About science; About the mountains of Venus

This is about science produced by the California Institute of Technology and originally broadcast by station KPCC Pasadena California. The programs are made available to the station by national educational radio. This program is about the mountains of Venus with host Dr. Albert Hibbs and his guest Dr. Richard Goldstein. Here now is Dr. hit the planet Venus and certainly one of the most mysterious or at least of the nearby planets in our solar system because it's always been hidden from us by its own veil of clouds. And no matter how hard we look or how often all we see is the cloudy atmosphere that circles the surface of the planet. But it is possible to penetrate through these clouds with radio signals. And even though this possibility has been theorized for many years it wasn't until rather recently that possibility was turned into reality with the development of sensitive receivers and large radio telescopes. One of the individuals who has helped in

this venture of exploring Venus and other planets with the use of radio and radar is our guest tonight Dr. Richard Goldstein. He is now manager of the communications systems research section at the Jet Propulsion Laboratory and also was part time he's on the campus of Cal Tech as a visiting associate professor in planetary sciences. Dr. Goldstein joined JPL in 1958 and shortly after he joined he went back to Cal Tech to do graduate work and got his Ph.D. there in 1962. His bachelor's degree prior to that had come from Purdue and electrical engineering. Well Dick perhaps we should start by setting the stage of radio exploration of the planets radar surely wasn't the first technique to be used how did we first get a radio look at a planet. Well I think the first time Venus was observed by radio is in 1956 at the Naval Research Laboratories. And it was

a very surprising thing at that time the temperature of Venus as measured in it was found to be very hot. And this information was classified classified and it was much later that came out to the general knowledge of the world and Venus was was much hotter than you could accommodate just by being so a little closer to the Sun than the Earth is. Why in the world was it classified was that just one of those accidents of the fact that it happened to have been done in a laboratory. That's correct. Well when you say it was observed by radio How can a radio How can a radio telescope observe a planet. Well any object which is hot and radiates as well as the radio waves and by using a large antenna on a very sensitive receiver the radio always make themselves known by the noise in the radio output similar to the his that you get in a normal radio when you're not tuned to a station. So if you point your antenna and Venus diskettes ever so slightly louder

and a measure of the extra loudness of static. When I looked at Dinas and that's unable to propagation of temperature while the hotter an object is the brighter is the radio emission. See it's not an ambiguous it's called brightness temperature rather than real temperature because there are several other possibilities. For instance lightning storms give off radio static but they're not associated with high temperature. And I think until just recently it was thought that Venus possibly could have lightning discharges in its atmosphere and account for the temperature. That was sort of I hope it was I hope there will be I do want to live. Everybody wanted Venus to be cold enough at least to have some kind of some chance for a form of life that we might recognize. But you have done I think quite a bit of your work with radar hundred one did the

radar work begin on a planet in the very first radar echo from a heavenly body would be that from the moon 1046 just an ordinary radar set that we use for latitude this takes. This was done in two groups the group in the US Signal Corps use some recently developed wartime radar and got an echo from the moon. Put another group in Hungary used a homemade. Set of equipment got an echo I think a month or two months before the U.S. 10 with homemade equipment. The ham comes to the fore. This man had his equipment I think destroyed or at least he had to take it down at least three times during the war and he finally got it together in forty six and detected an echo. But Venus is a much harder target than the moon of course and it's quite a bit further away. Is this farther away and the distance is what really exacts a penalty in radar. Venus is 10 million

times harder. Radar target. And and that's at closest approach when Venus is on the far side of the sun it's another factor of a thousand hard. Well what what kind of equipment is necessary to look at Venus I'm sure it's not the homemade set and I'm greeted. You know we've been using the radio tracking station and JPL is the same one that used to watch spacecraft right. It has an 85 foot dish antenna and extremely sensitive receivers. Their receiver is operated by a maser amplifier and is one of the most sensitive in the entire business. And also a strong transmitter is required we're presently running 100 kilowatts. I'm out of power for microwaves.

Yes and that's a big transmitter when I wanted to start the use of this equipment to look at the very first successful Venus experiment was done in 1981 at the time of closest approach. At that time the transmitter was 10 kilowatts so much smaller than the current one. What was the distance at that time. He has gained to about 30 million miles at that time. Was this primarily just an experiment to see whether it could work or not or could you actually learn something. You read that early radar experiment. As it turned out our hopes for for learning were well the actual experiment returned far more information than we had dared hope in the first place. That's not that's not typical of an experiment. Well it has been on our Venus work every time Venus comes close. We try out our latest gear and it's always been a surprise to us and we've always hoped to get a certain amount of information and Venus has always been very kind to us.

