Terrestrial Tip of the Cap

In B science fiction movies, a terrible force often pushes the Earth off its axis and spells disaster for all life on Earth. In reality, life would still be possible on Earth and any Earth-like planets if the axis tilt were greater than it is now, according to Penn State researchers.

Ultraviolet image of Venus obtained by Pioneer-1. Venus is a planet with no seasons, since it has no notable axial tilt.
Image Credit: BNSC

"We do not currently have observations of extrasolar planets, but I imagine that in the near future, we will uncover some of these small planets," says Darren M. Williams, assistant professor of physics and astronomy, Penn State Erie, the Behrend College. "The issue before us is what will they be like? Will they have moons? What will their climates be like? Will they be teaming with life or will life be rare?"

"I suspect, based on simulations and our own solar system, that many Earth-like planets will have spin axes that are tipped more severely than Earth’s axis."

Axis Shift

Williams, working with David Pollard, research associate in geoscience at Penn State, used general circulation climate models to simulate a variety of tilts, carbon dioxide levels and planets. They reported on their findings in the International Journal of Astrobiology.

The tilt of the Earth, at a 23.5-degree inclination, is the phenomenon responsible for the change of seasons. As the Earth orbits the Sun over the course of a year, the planet is exposed to patterns of solar radiation that mark the seasons. Much higher variation in the Earth’s tilt would generate greater extremes between summer and winter, and might be considered a major impediment to the evolution of complex life. To simulate these effects, the researchers first looked at present-day Earth with tilts varied between 23, 54, 70 and 85 degrees. The simulation that mimicked today’s Earth and tilt closely matched today’s climate, including regional precipitation patterns, snow and ice cover and drought.

"Tilts greater than the present produce global annual-mean temperatures higher than Earth’s present mean temperature of about 57 degrees Fahrenheit," says Williams. "Above 54 degrees of tilt, the trend is for the global annual-mean temperature to decrease as tilt increases."

The Penn State scientist explains that this decrease occurs because more land exists north of the equator in present-day Earth. Annual-mean temperatures, however, are not the best way to determine if a planet might be habitable, as seasonal temperature variations could be extreme.

This visible-light image, taken by NASA’s Viking spacecraft, shows a close-up Martian cliff with its striking mixture of dry-ice (carbon dioxide) and potentially water-ice nearer the northern cap Credit: NASA/JPL/Viking

The researchers also looked at these tilted Earths with 10 times the carbon dioxide in the atmosphere. Carbon dioxide as a greenhouse gas increases the temperatures on a planet. These models produced Earths with 11 to 18 degrees Fahrenheit higher annual-mean temperatures. This simulation is not far from present conditions on the planet Venus, but Venus has no tilted axis, and thus no winter, summer, nor any seasonal change.

Because all planets will not have Earth’s geography, the researchers took a page from Earth’s history and modeled a 750-million-year-old Earth representing the Sturtian glaciation and a 540-million-year-old Earth, the closest approximation available for the Varanger glaciation.

"During the Sturtian [named after the Sturt River Gorge, where a debris layer made up of angular to sub-rounded rocks of various types in a cement-like matrix were found], land masses were mainly equatorial and clumped mostly within 30 degrees of the equator," says the Penn State Erie researcher. "In the Varanger model, everything is close to the south pole."

While current day Earth is about 30 percent land to 70 percent water, these ancient geographies are about 22 percent land and 78 percent water.

"The highest temperatures and seasonal variations happen with the largest land areas at the mid to high latitudes," says Williams. The researchers also ran some of the model Earths with zero tilt.

"Present Earth is one of the most uninhabitable planets that we have simulated," says Williams. "Approximately 8.7 percent of the Earth’s surface is colder than 14 degrees Fahrenheit on average, and this percentage peaks at 13.2 percent in February owing to the large land masses at high latitude covered by snow."

The only planets colder than today’s Earth are those planets simulated with no tilt.

The Varanger simulation, with most land in the southern hemisphere, is the most extreme with 15.6 percent of the surface below 14 degrees Fahrenheit in July and 9.3 percent of the surface above 122 degrees Fahrenheit in January. On average, nearly 28 percent of this planet’s land mass is uninhabitable by Earth standards.

Life Without Seasons?

"This simulation suggests that planets with either large polar supercontinents or small inventories of water will be the most problematic for life at high obliquity," says Williams. Obliquity is the angle at which a planet is tilted on its axis. The higher the obliquity, the greater the tilt.

None of the planets with increased tilt had permanent ice sheets near the equator. This, however, does not guarantee that a world is suitable for life, the researchers note. The extremes of temperature on most of the simulated earths would make it difficult for all but the simplest Earth life forms to survive. Extremes caused because the tilt puts large portions of the planet in 24-hour darkness or 24-hour sunlight for long periods would also inhibit photosynthetic organisms.

The researchers suggest that even with high tilt, life can exist on the planets they modeled.

Scientists hypothesize that liquid water burst out from underground, eroded the gullies, and pooled at the bottom of the Newton Crater (shown above) as it froze and evaporated. If so, life-sustaining ice and water might exist even today below the Martian surface.
Credit: NASA

"Provided the life does not occupy continental surfaces plagued seasonally by the highest temperature, these planets could support more advanced life," the researchers say. "While such worlds exhibit climates that are very different from Earth’s, many will still be suitable for both simple and advanced forms of water-dependent life."

Today, both Mars and Earth are tilted at approximatelly the same angle: Earth at 23.5 degrees, Mars at 25.2. But unlike Earth, Mars’s obliquity changes over time. Over the course of tens of millions of years, Mars’s obliquity is believed to fluctuate between standing almost straight up and leaning over as much as 60 degrees. Scientists have long known the most important seasonal change on Mars is the autumn and winter "freezing out" of carbon dioxide from the atmosphere in the form of dry-ice frost and snow.

When its tilt is high, the poles get the lion’s share of the planet’s sunlight. And the poles just happen to be where most of Mars’ water is found. The axis of Mars slowly tilts over one hundred thousand-year cycles. As Mars tilts over, the poles warm and the mid-latitudes get colder, and then when the axis of Mars tilts back up again, the situation reverses itself, with the poles growing colder and the mid-latitudes growing warmer. During these cycles, water migrates between the polar regions and the mid-latitudes.

Today, of course, that water is frozen solid. But during periods of high obliquity, the polar regions get enough sunlight to raise the temperature significantly. Up to minus 20 degrees C, perhaps even higher. In other words, at periods of high obliquity, ground ice in the Martian polar and high-latitude regions should warm up enough for thin films of water to form. And those thin films of water, as some speculate, might provide a suitable environment in which Martian microbes could live.

So there is no reason to eliminate Earth-like planets with more tilt than Earth from future searches for life beyond the solar system. For instance, it is not only the Earth’s tilt but the proximity of its nearest celestial object that makes the planet hospitable: our own Moon is believed to play an important role in Earth’s habitability . Because the Moon helps stabilize the tilt of the Earth’s rotation, it prevents the Earth from wobbling between climatic extremes. Without the Moon, seasonal shifts would likely outpace even the most adaptable forms of life.

"We have one planet and we have a lot of species on this planet, but it is only one data point," says Williams. "Maybe one day we will figure out everything about life on our own planet, but no where near what is possible elsewhere."

The National Science Foundation supported this work.

Related Web Pages

Mars: Tilting Towards Life?
Evidence of Snow on Mars
Extrasolar Planets Catalog