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

Two Paths to Low Mass

A brown dwarf (also known as an infrared dwarf) is, in a way, a failed star.  Early in their lives, these ultra-low-mass stars (13+ MJ) fuse deuterium into helium-3, and in the highest mass brown dwarfs (65-80 MJ) lithium is depleted into helium-4, as shown below.

But the mass is too low for fusion to be sustained (the temperature and pressure in the core aren’t high enough), and soon the fusion reactions peter out.  Then, only the slow process of thermal contraction provides a source of heat for the wanna-be star.

There is another, very different, path to a brown dwarf star.  A cataclysmic variable usually consists of a white dwarf and a normal star in a close binary system.  As material is pulled off the “donor star” (as the normal star is called) onto the white dwarf, the donor star can eventually lose so much mass that it can no longer sustain fusion in its core, and it becomes a brown dwarf star.

When we see a white dwarf / brown dwarf binary system, how do we know that the brown dwarf wasn’t always a brown dwarf?  Strong X-ray and ultraviolet emission provides evidence of an accretion disk around the white dwarf, and astronomers can calculate the rate of mass transfer between the two stars.  Often, this is billions of tons per second!  Using other techniques to estimate the age of the binary system, we sometimes find that the donor star must have started out as a normal star with much more mass than we see today.