What did you learn with that first one. On the first one the important things we learned I think from JP Al's point of view was correction on the accepted value of the Astronomical Unit. Now in one thousand sixty two JPL sent Mariner Mariner 2 to the city of Venus to do this radio navigation one needs an accurate knowledge of the orbit and the distance away that Venus is and our radar measuring the distance improve the accuracy of the Astronomical Unit by almost a factor of a thousand. That means that if Mariner 2 had been aimed according to the wrong value or perhaps the older value. It would have missed by about 100000 miles. Not much of a change. Well that's certainly from a practical point of view. Space Engineering enormous benefit to get out of it but also of course the Astronomical Unit being at a fundamental scale of the solar system is of great value.

Just destroy me for the purposes of flying things around. If you get out it becomes important for spacecraft. The astronomical unit itself is so large that for most purposes this refinement isn't necessary but for the navigation of spacecraft it becomes very important. We also found out that Venus rotates very slowly. We are expecting Venus to be Earth like. Well at least I was. The Earth rotates once a day Mars rotates once every 24 hours. Jupiter every 10 hours. Nobody thought that Venus would be much different. And it turned out to be quite different rotated. Well it actually rotates once in about two hundred forty three days but in our first early measurements all we could find out was it are rotated very slowly but you found it did rotate and it was slow. Both of these. Which of course were uphill and completely unknown. I remember some of the astronomical work photographing the clouds of Venus would

occasionally show up patterns in the clouds and people would track the patterns month after month looking for some indication that they repeated try to get an estimate of the rotation rate from Lad was almost hopeless and the answer scattered all over the map backwards and forward slow and fast. If you dig up the old literature about Venus rotation you can get anything from. From about 10 hours to two hundred twenty five days there was a group of people that thought two hundred twenty five days would be appropriate because that's all long it takes Venus to rotate once around the sun so we keep one face toward the sun. And when you found a slow rotation right and it just seemed to cooperate that idea that seemed like a good a good possibility. But of course the measurements were conclusive and as I recall at the time publishing the fact that that it might be locked to the sun. How can you measure rotation by radar in radar we

have a Doppler shift that. As Venus rotates one side of it approaches earth and the echo from that side comes back higher in pitch just like the train whistle. But how can you tell whether you're looking at a side of Venus instead of just looking at Minas. Well now the antenna sends out a beam which completely illuminates Venus the diameter of the antenna beam when it gets to Venus is hundreds of times bigger than I am to the planet. So the whole planet is more or less uniformly illuminated but we process the echo for its spectral content. That is we transmitted as pure a spectral tone as we can and it comes back spectrally broadened and by looking at the width of this broadening there's enough information to determine the rotation rate. I see you mean our early measurements were noisy so it wasn't possible to to measure the with with very much accuracy as we tried a similar set

of experiments in one thousand sixty two a year and a half later when Venus again came close to Earth. And this time we were able to get a pretty good number for the rotation. We found it was 250 days plus or minus 50. So even though there was quite a bit of lack of precision in the measurement it was still a good number. But I surprising thing about that number was that it was rotating backwards. How could you tell a law that was rotating backwards. That becomes evident when you look at these echoes over a period of time. You see as Venus we see by the earth the apparent rotation as we see it changes in the same way that if a car drives by you on a straight highway first you look at the front and then you look at the back and there's been an effect of 180 degrees rotation even though nothing has been spinning. Well this serves to calibrate the spin of Venus conjunction. The

broadening of the spectrum was narrower than at any other time it's been had to be backwards if it was wider at conjunction then the spin would have to be forwards. So apparently parents spend to us was less when it was a junction another time. So you knew it was going slower you knew it was going retrograde Well then this no longer coincided with it at all with the idea that it was locked. That's right that pleat lead off the first hypothesis had to be discarded. But there was another possibility it could be locked to the sun but it could be minus two hundred twenty five days instead of plus. That is you could keep one face to the sun every spring say every spring on Venus. Seasonal lock out of the sun and that would also be a two hundred twenty five day period. It would still be within the bracket of 250 plus or minus 50. Right so in order to save face I was postulating the clock to the sun but backwards

since you had already come out with the suggestion that it was locked to the sun this was a fallback position. Right I had to do that once more in our studies because the next time we were able to measure the rotation it became two hundred forty three days plus or minus a very small fraction point 6. Well that doesn't that doesn't crack a 225 let me know so I have to give up this. But the next thing to consider is maybe a slog to the earth. How could that be. Well it's a very unlikely thing it could happen if Venus were not perfectly symmetric gravitationally if it had a sort of a dumbbell shape to it. And whenever it swings by the earth the Earth's gravity attraction on this dumbbell could unlock the rotation. So then once every conjunction in that part of Venus face the earth wouldn't the sun also have a tendency to lock onto any such bald with its gravitational field which is certainly

