Tuesday, 5 March 2013

Early Exploration of Venus - The Path to Venus Express

On February 22nd, Oxford physics invited Dr. Dmitrij Titov to present a colloquium on his studies of Venus from ESA's Venus Express mission. Titov was among the original proposers of this mission (along with Oxford's Fred Taylor, so AOPP has always has a close connection to Venus science), which has been returning new insights into our sister planet, revealing a world far removed from our original ideas. Inspired by his talk, I decided to look into Venusian exploration a little more closely.

To study Venus is to study our place in the solar system. Galileo was the first to see the phases of Venus and realise that the planet was rotating around the sun inwards of the Earths orbit. Recent images of transits of Venus across the face of the sun served to show how fragile our worlds are, but if you missed the 2012 transit then you'll have to wait until 2117 until the next one. Our understanding of Venusian weather really started with the observations of Lomonosov in 1761, who saw a tiny crescent around the planet during a transit and was the first to discover the planet's atmosphere. But early 20th century observations were misleading, showing cloud top temperatures of 240 K and an atmosphere of water vapour and carbon dioxide, making Venus appear as an Earth-like world. This period in the thirties and forties was a great period in Venus research, with poets and artists placing beautiful women in tropical pools, lakes and jungles on the surface of Venus. It gave the idea that the universe must be widely inhabited, with someone on the world right next to us. But then radio observations in the 1950s saw emission temperatures of 500 K, providing a dilemma that would last until the beginning of the space age.

First Steps to Venus

Russia was attempting to send missions to Venus even before the launch of Gagarin, but the missions apparently failed. The US flyby of Mariner 2 in 1962 ended our naive views of Venus, and the first data returned to Earth revealed a hellish world with temperatures exceeding 500 K. Venera 7 achieved the first soft landing in 1970, being incredibly over-designed as all previous attempts had failed several kilometres above the surface. Even Soviet U boat engineers were enlisted to ensure the lander could survive the extreme pressures, and resulted in huge spacecraft several tonnes in mass. The flotilla of Venera spacecraft between 1970 and 1980, two sent every ballistic opportunity, provided ground truth on Venusian temperatures, winds, chemistry, plasmas and radio waves... It was a wonderful time for space research, when no one seemed to be counting the money! Venera 13 and 14 provided the first panoramas of the stony desert surface, and left images of Lenin behind to bake in the extreme heat.

Pioneer Venus consisted of two components flown in 1978, one a dedicated orbiter and one with four entry probes. The in situ experiments taught us a lot about the Venusian climate, showing how different the atmosphere is to Earth - pressures of 90 bar (equivalent to an ocean 1 km deep) and temperatures of 460 K at the surface; an atmosphere of 96% carbon dioxide, clouds of sulphuric acid and so little water vapour that it would only be 3 cm deep if condensed as a liquid on the surface. Above the clouds, in the region known as the mesosphere, temperatures appeared to increase from the equator to the poles, exactly the reverse of what we'd expect, and ultimately explained by a Hadley cell of air moving towards higher latitudes, sinking and adiabatically heating. Seemingly persistent cold collars of air were discovered at mid latitudes in both hemispheres, sitting on top of the cloud decks and encircling the polar regions.

The visible appearance of the planet is due to sunlight reflected from the thick clouds between 50-70 km altitude, with characteristic dark patterns due to some unknown ultraviolet absorber within the clouds (possibly sulphur based). We don't see the surface, but if we were to fly within the clouds they would appear as a fog or light haze consisting of tiny particles. The sulphuric acid composition had been observed by both spectroscopy and polarimetry, but the chemistry within the clouds remains an enigma.

The Pioneer Venus mission revealed a rich dynamics, including a super rotating atmosphere, moving six times faster than the equatorial velocity of the surface, a global whirlpool. Project Vega placed two balloons into these clouds, which floated for fifty hours in 1984, and scientists are left dreaming of being able to do it again. Magellan was a US radar mission to unveil the surface beneath the clouds, showing unusual corona features, circular volcanic features and rift zones. Magellan showed the surprising result that the surface of Venus is very young, geologically speaking. The crater record shows an age of only half a billion years, suggesting catastrophic resurfacing, although the mechanism remains unclear.

Beyond this flotilla of missions, there was then a large gap in our exploration of Venus. The planet had been slowly unveiled, far beyond our naive perspective of the early 20th century. After such a deluge of interplanetary visitors, the next thing to attempt was a systematic survey of our sister planet, for which Titov and colleagues developed the Venus Express mission. This spacecraft is still operating at Venus today, and some of the key results will be the topic of a future blog post.

No comments:

Post a Comment