Radar observations of Venus were supplemented with data from the Arecibo ground-based radio telescope

(ORDO NEWS) — Scientists have processed a series of radar observations of Venus and mapped its surface from 1988 to 2020. We talk about the results of planetary radar data processing and the possibilities of radar astronomy.

The surface of Venus, the closest planet to us, is hidden from direct observation by a dense carbon dioxide atmosphere and a continuous layer of clouds consisting of concentrated sulfuric acid.

Flying over the night side of Venus, you can see the faint and blurry glow of its landscape, heated to 460 degrees Celsius, and in some parts of the infrared range, you can see details up to several kilometers in size from orbit – and that’s all you can count on.

Radar observations of Venus were supplemented with data from the Arecibo ground based radio telescope 2
A long exposure image of the night side of Venus taken by the Parker Solar Probe (left) and compared with a height map based on Magellan data (right). The land of Aphrodite indicated by the arrows is high mountains, it is heated a little less than the plains bordering it. The “blizzard” in the left photo is the result of cosmic rays

But there is one range of electromagnetic radiation in which the Venusian atmosphere is completely transparent – radio waves. In addition to penetrating ability, they have another unique advantage.

The natural glow of celestial bodies in the radio range is small, but the “brightness” and directivity of artificial radio wave transmitters can be huge. Unlike all other ranges, in the radio range, celestial bodies can be “highlighted” – both from the Earth and from spacecraft.

Not surprisingly, radar is the main method of mapping Venus. Ground-based radar studies of this planet began back in the 1960s and 1970s, and later they were joined by orbiters equipped with radars: the Soviet ” Venus ” and the American Pioneer-Venus and Magellan.

The data transmitted by the last of them made it possible to obtain the first and so far the only complete map of Venus with a resolution of up to 200 meters.

For a long time, the main radar observatory was the Arecibo radio telescope , which had the largest solid antenna (300 meters) and was equipped with a powerful radio wave transmitter. In 2020, it collapsed , but over its more than half a century of service , it managed to become an instrument of many radar research.

Radar observations of Venus were supplemented with data from the Arecibo ground based radio telescope 3
Arecibo radio telescope. The receiver was located above the antenna, where it was held by cables attached to three towers around it

Recently, a team of scientists led by Bruce Campbell released the results of all the radar observations of Venus made by the Arecibo and Green Bank telescopes from 1988 to 2020. Modern ground-based radar is slightly inferior to orbital in resolution, but it can be carried out much more often than orbital launches.

Scientists have processed data from observations in 1988, 2012, 2015, 2017 and 2020 and mapped the surface of Venus with a resolution of one to two kilometers. So far, no noticeable changes in the relief of the planet and the reflective properties of its surface have been identified.

Nevertheless, scientists have carefully characterized the properties of the reflected signals, which in the future will allow more accurate design and tuning of the radars of the planned VERITAS and EnVision spacecraft.

Radar observations of Venus were supplemented with data from the Arecibo ground based radio telescope 4
Intermediate result of Venus radar data processing in 2015 and 2017. Dark bands are excluded due to the difficulty of interpreting radar signals coming from areas near the equator of a celestial body. The bright spot near the upper edge of the disk is Maxwell’s Venusian mountains

In addition, scientists continue to process the data: careful noise removal, after which it will be possible to detect small changes in the surface and prepare a “list of targets” for the planned Venus flotilla . All interested researchers can join the processing; data is available here.

The technical details of the radar of celestial bodies are notable for their complexity, and yet they can be explained “on the fingers.”

First, the transmitter sends a specially calculated series of radio emission pulses with alternating phase towards the celestial body . The duration of each pulse is a few microseconds, and their number is several thousand.

The signal reflected from the closest point on the surface of Venus to the receiver arrives first. Then come the signals reflected from more and more distant parts of the planet’s disk, and the last – reflected from the relief at the edges of the visible disk.

Knowing the shape of the original signal, it is possible to accurately calculate the dependence of the reflection brightness on the distance to the reflecting areas from the shape of the reflected signal.

Due to the movement of the planets relative to each other, the signals also experience a Doppler shift, which can be measured in the same way that a radar measures the speed of a car.

This shift is superimposed by additional shifts caused by the daily rotation of Venus around its axis: surface areas moving “away from us” return a slightly lower frequency signal, and areas moving “towards us”, on the contrary, increase it.

Using this phenomenon, it is possible to eliminate orientation uncertainties of reflecting areas located at the same distance from the receiver.

In fact, the receiver registers “graphs” – the dependence of the brightness of the reflected signal and its Doppler shift on time.

The first is proportional to the reflectivity of the surface (stone placers reflect radio waves better than flat lava fields), and the second is the speed with which these areas move relative to us. And after processing, the signal, which at first looks like just waves on the oscilloscope screen, turns into a real image.

In the future, scientists plan to develop the capabilities of radar astronomy. Its resolution can exceed the resolution of the Hubble telescope by several times – for example, radar is often used to view near-Earth asteroids.

But earlier she faced a significant limitation on the “range”. When only one and the same radio telescope, Arecibo, could be used for radar and signal reception, the Earth, along with the receiver, had time to turn away during the round trip of the signal.

Now, sufficiently sensitive receivers are distributed throughout the planet and can work as a whole. The most impressive example of such an array is the future Square Kilometer Array observatory with a total area of ​​one square kilometer, which will be fully commissioned around 2027 .

The destruction of Arecibo will not delay the development of radar astronomy for long: in 2024, the Green Bank radio telescope, which has already demonstrated impressive radar capabilities, will receive a transmitter with a power of 500 kilowatts. Soon the entire solar system will be available for radar.

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