much stronger than the earth's The sun is much stronger and presents a much more disruptive force on Venus and the Earth can. But there's a resonance. If the rotation is close to the earth resonance point then the earth's effect is much stronger and the sun's effect while much stronger is first in one direction than in the other and in the other and so on so that it tends to balance out. So it is possible if Venus is which I say very resilient. For the Earth is a fact. Once it's spinning at this speed for the earth a fact to be stronger than the Sun's. Once it just happens to get into that residence position I magine the theory that I would see if it got out of that special speed of resonance it would fall into the saw in the sky. I don't mean falling on the sun fall out of the sun's influence and B lock them precisely in such a way. If it got away from the course we're still assuming that it is truly locked to the earth and measurements of more precision are awaiting us so that we can

decide how much more precise you have to get before you make that a more definite statement. Well now we can be sure that it's either add or very close to this residence. Perhaps in just one or two more conjunctions there will be enough. The lever on this measurement enough persuasion enough additional radar capability to establish conclusively whether or not Phoenix is tied to the earth. But if it did get out of this residence then the spin would gradually slow down until it would perhaps be trapped into this two hundred twenty five days backwards rotation under the influence of the sun. Has the radar work shown anything about the characteristics of the surface temperature. It's able it's able to give some idea of what the surface roughness is. Right now is a Venus turns out to be very much smoother to radar waves at any rate than the

moon is. But this is on a scale of the wavelength use. You can you can have you can have large craggy mountains. And that's rough compared to a wavelength of 12 and a half centimeters. But so at a field full of boulders and I would also be rough to our way of life. So we have to qualify the measurement according to the wavelength used. But we do find that Venus is very smooth relative to the moon because most of the energy that comes back is concentrated from the point. So if we had radar eyes we'd see a glint in the center of the desk where most of the energy is becoming it would be brighter things like that when Alesha Valetta's far like Apollo 12 centimeters away. Correct. But not perfectly polished because we can see the edges but dimly the moon is look more like a ping pong ball. I see. Not quite polished but at least some.

Can you tell anything else about any of the topography the fact that it's fairly small scale. Well we have found that there are prominent features on the surface of Venus that are relatively permanent that show off in the same place on Venus every year and a half. And that they are fairly large and in extent it is the strongest one that faces earth that's nearest the center of the disco and when the right conjunction is sort of round may be three hundred miles in diameter. On the specter that we take this specter shows an ice peaks there so you tend to call them mountains. You know there's five really quite strong ones and a whole host of minor ones that are a little harder to keep track of because they're not quite so distinctive. But they're not necessarily mountains all they are is

an area that reflects radar weighs more strongly than the surrounding area and it's rougher than the surrounding area because it has the ability to depolarize radar waves so they could be great craters. They could be mountain chains and fields of boulders would do it. Features such as the Grand Canyon on a grand scale could do it. But we call them mountains just to have a name. And these are the order you see some 300 miles in extent. This kind of feature the one who the works are able to map the vest is another one much weaker in its radar response tends to be sort of long like a linear chain of things. And there are some promises on the on the side of the disk that we don't see until Venus is farther away where the signal is too weak to determine precisely what it is.

This sounds a little mysterious that here you are illuminating the whole planet with a single beam and you're getting back just a signal from the planet. How can you tell size and shape and position of features of the order of a few hundred miles in size. Well hopefully we'll be able to see things in the order of 10 miles. And of course much finer was later but the basis for the technique is a combination of Doppler shift because different places on Venus have a different Doppler shift and also different time delays as it takes longer for a signal to get to the edge of Venus than it does to get to the front. So the echo is spread out both in frequency and in time and it's possible that the radar receiver to sort out these two effects separately from one another so on can do a two dimensional mapping. Our real problem here is signal to noise ratio so that limits the size of the cells

of resolution that we're able to get. But we're not limited in the same way that optical telescopes are. For example an optical telescope on the earth is forever limited by the turbulence of the atmosphere and an object that's farther away must have less resolution. Now in our case were time delay and Doppler don't change as you move an object farther away only signal to noise ratio. So we have the capability given enough signal power. To resolve it to whatever accuracy our equipment can be built This is then the primary. Direction of improvement is more signal power can you do something with the electronics to the receiver electronics to improve the signal. We have attacked this problem on all possible fronts. We need stronger transmitters. We have under development and perhaps will have on the air within a few weeks. A 400 kilowatt transmitter

