Category The Earth, Earth Science, Planet Earth

WILL LIFE ON EARTH GO ON FOR EVER?

Life on Earth cannot go on forever because it depends on the Sun and, like all stars, our Sun will eventually die. However, that will happen billions of years in the future. In the meantime, we need to be concerned about the way in which we are using our planet now, so that it will continue to provide a home for all the living things that share it with us in the next century and beyond.

The biological and geological future of Earth can be extrapolated based upon the estimated effects of several long-term influences. These include the chemistry at Earth’s surface, the rate of cooling of the plant’s interior, the gravitational interactions with other objects in the Solar System, and a steady increase in the Sun’s luminosity. An uncertain factor in this extrapolation is the ongoing influence of technology introduced by humans, such as climate engineering, which could cause significant changes to the planet. The current Holocene extinction is being caused by technology and the effects may last for up to five million years. In turn, technology may result in the extinction of humanity, leaving the planet to gradually return to a slower evolutionary pace resulting solely from long-term natural processes.

The luminosity of the Sun will steadily increase, resulting in a rise in the solar radiation reaching the Earth. This will result in a higher rate of weathering of silicate minerals, which will cause a decrease in the level of carbon dioxide in the atmosphere. In about 600 million years from now, the level of carbon dioxide will fall below the level needed to sustain C3 carbon fixation photosynthesis used by trees. Some plants use the C4 carbon fixation method, allowing them to persist at carbon dioxide concentrations as low as 10 parts per million. However, the long-term trend is for plant fe to die off altogether. The extinction of plants will be the demise of almost all animal life, since plants are the base of the food chain on Earth.

In about one billion years, the solar luminosity will be 10% higher than at present. This will cause the atmosphere to become a “moist greenhouse”, resulting in a runaway evaporation of the oceans. As a likely consequence, plate tectonics will come to an end and with them the entire carbon cycle. Following this event, in about 2–3 billion years, the planet’s magnetic dynamo may cease, causing the magnetosphere to decay and leading to an accelerated loss of volatiles from the outer atmosphere. Four billion years from now, the increase in the Earth’s surface temperature will cause a runaway greenhouse effect, heating the surface enough to melt it. By that point, all life on the Earth will be extinct. The most probable fate of the planet is absorption by the Sun in about 7.5 billion years, after the star has entered the red giant phase and expanded beyond the planet’s current orbit.

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WHAT HAPPENS DURING A LUNAR ECLIPSE?

          A lunar eclipse occurs when the Earth comes directly between the Sun and the Moon. As the Moon moves through Earth’s shadow, the planet prevents direct sunlight from reaching the surface of the Moon. The Moon does not disappear but turns red because Earth’s atmosphere bends the Sun’s rays. A lunar eclipse can occur only on the night of a full moon. The type and length of a lunar eclipse depend on the Moon’s proximity to either node of its orbit.

          During a total lunar eclipse, Earth completely blocks direct sunlight from reaching the Moon. The only light reflected from the lunar surface has been refracted by Earth’s atmosphere. This light appears reddish for the same reason that a sunset or sunrise does: the Rayleigh scattering of bluer light. Due to this reddish color, a totally eclipsed Moon is sometimes called a blood moon.

          Unlike a solar eclipse, which can only be viewed from a relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of Earth. A total lunar eclipse can last up to nearly 2 hours, while a total solar eclipse lasts only up to a few minutes at any given place, due to the smaller size of the Moon’s shadow. Also unlike solar eclipses, lunar eclipses are safe to view without any eye protection or special precautions, as they are dimmer than the full Moon.

          Earth’s shadow can be divided into two distinctive parts: the umbra and penumbra. Earth totally occludes direct solar radiation within the umbra, the central region of the shadow. However, since the Sun’s diameter appears about one-quarter of Earth’s in the lunar sky, the planet only partially blocks direct sunlight within the penumbra, the outer portion of the shadow.

          A penumbral lunar eclipse occurs when the Moon passes through Earth’s penumbra. The penumbra causes a subtle dimming of the lunar surface. A special type of penumbral eclipse is a total penumbral lunar eclipse, during which the Moon lies exclusively within Earth’s penumbra. Total penumbral eclipses are rare, and when these occur, the portion of the Moon closest to the umbra may appear slightly darker than the rest of the lunar disk.

          A partial lunar eclipse occurs when only a portion of the Moon enters Earth’s umbra, while a total lunar eclipse occurs when the entire Moon enters the planet’s umbra. The Moon’s average orbital speed is about 1.03 km/s (2,300 mph), or a little more than its diameter per hour, so totality may last up to nearly 107 minutes. Nevertheless, the total time between the first and the last contacts of the Moon’s limb with Earth’s shadow is much longer and could last up to four hours.

