<p>It did not detect methane lakes. </p>

<p>It found that Titan was shiny (in radar terms): that is, the reflections were from a smooth surface rather than a rough one, and at the same time not very intense. </p>

<p>As a result (quoting the 2003 New Scientist article <a href="https://www.newscientist.com/article/dn4227-radar-reveals-titans-methane-lakes/" rel="noreferrer">Radar reveals Titan's methane lakes</a> linked in one of the comments to your question), “<em>some researchers believe that these are methane lakes, sitting in impact craters.</em>” Other researchers may think of different explanations.</p>

<p>If we come to know, from other evidence and reasoning or just by elimination, that there <em>are</em> methane lakes on Titan, then Arecibo can claim to have seen them. But by itself it is providing no evidence that they are methane, or even that they are lakes. </p>

<p>Equally, if someone had launched a space telescope in 1961 and produced high-quality colour images of Mars, they could have claimed, on the Arecibo principle, “First detection of the seasonal vegetation cycle on Mars”, since at that time most scientists believed that that was what the seasonal colour changes were. </p>

CC BY-SA 4.0
2019-08-14T12:38:24.593Z
There _is_ by now quite strong independent evidence of [lakes on Titan](https://en.wikipedia.org/wiki/Lakes_of_Titan). – And, ultimately _all_ “detections” are just (more or less) indirect observations of evidence that is consistent with some theory. If there is one simple theory that's consistent with all observations, then I'd call it fair to speak of “Arecibo _detected_ methane lakes”.
leftaroundabout
CC BY-SA 4.0
2019-08-14T15:19:07.257Z
So you mean, "detected but not discovered"? That is fair enough.
Martin Kochanski
CC BY-SA 4.0
2019-08-14T07:20:50.817Z
2019-08-14T08:13:35.843Z
2019-08-14T08:13:35.843Z
Martin Kochanski
<h2>Titan "lakes":</h2>

<p>Published Open Access in Science: <a href="https://science.sciencemag.org/content/302/5644/431" rel="noreferrer">Radar Evidence for Liquid Surfaces on Titan</a> Campbell, D. B., Black, G. J., Carter, L. M., and Ostro, S. J., Science <strong>302</strong>, 5644, pp. 431-434, 17 Oct 2003 DOI: 10.1126/science.1088969</p>

<p><em>This was a</em> <strong><em>really elegant experiment!</em></strong> A continuous, unmodulated, circularly polarized 13 cm wave was broadcast from Arecibo towards the Saturn/Titan system, and the Doppler shift was used to isolate the returned signal from Titan.</p>

<p>Most of the surface is rough, so there is signal returned from areas all over Titan's disk, and since the moon rotates, albeit slowly, returned power from the "left" and "right" sides are shifted to higher and lower frequencies. </p>

<p>However during some observing times there was a very strong and pronounced reflection with zero Doppler shift with respect to Titan's known radial velocity, and this peak is attributed to specular reflection. Checks on received polarization confirm that while the power from the rough surface is returned in both circular polarization state, the presumed specular component is only in the expected circular polarization state.</p>

<p>As pointed out in <a href="https://astronomy.stackexchange.com/a/33027/7982">@Martin Kochanski's thoughtful answer</a> there is no determination from the radar observation that the returned specular reflection comes from methane. This is simply a presumed component of the presumed lakes, based on known information about Titan's chemistry at the time (2003).</p>

<blockquote>
  <p>We observed Titan on 16 nights in November and December 2001 and on 9 nights in November and December 2002, transmitting at 13-cm wavelength with the 305-m Arecibo telescope and receiving the echo with Arecibo. Titan's rotational and orbital periods are 15.9 days, and our 2001 observations were obtained at a uniform 22.6° (∼800 km) interval in longitude. The 9 observations in 2002 did not provide uniform coverage. The latitude of the subearth track was 25.9°S in 2001 and 26.2°S in 2002, its farthest southern excursion. The round-trip light time to the Saturn system during the observations was 2 hours 15 min, and the limited tracking time of the Arecibo telescope meant that signal reception was restricted to ∼30 min per day, corresponding to 0.5° of Titan rotation (20 km of motion of the subearth point). On one night in 2001 and for most of the 2002 observations (as well as others when we were attempting ranging measurements to Titan), the 100-m Green Bank Telescope (GBT) was also used to receive the echo for the full round-trip time. These data have lower signal-to-noise ratios than those obtained with Arecibo receiving the echo, but the longer receive time corresponding to 2.1° of Titan rotation allowed more subearth locations to be studied.</p>
</blockquote>

