The last decade has been an exciting one for planetary science as space missions have revealed some tremendous discoveries about the planets in our Solar System. Yet Venus, the closest planet to Earth, stubbornly refuses to reveal its secrets. Long since known as the planet of mystery, Venus has tantalised astronomers for centuries.
Trying to peer through the ever-present clouds to learn about the baking world beneath has proved to be both challenging and frustrating.
In spite of that, some early amateur astronomers made observations and formed theories that now seem strangely prescient.
Their ideas are largely forgotten, yet there seems to be a nugget of truth in one enduring mystery from the 18th century – the supposed sightings of the Himalayas of Venus.
As far as we know, Galileo was the first person to turn a telescope towards Venus, in the winter of 1609–10. Although his telescope only had a power of x30 (less than a pair of modern binoculars), what he saw marked the beginning of the end for the old Earth-centred model of the Solar System.
As he followed Venus, he noticed that the planet had phases ranging from full to a thin crescent – a phenomena that could only occur if Venus orbited the Sun.
An image showing the phases of Venus, captured by Roger Hutchinson, London, UK, 25 March 2017. This image won the Planets, Comets & Asteroids category of the 2017 Astronomy Photographer of the Year competition.
As telescopes became more refined, the planets slowly gave up their secrets: the rings of Saturn were revealed, along with the ice caps of Mars.
Yet Venus showed little surface detail except for some vague elusive streaks. In 1698, Christiaan Huygens was the first to realise that the planet had a thick impenetrable atmosphere that prevented astronomers from seeing anything.
Yet interest in Venus did not diminish, and in particular the German astronomer Johann Hieronymus Schröter observed the planet in great detail.
At about five in the evening on 28 December 1789, Schröter turned his 6.5-inch reflector towards Venus. What he saw was quite unexpected.
The seeing was good and Venus appeared nearly 50 per cent illuminated. The terminator appeared somewhat jagged, but it was the southern pole that caught his attention.
Here, the southern cusp appeared rounded or blunt, but even more startling was the appearance of tiny specks of light on the night side of the southern pole.
Schröter had observed similar things on the Moon – the tops of high mountains catching the Sun’s rays while the rest lay in shadow was a familiar sight – and he wondered: was he seeing the top of a vast mountain piercing the Venusian clouds?
He estimated such a mountain would need to be 16-30km high, overshadowing the highest mountains here on Earth. He had a similar view on 31 January 1790 and three more times in December 1791.
William Herschel (who had in fact made Schröter’s telescope) was somewhat sceptical, not having made any such observations himself. This was not the only account, however.
In 1813 and 1814 the German observer Franz von Paula Gruithuisen noted the caps seemed to be particularly brilliant.
He also observed spots and made comparisons with the polar ice caps on Mars, which looked very similar.
In 1878 the French astronomer Étienne Léopold Trouvelot observed ‘sparkling stars’ at the base of the southern polar regions, which again were interpreted as the peaks of the ‘Venusian Himalayas’.
Although the reports of bright, detached peaks vanished by the end of the 19th century, there are still a number of 20th-century observations showing Venus with a deformed terminator or blunted caps.
With the dawning of the Space Age, there was much speculation about what our robotic emissaries would reveal about Venus.
Would we discover a tropical paradise covered in oceans, or would the planet be an oil prospector’s dream, rich with oceans of hydrocarbons?
These were some of the competing theories about what might lie below the Venusian cloud tops.
A close-up of Venus captured by NASA’s Mariner 10 probe on 5 February 1974. Credit: NASA
On 14 December 1962, NASA’s Mariner 2 passed within 34,773km of Venus and revealed a desolate world with a surface temperature of 500°C – hot enough to melt lead.
The Russian Venera probes provided tantalising images of the surface, showing a volcanic landscape under a dull yellow sky.
Further missions started to reveal some surprising results – lightning was detected in the sulphuric acid clouds and violent winds dominated the upper atmosphere.
In 1990, NASA’s Magellan probe used radar to pierce the clouds and map the surface. Those striking images revealed a surface dominated by vast shield volcanoes and strange geological faults.
Perhaps the biggest mystery was the age of the surface. On the Moon and Mars, craters show that some parts of the surface are much older than others, and yet the surface of Venus appears to be all the same age.
Could there have been some violent resurfacing in the distant past?
During the 1970s and ’80s, NASA’s Pioneer Venus spacecraft recorded large variations in sulphur dioxide levels in Venus’s atmosphere.
This gas is usually a by-product of volcanic activity and, taken with the Magellan data, led many scientists to believe that Venus is probably a volcanically active world – we just haven’t caught it in the act yet.
This brings us to amateur astronomers today. In 2017, Australian amateur astronomers Phil Miles and Anthony Wesley successfully imaged the night side of Venus in infrared.
They captured the glowing surface and something else: small bright spots on the surface. Is it possible these spots are the smoking gun, images of active volcanoes?
“The bright spot Phil recorded in 2017 was very interesting and quite unexpected,” says Anthony Wesley. “We’re both very keen to image that region again next year and see if it appears the same or different.”
If it is the case that Miles and Wesley have indeed been the first people to capture active volcanoes on the surface, then this is an entirely new line of research that astronomers can pursue.
We will of course need many more similar observations before we can be certain. That said, it does give the amateur a new purpose as we finally have the tools to peer beneath the Venusian clouds.
The Himalayas of Venus as recorded by Schröter and others were probably illusions caused by poor seeing, and yet these early attempts to glimpse beneath the clouds of our sister world seem to be in some small way commemorated by the possibility of active volcanoes on the surface of this enigmatic planet.
Capturing Venus in infrared
Trying to search for active volcanoes on the surface of Venus is no easy task, but it is now within the means of amateur astronomy.
The first requirement is a large aperture scope (at least 200mm). The next essential items are filters and cameras that allow you to image Venus in infrared.
The surface of Venus is about 500°C and you’re going to need to image the planet in the 1020nm band.
You will need to find either a filter or camera in this range (or stack two separate filters together). Hotter regions should appear as brighter spots.
The faint images will require careful processing and you will need to take several images over the course of an evening to confirm any bright spots are genuine.
The best time to start your search is when Venus is in the crescent stage, otherwise the sunlit clouds will wash out the faint nightside image.
In 2017, small bright spots were observed on the night side of Venus, as captured by amateur astronomers Phil Miles and Anthony Wesley with their 508mm Newtonian telescope (see above). These may be evidence of volcanic activity.
For more information about the Venus night side project, see www.astrogem.com.au/Venus/nightside.
This article originally appeared in the December 2019 issue of BBC Sky at Night Magazine. Paul G Abel is director of the British Astronomical Society’s Mercury and Venus section.