receiver. Its its main contribution is great amounts of amplification while providing the smallest amount of extra noise possible. The receiver we have is amazingly sensitive yet we're developing a receiver that's more sensitive still. Here we are approaching an absolute limit however because soon the noise that one receives will be galactic radiation so that even if the receiver produces no additional noise itself. You're limited by this background but we're not at that limit yet although we're closing in on it. Then antenna size is another. Variable what antenna what's what what's a size and how are you working with now. Currently we use an 85 foot daish. We have done some experiments lately with Jay feels new 210 for days. The difference for me two hundred ten and 85 may not seem very much is two numbers but a

radar performance is proportional to the fourth power of the antenna diameter. I see because you get it you get a square advantage going out and coming back most of it. So far we have only received on the 210 and that gives an advantage of a factor of about oh let me express it this way one hour's observation on the two tan produces as good a radar return as 36 hours on the 85 hours of measurement is this then a time exposure sort of thing that you are doing will very much you send out a pulse and then look at that another pulse and look at that as a steady signal you send out receive the signal as is a complicated signal it's wideband modulators signal and we transmit it for the full time of flight. When Venus is close to the time of flight is maybe four and a half minutes when Venus is far this can run up to 30 minutes. And as soon as the first part of the signal starts returning we switch to receive because we cannot

receive on the transmitters on the signal is so powerful and the Echo is so weak. Then we can receive for this four and a half minutes or thirty minutes depending. You said you say you sent out a broadband modulation signal. We first started talking about something on a very narrow That's a mano chromatic single frequency. That's because we have more than one kind of experiment one of which is a very narrow band monochromatic signal. But we were discussing mapping later than that. And the best mapping is done with a modulation signal which is wideband. Why is not it's necessary to have some kind of structure on your transmitter signal if you're going to recover the time delay information. You have a completely steady state signal that never changes and time delay doesn't mean anything. You can't know it started from the other. I see so by knowing the structure of the signal you sent out you then look at the structure what comes back in you know the details of the

time. Correct we would build a local replica of what was transmitted and delay it very carefully until it matches what comes back. And we know how long the time the flight was in and sort out the parts that came back at different times. This is this one large feature you say appeared to be facing Earth as Venus came closest to ours now is there any chance that this corresponds to this hypothetical bulge in Venus which would make. Unfortunately it's not at all it's not in the right place and it's probably not big enough I think. Why is that we have a place. Well it's not it's not. It's not near the equator for one it's off the equator by a fair amount and it's not center Duffey there it's off the meridian. But that's not exact we have using this mapping but with. Faster modulation We range Venus and

lately we've been able to arrange Venus to just about one part in ten of the eight which is one of them. How does that work out intreat. Or Miles I want to see thirty million miles and that's thirty times ten of the six miles we're down talking about feet and ran distance. Not really it's about. Say a quarter of a mile or maybe even a tenth of a mile. When the Venus is close but this is at a at a distance of thirty million miles so the percentage accuracy is phenomenal. But this ranging accuracy is enough to pick up a ball John. You know yes indeed 500 feet if the bulge is say as big as a mile. Unfortunately. The balls made their ranging measurements not only show the balls but also the orbital variations of Venus so that in order to find the balls you have to set the orbit straight and there's about six variables that are all

tied up together that we have to solve simultaneously. That works in progress and we don't have enough data now. I don't think to determine this balls can be seen or not. But we expect that shortly. So the status of your latest prediction ride's what the computer says about the orbit of Venus. That's right. Well I think I just have time for one more question and let me ask you this. As the as the amateur and Hungary beat the U.S. Navy to radar off the moon. Did anybody beat you to radar measurements of Venus. Well it appeared so because the first echo from Venus was published in 1958. And that group try to get in one thousand fifty nine. And we're not able to succeed but we thought the nine hundred fifty eight results were more valid and we went on in 1981 and we got an echo and it turned out that ours was really the first echo and I wanted 58

was a misinterpretation a system noise. The other group was also on the air in 61 and about two weeks after we were on they also got echoes as a matter of fact there's a number of radar observatories on the air that are probing Venus now so they didn't for three years you beat by two weeks. Dick thank you very much for coming and telling us about your observations of Venus and how to map something out of the clouds with radar. This was about science with host Dr. Albert Hibbs and his guest Dr. Richard Goldstein join us again for our next program what Dr. Hibbs will lead a discussion about the new chemistry about science is produced by the California Institute of Technology and is originally broadcast by station KPCC in Pasadena California. The programs are made available to this station by the national educational radio network.