Preparing to study the acid clouds of Venus

Radar image of the surface of Venus

Venus is a planet similar to Earth in size, but it has a very thick atmosphere and, due to the combination of this thick atmosphere and closer proximity to the Sun, a very high surface temperature of over 400 degrees centigrade, complemented by sulfuric acid clouds. In this blog post we will talk about what would be needed to find any habitable places on Venus and how to design a mission to visit them, as well as Tartu Observatory’s team’s part in the mission.

Why to study venus and the VLF mission study

The Venusian atmosphere mostly consists of CO2 and has clouds of sulfuric acid, so it has long been thought to be uninhabitable for life. In 2020, traces of phosphine were found in the Venusian atmosphere, and earlier measurements seem to enforce this. As there is no known process that could produce this gas on Venus in these quantities without life, phosphine has the potential of being a sign of life. To investigate this further, Venus Life Finder mission study, mainly sponsored by the Breakthrough Initiatives (a final report with all the details can be found here) was performed with more than 50 scientists and engineers from around the world (led by Prof. Sara Seager from MIT), and in it results from previous missions to Venus were analyzed and a set of three missions was proposed to investigate the situation further and to figure out what is actually going on there.

The main region of interest comprises the clouds of Venus at around 60 km from its surface, where the temperature is close to room temperature on Earth and pressure close to the ambient pressure on Earth’s surface, but the exact conditions there are unknown.

The VLF mission study proposes a set of three missions to gradually assess the habitability of Venusian clouds and to eventually bring back a sample for study on Earth. For a detailed description of the missions, instruments, and institutions and people involved, see the report. In the present blog post, we mostly concentrate on Tartu Observatory’s contribution.

A balloon probe floating in the Venusian atmosphere, similar to what the second mission within the VLF missions is supposed to look like. Illustration credit: Mihkel Pajusalu

How Tartu Observatory was included in the mission

Tartu Observatory has been tasked with building an acidity sensor to measure the sulfuric acid content of the Venusian cloud droplets and potentially an oxygen sensor. Here, we will mostly discuss the development of the acidity sensor, as it has been assigned to the second probe within the series of missions outlined in the VLF mission study report.

The acidity sensor has been designed to do a statistical study of the acidities of Venusian cloud droplets, because it is expected that different cloud particles could have very different acidities. The most prevalent theory thus far has been that the Venusian clouds consist of almost concentrated sulfuric acid, and this would be a significant hurdle for life. But newer analysis of data (see the report) proposes that there could be droplets in the Venusian atmosphere, the pH of which could be around 1. This acidity range is already habitable on Earth. For life to be possible, of course, other factors have to align also.

The idea to start the development of the sensor in Tartu emerged because Dr. Mihkel Pajusalu had previously worked at MIT during his post-doc times and developed various sensors for astrobiology studies, including luminescence-based oxygen sensors. Several scientific studies were performed (for example,,, and ). The application of the same general principle was then studied for the task at hand, looking for fluorescent pigments that would fluoresce differently in different acidities, and a small workgroup was formed (including also Dr. Ida Rahu and Laila Kaasik) to study this during the summer of 2021. In addition, secondary school students were involved in aspects of the project.

Laila Kaasik demonstrating the fluorescent compound fluorescein. Image credit: Laila Kaasik

The preliminary study results (included in the VLF mission study report), showed that a fluorescent pigment fluorescein is very well suited for this application, and the Tartu Observatory workgroup figured out a way of producing fluorescent films that could be coated on glass plates. Preliminary tests performed in laboratory conditions showed that this method is viable.

The path towards Venus

The work done for the VLF mission study was just the beginning. The next steps have to produce an actual scientific instrument that could be mounted on a spacecraft and sent to the clouds of Venus. The sensor developed at TO is intended for the second probe in the series of VLF missions, tentatively aimed to launch in 2026.

The current plan for the instrument is to use a camera system that would take photos of the fluorescent film in quick succession using different illuminating LEDs. The camera system could be similar to the one developed at Tartu Observatory currently for the Comet Interceptor ESA F-class mission and its OPIC (Optical Periscopic Imager for Comets).

Another important aspect of developing the sensor is how to simulate the Venusian cloud particles to test the sensor and calibrate it and to evaluate its expected performance during the mission. For this, we need to develop a system that could generate sulfuric acid clouds in controlled conditions. Currently, this setup is being developed.

The way towards a full instrument. Left: OPIC instrument for ESA F-class mission Comet Interceptor. Right: modified version to accommodate observation of a sensor plate. Illustration credit: Mihkel Pajusalu

We hope to develop a new version of the sensor and to publish the current sensor design as an article in 2022.

Also, there are uncertainties regarding the funding of the missions. The first set of the VLF missions, due to launch in 2023, is close to being funded. Some potential sponsors are being talked to about the follow-up missions. For more details, please see the full mission study report.

Mihkel Pajusalu. Image credit: Laila Kaasik

Dr. Mihkel Pajusalu, Head of the Space Technology Department and Associate Professor in Space Technology at Tartu Observatory, University of Tartu. Leader of the development of the acidity sensor for the Venus mission and PI of the OPIC instrument for Comet Interceptor.

Some media coverage and press releases for the full mission:

Space Travel Blog

Various examples of media coverage in Estonia:

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