We're in for a cold snap here in northern Minnesota. Gusty winds and temperatures in the double digits below zero are forecast for the coming days and nights. Cold like that can be dangerous, so many plan to stay inside. For those headed outdoors — and that includes us skywatchers — cover up and keep moving.

Vostok Station in Antarctica seems an appropriate place for Earth's record low temperature. Occasional colder readings have been detected remotely by satellites along Antarctic mountain ridges since that time, but the Vostok temperature remains the lowest direct measurement. (NASA)
Vostok Station in Antarctica seems an appropriate place for Earth's record low temperature. Occasional colder readings have been detected remotely by satellites along Antarctic mountain ridges since that time, but the Vostok temperature remains the lowest direct measurement. (NASA)

Cold is relative, of course. Here in Duluth in February, the average daily low is 9° (-13° C). In Fairbanks, Alaska it's -8° (-22° C), while in Yakutsk, Russia, considered the coldest large city on Earth, the average high temperature during the shortest month is -20° (-29° C) with lows around -38° (-39° C). Heck, that's cold enough to freeze mercury!

While those numbers may make you shiver, the lowest temperature directly recorded on Earth is −128.6° F (-89.2° C) at the Soviet Vostok Station in Antarctica on July 21, 1983. Since carbon dioxide freezes at -109°, I wondered whether it might have fallen as snow or condensed into frost that July day. It turns out that there's so little CO2 in the atmosphere that under earthly air pressures, it remains a gas well below its freezing point. Darn!

During fall and winter, water and carbon dioxide fall and condense as flakes and ice in the Martian polar regions. This is the north polar cap which is overlain by a layer of dry ice with water ice underneath. (NASA / JPL-Caltech / Malin Space Science Systems)
During fall and winter, water and carbon dioxide fall and condense as flakes and ice in the Martian polar regions. This is the north polar cap which is overlain by a layer of dry ice with water ice underneath. (NASA / JPL-Caltech / Malin Space Science Systems)

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That's not true on Mars, where carbon dioxide routinely condenses as frost or falls from clouds that envelop the polar regions during the winter months. The Red Planet is not only much colder than Earth with wintertime polar temperatures around -220° F / -140° C, but its thin atmosphere exerts very little pressure.

As a disturbing aside of what can happen in extreme environments, in 1959, a game of chess between two scientists at Vostok turned deadly when the loser grabbed an ice axe and murdered his opponent. The Russian government responded by banning chess at all Russian Antarctic research stations.

Mars gets colder than the Earth for a couple reasons. It's farther from the sun and has an extremely thin, dry atmosphere just 1 percent as thick as Earth's. The air warms up a modest amount during the day but quickly loses heat at night. The planet also lacks large bodies of water that would help retain heat the solar heat. Earth has all of the above in abundance.

During the day, some of the sun's energy bounces off the Earth back into space, but much of it penetrates the atmosphere and warms the planet. At night, much of the energy escapes back into space, but some is trapped inside the atmosphere by greenhouse gases and further warms the Earth. (NASA)
During the day, some of the sun's energy bounces off the Earth back into space, but much of it penetrates the atmosphere and warms the planet. At night, much of the energy escapes back into space, but some is trapped inside the atmosphere by greenhouse gases and further warms the Earth. (NASA)

Thanks to carbon dioxide, methane and water vapor, which absorb and trap the heat from the sun, the average global temperature on our home planet is 60° F (16° C). Without these gases it would plummet to 0° F (-18° C). With chocolate as a possible exception, we all know that too much of a good thing can sometimes become a bad thing. Increased levels of CO2 pumped into the atmosphere since the industrial revolution have led to global warming and its consequences.

Outer space is an altogether different place. Populated by scant atoms, it's essentially a vacuum. With no air to distribute heat and equalize temperatures, space is both Death Valley and Antarctic at the same time.

ISS crewmembers keep an eye on a SpaceX Dragon cargo ship from inside the station's cupola. (NASA)
ISS crewmembers keep an eye on a SpaceX Dragon cargo ship from inside the station's cupola. (NASA)

Take the International Space Station (ISS), for example. It orbits 250 miles (402 km) above the Earth in outer space. Infrared light (heat energy) from the sun warms the sun-facing side of the ISS to 250° F (121° C), hotter than a raging boil. At the same time, temperatures on the shadowed side drop to -250° F (-157° C), a difference of 500° F. To keep the astronauts comfortable, the ISS uses a thermal control system and multilayer Mylar-Dacron insulation.

Because space is so empty it doesn't conduct heat like the air or ground does on Earth. Instead, it's dominated by radiation. Things heat up when they absorb solar radiation (sunlight) and then re-radiate the heat back into space. Earth's greenhouse gases trap some of that heat to keep us cozy, but in the case of Mercury, which has no atmosphere, the surface temperature skyrockets to 800° (430° C) during the day and bottoms out at -290° F (-180° C) at night.

We've been talking about outer space near a star like the sun, but how cold is it in a dark corner of the universe far from the toasty stars? Strictly speaking, space has no temperature since it has no mass. Temperature is a measure of how warm or cold an object is like the atmosphere, a popsicle or a person.

While not as cold as outer space, sometimes it feels that way when observing in winter. (Bob King)
While not as cold as outer space, sometimes it feels that way when observing in winter. (Bob King)

But just for giggles, let's take a thermometer out in our starless void and check the reading. It will quickly tumble below zero and keep going until it reaches -454.67° F (-270.43° C). While that's pretty dang cold, it's not absolute zero, the coldest possible temperature. To reach that numbing number we'll need to keep going to -459.67° (-273.15° C). By the way, the sensation we call heat is created by wriggling molecules. If you overheat a meal in the microwave, the water molecules in your food will jiggle with enough fury to burn your tongue! At absolute zero atoms are essentially motionless.

Notice that our coldest parcel of outer space is 4.9° (2.7° C) warmer than absolute zero. The source of that heat takes us back to the Big Bang 13.8 billion years ago, when the universe began as a microscopic "singularity" with a temperature north of 1,000 trillion degrees. As the fireball expanded and cooled, the first subatomic particles could form. When these gathered into the first neutral atoms — a pivotal event that occurred about 380,000 years after the Big Bang — the radiant energy (light) bound up in the expanding universe broke free and streamed across all of space.

Back then, that light was HOT (around 5,000° F) and brilliant, but the expanding universe stretched out its waves, gradually cooling the light down to its current temperature of just 4.9° F (2.7° C) above absolute zero. Astronomers call the ubiquitous leftover radiation the Cosmic Microwave Background (CMB).

This map of the entire sky shows tiny variations in the temperature of the Cosmic Microwave Background captured by the Wilkinson Microwave Anisotropy Probe (WMAP) space probe. (NASA)
This map of the entire sky shows tiny variations in the temperature of the Cosmic Microwave Background captured by the Wilkinson Microwave Anisotropy Probe (WMAP) space probe. (NASA)

We can't see the CMB because as the name implies, it glows in the microwave part of the spectrum. But we've confirmed it's there and nailed down the temperature to five decimal places. Incredibly, physicists and astronomers predicted its existence and temperature before this remnant glow of the Big Bang was discovered in 1964. Finding it helped to confirm the origin of the universe in a blazingly hot, singular event billions of years ago.

All cold ultimately derives from outer space. The sun keeps our planet habitable. Remove it and within a week the average global surface temperature would drop to 0° F (-18° C). In a year it would plummet to -100° (-73° C). Long after that it would settle down to -400° F (-240°), the temperature at which the heat radiating from the Earth's core would be in equilibrium with heat escaping into space.

Stay warm, everybody!

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.