Can Venus or Mercury sustain life?
By Meagan Jorgenson, Kelsey Weninger, Dana Macpherson, Dylan Fafard, Cody Georget and Yuliya Kozachok
The atmospheres of Venus and Mercury, when compared to Earth, are significantly different; these differences create environments that are unsuitable for life.
An Introduction to Venus and Mercury
Venus and Mercury, along with Mars and Earth, are considered terrestrial planets; they are composed primarily of silicate rock and metals. These four planets are also considered the inner planets because they orbit close to the Sun, especially compared to the outer planets. These similarities often lead to Venus and Mercury being referred to as ‘Earth-like’. However, there are significant differences in the atmospheres of Venus and Mercury, as compared to Earth, which make them unsuitable for life.
Venus is similar to Earth in respect to density, size, mass, and volume. However, the two planets differ when it comes to their atmosphere, which could explain why Venus has been, to our knowledge, unable to sustain life. The axial tilt of Venus is 177°, equivalently an axial tilt of 3° with the rotation backwards; for comparison, Earth has an axial tilt of 23°. The axial tilt of a planet determines the duration and severity of its seasons. Earth’s axial tilt is significant, meaning that the hemisphere that is tilted towards the Sun receives more energy than the one tilted away. In contrast, Venus’ axial tilt is insignificant, meaning that no hemisphere receives more energy from the Sun than the other. The daytime and nighttime sides of Venus remain at an average of 470°C at the surface. This is interesting given that Venus rotates very slowly, about once every 243 Earth days. This leaves the nighttime side of the planet without sunlight for an extended period. However, due to extreme winds of up to 224 mph, clouds are able to circulate the planet every 4 days which distributes the heat of the planet more evenly.1 This prevents the nighttime side of the planet from cooling too much. In addition to the winds, Venus also has a very thick atmosphere, and the greenhouse gases in it (mainly carbon dioxide) act as an insulating blanket; this keeps surface temperatures roughly the same everywhere on the planet, regardless of whether it is day or night. Thus, there are no cooler spots on Venus, and life would be forced to deal with the extreme heat, without reprieve.
In 1966, Venus experienced the first impact of an artifact on the surface of another planet. That artifact was the unmanned Venera 3 atmospheric probe. Between 1966 and 1982, the former Soviet Union conducted the Venera series of missions that sent atmospheric and lander probes to Venus. The data collected during these missions helped scientists develop new and exciting theories about the planet.
The Venera Missions
Science from Venera 419
Carbon dioxide 90-95%
Molecular oxygen 0.4-0.8%
Water vapor 0.1-1.6%
Temperature 270-280°C @ point of crash
Pressure 20 kg/cm2 or 15-22 atm @ point of crash
No radiation belts, magnetic fields found
The Venera 4 mission was the first of Soviet Union’s many probes that successfully studied Venus’ surface and atmosphere.2 It was the first probe to provide in-place analysis of another planet’s environment. It provided data that showed the Venusian atmosphere consists primarily of carbon dioxide, as well as nitrogen, oxygen, and water vapor. Upon first inspection, this atmospheric composition does not preclude the suitability of life. However, the Venera 4 also provided direct measurements that demonstrated the extreme heat of Venus and the tremendous density of the atmosphere. Although the capsule was designed to withstand extreme g-forces and temperatures, Venera 4 experienced a malfunction and stopped sending data before it landed on the surface. Venera 5 and 6 experienced similar fates. However, Venera 5 was able to detect a light level of 250 Watts per square meter inside the atmosphere, which is roughly one-quarter that of Earth’s average.3 This smaller amount of light reaching the surface of Venus could hamper the productivity of photochemical processes needed for some types of life.The transmitter of the Venera 7 probe was affected by the dense atmosphere and, as a result, sent very weak signals. It wasn’t until a month later that the descent signal tapes were reviewed, and it was found that Venera 7 had transmitted information from the surface of Venus. This made it the first probe to safely reach the surface of Venus. It found that Venus has a very dense atmosphere and a pressure at the surface of 92 standard atmospheres, which is far greater than scientists had originally estimated. To give some perspective on this value, a pressure of 92 standard atmospheres is similar to being under 1000 meters of water.4 Venera 7 also measured a surface temperature of 475 °C and surface winds of 2.5 m/s.3
The Venera missions found that carbon dioxide, a greenhouse gas, makes up 95% of Venus’s atmosphere; this is the main cause of a greenhouse effect on Venus. The Bond albedo of Venus is 0.90, so only 10% of light penetrates the atmosphere. Venus’ albedo makes it difficult for longwave radiation to penetrate its atmosphere, however, shortwave radiation is met with much less resistance. The shortwave radiation that reaches the surface of the planet heats the ground and is re-emitted as infrared radiation. The greenhouse gases in the atmosphere absorb and trap this infrared radiation.5 Although this occurs on Earth as well, the extreme amounts of carbon dioxide in the Venusian atmosphere only let a much smaller fraction of the infrared radiation escape into space. This effect creates an extremely hot and steady surface temperature that is enough to easily melt lead, and makes the planet unsuitable for life. The melting temperatures of different proteins, an essential part of all living organisms, varies, but temperatures above 41 °C cause them to break down; thus, organisms would require extreme survival mechanisms to combat the heat on Venus.6
Venera 11 found sulfuric acid and chlorine in the cloud layers, as well as carbon monoxide at low altitudes in the atmosphere.7 Each of these chemicals poses a danger to organic compounds and/or organic processes.
