Month: February 2018

Don’t Talk to Me About a Face on Mars

I couldn’t decide which paper to discuss this week, so I’m talking about both. One made me think, the other made me angry. I suspect that this was the intent of the assignment.

Solar system artifact SETI might be one of the most giggle-inducing subsets of SETI. This seemingly wasn’t always true (throwback to the Martian “canals”), but it suffers from a series of issues. One is that there is a popular misconception that the solar system is a relatively well-explored piece of “territory” and we haven’t found anything yet. So solar system artifact SETI, in that light, seems antiquated. It always surprises me to learn how little we know as I progress through my education, and I think that feeling is relevant here. In addition, solar system artifact SETI makes SETI seem so close to home that the only conceptual guideposts people have are (generally cheesy and terrible) science fiction. Where a remote detection or a long timescale exchange of radio signals would be distant, sterile, and narratively boring, the discovery of an alien probe in the solar system or a city and giant face on Mars feels like fiction, so it’s treated as such, instead of legitimate science.

Now, to the articles!

The Freitas paper very logically stepped through the process of finding an answer to the question “which surveys on which parts of parameter space would have to be performed to disprove the existence of probes in the solar system?”. I take issue to a couple of framing assumptions that Freitas makes. Firstly, I don’t think it’s reasonable to assume that any probes we would find would be made for a neutral-to-discovery purpose (because we would’ve found one already if it wanted to contact us and we’d never see it if it was trying to hide, the logic goes). I can imagine many exceptions to this idea: a probe that was meant for communication but was damaged, a probe that is trying its best to contact us but by a method that we don’t have access to yet (perhaps on purpose, so we only see it at a reasonable point in our technological development), or a probe that was only partly for communication (mostly for another purpose, with only minor energy put into a beacon). The other thing that bothered me is that all of the constraints for the observational probe were constructed under the assumption that the probe wanted to observe Earth. That looking at a habitable planet, and that having the resolution to watch civilization arise on it, were valuable to our hypothetical watchers. I think that’s a bit anthropocentric – it could very well be that our asteroid belt is absolutely fascinating and ugh look at the primitives mucking up that third planet, they’re everywhere in this sector.

All of that said: you can’t do this work without making some assumptions to reduce the scale of the problem, and these were relatively minor ones. I loved the structure of the paper: consider the construction and purpose of the probe, then consider where it could be placed, note previously completed searches and their incompleteness (chock-full of references), then look at detection probabilities with current instruments and reasonable times. This is a methodical and scientific way to go about the stated problem in the paper.

The Carlotto paper, on the other hand, was rage-inducing. It would take a very, very convincing landscape artifact for me to feel comfortable announcing a “non-natural” origin. If Europa was covered in a giant swastika a la Armada (Ernest Cline’s less successful follow-up novel to Ready Player One), that would probably be sufficient. The 3D face is reconstructed from only two relatively low-resolution images, which makes me uncomfortable. The feature has since been imaged from other angles (by, among others, the Mars Global Surveyor) and, spoiler alert, doesn’t actually look like a face.

This is an example of textbook pareidolia: humans have a tendency to see patterns in random data, especially and specifically faces. Being good at recognizing and reading faces is vital for a social creature like a human being, so better to have some false positives than to accidentally mistake one’s significant other for a coat rack. But a base and known brain-stem bias like this should NOT cause us to write horribly misleading papers about the possible existence of an extinct Martian civilization. I don’t know how couth it is to consider the political ramifications of other people’s research, but it’s frustrating to see a vibrant and important sub-field continually shooting itself in the foot with scientifically sketchy bold claims and over-speculation from a few members of the community.

/endrant

The face of a facies: when a face is just a (rock) face

In 1976, the Viking spacecraft orbiting Mars took a picture. Well, it actually took many pictures, but there was one of particular interest to some people at Goddard and then the general public. This picture shows a structure that looks kind of like a human face:

The paper originally suggesting this feature looked like a face was originally dismissed for a few reasons, but people did continue to study it. Carlotto (my guess is for his own curiosity and for the interest of the public) performed a fairly extensive analysis of the image above along with one taken 35 orbits later. He cleaned up the data as best he could and then, using the two images, attempted to make a 3-D reconfiguration of the rock to see if the features persisted. I must say, I am fairly impressed with this paper. Carlotto only presents his opinion, that the feature might be artificial, near the end of the paper and only in one sentence. During the rest of the paper, he focuses mostly on explaining his methods and analysis.