<hr>

<p>Here is some of the Titan data:</p>

<p><img src="https://i.sstatic.net/leUsW.jpg" alt="Fig. 3. The OC radar echo spectrum at 1.0-Hz resolution for the 2002 observation "></p>

<blockquote>
  <p>Fig. 3. The OC radar echo spectrum at 1.0-Hz resolution for the 2002 observation at the subearth longitude of 80°. The normalized cross section for the specular component of the echo and the RMS slope are 0.023 and 0.2°, respectively.</p>
</blockquote>

<p><a href="https://i.sstatic.net/Fj6JY.jpg" rel="noreferrer"><img src="https://i.sstatic.net/Fj6JYm.jpg" alt="Fig. 1. Arecibo radar echo spectra from the 2001 data for five subearth longitudes on Titan."></a> Click for full size</p>

<blockquote>
  <p>Fig. 1. Arecibo radar echo spectra from the 2001 data for five subearth longitudes on Titan. Spectra are shown for both the expected (OC) sense of received circular polarization and the cross-polarized (SC) sense. The ordinate is in standard deviations of the noise. The limb-to-limb Doppler-broadened bandwidth for Titan is 325 Hz. Four of the OC spectra show evidence of a specular component at 0 Hz.</p>
</blockquote>

<hr>

<h2>Saturn's rings "imaged" (delay-Doppler):</h2>

<p>From <a href="http://sun3.oulu.fi/~hsalo/HTML/nicholson2005_radar.pdf" rel="noreferrer">Radar imaging of Saturn’s rings</a> Nicholson, P. D. et al., Icarus 177 (2005) 32–62, doi:10.1016/j.icarus.2005.03.023</p>

<p>The "image" below is not a conventional image, since the Arecibo dish has no way to spatially resolve the transverse expanse of Saturn and its rings. It is a "delay-Doppler" image, using 12.6 cm, ~500 kW radar broadcasts transmitted by Arecibo. The round-trip light time was about 135 minutes. Because Arecibo has limited steering away from the zenith (&lt;19.7 degrees) max Saturn was only available to the dish for 166 minutes even under ideal conditions.</p>

<p>The vertical axis shows a delay of about +/- 800 milliseconds which demonstrates spatial resolution, but in the radial or depth direction. The horizontal axis is Doppler shift. The +/- 300 kHz shift represents the orbital velocity of particles in the rings.</p>

<p>While the Titan specular reflection above was done with a continuous or CW beam, the delay-doppler imaging technique requires a frequency modulation of the beam with a frequency-hopping pattern. By applying a correlation function using the known  pattern to the recorded received signals, components with different return times and different Doppler shifts can be extracted, and the results is then hzstogramed, producing the delay-Doppler image below.</p>

<p>This is a standard technique and has been used to image other planets and asteroids:  See the following items and refereces within:</p>

<ul>
<li><a href="https://astronomy.stackexchange.com/a/30267/7982">this answer</a> to <em>What causes “North-South ambiguity” when doppler radar imaging a planet surface equator on?</em></li>
<li><a href="https://space.stackexchange.com/q/22493/12102">Why do radar maps of the surface of Venus have missing slices?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/22480/7982">What is the physical geometry of this apparent “eclipse” of a tiny moon of Asteroid Florence?</a></li>
<li><a href="https://space.stackexchange.com/q/21158/12102">Is delay-doppler radar imaging of NEO asteroids possible only if it spins fast enough?</a></li>
</ul>

<p><img src="https://i.sstatic.net/s1TTm.jpg" alt="Fig. 2. Delay–Doppler images of Saturn&#39;s rings"></p>

<blockquote>
  <p>Fig. 2. Delay–Doppler images constructed from data obtained in (a) October 1999, (b) November 2000, (c) December 2001, and (d) January 2003.
  Both OC and SC polarizations were combined to maximize the signal to
  noise ratio. Note the four bright regions in each image where the delay and
  Doppler cells are parallel and where the A and B rings appear to cross over
  one another.</p>
</blockquote>

