Small green men live on Mars and the inhabitants of Venus feed on nothing but air and love. These are some of the ideas that people had about Earth’s celestial neighbors towards the end of the 19th century. In 1877, the Italian astronomer Giovanni Schiaparelli observed dark lines on the surface of Mars, which were interpreted as artificially created irrigation channels. The supposed existence of these channels gave rise to the idea that Mars was a planet inflicted by drought whose civilization was struggling to survive. Venus, on the other hand, was seen as a younger version of Earth with a mild climate and rich vegetation because of its opaque atmosphere. There was even conjecture that Venus might be home to dinosaurs. It was only with the improvement of the available remote sensing instruments and, finally, the deployment of space probes that it became clear that there are no civilizations on Mars and Venus. However, to this very day, scientists have not given up their search for life.
Why have no astronauts been to Mars and Venus yet?
In theory, answering the question as to whether there is life on Earth’s neighboring planets is quite simple: Astronauts simply have to fly there, have a good look at what they find, collect samples and then return to Earth. Scientists subsequently analyze the samples and make the discovery of the century. Or not, as the case may be.
In practice, the whole thing is a little more complicated. The distance alone makes astronautical missions to Mars and Venus technically challenging. A journey to Earth’s neighboring planets may sound like merely a stone’s throw, but with the propulsion technology available today, it comes close to the boundaries of feasibility. Our earthly ideas of “near” and “far away” quickly reach their limits when it comes to distances in space.
On average, the Moon is a good 384,000 kilometers away from Earth. That this counts as extremely close on a cosmic scale can already be seen from the fact that the distance can still be expressed relatively easily in terms of kilometers. When it comes to expressing the distances between Earth and its neighboring planets, using kilometers becomes much more unwieldy. Thus, the smallest distance between Earth and Mars is 55 million kilometers, while the closest Earth and Venus ever come to each other is 38 million kilometers.
To avoid having to deal with huge numbers to describe cosmic distances, the light year is used as a unit of measurement in astronomy, especially in the context of popular science.
Expressed in these terms, the smallest distance between Earth and Mars is 0.0000058 light years or 3.05 light minutes. Earth and Venus are 0.0000040 light years or 2.10 light minutes apart. Consequently, it takes light several minutes to travel from Mars and Venus to Earth. What this means for manned missions is that with the propulsion technologies available today a journey to Venus would, at best, take several months and the journey to Mars more than a year. A particular problem is the fact that outside Earth’s protective atmosphere, astronauts are exposed to elevated levels of radiation. Thus, the amount of radiation on a flight to Mars is roughly the same as lifelong exposure on Earth.
However, the fact that no astronauts have ever visited Venus and Mars is not only due to the duration of the journey and exposure to radiation.
Hot, hotter, Venus
It is repeatedly said that of all the planets in our solar system Venus has the greatest similarity to Earth. Like Earth, Venus is a terrestrial planet and the two planets differ only slightly in size and density. This means that the gravitational acceleration of both planets is roughly the same. But that is about as far as the similarities go.
As the planet in our solar system the second closest to the sun after Mercury, Venus lies just outside the habitable zone. Surface water in liquid form, which is a prerequisite for the development of life, does not occur due to the planet’s proximity to the sun.
A year, i.e. one orbit around the sun, is shorter on Venus than on Earth, meaning that the Venus year has a length of only a good 224 Earth days. There are no seasons as the equator is only slightly inclined in relation to the orbital plane. Venus rotates extremely slowly, meaning that one day on this planet has the same length as 116 Earth days and is thus roughly equivalent to half a Venus year. Venus has a retrograde rotation, meaning that it rotates in the opposite direction to almost all other planets in the solar system. Venus does not have any moons.
Venus is the only terrestrial planet in our solar system to have a constantly opaque atmosphere consisting of a good 96% carbon dioxide, the rest being primarily nitrogen. Compared to Earth’s atmosphere, the atmosphere on Venus has a significantly greater mass and thus has a pressure of 92 bar at the mean ground level. This corresponds to the pressure which Earth has at a sea depth of almost 920 meters.
The opacity of the atmosphere is caused by the constantly closed cloud layer. It starts at about 50 kilometers above the surface of Venus and extends to a height of 70 kilometers. Unlike clouds on Earth, which consist primarily of water vapor, the clouds on Venus are composed mainly of droplets of sulfuric acid. There are also aerosols containing chlorine and phosphorus. The sulfuric acid rains down regularly but does not reach the surface of Venus due to the high temperatures. The droplets evaporate just below the cloud cover and decompose into sulfur dioxide, water vapor and oxygen. These gases then rise to the top cloud areas, where they again react with each other and condense to form sulfuric acid.
Although the dense cloud cover reflects a large part of the solar radiation, the average surface temperatures of 460 °C exceed even those on Mercury, which is closer to the sun. This is due to the strong greenhouse effect, which is primarily caused by the high quantities of carbon dioxide in the atmosphere. Without this greenhouse effect, an equilibrium temperature of about -50 °C would prevail on Venus.