Venera 11 and 12 were equipped with the GROZA instrument, which was used to measure the sounds on Venus; wind, thunder, and lightning were detected. However, Venus is the only planet whose lightning is not associated with water clouds, but clouds of sulfur dioxide and droplets of sulfuric acid.8 Venera 11 found that lightning flashes every 25 seconds somewhere in the planet’s atmosphere, and Venera 12 identified 1,200 strikes altogether.1 The differences in atmospheric composition, pressure, temperature, wind speeds, and source of lightning, compared to Earth, combine to create a very hostile environment, which would be unsuitable for life as we currently understand it.
Developments since Venera
Many new astronomical techniques have been developed since the Venera missions. One technique, infrared heterodyne spectroscopy, has been particularly useful in observing weather characteristics of Venus with unprecedented accuracy. This technique focuses solely on incoming light in the infrared frequency range, then mixes that light with laser light at a similar (but not the same) frequency, and generates a difference pattern between the two frequencies in the radio-frequency range.9 This allows astronomers to take a very close look at the infrared frequencies of an object. This is akin to using a microscope to study an object; only a small portion of it is visible, but at a large magnification level, which reveals very fine details of the object. Using this technique, the Heterodyne Instrument for Planetary Wind and Composition (HIPWAC) project can determine the chemical composition of planetary atmospheres, measure planetary winds, determine atmospheric profiles (i.e., how gas abundance, pressure, and temperature change with altitude), and measure photochemical processes.9 HIPWAC was involved in the first direct measurement of sub-solar and anti-solar winds, winds that are not directly caused by solar winds, on Venus, which were measured to an accuracy of roughly 2 m/s at an altitude of 110 km.10 Infrared spectroscopy was also used to measure and remotely monitor the abundance of sulfur dioxide below the clouds of Venus, between altitudes of 35-45 km; this sulfur dioxide is a likely tracer of Venusian volcanism. The results of this new spectroscopy have been consistent with laboratory and modeling studies.11 They are also consistent with, and often more accurate than, the findings from the Venera missions.
An Introduction to Mercury
Mercury, like Venus, is categorized as both an inner planet and a terrestrial planet. Of the four terrestrial planets within our Solar System, Mercury orbits closest to the Sun. It has a very thin atmosphere; the density and composition of its atmosphere is a result of its physical size and the strength of its magnetic field. Mercury’s radius, which can be measured using a telescope, is approximately one-third that of Earth’s, making it barely larger than our moon. As a result, Mercury’s surface area is only 15% that of Earth’s. The surface consists primarily of plains and craters from the collision of asteroids and comets.12 The persistence of craters is evidence that the planet has a virtually nonexistent atmosphere, and remains unprotected from large or high energy impacts.
NASA’s MESSENGER probe collected a massive amount of data about Mercury during its three flybys and four years in orbit of the planet. Scientists were able to combine radio tracking and topographic data to determine that Mercury has a large, metallic, partially liquid, rotating core.13,14 Scientists were then able to model Mercury’s magnetic field using their knowledge of its core composition and dynamo theory, which says that a rotating, convecting, and electrically conducting fluid can maintain a magnetic field. A strong magnetic field is important for the development and maintenance of an atmosphere. Earth’s magnetic field is able to slow down and trap high-energy charged particles from the Sun, creating what is called the Van Allen Belt.15 Due to Mercury’s core composition and small size, the strength of its magnetic field is more than 100 times weaker than Earth’s.16 This is far too weak to produce a similarly protective belt, so damaging radiation from the Sun is able to reach the surface of the planet. Charged particles that reach the surface can cause surface material to be ejected high above the planet, but not high enough to escape Mercury’s gravity. This results in heavier elements (e.g. sodium and magnesium) being added to the atmosphere.17 NASA’s MESSENGER and Mariner 10 probes provided an abundance of data that scientists could use to explain why Mercury has an atmosphere, and why the atmosphere is made up of heavier elements. Mercury’s thin atmosphere is created by the combination of solar winds, its weak – but dynamic – magnetosphere, and its gravity.
The Mariner 10 was launched in 1973 and flew by Mercury three times. It was equipped with an onboard ultraviolet spectrometer, which was able to collect atmospheric data from airglow and occultation. Using these data, an upper bound of the atmospheric pressure at Mercury’s surface was calculated to be about 5 quadrillion times less than Earth’s.12 Mercury’s atmosphere is so thin that it has been classified as a surface-bounded exosphere. This means that the density is so low that a particle has a higher chance of escaping into space than colliding with another particle. Its atmosphere is constantly in a state of flux; it’s being stripped away by strong solar winds and replenished by a bombardment of charged particles, radioactive decay of elements, and dust from meteorite impacts.
