Venus: Future Earth?

In terms of bulk properties, Venus is the most Earthlike planet in the solar system. The diameter of Venus is 95% of Earth’s diameter. The mass of Venus is 82% of Earth’s mass. It has a nearly identical composition.

But…the average surface temperature of Venus is 735 K (863˚ F) and the surface atmospheric pressure is 91 times greater than Earth’s—equivalent to the pressure 3,000 ft. below the ocean’s surface. The present atmosphere of Venus is composed of 96.5% carbon dioxide (CO2) and 3.5% nitrogen (N2), plus a number of trace elements and compounds.

Venus was not always so inhospitable. What happened?

The cratering record suggests that nearly all of Venus has been resurfaced within the last 300 – 800 Myr. Before that, Venus probably was much more hospitable, even habitable, perhaps. The Pioneer Venus large probe and infrared spectral observations from Earth of H2O and HDO (deuterated isotope of water) indicate that the deuterium-to-hydrogen ratio in the Venusian atmosphere is 120 – 157 times higher than in water on Earth, strongly suggesting that Venus was once much wetter than it is today and that it has lost much of the water it once had to space. (Hydrogen is lighter than deuterium and therefore more easily escapes to space.) In addition to deuterium abundance, measuring the isotopic abundance ratios of the noble gases krypton and xenon would help us better understand the water history of Venus. These cannot be measured remotely and requires at-Venus sampling.

Venus receives 1.92 times as much solar radiation as the Earth, and this was undoubtedly a catalyst for the runaway greenhouse effect that transformed the Venusian climate millions of years ago.

We know that CO2 is a potent greenhouse gas, but anything that increases the amount of water vapor (H2O) in the atmosphere leads to global warming as well. As do clouds.

Climate modeling shows us that that the hothouse on the surface of Venus today is due to CO2 (66.6%), the continual cloud cover (22.5%), and what little water vapor remains in the atmosphere (10.9%).

Interestingly, if all the CO2 and N2 in the Earth’s crust were somehow liberated into the atmosphere, our planet would have an atmosphere very similar to Venus.

Venus is the easiest planet to get to from Earth, requiring the least amount of rocket fuel. There is so much we still don’t understand about how Venus transformed into a hellish world, and we would be well-advised to learn more about Venus because it may inform us about Earth’s future as well.

Tessera terrain covers about 7% of the surface of Venus. These highly deformed landforms, perhaps unique in the solar system, may allow us to someday sample the only materials that existed prior to the great resurfacing event.

COLORIZED TOPOGRAPHIC DATA OVERLAID UPON FORTUNA TESSERA TERRAIN IMAGE
In this radar image, blue represents the lowest elevations, white the intermediate elevations, and red the highest elevations. Source: Emily Lakdawalla, https://www.planetary.org/blogs/emily-lakdawalla/2013/02071317-venus-tessera.html .

If living organisms ever developed on Venus, the only place they could still survive today is 30 miles or so above the surface where the atmospheric temperature and pressure are similar to the surface of the Earth.

Even four billion years ago, Venus may have been too close to the Sun for life to develop, but if it did, Venus probably remained habitable up to at least 715 Myr ago.

Now for the bad news. All main-sequence stars, including our Sun, slowly brighten as they age, and their habitable zones move outward from their original locations. Our brightening Sun will eventually render the Earth uninhabitable, certainly within the next two billion years, and our water could be lost to the atmosphere and then space within the next 13o million years, leading to a thermal runaway event and an environment similar to that of Venus. Human-induced climate change could make the Earth uninhabitable for humans and many other species long before that.

One indication that water is being lost to space and surface warming is occurring is water vapor in the stratosphere. The more water vapor that is in the stratosphere, the more water is being forever lost to space and the greater the surface warming. Careful and continuous monitoring of water vapor levels in the Earth’s stratosphere is important to our understanding of climate change on Earth.

To conclude, Arney and Kane write:

“Venus teaches us that habitability is not a static state that planets remain in throughout their entire lives. Habitability can be lost, and the runaway greenhouse is the final resting place of once watery worlds.”

References

Arney, G., & Kane, S. 2018, arXiv e-prints, arXiv: 1804.05889

Bézard, B., & de Bergh, C. 2007, J. Geophys. Res., 112, E04S07, doi: 10.1029/2006JE002794.

Ostberg, C., & Kane, S. R. 2019, arXiv e-prints,arXiv: 1909.07456

Way, M.J. 2019, EPSC Abstracts, 13, EPSC-DPS2019-1846-1

Way, M. J., Del Genio, A. D., Kiang, N. Y., et al. 2016, Geophys. Res. Lett., 43, 8376

Habitable Zones

One common definition of the habitable zone of a star is the range of distances from the star where liquid water could exist on the surface of a planet (where the planetary surface temperature ranges between 0° and 100° C [273.15 – 373.15 K]).

Of course, atmospheric pressure affects the temperature range for liquid water.  For example, at 3% of sea level atmospheric pressure, water boils at 26.4° C, not 100° C.  But at 68 atmospheres, water stays liquid until it reaches a scalding temperature of 285° C.  At the other end of the liquid water spectrum of temperatures, the freezing point of water only increases to 0.01° C from 1 atm all the way down to 0.006 atm.  At atmospheric pressures below 0.006 atm, liquid water can’t exist: the only phases that can be present are solid and gas.  At higher pressures, all the way up to about 99 atm, the freezing point of water remains at 0° C.  Then, from 99 atm up to 2,072 atm, the freezing point of water lowers to -21.9° C.  Then it goes back up to 0° C again at 6,241 atm.  Above 70,000 atm, H2O can exist only in solid form.

So, the range of temperature where liquid water can exist is generally smaller at lower atmospheric pressure, and greater at higher atmospheric pressure.

Substances dissolved in the water, called solutes, can also change the range of temperatures where liquid water can exist.  And, who’s to say that life couldn’t exist with only water ice or water vapor in the environment?

And what about life beneath the surface of a planet, moon, asteroid, comet, etc.?  It seems reasonable to suggest that subsurface liquid water exists on more worlds than liquid water on the surface.

And does life always require H2O to exist?

Determining the “habitable zone” of a star is complicated.  That’s why we often narrow it down to just where terrestrial life could exist.

So, for now, let’s stick with that.

As you might expect, many factors enter into the equation: some relate to the star (e.g. size and surface temperature and hence bolometric luminosity), and some relate to the planet (e.g. atmospheric composition & density, and albedo).  A liberal definition might say that the habitable zone in our solar system lies between the orbits of Venus (0.7 AU) and Mars (1.5 AU).

If one accepts this, then the calculation of the habitable zone around any other star is straightforward:

where

R1 is the inner radius of the habitable zone, in astronomical units
R2 is the outer radius of the habitable zone, in astronomical units
r* is the radius of the star, in solar radii
t* is the effective temperature of the star’s photosphere, in Kelvin

Here’s an example that’s made big news lately: seven planets very similar in size to the Earth have been discovered orbiting the red dwarf star TRAPPIST-1, located 39 light years from our solar system in the direction of the constellation Aquarius.  The estimated size of the star is 0.117 solar radii, and the estimated effective temperature 2559 K.  Using the above equations, we get R1 = 0.016 AU and R2 = 0.034 AU. Thus, using our approach, it appears that planets TRAPPIST-1d (0.772 R) and TRAPPIST-1e (0.918 R) are most likely to be within the star’s habitable zone.