I would say that his only flaw would be trying to pull too much information out of his crappy data. I also feel he should have used the other two images (although low resolution), because two data points is simply not enough to make a proper 3-D model of anything, especially if they two data points are from roughly the same angle at roughly the same time of day (Mars day). His 3-D model results (which he displays from different simulated camera positions) look near exact to the original image:

I agree with the original non-believers though. We humans are great at finding faces in just about any kind of rock:  Old Man of the Mountain, the Romanian Sphinx, the Pedra da Gávea, the Old Man of Hoy, Stac Levenish, and the Badlands Guardian.

For anyone interested, here’s what the face looks like when you have nice data:

I mean, it looks really neat! It is still a nicely shaped, elliptical-ish mound. There even appear to be sand-drift (or maybe water) channels running off it. It just definitely looks like a rock though. This picture is courtesy of the Mars Global Surveyor from 1996.  The Mars Reconnaissance Orbiter (2005) also imaged this area, but it looks about the same as above: no face. And this kind of makes sense! If life were to take as long to arise on Mars as it did on Earth (which hopefully would have occurred when it was still wet), then any civilization would have been recently died out, and anything they left behind would be visible yet subject to weathering. With its sad atmosphere and lack of water cycle, Mars no longer has as much weathering as it maybe did at one point, but any “face” made by a civilization would not last. (Also, who would make a mile long face?? At least our large structures on Earth are visually appealing when you’re next to them and don’t require spacecraft to admire.)

Feritas (1983) Summary

This paper discusses the possibility of detecting Bracewell probes in the Earth and Moon vicinity.

The author begins by classifying the probes into three categories: class one, objects intended to be found, class two, objects intended not to be found and class three, objects for which detection is not important or relevant. Since we have not detected any probes yet, it is likely we will not detect any class one or class two objects. Therefore, the author argues that the only observable objects will be class 3 objects.

Further, the author argues the sizes of those probes to be 1 to 10 m, taking into account 1) the long-duration of exploration 2) capability of withstanding meteoroid 3) radiation pressure 4) capability of returning information.

Next, the author discusses the search space in geocentric orbits, selenocentric orbits, Earth-Moon libration orbits, and Earth-Moon halo orbits. In each of the orbits, the author discusses the size of the search space and also the search speed.

Finally, the author argues that it is possible to detect such probes in a SETI program which extends for 2 years both from the space or ground. Additionally, the author also points out the possibility of detecting those probes in radio and infrared band.

The reason this paper is important is because this is the first time people have tried quantitatively to define where to search for Bracewell probes near the Earth and Moon.

SETA: Who? What? When? Where? Why?

The title may appear bizarre to most readers but, if one recently finished perusing a 1983 article by nanotechnologist Robert A. Freitas Jr., the answer would be simple: Extraterrestrials. Artifacts. Now. Solar System. Why not? Freitas Jr. attempts to build upon the legacy of Bracewell to unite fiction with science. This article itself can be viewed as but a small contribution Freitas Jr. to foment the idea of xenology, or the scientific study of imaginary extraterrestrial life. Freitas Jr. gives the reader three classes of extraterrestrial artifacts (ETA) to consider, each having the technology necessary to successfully complete its mission:

  1. objects intended to be found,
  2. objects intended to not be found, and
  3. objects for which detection by us is irrelevant or unimportant.

He argues that class I objects do not exist while class II objects are impossible to detect given our technological prowess. His paper emphasizes the possibility of class III objects existing within our Solar System, potentially near the Earth. Freitas Jr. postulates such an object would monitor “phenomena relevant to [its] mission”, which ostensibly wpuld be to study life on Earth. Such an object, while somehow having the technology necessary for interstellar travel, would not have the technology to (i) properly adapt to the terrestrial surface and environment or (ii) power itself without the Sun. Ergo, the best place for uninterrupted observations of life would be in a near-Earth orbit.

For some unexplained reason, the ETA would conduct “diffraction-limited optical” observations of the Earth which, for a near-lunear orbit, would suggest a size of ~3-30 meters. In order to survive catastrophic meteoric impacts, Freitas Jr. limits the size to >0.2-20 meters for a lifetime of on-order 1 million years in a near-Earth orbit. He posits the ETA would use the waterhole to communicate with its home, placing another constraint on size. When all constraints are considered, Freitas Jr. decides a ~1-10 meter object with an albedo comparable to an asteroid would be the limiting artifact we could fine. He goes on to posit regions near Earth that should be explored (see Figure 1) at visible, radio, and infrared wavelengths.