CC BY-SA 4.0
2019-08-14T14:51:14.623Z
Very thorough and well-sourced answer!
antlersoft
CC BY-SA 4.0
2019-08-15T22:54:29.817Z
I've just asked [Why is Saturn invisible in this radar image of its rings?](https://astronomy.stackexchange.com/q/33060/7982)
uhoh
CC BY-SA 4.0
2019-08-14T08:11:52.957Z
2019-08-15T22:41:10.783Z
2019-08-15T22:41:10.783Z
uhoh
<p><a href="https://space.stackexchange.com/a/37817/12102">This answer</a> to <em>Farthest distance to a solar system object that's been measured by radar?</em> mentions that Saturn's rings, and the Uncover Travel post <a href="http://uncover.travel/arecibo-observatory-puerto-rico-the-worlds-largest-radio-telescope-for-over-50-years/" rel="noreferrer">Arecibo Observatory, Puerto Rico – The World’s Largest Radio Telescope For Over 50 Years</a> mentions:</p>
<blockquote>
<p>Among other accomplishments of the Arecibo Observatory are:</p>
<ul>
<li>Direct imaging of an asteroid for the first time in history.</li>
<li>Discovery of water ice deposits at Mercury’s poles.</li>
<li>Tracking of near-Earth asteroids to monitor impact risks.</li>
<li>Mapping the cloud-covered surface of Venus.</li>
<li><strong>Radar imaging of the rings of Saturn, revealing new details of the ring structure.</strong></li>
<li><strong>First detection of methane lakes on Titan, a moon of Saturn.</strong></li>
<li>First detection of an asteroid with a moon.</li>
</ul>
</blockquote>
<p><strong>Question:</strong> How did Arecibo detect methane lakes on Titan, and image Saturn's rings? These are pretty remarkable feats from Earth for a single radiotelescope. How were they done? Can citations be found and the examples of the image of the rings and methane lake evidence be shown?</p>

CC BY-SA 4.0
2019-08-14T06:58:38.557Z
First Google of "arecibo methane titan". https://www.google.com/amp/s/www.newscientist.com/article/dn4227-radar-reveals-titans-methane-lakes/amp/.  This is a poor question
ProfRob
CC BY-SA 4.0
2019-08-14T08:15:21.630Z
No I don't think that it is @RobJeffries. That google returns to you some blurb does not make my question poor. You can guess that a "How..." question can have an excellent answer, and questions that generate excellent answers are not poor, they are what make SE thrive. I'll write an answer myself to show you what I mean, and to get the ball rolling.
uhoh
CC BY-SA 4.0
2019-08-14T08:56:21.623Z
Fair enough, though that brief article answers (that part of) your question perfectly well - as illustrated by Martin Kochanski's answer.
ProfRob
CC BY-SA 4.0
2019-08-14T12:18:34.017Z
@RobJeffries luckily it's the OP (in this case me) that has the luxury of deciding what does or doesn't answer a question in SE.
uhoh
CC BY-SA 4.0
2019-08-14T16:50:43.923Z
If there's substantial disagreement on what does or doesn't answer your question, that suggests that your question may be unclear and/or too broad. Both are valid reasons to close a question on SE until it's clarified.
Ilmari Karonen
CC BY-SA 4.0
2019-08-14T16:53:59.457Z
@IlmariKaronen there's no such thing as "substantial disagreement on what does or doesn't answer (a Stack Exchange) question". In Stack Exchange the OP (in this case me) has **100% absolute authority on this matter**. That is a pillar of SE. The question in question (this question, my question) is both quite narrowly and carefully scoped, and absolutely clear.
uhoh
<radio-astronomy><saturn><titan><radar><saturn-rings>
How did Arecibo detect methane lakes on Titan, and image Saturn's rings?
uhoh
# How did Arecibo detect methane lakes on Titan, and image Saturn's rings?

(Asked by: username_0 on 2019-08-14)

[This answer](https://space.stackexchange.com/a/37817/12102) to *Farthest distance to a solar system object that's been measured by radar?* mentions that Saturn's rings, and the Uncover Travel post [Arecibo Observatory, Puerto Rico – The World’s Largest Radio Telescope For Over 50 Years](http://uncover.travel/arecibo-observatory-puerto-rico-the-worlds-largest-radio-telescope-for-over-50-years/) mentions:

> Among other accomplishments of the Arecibo Observatory are:
>
> * Direct imaging of an asteroid for the first time in history.
> * Discovery of water ice deposits at Mercury’s poles.
> * Tracking of near-Earth asteroids to monitor impact risks.
> * Mapping the cloud-covered surface of Venus.
> * **Radar imaging of the rings of Saturn, revealing new details of the ring structure.**
> * **First detection of methane lakes on Titan, a moon of Saturn.**
> * First detection of an asteroid with a moon.

**Question:** How did Arecibo detect methane lakes on Titan, and image Saturn's rings? These are pretty remarkable feats from Earth for a single radiotelescope. How were they done? Can citations be found and the examples of the image of the rings and methane lake evidence be shown?

