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Thoughts & Observations of a Free Range Astrophysicist

Saguaro at Night

A Saguaro’s universe

Building a cactus starts with the Big Bang

The iconic saguaro cactus gives the desert an otherwordly beauty. That beauty does not exist in isolation. It embodies the fascinating and awe-inspiring processes that have shaped the universe, going all the way back to the Big Bang itself.

This article originally appeared in my Astronomy Magazine column, For Your Consideration.


I live in the desert near the Superstition Mountains, east of Phoenix, Arizona. Not far from my house is a gorgeous Saguaro cactus that I drive by daily. I’ll be honest: I had no clue how big those things were until I stood beside one and looked up! This particular monster has eight arms growing from its 60-foot-tall body.
Wind Patterns

The desert home of the Saguaro owes its existence to many factors, including solar-driven convection, the heat of vaporization of water, the Coriolis effects due to Earth’s rotation, the thermal properties of land and water, ocean currents, and mountain ranges and other terrain that control global wind and weather patterns. The top image shows surface winds on a day in late April. The effects of surface features on wind patterns are clear. The lower frame shows high-altitude winds for the same day that more closely track the large-scale convection patterns that carry heat from the equator toward the poles. (Data from Cameron Beccario. Interactive maps available at https://earth.nullschool.net)

A Saguaro doesn’t even get its first arm until it’s at least 75 years old, and it can live for 200 years or longer. Judging by its splendor, this guy has been around long enough to see European settlement in the Valley of the Sun grow from a few buildings along the Salt River into a sprawling metropolis of 4.6 million people spanning 9,000 square miles.

Which came first, the desert or the cactus?

The Sonoran Desert is the only place on Earth where Saguaros grow. That’s because the cactus and the desert grew up together. As the climate around here became what it is today, plants that fared better survived, reproduced, and passed along their genes. Less successful plants didn’t. Generation after generation, as the climate changed, the Saguaro evolved to keep up.

But wait a minute! Why is this a desert at all? That has a lot to do with the physics of water and convection. Sunlight heats the tropics, driving planetwide convection that carries much of that thermal energy toward the poles. As warm, moist tropical air rises, it also cools, dumping much of its moisture as rain. As water vapor condenses, it releases heat. By the time that once-moist tropical air completes its upward path, it is both dry and a lot warmer than it might have been.

What goes up must come down. As convection carries that air back to lower altitudes, it compresses, turning already relatively warm, dry air into a veritable blast furnace. Welcome to the horse latitudes, latitudes around 30° along which many of the planet’s deserts are located.

A home for the saguaro, spinning in space.

Now we have to worry about Earth’s rotation. As convection carries air north and south, the Coriolis effect diverts that air into strong bands of easterly and westerly winds and the powerful jet streams that carry weather systems west to east around the planet.

And then there’s the star that formed at the center of that rotating, collapsing interstellar cloud. Nuclear reactions in the Sun’s core power the whole show of climate and life.

Anyway, there is a nice jet stream drawing moist air from the Pacific eastward toward the Sonoran Desert. So why is it still dry? The culprits this time are the mountains that surround it. When moist air flows into the side of a mountain, it moves upward. And just like what happens in the tropics, rising air loses its moisture as precipitation and is heated by condensation. When that now dry and relatively warm air comes flowing down the backside of the mountain, well, there’s that blast furnace again!

Crab Nebula

The Crab Nebula is the expanding remnant of a supernova explosion witnessed by Chinese astrologers in 1054 AD. Like you and I, the saguaro is made of atoms that were formed in starts that lived and died long before the Sun and Earth formed.

A living planet is a legacy of stars long dead.

If you want to know about that cactus I smile at when I drive by, you have to know about mountains. That gets us into the forces that shape Earth’s surface. Convection of hot, viscous rock in our planet’s interior carries heat outward from the center toward the surface. The scum floating on top, otherwise known as the crust, gets shoved around by those convection currents. Tectonic plates crash together, forcing mountains skyward. Friction near plate boundaries heats rock that ultimately reaches the surface, building towering volcanoes.

By the way, the energy powering that convection comes from the radioactive decay of atomic nuclei that were synthesized in the hearts of stars that lived and died long before the Sun and Earth formed. The same can be said for the carbon, nitrogen, and every other chemical element apart from hydrogen that makes up my friend the Saguaro.

To know the saguaro is to know the universe.

And so the story goes. If you really want to know about that cactus, ask biologists studying evolution. Ask oceanographers or climate scientists. Ask geologists or planetary scientists. Ask astrophysicists studying the life cycles of stars. Ask cosmologists studying the origin and evolution of the universe. Ask particle physicists who are grappling with the nature of the dark matter, without which our galaxy would have never formed.

There is a gorgeous Saguaro cactus near my house. If you really want to know about it, you kind of have to talk to everybody. To really understand that cactus is to understand the universe.

A Saguaro’s universe ^ Building a cactus starts with the Big Bang  © Dr. Jeff Hester
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Over his 30 year career as an internationally known astrophysicist, Dr. Jeff Hester was a key member of the team that repaired the Hubble Space Telescope. With one foot always on the frontiers of knowledge, he has been mentor, coach, team leader, award-winning teacher, administrator and speaker, to name a few of the hats he has worn. His Hubble image, the Pillars of Creation, was chosen by Time Magazine as among the 100 most influential photographs in history.
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