Figure 1: Above are the orbital search regions for ETAs proposed by Freitas Jr. The Sun-Earth Lagrangian L4 tadpole is at 10x scale. The Earth-Moon system is at 200x scale for reference. Freitas Jr. argues a 1-10 meter object could be watching us in a stable/pseudo-stable orbit in the gray regions. Source: The Search for Extraterrestrial Artifacts (SETA) Freitas Jr. & Valdes, 1985

To this blogger, the ideas postulated by Freitas Jr. are fiction wrapped in physics. One should be wary of astrophysical programs proposed by an individual contemplating alien sex or emotions. If we assume an ETA can appear as an asteroid, then we should proceed with certain observations, such as the database of fast-moving asteroids with IRAS. However, contemplating an arguably biased set of objects and proposing how to observe said objects would be the height of folly for the scientific community. There are layers of assumptions that must hold true for the search for ETA (SETA) to be a viable program. Some of the requirements placed on ETA are arbitrary and appear only limited by the imagination. SETA, like most xenoarcheology, will only be successful in the realm of science fiction (i.e. Halo) until it can properly motivate its goals with science.

Reaction to Arnold (2005)

Is it possible that an advanced extraterrestrial intelligence (ETI) would have knowledge of the astronomical development of more primitive societies, and hence pre-emptively manufacture artificial megastructures around stars within their domain in an attempt to make their presence known? This is precisely the question that astronomer Luc Arnold sought to answer in his paper which laid down the groundwork for the idea that such structures would be detectable by modern astronomical instruments. Several years in advance of the deployment of the Kepler space observatory, a landmark mission which aimed to quantify the frequency of Earth-like planets orbiting Sun-like stars using a detection technique called “transit photometry,” Arnold posited that the high precision photometric monitoring of stars afforded by Kepler would be sufficient to distinguish between artificial structures embodying a variety of geometries and extrasolar planets, which are approximately spherical. If an ETI had the desire to reveal themselves, they would take advantage of the fact that societies with emerging science would perform routine astronomical observations of stars (in an attempt to detect worlds orbiting them) and hence place something less obviously natural in front of them! Arnold examined the possibility of three geometries of objects that may serve this purpose: a pure equilateral triangle, a double-screened object, and a series of screens on a louver. In all three cases, by subtracting the best fit circular aspect (assuredly that of a planet) from the artificial lightcurves of these geometries, the residuals were above the \textit{Kepler} photometric sensitivity and hence theoretically distinguishable. The louvre system may be actuated in such as a way that it could also convey information, and so Arnold quantified the effectiveness of a megastructure signaling system by examining its spatial data rate, which he showed to be comparable to that of laser (but without the requirements and shortcomings that come with laser signaling, such as precise knowledge of the system’s future position at the time of receipt). Therefore he concludes that such a signal system is feasible. However there are some problems that would have to be addressed, such as perhaps the scale of the engineering project. Even granting the alien intelligence the benefit of the doubt and ascribing to them an advanced knowledge of astroengineering, I still had a few concerns. Wouldn’t the triangle have to not significantly rotate along the transit arc in order to maintain its projected equilateral aspect from our vantage point? Would such an object be three-dimensional or 2D planar, and in either case, what would happen if it spun on its axis? One could imagine that these structures could be statites, objects that are stationary with respect to the host star supported by radiation pressure. If the triangle was composed of solar sail material, then every time it minimizes its aspect during rotation (i.e. when it is parallel to our line of sight) then wouldn’t it fall inward towards the star? Setting these problems aside, this is nonetheless a brave submission by Arnold and worth taking into consideration as more and more photometric data becomes available.

Interstellar Semaphores?

This article by Luc Arnold (2005) explores light curves from aspherical transiting objects – namely a triangle, two screen, and a six screen. By analyzing the resultant light curve and its deviation from a standard light curve (due to a spherical object), it argues  that these can be potential signatures of Extra Terrestrial Intelligence (ETI) and even as a means of interstellar communication .

Louver like six screen object

Arnold first establishes the premise for the detectability by space based telescopes of such irregular shaped objects (shown above), and then the ability to distinguish these asymmetric shapes from spherical planets. He then considers not one such object, but a series of objects of different sizes and spacing (gaps). These objects could encode a pattern (say prime numbers) to be used as a means of transmitting information. This is reminiscent of late 18th century communication during the Anglo – Franco wars using optical telegraphs and semaphores (as shown below).

Here, they used various ON and OFF positions of the shutter to transfer 6 bits of information – 2^6 – 1 = 63.