**Tags:** radio-astronomy, saturn, titan, radar, saturn-rings

> **Comments:**
> **username_1**: First Google of "arecibo methane titan". https://www.google.com/amp/s/www.newscientist.com/article/dn4227-radar-reveals-titans-methane-lakes/amp/.  This is a poor question  
> **username_0**: No I don't think that it is @RobJeffries. That google returns to you some blurb does not make my question poor. You can guess that a "How..." question can have an excellent answer, and questions that generate excellent answers are not poor, they are what make SE thrive. I'll write an answer myself to show you what I mean, and to get the ball rolling.  
> **username_1**: Fair enough, though that brief article answers (that part of) your question perfectly well - as illustrated by username_3's answer.  
> **username_0**: @RobJeffries luckily it's the OP (in this case me) that has the luxury of deciding what does or doesn't answer a question in SE.  
> **username_2**: If there's substantial disagreement on what does or doesn't answer your question, that suggests that your question may be unclear and/or too broad. Both are valid reasons to close a question on SE until it's clarified.  
> **username_0**: @username_2 there's no such thing as "substantial disagreement on what does or doesn't answer (a Stack Exchange) question". In Stack Exchange the OP (in this case me) has **100% absolute authority on this matter**. That is a pillar of SE. The question in question (this question, my question) is both quite narrowly and carefully scoped, and absolutely clear.  

---

## 2 answers

### Answer by username_0 on 2019-08-14. Score: 25 | Accepted answer

## Titan "lakes":

Published Open Access in Science: [Radar Evidence for Liquid Surfaces on Titan](https://science.sciencemag.org/content/302/5644/431) Campbell, D. B., Black, G. J., Carter, L. M., and Ostro, S. J., Science **302**, 5644, pp. 431-434, 17 Oct 2003 DOI: 10.1126/science.1088969

*This was a* ***really elegant experiment!*** A continuous, unmodulated, circularly polarized 13 cm wave was broadcast from Arecibo towards the Saturn/Titan system, and the Doppler shift was used to isolate the returned signal from Titan.

Most of the surface is rough, so there is signal returned from areas all over Titan's disk, and since the moon rotates, albeit slowly, returned power from the "left" and "right" sides are shifted to higher and lower frequencies.

However during some observing times there was a very strong and pronounced reflection with zero Doppler shift with respect to Titan's known radial velocity, and this peak is attributed to specular reflection. Checks on received polarization confirm that while the power from the rough surface is returned in both circular polarization state, the presumed specular component is only in the expected circular polarization state.

As pointed out in [@username_3's thoughtful answer](https://astronomy.stackexchange.com/a/33027/7982) there is no determination from the radar observation that the returned specular reflection comes from methane. This is simply a presumed component of the presumed lakes, based on known information about Titan's chemistry at the time (2003).

> We observed Titan on 16 nights in November and December 2001 and on 9 nights in November and December 2002, transmitting at 13-cm wavelength with the 305-m Arecibo telescope and receiving the echo with Arecibo. Titan's rotational and orbital periods are 15.9 days, and our 2001 observations were obtained at a uniform 22.6° (∼800 km) interval in longitude. The 9 observations in 2002 did not provide uniform coverage. The latitude of the subearth track was 25.9°S in 2001 and 26.2°S in 2002, its farthest southern excursion. The round-trip light time to the Saturn system during the observations was 2 hours 15 min, and the limited tracking time of the Arecibo telescope meant that signal reception was restricted to ∼30 min per day, corresponding to 0.5° of Titan rotation (20 km of motion of the subearth point). On one night in 2001 and for most of the 2002 observations (as well as others when we were attempting ranging measurements to Titan), the 100-m Green Bank Telescope (GBT) was also used to receive the echo for the full round-trip time. These data have lower signal-to-noise ratios than those obtained with Arecibo receiving the echo, but the longer receive time corresponding to 2.1° of Titan rotation allowed more subearth locations to be studied.

---

Here is some of the Titan data:

![Fig. 3. The OC radar echo spectrum at 1.0-Hz resolution for the 2002 observation ](https://i.sstatic.net/leUsW.jpg)

> Fig. 3. The OC radar echo spectrum at 1.0-Hz resolution for the 2002 observation at the subearth longitude of 80°. The normalized cross section for the specular component of the echo and the RMS slope are 0.023 and 0.2°, respectively.

