Another Way to Search for Biosignatures of Alien Life. The Material Blasted out of Asteroid Impacts

In recent years, the number of confirmed extra-solar planets has risen exponentially. As of the penning of the article, a total of 3,777 exoplanets have been confirmed in 2,817 star systems, with an additional 2,737 candidates awaiting confirmation. What’s more, the number of terrestrial (i.e. rocky) planets has increased steadily, increasing the likelihood that astronomers will find evidence of life beyond our Solar System.

Unfortunately, the technology does not yet exist to explore these planets directly. As a result, scientists are forced to look for what are known as “biosignatures”, a chemical or element that is associated with the existence of past or present life. According to a new study by an international team of researchers, one way to look for these signatures would be to examine material ejected from the surface of exoplanets during an impact event.

The study – titled “Searching for biosignatures in exoplanetary impact ejecta“, was published in the scientific journal Astrobiology and recently appeared online. It was led by Gianni Cataldi, a researcher from Stockholm University’s Astrobiology Center. He was joined by scientists from the LESIA-Observatoire de Paris, the Southwest Research Institute (SwRI), the Royal Institute of Technology (KTH), and the European Space Research and Technology Center (ESA/ESTEC).

Artist’s impression of what an asteroid hitting the Earth might look like. Credit: NASA/Don Davis.

As they indicate in their study, most efforts to characterize exoplanet biospheres have focused on the planets’ atmospheres. This consists of looking for evidence of gases that are associated with life here on Earth – e.g. carbon dioxide, nitrogen, etc. – as well as water. AsĀ Cataldi told Universe Today via email:

“We know from Earth that life can have a strong impact on the composition of the atmosphere. For example, all the oxygen in our atmosphere is of biological origin. Also, oxygen and methane are strongly out of chemical equilibrium because of the presence of life. Currently, it is not yet possible to study the atmospheric composition of Earth-like exoplanets, however, such a measurement is expected to become possible in the foreseeable future. Thus, atmospheric biosignatures are the most promising way to search for extraterrestrial life.”

However, Cataldi and his colleagues considered the possibility of characterizing a planet’s habitability by looking for signs of impacts and examining the ejecta. One of the benefits of this approach is that ejecta escapes lower gravity bodies, such as rocky planets and moons, with the greatest ease. The atmospheres of these types of bodies are also very difficult to characterize, so this method would allow for characterizations that would not otherwise be possible.

And as Cataldi indicated, it would also be complimentary to the atmospheric approach in a number of ways:

“First, the smaller the exoplanet, the more difficult it is to study its atmosphere. On the contrary, smaller exoplanets produce larger amounts of escaping ejecta because their surface gravity is lower, making ejecta from smaller exoplanet easier to detect. Second, when thinking about biosignatures in impact ejecta, we think primarily of certain minerals. This is because life can influence the mineralogy of a planet either indirectly (e.g. by changing the composition of the atmosphere and thus allowing new minerals to form) or directly (by producing minerals, e.g. skeletons). Impact ejecta would thus allow us to study a different sort of biosignature, complementary to atmospheric signatures.”

Another benefit to this method is the fact that it takes advantage of existing studies that have examined the impacts of collisions between astronomical objects. For instance, multiple studies have been conducted that have attempted to place constraints on the giant impact that is believed to have formed the Earth-Moon system 4.5 billion years ago (aka. the Giant Impact Hypothesis).

While such giant collisions are thought to have been common during the final stage of terrestrial planet formation (lasting for approximately 100 million years), the team focused on impacts of asteroidal or cometary bodies, which are believed to occur over the entire lifetime of an exoplanetary system. Relying on these studies, Cataldi and his colleagues were able to create models for exoplanet ejecta.

As Cataldi explained, they used the results from the impact cratering literature to estimate the amount of ejecta created. To estimate the signal strength of circumstellar dust disks created by the ejecta, they used the results from debris disk (i.e. extrasolar analogues of the Solar System’s Main Asteroid Belt) literature. In the end, the results proved rather interesting:

“We found that an impact of a 20 km diameter body produces enough dust to be detectable with current telescopes (for comparison, the size of the impactor that killed the dinosaurs 65 million years ago is though to be around 10 km). However, studying the compositionĀ of the ejected dust (e.g. search for biosignatures) is not in the reach of current telescopes. In other words, with current telescopes, we could confirm the presence of ejected dust, but not study its composition.”

Perspective view looking from an unnamed crater (bottom right) towards the Worcester Crater. The region sits at the mouth of Kasei Valles, where fierce floodwaters emptied into Chryse Planitia. Credit: ESA/DLR/FU Berlin

In short, studying material ejected from exoplanets is within our reach and the ability to study its composition someday will allow astronomers to be able to characterize the geology of an exoplanet – and thus place more accurate constraints on its potential habitability. At present, astronomers are forced to make educated guesses about a planet’s composition based on its apparent size and mass.

Unfortunately, a more detailed study that could determine the presence of biosignatures in ejecta is not currently possible, and will be very difficult for even next-generation telescopes like the James Webb Space Telescope (JWSB) or Darwin. In the meantime, the study of ejecta from exoplanets presents some very interesting possibilities when it comes to exoplanet studies and characterization. As Cataldi indicated:

“By studying the ejecta from an impact event, we could learn something about the geology and habitability of the exoplanet and potentially detect a biosphere. The method is the only way I know to access the subsurface of an exoplanet. In this sense, the impact can be seen as a drilling experiment provided by nature. Our study shows that dust produced in an impact event is in principle detectable, and future telescopes might be able to constrain the composition of the dust, and therefore the composition of the planet.”

