What if Earth had no atmosphere?

Air is all around us. We can't see it, but it is there.

Air keeps Earth warm, like a jacket. What if Earth had no air at all?

You can't see it, but you live at the bottom of an ocean of air. NASA describes Earth's atmosphere as a jacket for the planet: it keeps us warm, gives us oxygen to breathe and is where all our weather happens.

So what would happen if that jacket disappeared? In this lab you can switch the air off in a model of a patch of ground and watch its temperature through a day and a night.

Earth's atmosphere does several jobs at once: its greenhouse gases slow the escape of heat to space, and its winds, together with ocean circulation, carry solar heat from the equator toward the poles (NASA Earth Observatory).

This lab uses a model of one patch of equatorial ground at an equinox. Switching 'air and oceans' off removes both effects at once, leaving bare rock that absorbs sunlight by day and radiates freely at night. How do the day's hottest and coldest temperatures change?

In the day, the Sun warms the ground. At night, the ground cools down.

The air keeps some heat in, so nights don't get too cold.

Near the ground, Earth's air is 78 percent nitrogen and 21 percent oxygen, with a little argon, carbon dioxide and other gases. Some of those gases, called greenhouse gases, hold heat in like a blanket.

In the model, a patch of dry sand on the equator with air around it warms to about 37 °C in the afternoon and cools to about 13 °C before dawn. That is a swing of about 24 degrees between day and night.

NASA gives the near-surface composition as 78 percent nitrogen, 21 percent oxygen and 1 percent other gases, including argon and carbon dioxide. The atmosphere shapes climate and weather, shields the surface from much harmful solar radiation and burns up most meteoroids.

The model's 'with air' case uses albedo 0.30, an effective emissivity of 0.61 and heat sharing of 3 W m⁻² K⁻¹ toward a 288 K mean. With dry sand (0.6 MJ m⁻² K⁻¹) it gives a repeating day from 13.3 °C to 37.0 °C, a swing of 23.7 K.

In our pretend lab, Olly takes the air away.

Now the nights get much colder! The ground loses its heat to space.

Switch 'Air and oceans' off. Now the sand is bare rock under the sky, like the Moon's surface. In the model, the coldest part of the night drops to about 2 °C, and even the afternoon only reaches about 32 °C. The day-night swing grows to about 30 degrees.

Make the ground store less heat, like the Moon's dusty soil, and the change is huge: the model swings from about 84 °C to about −58 °C in a single day.

With air off the model uses albedo 0.12, emissivity 0.95 and no heat sharing. The patch now absorbs more sunlight, but it radiates much more freely and receives no heat from elsewhere, so both extremes fall: the maximum to 31.7 °C and the minimum to 1.8 °C, a 30.0 K swing.

At a heat capacity of 0.1 MJ m⁻² K⁻¹, close to lunar soil, the repeating day runs from 83.6 °C to −58.2 °C. This is a thought experiment; nothing could actually strip Earth's air away in the model's sense.

Warm things give off heat, even at night.

Air catches some of that heat and keeps it close. Without air, the heat zooms away.

Everything warm glows invisible heat, called infrared. At night, the ground keeps glowing heat upward. Greenhouse gases in the air catch some of it and send part back down, so the ground cools slowly.

Winds and ocean currents help too: they carry heat from the warm equator toward the cold poles. In the model, taking all the air away means nothing catches the heat or brings more in, so the ground cools much faster after sunset.

The model integrates C dT/dt = (1 − α)S₀ max(0, cos 2πt/S) − εσT⁴ + k(T̄ − T). Removing the air raises the effective ε from 0.61 to 0.95, so outgoing infrared grows at any temperature, and sets k to zero, cutting the supply of heat from the planet-wide mean.

The model's limits: one patch on the equator at an equinox, no clouds, rain, seasons or real atmospheric layers, and fixed heat sharing. Its temperatures are for comparing worlds, not a climate prediction.

The Moon has almost no air. Its days are very hot and its nights are very cold.

We are lucky to have our air jacket!

We can see a world without air close by: the Moon. It has only a very thin layer of gas, called an exosphere, that you could never breathe. NASA reports that the Moon's surface reaches about 127 °C in full sunlight and plunges to about −173 °C in darkness.

Air does other jobs too. It shields us from much of the Sun's harmful radiation, and most meteoroids burn up in it as shooting stars before they reach the ground.

The Moon's exosphere is not breathable and too sparse to impede impacts, and its surface ranges from about 127 °C in full Sun to about −173 °C in darkness (NASA). That 300 K range is about twice the model's widest 24-hour swing (142 K with lunar-like heat storage); the model does not try to reproduce the real Moon.

NASA also notes that removing carbon dioxide alone would collapse Earth's greenhouse effect and drop its surface temperature by about 33 °C from today's average of about 15 °C.