[![Fig. 1. Arecibo radar echo spectra from the 2001 data for five subearth longitudes on Titan.](https://i.sstatic.net/Fj6JYm.jpg)](https://i.sstatic.net/Fj6JY.jpg) Click for full size

> Fig. 1. Arecibo radar echo spectra from the 2001 data for five subearth longitudes on Titan. Spectra are shown for both the expected (OC) sense of received circular polarization and the cross-polarized (SC) sense. The ordinate is in standard deviations of the noise. The limb-to-limb Doppler-broadened bandwidth for Titan is 325 Hz. Four of the OC spectra show evidence of a specular component at 0 Hz.

---

## Saturn's rings "imaged" (delay-Doppler):

From [Radar imaging of Saturn’s rings](http://sun3.oulu.fi/~hsalo/HTML/nicholson2005_radar.pdf) Nicholson, P. D. et al., Icarus 177 (2005) 32–62, doi:10.1016/j.icarus.2005.03.023

The "image" below is not a conventional image, since the Arecibo dish has no way to spatially resolve the transverse expanse of Saturn and its rings. It is a "delay-Doppler" image, using 12.6 cm, ~500 kW radar broadcasts transmitted by Arecibo. The round-trip light time was about 135 minutes. Because Arecibo has limited steering away from the zenith (<19.7 degrees) max Saturn was only available to the dish for 166 minutes even under ideal conditions.

The vertical axis shows a delay of about +/- 800 milliseconds which demonstrates spatial resolution, but in the radial or depth direction. The horizontal axis is Doppler shift. The +/- 300 kHz shift represents the orbital velocity of particles in the rings.

While the Titan specular reflection above was done with a continuous or CW beam, the delay-doppler imaging technique requires a frequency modulation of the beam with a frequency-hopping pattern. By applying a correlation function using the known pattern to the recorded received signals, components with different return times and different Doppler shifts can be extracted, and the results is then hzstogramed, producing the delay-Doppler image below.

This is a standard technique and has been used to image other planets and asteroids: See the following items and refereces within:

* [this answer](https://astronomy.stackexchange.com/a/30267/7982) to *What causes “North-South ambiguity” when doppler radar imaging a planet surface equator on?*
* [Why do radar maps of the surface of Venus have missing slices?](https://space.stackexchange.com/q/22493/12102)
* [What is the physical geometry of this apparent “eclipse” of a tiny moon of Asteroid Florence?](https://astronomy.stackexchange.com/q/22480/7982)
* [Is delay-doppler radar imaging of NEO asteroids possible only if it spins fast enough?](https://space.stackexchange.com/q/21158/12102)

![Fig. 2. Delay–Doppler images of Saturn's rings](https://i.sstatic.net/s1TTm.jpg)

> Fig. 2. Delay–Doppler images constructed from data obtained in (a) October 1999, (b) November 2000, (c) December 2001, and (d) January 2003.
> Both OC and SC polarizations were combined to maximize the signal to
> noise ratio. Note the four bright regions in each image where the delay and
> Doppler cells are parallel and where the A and B rings appear to cross over
> one another.

> **Comments:**
> **username_5**: Very thorough and well-sourced answer!  
> **username_0**: I've just asked [Why is Saturn invisible in this radar image of its rings?](https://astronomy.stackexchange.com/q/33060/7982)  

---

### Answer by username_3 on 2019-08-14. Score: 12

It did not detect methane lakes.

It found that Titan was shiny (in radar terms): that is, the reflections were from a smooth surface rather than a rough one, and at the same time not very intense.

As a result (quoting the 2003 New Scientist article [Radar reveals Titan's methane lakes](https://www.newscientist.com/article/dn4227-radar-reveals-titans-methane-lakes/) linked in one of the comments to your question), “*some researchers believe that these are methane lakes, sitting in impact craters.*” Other researchers may think of different explanations.

If we come to know, from other evidence and reasoning or just by elimination, that there *are* methane lakes on Titan, then Arecibo can claim to have seen them. But by itself it is providing no evidence that they are methane, or even that they are lakes.

Equally, if someone had launched a space telescope in 1961 and produced high-quality colour images of Mars, they could have claimed, on the Arecibo principle, “First detection of the seasonal vegetation cycle on Mars”, since at that time most scientists believed that that was what the seasonal colour changes were.

> **Comments:**
> **username_4**: There _is_ by now quite strong independent evidence of [lakes on Titan](https://en.wikipedia.org/wiki/Lakes_of_Titan). – And, ultimately _all_ “detections” are just (more or less) indirect observations of evidence that is consistent with some theory. If there is one simple theory that's consistent with all observations, then I'd call it fair to speak of “Arecibo _detected_ methane lakes”.  
> **username_3**: So you mean, "detected but not discovered"? That is fair enough.  

---