 

Murray’s 6 shutter telegraph 

 

A similar concept is hypothesized by Arnold, as a means of interstellar information transfer. The variations in the light curve will be akin to the shutters in the telegraph changing state. He then compares the data transfer rate per steradian of this method with that of laser beacons. Though they have a higher transfer rate if focussed on one star due to their small cone angle, if one aims to transmit over a greater area, then the interstellar semaphores are faster.

I think it is a very novel method for communication that has parallels with techniques used by humanity a few centuries ago. Further, using this technique we may communicate with civilizations which are not in the Earth Transit Zone by giant sails and solar panels in polar orbits of the Sun, perhaps a viable alternative to energy intensive beamed transmission?

 

 

Is It a Megastructure? (No)

The G-HAT papers have definitely been dense reading. I think Part IV (the paper I’ll be discussing in this blog post) has been the most accessible one to me so far.

My favourite table/figure in this work is simply Table 1, so I’m going to talk about it a lot.

The aforementioned Table I

Firstly, I think it encapsulates the fundamental challenges in artifact SETI: 1) the artifact has to exist and 2) we have to be able to tell that there’s an artifact. This sounds simple, but I think is a useful comparison to the fundamental challenges in communication SETI: 1) the (intentional!) signal has to exist and 2) we have to be able to tell that it’s a signal.

Secondly, it’s good to keep perspective in SETI research: not every anomaly is ETI. In fact, all of them so far are not. The table illustrates that for every megastructure-y looking object, there are reasonable (and plausible) “natural confounders”. This makes sense; though we’re trying to wring as much information out of a single lightcurve as we can (and we’re quite good at it – we can even tell the full 3D stellar rotation and planetary orbit geometry) it’s still just a lightcurve, and there are many inputs that produce the same output. Perspective is vital, especially in a paper that is ~hunting for alien megastructures~

Thirdly, I just love lists, as you can probably tell by the way I write my blog posts. The section following the table goes through the six physical “Distinguishing Features of Megastructures” (ex. anomalous masses, aspects, or orbits).  The section after that talks about the nine physical “Confounding Natural Sources of Megastructure Signatures” (ex. starspots, ring systems, or non-transited stars in the field of view aka. “blends”). I’m italicizing the word physical to illustrate exactly what it is that I like about the structure of this section: it shows what distinguishing properties of the systems being observed would be visible to an observer within the system. But we are not within the system – we’re working from lightcurves. And that’s where Table 1 comes in: showing exactly which of the physical properties discussed in (perhaps agonizing) detail would cause which of the 10 lightcurve anomalies in the Table.

I will now briefly summarize the rest of the paper, which I found generally less interesting to me. The next section talks about a few objects in particular that show some of the transit anomalies discussed in the previous section. The section after that discusses how to distinguish a signal beacon from a constant source from an information-rich signal by doing statistical analyses in both the frequency and time domains. The authors quantify it with a “normalized information content statistic”.  I’ll admit that the methods in this section were mostly over my head, but I think (hypothetically) that the uniform application of them to future SETI studies would be a fruitful pursuit.

Is that a ringed planet or a spiffy pyramidal satellite?

Ground- and space-based photometric missions have proven that observing the light from a star can reveal exciting orbital companions, such as extrasolar planets. Luc F. A. Arnold, an astronomer at the Observatoire de Haute-Provence, also thinks photometry can reveal artificial, transiting objects. In his 2005 paper, Arnold proposes a new method to detect alien mega-structures by examining transiting light curves from space-based photometry for deviations from the expected transit of a spherical body (see Figure 1). Arnold has proclaimed that:

“Artificial structures may be the best way for an advanced extraterrestrial civilisation to signal its presence to an emerging technology like ours”

Figure 1: Above is the expected signal for a transiting exoplanet (HAT-P-3b). As a natural, spherical body, the light curve has a predictable shape. Arnold performed simulations for other transiting shapes to verify if there would be any noticeable change. Source: Jason Eastmann

Arnold considered the capabilities of space-based missions, such as ESA’s Corot telescope and NASA’s Kepler telescope, and assumed a photometric precision of 10-4h-0.5, where h is the per point integration time in hours. It is important to note that this was not an analytical treatment of the transit signal for an arbitrarily shaped objects as Arnold did not predict the functional form such a transit would follow. To estimate how the signals would vary for various shapes (see Figure 2), Arnold performed simulations on a strip of solar surface. The strip would vary depending on the impact parameter of the transiting object, but in each case the limb darkening parameters were interpolated to create a realistic stellar surface. In each simulation, the transiting object was assumed to have a semi-major axis of 1 [AU], a cross-section of 1.16 Jupiter radii, and orbited HD 209458, a sun-like star. The artificial shapes considered in the paper were (i) a triangle, (ii) a louvre-like two-screen object, and (iii) a louvre-like six-screen object. The louvre shape was intended to mimic that of known man-made objects.