Life in the clouds
Despite its location outside the habitable zone and the extremely hostile environment, some scientists still suspect that there might be life on Venus. But not on the surface – the temperatures there are too extreme even for the most heat-resistant thermophilic life forms – but in the cooler clouds. In the lower cloud layer, the temperatures are a comparatively mild 60 °C and the pressure of only 1 bar is bearable. In addition, the planet’s gaseous coat filters most of the harmful radiation from outer space, making life theoretically possible.
The instruments on board various Venus probes have recorded spectral signatures in Venus’ atmosphere resembling the absorption characteristics of biological molecules on Earth. Bacteria and viruses can be found in Earth’s atmosphere at altitudes of up to about 40 kilometers.
In order to find out whether there is life in the atmosphere of Venus, samples would ideally have to be taken there. So far, no Venus mission has succeeded in doing this. Several planned landing modules were destroyed while still in the atmosphere and even after successful landings the technical equipment withstood conditions on the surface for only a few minutes or hours.
In 2014, NASA presented a plan for a three-phase mission to Venus known as HAVOC. The abbreviation stands for High Altitude Venus Operational Concept. In this project, first robots and then finally astronauts will be exploring Venus from airships hovering in the less hostile atmosphere. Due to its technical complexity, however, the project has so far not been pursued in any greater detail.
Mars as Planet B?
Compared to Venus, Mars appears to be much more hospitable. Alongside Mercury, Venus and Earth, it is the fourth terrestrial planet, with a diameter of just under 6,800 kilometers, making it only about half the size of Earth. In combination with its comparatively low density, this small size causes gravitational acceleration of 3.69 meters per second squared, which is only a good third of gravitational acceleration on Earth.
Like Venus, Mars is just outside the habitable zone, but on the opposite side far from the sun. The planet takes just under 687 Earth days to orbit the sun. At the same time, its orbit deviates relatively sharply from a circular shape: After Mercury, Mars has the most eccentric orbit of all the planets in our solar system. A Mars day is also referred to as a sol and, at 24 hours, 39 minutes and 35 seconds, is roughly as long as an Earth day. The planet’s equatorial plane is inclined relative to its orbital plane, which is why different seasons can be observed. However, the eccentricity of its orbit means that these seasons are of different lengths in the northern and southern hemispheres. Thus, the summer lasts 179 sols in the northern hemisphere but only 154 sols in the southern hemisphere.
Mars is orbited by two small moons known as Phobos and Deimos (Greek for “fear” and “terror”, respectively). They are probably both former asteroids trapped by Mars’ gravitational force.
Mars has no magnetic field and its atmosphere, which consists mainly of carbon dioxide, is extremely thin. On the surface of the planet, the atmospheric pressure is only 0.006 bar. This is equivalent to the pressure on Earth measured at an altitude of more than 35 kilometers. The low atmospheric pressure means that water evaporates even at low temperatures and cannot exist on the surface in liquid form. In addition, the thin atmosphere is largely permeable to cosmic rays. During the day, solar radiation reaches the surface almost unfiltered and is released back into space at night, something which is accompanied by strong temperature fluctuations. Close to the equator, temperatures reach about 20 °C during the day but drop to -85 °C at night. The average temperature on Mars is -55 °C.
One consequence of the daily and seasonal temperature fluctuations are strong winds that regularly envelop Mars in dust clouds, occasionally even assuming global dimensions. Despite in some cases extremely high wind speeds, storms on Mars have considerably less force than on Earth and carry only small particles with them. The reason for this is again the thin atmosphere. Even so, the dust storms pose a threat to exploration missions on Mars. In extreme cases, sun light can be blocked for months, cutting off probes and autonomous vehicles from their source of energy and resulting in their loss.
Mars’ reddish color is due to hematite, a mineral that contains iron and is formed from the originally dark mineral magnetite as a result of oxidation processes. In other words, the surface of Mars is covered in rust. Whether this was caused by liquid water once present on Mars or solely by mechanical effects has not yet been conclusively determined.
Life under the surface?
The surface of Mars is constantly being sterilized by the cosmic rays which enter the atmosphere almost unfiltered. Accordingly, higher or even intelligent forms of life are not possible on the planet. On the other hand, the existence of simple organisms is quite conceivable given that life can be found in similarly extreme conditions on Earth. Examples are the tardigrades or cyanobacteria of the genus Croococcidiopsis. As polyextremophilic life forms, they tolerate extreme dryness, sharp temperature fluctuations, high radiation and other hostile environmental conditions. Experiments with tardigrades have shown that they can survive a stay in space unprotected. On Mars, similar organisms could also exist in the ground or under the ice caps at the poles.
The ExoMars-mission, which is a joint project of ESA and the Russian space agency Roscosmos, is currently actively searching for traces of life on Mars. While early work was confined to observation from the surface of Earth together with speculation, modern scientific inquiry has emphasized the search for water, chemical biosignatures in the soil and rocks of the planet’s surface and biomarker gases in the atmosphere.