In the coming decades, astronomers will be studying extra-solar planets with instruments of increasing sensitivity and power in the hopes of finding indications of life. Given time, searching for biosignatures in the debris around exoplanets created by asteroid impacts could be done in tandem with searchers for atmospheric biosignatures.

With these two methods combined, scientists will be able to say with greater certainty that distant planets are not only capable of supporting life, but are actively doing so!

Further Reading: Astrobiology, arXiv

There’s a Hard Rock Rain on the Moon, We Can See it From Earth

In February of 2015, the National Observatory of Athens and the European Space Agency launched the Near-Earth object Lunar Impacts and Optical TrAnsients (NELIOTA) project. Using the 1.2 meter telescope at the Kryoneri Observatory, the purpose of this project is to the determine the frequency and distribution of Near-Earth Objects (NEOs) by monitoring how often they impact the Moon.

Last week, on May 24th, 2017, the ESA announced that the project had begun to detect impacts, which were made possible thanks to the flashes of light detected on the lunar surface. Whereas other observatories that monitor the Moon’s surface are able to detect these impacts, NELIOTA is unique in that it is capable of not only spotting fainter flashes, but also measuring the temperatures of they create.

Projects like NELIOTA are important because the Earth and the Moon are constantly being bombarded by natural space debris – which ranges in size from dust and pebbles to larger objects. While larger objects are rare, they can cause considerable damage, like the 20-meter object that disintegrated above the Russian city of Chelyabinsk in February of 2013, causing extensive injuries and destruction of property.

The two main smoke trails left by the Russian meteorite as it passed over the city of Chelyabinsk. Credit: AP Photo/Chelyabinsk.ru

What’s more, whereas particulate matter rains down on Earth and the Moon quite regularly, the frequency of pebble-sized or meter-sized objects is not well known. These objects remain too small to be detected by telescopes directly, and cameras are rarely able to picture them before they break up in Earth’s atmosphere. Hence, scientists have been looking for other ways to determine just how frequent these potentially-threatening objects are.

One way is to observe the areas of the lunar surface that are not illuminated by the Sun, where the impact of a small object at high speed will cause a bright flash. These flashes are created by the object burning up on impact, and are bright enough to be seen from Earth. Assuming the objects have a density and velocity common to NEOs, the brightness of the impact can be used to determine the size and mass of the object.

As Detlef Koschny – the co-manager of the near-Earth object segment of the ESA’s Space Situational Awareness Program, and a scientist in the Science Support Office – said in an ESA press release:

These observations are very relevant for our Space Situational Awareness program. In particular, in the size range we can observe here, the number of objects is not very well known. Performing these observations over a longer period of time will help us to better understand this number.

Tiny pieces of rock striking the Moon’s surface were witnessed by the NELIOTA project, which was monitoring the dark side of the Moon. Credit: NELIOTA project

After being taken offline in 2016 for the sake of making upgrades, the NELIOTA project officially began conducting operations on March 8th, 2017. Using this refurbished telescope, which is operated by the National Observatory of Athens, NELIOTA is capable of detecting flashes that are much fainter than any current, small-aperture, lunar monitoring telescopes.

The telescope does this by observing the Moon’s night hemisphere whenever it is above the horizon and between phases. At these times – i.e. between a New Moon and the First Quarter, or between the Last Quarter and a New Moon – the surface is mostly dark and flashes are most visible. Incoming light is then split into two colors and the data is recorded by two advanced digital cameras that operate in different color ranges.

This data is then analyzed by automated software, which extrapolates temperatures based on the color data obtained by the cameras. As Alceste Bonanos – the Principal Investigator for NELIOTA – explained, all this sets the 1.2 meter telescope apart:

Its large telescope aperture enables NELIOTA to detect fainter flashes than other lunar monitoring surveys and provides precise color information not currently available from other project. Our twin camera system allows us to confirm lunar impact events with a single telescope, something that has not been done before. Once data have been collected over the 22-month long operational period, we will be able to better constrain the number of NEOs (near-Earth objects) in the decimetre to metre size range.

Images showing the lunar impact flash caught by NELIOTA. Credit: NELIOTA project

The NELIOTA project scientists are currently collaborating with the Science Support Office of ESA to analyze the flashes and measure the temperatures of each flash. From this, they hope to be able to make accurate estimates of the mass and size of each impactor, which they will further corroborate by analyzing the size of the craters these impacts leave behind.

The study of impacts on the Moon will ultimately let scientists know exactly how often larger objects are raining down on Earth. Armed with this information, we will be able to make better predictions on when and how a potentially-threatening object could be entering our atmosphere. As the Chelyabinsk meteor demonstrated, one of the greatest dangers posed by meteorites is a general lack of preparedness. Where people can be forewarned, injury, damage and even deaths can be prevented.

NELIOTA is also contributing to public outreach and education through a number of initiatives. These include public tours of the Kryoneri Observatory – in which the details of the NELIOTA project are shared – as well as presentations to students and the general public about Near-Earth Asteroids. The project team are also training two PhD students in how to operate the Kryoneri telescope and conduct lunar observing, thus creating the next-generation of NEO observers.

This summer (Friday, June 30th), the Observatory will also be hosting a public event to coincide with Asteroid Day 2017. This international event will feature presentations, speeches and educational seminars hosted by astronomical institutions and organizations from all around the world. Save the date!

Further Reading: ESA