Figure 2: Above is an artistic representation of possible, artificial transiting objects. From top to bottom: .a series of spherical objects, a two panel satellite, a two panel solar screen, a pyramid. Source: Jimmy Paillet 

The results from Arnold are encouraging for proponents of artifact SETI. The different shapes, when compared to that of a planet, produced noticeable residuals. If the different shape rotates, the transit shows further differences (see Figure 3). Furthermore, if the host star were a smaller, cooler dwarf, there would be a stronger transit signal. Arnold does note the degeneracies between the curve of a ringed, transiting planet and a non-rotating, artificial triangular object. Given the precision of Kepler and Corot, this would be a concern for future, more sensitive photometric missions. Things get easier to distinguish if we assume a louvre-shaped satellite, as each screen can be considered as a single object transiting and would collectively alter ingress and egress.

Figure 3: Above is an example of a simulation by Arnold. The top plot is a transiting, rotating triangle. The bottom is a plot of the deviations from a transiting exoplanet caused by the rotating triangle. There is a distinct signature that, if observed, would help disentangle an artificial transit from that of a planet. Source: Luc F. A. Arnold

Perhaps the most important thing to note is the implication of using transits as beacons to signal others. A set of close objects would transit quasi-simultaneously and, if they were the same size, the light curve would show variable depth. Reminiscent of the prime number sequence a Bracewell probe would send, Arnold posits the time between the altering transits could be used to encode information as a message. Most importantly, any observer to the transit would receive the message. This blogger maintains skepticism about the applicability of this analysis to transit searches. For reference, the Kepler mission has produced 40,726,580 light curves. For the robust approach presented by Arnold, this would require carefully scrutinizing all light curves for anomalies and then properly vetting such signal. Perhaps future data scientists will properly address this problem and the search for a transiting triangle will become feasible.

Just stick with lasers

Arnold (2005) is the first mention (I think) of using transits as a potential form of communication. Arnold suggests that advanced civilizations could embark on ridiculous engineering adventures and launch one or multiple crafts into orbit as a way to long-term communicate their existence. A civilization could launch a single, large object that would block out a substantial amount of the star (like a Dyson sphere), or they could launch objects far from circular, whose transit curves would have different ingresses and/or egresses.

Personally, I doubt this would ever happen with the intent of communication. Such a project would take so long to complete, and so many resources, that I doubt a civilization would bother. If they were to bother, I don’t think the intent would be as a form of communication.

Arnold mentions that civilizations could group multiple objects into prime numbers (his example is 11 objects with 1,2,3, then 5 objects). This just seems like pure fiction to me. I haven’t done any math or simulations, but I’m skeptical that it is possible to keep this many objects gravitationally stable while still maintaining transit alignment. And even if this could be done, it would quickly go unstable, with orbits deteriorating.

If people search trasist data for megastructures, which they have and will hopefully continue to do, I feel that they should just look for odd or anomalous transits, possibly even swarms. But I don’t think people should bother with looking for messages in transits.

Arnold (2005) Summary

The author in this paper discusses the possibility of detecting artificial non-spherical objects with Kepler and COROT, including single objects and multiple objects. The three single object cases discussed in this paper are triangle, two-screen and louver-like six screen shaped objects. The transit depths generated by their simulations are on the order of 100ppm which is detectable with Kepler. Then the author moves on to discuss detecting multiple-object transit signals, specifically, multiple transits, grouped by prime numbers.

Further, the author discusses the efficiency of using the above mentioned artificial transits as a communication tool. The author compares this way of communication with laser beacons and finds similar communicating efficiencies. The author additionally shows that with our current technology, to communicate with each target star using laser pulses, the time required is on the order of days. The communication efficiency will be much improved now since the launch of GAIA satellite.

Finally, the author argues that transit signals will be used for attention-getting and laser pulses will be used for data transfer since it is more directional.

There are several limitations to this paper, first, in the communication efficiency part, the author does not take into account the factor of distance. Second, in the communication efficiency part, the author assumes ETI has knowledge of proper motions and distances to the target stars which is not always the case.

The main concern I have with this paper is that the author does not at all discuss what could mimic the artificial signals during the limb-darkening period for natural sources such as stellar activities.