          The relative distance of the Moon from Earth at the time of an eclipse can affect the eclipse’s duration. In particular, when the Moon is near apogee, the farthest point from Earth in its orbit, its orbital speed is the slowest. The diameter of Earth’s umbra does not decrease appreciably within the changes in the Moon’s orbital distance. Thus, the concurrence of a totally eclipsed Moon near apogee will lengthen the duration of totality.

          A central lunar eclipse is a total lunar eclipse during which the Moon passes through the centre of Earth’s shadow, contacting the anti-solar point. This type of lunar eclipse is relatively rare.

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WHY ARE THERE SO MANY VOLCANOES ON VENUS?

          Venus is covered by hundreds of thousands of volcanoes. This is because the surface of the planet is a thin skin floating on hot molten rock. This lava is vented wherever possible, meaning that, unlike Earth, Venus has volcanoes everywhere. Most of these volcanoes are around 3km (2 miles) wide and 90m (395ft) high, but there are over 160 much larger than this. Some volcanoes on Venus are over 100km (60 miles) in diameter! The volcanic activity on Venus means that the surface of the planet is always changing.

          The surface of Venus is dominated by volcanic features and has more volcanoes than any other planet in the Solar System. It has a surface that is 90% basalt, and about 65% of the planet consists of a mosaic of volcanic lava plains, indicating that volcanism played a major role in shaping its surface. There are more than 1,000 volcanic structures and possible periodic resurfacing of Venus by floods of lava. The planet may have had a major global resurfacing event about 500 million years ago, from what scientists can tell from the density of impact craters on the surface. Venus has an atmosphere rich in carbon dioxide, with a density that is 90 times greater than Earth’s atmosphere.

          Even though there are over 1,600 major volcanoes on Venus, none are known to be erupting at present and most are probably long extinct. However, radar sounding by the Magellan probe revealed evidence for comparatively recent volcanic activity at Venus’s highest volcano Maat Mons, in the form of ash flows near the summit and on the northern flank. Although many lines of evidence suggest that Venus is likely to be volcanically active, present-day eruptions at Maat Mons have not been confirmed.

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WHY IS VENUS LIKE A GREENHOUSE?

          Less than 20% of sunlight falling on Venus breaks through the clouds. Despite this, Venus has the hottest surface temperature of any planet in the Solar System. This is because infrared radiation (heat) released from the planet cannot escape back into space. The atmosphere traps heat inside, like the glass in a green-house, meaning that the temperature is over 400°C (750°F), greater than it would he if Venus had no atmosphere.

          Greenhouse involving carbon dioxide and water vapor may have occurred on Venus. In this scenario, early Venus may have had a global ocean if the outgoing thermal radiation was below the Simpson-Nakajima limit but above the moist greenhouse limit. As the brightness of the early Sun increased, the amount of water vapor in the atmosphere increased, increasing the temperature and consequently increasing the evaporation of the ocean, leading eventually to the situation in which the oceans boiled, and all of the water vapor entered the atmosphere. This scenario helps to explain why there is little water vapor in the atmosphere of Venus today. If Venus initially formed with water, the greenhouse would have hydrated Venus’ stratosphere, and the water would have escaped to space. Some evidence for this scenario comes from the extremely high deuterium to hydrogen ratio in Venus’ atmosphere, roughly 150 times that of Earth, since light hydrogen would escape from the atmosphere more readily than its heavier isotope, deuterium. Venus is sufficiently strongly heated by the Sun that water vapor can rise much higher in the atmosphere and be split into hydrogen and oxygen by ultraviolet light. The hydrogen can then escape from the atmosphere while the oxygen recombines or bonds to iron on the planet’s surface. The deficit of water on Venus due to the runaway greenhouse effect is thought to explain why Venus does not exhibit surface features consistent with plate tectonics, meaning it would be a stagnant lid planet. Carbon dioxide, the dominant greenhouse gas in the current Venusian atmosphere, owes its larger concentration to the weakness of carbon recycling as compared to Earth, where the carbon dioxide emitted from volcanoes is efficiently sub ducted into the Earth by plate tectonics on geologic time scales through the carbonate-silicate cycle, which requires precipitation to function.

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WHAT WAS THE MAGELLAN MISSION?

          The most detailed information about Venus was acquired by a space probe called Magellan. Launched in 1989, Magellan travelled to Earth’s neighbour and spent three years building a complete map of the planet. Flying as low as 294km (183 miles) above the surface, Magellan bounced radar pulses off the solid ground beneath and sent the data back to Earth to he analyzed. It measured strips of land 24km (14 miles) wide and 10,000km (6000 miles) long each time it circled the planet, while its altimeter measured its height above the surface.

