scientific studies

[OPINION] Paying respect to our own star

Kelvin S. Rodolfo
[OPINION] Paying respect to our own star

SUNSET. A view of skyscrapers Shanghai Jin Mao Tower and Shanghai World Financial Center during a sunset in Shanghai, China, November 23, 2021.

Aly Song/Reuters

Virtually all the energy that powers Earth’s surface and its living cargo is solar

The following is the 36th in a series of excerpts from Kelvin Rodolfo’s ongoing book project “Tilting at the Monster of Morong: Forays Against the Bataan Nuclear Power Plant and Global Nuclear Energy.

Virtually all the energy that powers Earth’s surface and its living cargo is solar, so let’s start exploring Earth’s surface environment and its history by paying respect to our own star.  A few basic facts:

Three spheres govern how much solar energy each planet receives. First, the Sun’s surface, which emits a huge amount of heat every second: 1026 calories (written out, the number is a 1 followed by 26 zeros).  

For reference, a calorie is how much heat raises the temperature of one milliliter of water by one degree Celsius. In practical terms, to heat a liter of water from room temperature to boiling takes about 75,000 calories. (This is not the diet Calorie, which is a thousand little calories, perhaps so dieters won’t feel too intimidated.)

Those 1026 calories radiate out into space in all directions.  Reaching Earth almost 150 million kilometers away, think of all this heat as having been spread out over a vast sphere in which Earth, our third sphere, sits and receives her tiny share.  

We call the intensity of sunlight reaching Earth its “solar constant.”  The last picture does the math: all the Sun’s radiation divided by the area of the Sun-Earth sphere yields our solar constant, 0.033 calorie per square centimeter per second.  

Under this solar heating Earth was neither too hot nor too cold for life to begin and endure.  Could the solar constants of Venus or Mars also support life now or in the past? Let’s compare them. Keeping things simple, let’s just refer to Earth’s solar constant as 100% of the solar energy it receives.  

Comparing Earth’s solar constants with those of Venus and Mars

Mercury, the planet closest to the Sun, plays only a minor role here.  It has no atmosphere, so facing the Sun at noon its temperature is 427°C. At midnight it is -173°C, horribly, inconceivably frigid. Common sense tells us that how much and how intensely solar energy arrives at each planet depends on how far it is from the Sun.  But doubling the distance doesn’t simply cut the energy in half; it reduces the solar constant to 1/4th; tripling the distance reduces it to 1/9th; quadrupling it reduces it to 1/16th, and so on.  (Some readers may recognize this as the “inverse square of distance” rule that also governs gravitational and magnetic attractions and other forces.) That is why the solar-intensity curve drops off so drastically with distance from the Sun.  Mercury receives almost seven times as much solar heating as Earth; Venus almost twice as much; and Mars a little less than half.

Now let’s compare how hot the inner planets actually are.  The green dots in this next diagram would be the surface temperatures of the planets if they were controlled only by their solar constants. But compare them to their actual surface temperatures, the red dots.

We see that Earth would be well below the freezing point of water if its solar constant was the only controlling factor.  Fortunately, our Greenhouse Effect keeps our actual average surface temperature comfortably in the range of liquid water.  

Mars has a very thin atmosphere.  At its surface it weighs only 0.65% of Earth’s. Its small Greenhouse Effect does warm it a little, but for humans, Mars would be still viciously frigid. 

Venus is the real shocker, hotter even than Mercury at high noon! The immediate lesson? The planets are far from passive in how they respond to solar heating.

Earth and Venus: ‘Twin planets?’

During my adolescent years of the early 1950’s, science-fiction magazines published stories about Venus, the “Planet of Love”. One fondly-remembered magazine cover sported playful, beautiful Venusian maidens scantily clad in filmy attire…

Back then, the stories didn’t seem too far-fetched. After all, the Venus solar constant, only about twice ours, predicted a 40°C surface temperature – an uncomfortably hot 104° Fahrenheit, yes, but still livable (if people wore filmy garments).

Back then, all we could see of Venus through our strongest telescopes was the cool top of its atmosphere, white, faintly tinted yellow.  Surely, it would reflect some of the sunlight back 0ut to space, keeping the planet below 40°C.

We didn’t need to go to Venus to know its dimensions; astronomers have been measuring them long-distance for centuries. And in all the big ways, Earth and Venus are twins. Compare them:

Even the pull of gravity on Venus is only a little less than ours. A fictional Venusian maiden could probably leap a little higher than one of our ballerinas of similar size and strength could on Earth.     

Our understanding of Venus changed drastically in the 1960s as the Space Age began. In 1962 Mariner II from the US made the first “flyby” past Venus, radioing back that it has no magnetic field, and its surface is very hot.  By 2007, four other US satellites had gathered data during flybys past Venus enroute to other planets. 

In 1978 the NASA Pioneer program sent four small probes through the Venus atmosphere down to the planet surface, and put a satellite in Venusian orbit that radioed back atmospheric and surface data until 1992. NASA’s Magellan orbiter made accurate maps of the surface gravitational field and topography from 1990 to 1994.

From 1961 to 1984 the Soviet Union explored Venus much more elaborately, furtively and expensively, seemingly obsessed with conquering the planet.  I was reminded of this in 2022 when Russia, trying to overwhelm Ukraine, threw thousands of troops and vehicles into lethal combat, casualties be damned. 

The USSR’s Venera program included at least 18 failed missions.  But they did land many probes. Some sent back much data before the enormous heat and pressure on the planet surface would crumple them in less than an hour.

So, after all the space missions, what do we know about Venus now? Let’s compare Earth and Venus some more.

Earth’s record hottest and coolest surface temperatures are 58 and -88 ° C. The average is 22° C, so much of Earth’s water is liquid, fortunately for us.

The hellish 464°C surface temperature of Venus is hot enough to melt tin, lead and even zinc.  Apart from that heat, volcanoes are active, inspiring this artistic landscape:

The picture says nothing about how powerfully the Venus atmosphere presses down on the planet’s surface. Earth’s atmospheric pressure is 14.7 pounds per square inch, about a kilogram per square centimeter. We don’t notice it because it’s the same both outside and inside our bodies.

But the Venus atmosphere is 93 times more massive! To experience similar pressure on Earth, dive a kilometer below our ocean surface.

Earth’s atmospheric composition is also very different from those of her neighbors: 

Why these great differences? Life on Earth! Gaia! We see why and how in our next foray. –

Born in Manila and educated at UP Diliman and the University of Southern California, Dr. Kelvin Rodolfo taught geology and environmental science at the University of Illinois at Chicago since 1966. He specialized in Philippine natural hazards since the 1980s.

Keep posted on Rappler for the next installment of Rodolfo’s series.

Previous pieces from Tilting at the Monster of Morong:

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