°C near the core, consisting of molten metallic hydrogen (i.e., behaving like a molten metal, for example mercury), which produces its intense magnetic field (20 to 50 times Earth’s and more than twice the Sun’s, except in sunspots, where it is about 300 times stronger). The pressure, which on the outside is of the same order of magnitude as ours, likely reaches 100 million times that toward the centre.
Jupiter is basically a failed star, a terrible place.
Figure 3 – Jupiter (source: NASA / Space Telescope Science Institute, 2017)
