NASA’s Dawn spacecraft took this image of the dwarf planet Ceres in 2015. Credit: Justin Cowart, Public domain, via Wikimedia Commons.

The Dawn mission and the search for organics in the Solar System: the dwarf planet Ceres and the building blocks of life delivered from space.

The dwarf planet Ceres is a bizarre, cryovolcanic world. However, the organic deposits discovered on its surface so far are unlikely to originate from its interior.

Organic molecules are among the necessary inventory of life-friendly worlds.

On Earth, the compounds of carbon, hydrogen and – in smaller quantities – other elements form the basic building blocks of all life.

In recent years, researchers have found such molecules at great distances from the Sun: on trans-Neptunian objects, comets, and far-away asteroids. These bodies are thought to be largely unaltered remnants from the early days of the Solar System.

The building blocks of life may therefore have been part of their ‘basic configuration’ from the very beginning and possibly reached the inner Solar System only later.

For the current study, the researchers looked for previously unknown deposits of organic material on dwarf planet Ceres.

With its location in the middle of the asteroid belt between the orbits of Mars and Jupiter, the body is neither clearly native to the inner nor the outer Solar System. According to earlier studies, this location could even be its birthplace.

Scientists are therefore interested in the origin of Ceres’s organic components: Did they originate locally in the asteroid belt? Or did they arrive later?

The surface of the dwarf planet Ceres. The sites of organic material are shown as or in red boxes. The vast majority of sites are found near the Ernutet crater in the northern hemisphere. Credit: MPS. CC BY-NC-ND.

Searching for organics from afar

Evidence of deposits of organic material was already found during the early stages of the Dawn mission.

The Dawn spacecraft reached Ceres in March 2015 and accompanied it for about three and a half years.

During this time, the scientific camera system and the spectrometer on board scanned the entire surface of the dwarf planet.

Potential patches of organic material can be detected from the camera data: the brightness of the light reflected from these areas increases noticeably with increasing wavelength.

The spectrometer splits the light into many more wavelengths than the camera and can therefore prove or disprove the presence of organics.

Unfortunately, remote data is not sufficient to identify individual types of molecules beyond doubt.

However, it is certain that the discovered deposits consist of organic compounds that have a chain-like structure: researchers refer to such molecules as aliphatic hydrocarbons.

The authors of the current study have now used artificial intelligence to comb the entire surface of the dwarf planet for traces of aliphatic organic molecules.

First author Ranjan Sarkar from MPS, said: “Sites of such organic molecules are actually rare on Ceres, and devoid of any cryovolcanic signatures.”

The vast majority of deposits can be found along the edge or near the large Ernutet crater in the northern hemisphere of the dwarf planet. Only three are located at a greater distance from it. Two patches were not previously known.

A closer look at the geological structures at the locations of the organic material allows further conclusions.

MPS’s Martin Hoffman said: “At none of the deposits do we find evidence of current or past volcanic or tectonic activity: no trenches, canyons, volcanic domes or vents. Furthermore, there are no deep impact craters nearby.”

Impacts from distant neighbours

During the Dawn mission, Ceres had turned out to be an extraordinary, cryovolcanic world.

Under its surface, a watery brine is hidden, which in some places has been seeping to the surface until recently.

Andreas Nathues from MPS, who is head of the camera team, said: “Of course, the first assumption is that Ceres’ unique cryovolcanism has transported the organic material from the interior of the body to the surface, but our results show otherwise.”

At the sites of cryovolcanic activity, there is no proof of organic matter. And where organic compounds have been reliably detected, there is no evidence of deep or surface activity.

The researchers therefore argue that the impact of one or more asteroids from the outer asteroid belt introduced the organic material.

Computer simulations show that these bodies are among the ones that most frequently collided with Ceres.

Since the not-too-distant neighbours do not pick up much speed, little heat is generated upon impact. Organic compounds can survive these temperatures.

Nathues said: “Unfortunately, Dawn can’t detect all types of organic compounds.”

It is quite likely that building blocks of life were also formed in Ceres’ underground ocean and perhaps even reached the surface – or are still doing so.

Nathues explained: “However, the organic deposits that have been reliably detected with Dawn so far likely do not originate Ceres itself.”

A future lander mission would be needed to detect organic material from the interior of Ceres.

The study is published in AGU Advances.