          The Magellan spacecraft was the first planetary explorer to be launched by a space shuttle when it was carried aloft by the shuttle Atlantis from Kennedy Space Center in Florida on May 4, 1989. Atlantis took Magellan into low Earth orbit, where it was released from the shuttle’s cargo bay and fired by a solid-fuel motor called the Inertial Upper Stage (IUS) on its way to Venus. Magellan looped around the Sun one-and-a-half times before arriving at Venus on August 10, 1990. A solid-fuel motor on the spacecraft then fired, placing Magellan into a near-polar elliptical orbit around Venus.

          Spacecraft carried a sophisticated imaging radar, which was used to make the most highly detailed map of Venus ever captured during its four years in orbit around Venus from 1990 to 1994. After concluding its radar mapping, Magellan also made global maps of Venus’s gravity field. Flight controllers then tested a new maneuvering technique called aero braking, which uses a planet’s atmosphere to slow or steer a spacecraft. The spacecraft made a dramatic plunge into the thick, hot Venusian atmosphere on October 12, 1994, and was crushed by the pressure of Venus’s atmosphere. Magellan’s signal was lost at 10:02 Universal Time (3:02 a.m. Pacific Daylight Time) that day.

          The Magellan mission was divided up into “cycles” with each cycle lasting 243 days (the time necessary for Venus to rotate once under the Magellan orbit). The mission proceeded as follows:

Magellan Assembly

          On May 4, 1989, the Magellan spacecraft was deployed from the shuttle. The spacecraft is topped by a 3.7-meter (12-foot) diameter dish-shaped antenna that was a spare part left over from the Voyager program. The long, white, horn-shaped antenna, attached just to the left of the dish antenna, is the altimeter antenna that gathers data concerning the surface height of features on Venus. Most of the spacecraft is wrapped in reflective white thermal blankets that protect its sensitive instruments from solar radiation. 

Deployment

          The Magellan spacecraft’s deployment from the shuttle Atlantis’ cargo bay was captured by an astronaut with a hand-held camera pointed through the shuttle’s aft flight deck windows. Deployment occurred in the early evening of May 4, 1989, after Atlantis had carried Magellan and its Inertial Upper Stage (IUS) booster rocket, into low Earth orbit. Once the shuttle was safely away from the spacecraft, the IUS ignited and placed Magellan on course for its 15-month journey to Venus.

Magellan Orbiting Venus

          On August 10, 1990, Magellan entered into orbit about Venus, as depicted in this artist’s view. During its 243-day primary mission, referred to as Cycle 1, the spacecraft mapped well over 80 percent of the planet with its high-resolution Synthetic Aperture Radar (SAR). The spacecraft returned more digital data in the first cycle than all previous U.S. planetary missions combined.

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HOW CAN WE SEE PAST VENUS’ CLOUDS?

          Venus’ atmosphere is formed from clouds of carbon dioxide, nitrogen and sulphuric acid. This heavy layer of clouds is over 30km (18 miles) deep in some places, meaning that no part of the planet’s surface can be seen with the naked eye. Only since the 1970s have scientists been able to “look” past these clouds to see the solid ground beneath. This has mainly been done with equipment mounted on space probes. Radar technology allows probes to record the geography of the planet, and to produce a map of surface features.

          The clouds of Venus are its defining characteristic. We can see the surface of Mars and Mercury, but the surface of Venus is shrouded by thick clouds. For most of history, astronomers had no idea what was beneath those clouds, and they imagined a tropical world with overgrown vegetation and constant rainfall. They couldn’t have been more wrong.

          The climate of Venus isn’t tropical at all; it’s hellish. Temperatures on the surface of Venus approach 475°C and the atmospheric pressure is 93 times what you experience here on Earth. To experience that kind of pressure, you would need to swim down 1 km beneath the surface of the ocean. Venus’ atmosphere is made almost entirely of carbon dioxide, and not the oxygen/nitrogen mix we have here on Earth.

          The clouds we see on Venus are made up of sulfur dioxide and drops of sulfuric acid. They reflect about 75% of the sunlight that falls on them, and are completely opaque. It’s these clouds that block our view to the surface of Venus. Beneath these clouds, only a fraction of sunlight reaches the surface. If you could stand on the surface of Venus, everything would look dimly lit, with a maximum visibility of about 3 km.

          The upper cloud deck of Venus is between 60-70 km altitudes. This is the part of Venus that we see in telescopes and visible light photographs of the planet. The clouds on Venus rain sulfuric acid. This rain never reaches the ground, however. The high temperatures evaporate the sulfuric acid drops, causing them to rise up again into the clouds again.

          Venus spacecraft have detected lightning on Venus, coming out of the clouds with a similar process to what we have on Earth. The first bursts of lightning were detected by the Soviet Venera probes and later confirmed by ESA’s Venus Express spacecraft.

Picture Credit : Google