The Mariner 10 and MESSENGER probes were used to observe ultraviolet radiation through a photometer and an imaging mass spectrometer. This provided evidence of oxygen, hydrogen, helium, potassium, water vapor, and silicon in Mercury’s atmosphere.18 Because Mercury has such a thin atmosphere, it is unable to store and regulate incoming solar radiation; therefore, the surface temperature fluctuates between 427 °C on the side that faces the Sun, and -173 °C on the night time side.12 This could be why Mercury has been nicknamed “the planet of extremes”. These extreme temperature fluctuations, along with a weak magnetosphere and thin atmosphere, create a very hostile and inhospitable environment on Mercury.
Studying Mercury and Venus’ atmospheres, and comparing them to Earth’s, can help us appreciate that certain atmospheric conditions are essential to making a planet hospitable for life. Venus’ atmosphere contains some components vital for life, such as carbon dioxide and nitrogen; however, its extremely high density and convection currents create an environment so hot that unprotected organic materials would melt, boil, and vaporize. Similarly, Mercury’s atmosphere, or lack thereof, contains some components vital for life, such as oxygen and water vapor. However, Mercury’s extremely thin atmosphere and lack of protection from solar winds create an environment that is simply too barren and fluctuating for life to begin or survive.
1N.T. Redd, Space.com (2012), (http://www.space.com/18527-venus-atmosphere.html), Web. Mar. 2016
2D.E. Reese and P.R. Swan, Science 159, 1228 (1968).
3B. Harvey, Astronomische Nachrichten 34, 367 (1996).
4 Wikibooks, Open Books for an Open World (2010), (https://en.wikibooks.org/wiki/Solar_System/Venus), Web. Mar. 2016
5M. Snels, S. Stefani, D. Grassi, G. Piccioni, and A. Adriani, Planetary And Space Science 103, 347 (2014).
6 T.A. Holme, Encyclopedia.Com (2004), (http://www.encyclopedia.com/topic/denaturation.aspx), Web. Mar. 2016
7N.L. Johnson, American Astronautical Society 47, 276 (1979).
8D. Brown and M. Talevi, NASA (2007). (http://www.nasa.gov/vision/universe/solarsystem/venus-20071128.htm), Web. Mar. 2016.
9 Goddard Space Flight Centre, HIPWAC. (http://ssed.gsfc.nasa.gov/hipwac/howitworks.html), Web. Mar. 2016.
10 NASA Goddard Space Flight Centre, – Lasers And the Dynamic Mesosphere/Thermosphere of Venus, (2010), (http://ntrs.nasa.gov/search.jsp?R=20100031081), Web. Mar. 28, 2016.
11 B. Beard, C. de Bergh, F. Bruce, D. Crisp, J. Maillard, T. Owen, J.B. Pollack, and D. Grin spoon, Geophysical Research Letters 20, 1587 (1993).
12 Universe Today (2015), (http://www.universetoday.com/13943/mercury/), Web. Mar. 2016.
13 T. Tolbert, NASA (2016). (http://www.nasa.gov/mission_pages/messenger/media/PressConf20120321.html), Web. Mar. 2016.
14 N.L. Chabot, E.A. Wollack, R.L. Klima, and M.E. Minitti, Earth And Planetary Science Letters 390, 199 (2014).
15P. Grego, Venus And Mercury, and How to Observe Them (Springer, New York, 2008).
16 Y. Kasaba, Jaxa, (http://www.stp.isas.jaxa.jp/mercury/document/Mercury_quantities.pdf), Web. Mar. 28, 2016.
17 Student Science (2016), (https://student.societyforscience.org/article/mercurys-magnetic-twisters), Web. Mar. 2016.
18D.J. Stevenson, Nature 485, 52 (2012).
19B. Harvey, Russian Planetary Exploration: History, Development, Legacy, Prospects (Springer, Berlin, 2007).
20G. Elert, Pressure On the Surface of Venus (2000), (http://hypertextbook.com/facts/2000/NangMiu.shtml), Web. Mar. 2016.
21B. Steigerwald, NASA (2011), (http://www.nasa.gov/topics/solarsystem/features/venus-temp20110926.html), Web. Mar. 2016.
22 Pamela Elizabeth Clarke (2007), Dynamic Planet-mercury in the context of its environment. Springer.
23F. Cain, Universe Today (2009), (http://www.universetoday.com/36721/weather-on-venus/), Web. Mar. 2016.
24 D.A. Rothery, Planet Mercury: From Pale Pink Dot to Dynamic World (Springer, Cham, 2015), Web. Mar. 2016.
25 B. Dunbar, NASA (2009), (http://www.nasa.gov/mission_pages/messenger/multimedia/magnetic_tornadoes.html), Web. Mar. 2016.