Thirty-year-old Data Offers New View Of Venus
(Inside Science) -- In 2010, the European Space Agency's Venus Express orbiter observed that twice as many hydrogen atoms as oxygen atoms were escaping from Venus into space. This was the first evidence that Venus might once have harbored puddles, pools and even lakes of liquid water on its surface. Now, a new study suggests that Venus could be storing some amount of intact water molecules within its mantle.
To determine this, Justin Filiberto, a geologist at Southern Illinois University in Carbondale, compared what geologists know about the composition of rocks on the surface of Venus with rock formation processes here on Earth. His results, which appeared in the December issue of the journal ICARUS, suggest that some types of rocks on Venus could only have formed in the presence of water and carbon dioxide.
Between 1981 and 1984, the USSR launched six missions to Venus. Three of those missions, Venera 13 and 14 and Vega 2, compiled what today remains the most complete chemical composition analyses of rock and soil on Earth's sister planet.
The Venera and Vega probes landed at different points on the surface and each used a robotic drilling arm to collect samples of rock and soil. The samples are representative of the three main types of terrain on Venus. Venera 13 collected samples from the planet's rolling upland plains, which cover about 70 percent of the surface, and Venera 14 sampled rock from flat lowlands, which comprise 20 percent of the surface. About ten percent of the surface is mountainous terrain. Vega 2 landed at one of those regions, known as Aphrodite Terra.
Onboard instruments then analyzed the samples' chemical makeup before the probes melted and crumbled under Venus' extreme temperatures and pressures--but not before they first relayed the data to Earth describing the percentage, by weight, of chemical compounds like titanium dioxide and silicon dioxide.
Although geologists determined that all of the rock samples were igneous, meaning they formed from the cooling and solidification of magma or lava, the data lacked some important information. For example, the total weight of the rock samples from the Vega 2 analyses adds up to 89.6 percent, suggesting that the analyses are missing some elements.
Moreover, the probes were unable to analyze certain elements like sodium, which is a crucial ingredient for classifying types of igneous rocks on Earth. Without this piece of the puzzle, there is no guarantee they can accurately deduce the conditions under which the rocks formed, or develop a better understanding of volcanic activity in the mantle of Venus.
Geologists do know, however, what kind of role sodium and other elements like magnesium and silicon play in the formation of igneous rocks on Earth. And using a process called crystallization, geologists can determine the kinds of pressure, temperature and water conditions of the magma that produce the many different types of igneous rocks on Earth and the weight percentages of different elements produced as a result. Using this information about Earth rocks as an analog, Filiberto set constraints on the different pressure, temperature and water conditions that Venus rocks might have formed under.
Depending on its location in Earth's mantle, magma contains different constituents such as dissolved carbon dioxide or hydrous minerals, minerals with water molecules incorporated into their molecular structures. Filiberto found that the Venera 14 and Vega 2 samples contained crystal structures much like basalts on Earth that formed from hydrous sources near the upper mantle, where pressure is lower.
Venera 13 samples, on the other hand, likely formed at higher pressures, deep within the planet's mantle, from magma rich in carbon dioxide. This is the first study to show that rocks on Venus formed under different pressures at different depths of the planet, Filiberto said.
"This is a new conclusion about the constraints on the water and carbon dioxide contents of the magmas," Filiberto said. "And it shows that the interior of Venus is not just Earth-like in bulk chemistry but in water and carbon dioxide as well."
While the study shines more light on the geological story of Venus, the results are limited by the thirty-year-old, incomplete data, said Greg Shellnutt, who is a geochemist at the National Taiwan Normal University and also studies the geological origins of Venusian rocks.
"This could be very exciting results but there are still so many unknowns. We've reached the limit of our data," Shellnutt said, "but we're doing the best with the data we have to work with."
Both Shellnutt and Lori Glaze, Deputy Director of NASA's Solar System Exploration Division, agree that confirming results like Filiberto's will require dispatching the first Venus lander of the 21st century. Glaze said that for her work on conceptualizing future Venus missions, the most important points in Filiberto's paper are his models. The models detail what type of information is still needed in the different regions on Venus. These include, for example, the rocks' sodium, magnesium and silicon contents, at any point on the surface of Venus, to understand the rocks' origins.
"This type of work in Justin Filiberto's paper helps to better define what kind of instruments and what types of sensitivity we need to make when we go back to the surface," Glaze said. "One of the key ways to get at the surface history of Venus is to land there and do a better job of the chemistry analyses."
Such a mission would paint geologists a more complete picture of a long-lost Venus when large amounts of liquid water may have adorned its surface. Moreover, determining if water or carbon dioxide is a dominant presence in magma is important for understanding Venus' climate evolution and atmospheric chemistry, Filiberto said.
Jessica Orwig is a contributing writer to Inside Science News Service.