OFFICIAL24 Why does the professor mention the fact that Venus has no surface water?

[00:00.00]NARRATOR: Listen to part of a lecture in an astronomy class.
[00:08.60]MALE PROFESSOR: Uh, many people have been fascinated about Venus for centuries because of its thick cloud cover...the so-called "planet of mystery" and all of that. [00:18.17]Well, what's under those clouds? [00:20.89] What's the surface of the planet like? [00:23.12]Uh, some questions about the surface are still unresolved, but...but we have learned a lot about it in the past several years. 
[00:30.62] First of all, let me talk about how we've been able to get past those clouds. [00:35.78] First there were Soviet modules that landed directly on the surface and sent back some images of what was around them. [00:42.74]Second, we did some radar imaging from satellites from above. [00:46.85] Radar can get through the clouds. [00:49.35]So, what have we learned? [00:52.27]Yes, Karen?[00:54.41]FEMALE STUDENT: Well, I remember reading that there's not really a lot going on...that the surface of Venus is just flat and smooth in a lot of places...[01:06.34]MALE PROFESSOR: Uh, yeah, it's smooth in a lot of places, [01:09.36] but that's not...uh, that's not the whole picture. [01:12.61]In other areas, you've got canyons, rift valleys, meteoric craters, uh, lava domes—these lava formations that look like giant pancakes—[01:22.81]and also volcanoes...
[01:24.64] well, one of the most interesting features on the surface are, in fact, the shield volcanoes.[01:29.68]Shield volcanoes form when magma comes out of the ground in the same spot over and over again. [01:40.08]Remember, magma is hot, molten rock that's underground and it's called lava when it reaches the surface—[01:46.82]uh, so the lava builds up and hardens, and a volcano forms.
[01:51.23]Now, the lava on Venus is thin. [01:54.00It spreads out easily. [01:55.91]So shield volcanoes have very gentle, sloping sides. [01:59.91]They're called shield volcanoes because viewed from above, they kind of resemble shields, you know, like a warrior's shield.
[02:07.19]But what's particularly interesting about these volcanoes is that most of the volcanoes here on Earth are not shield volcanoes. [02:20.62]Instead, they're other volcano types, like stratovolcanoes for example, which are a result of tectonic plate movement. [02:28.44]Remember tectonic plates?
[02:30.37]Underneath the Earth's crust, there are a number of shifting slabs or plates that are slowly moving. [02:36.18]And in the zones on the edges of the plates, where different plates meet and interact, that's where we get most of Earth volcanoes.
[02:44.25] On Venus, however, volcanoes are not clustered in discrete zones like they're on Earth. [02:50.25]Instead, they're more or less randomly scattered over Venus' surface. [02:55.52]Well, that's significant: [02:58.29]Venus has mostly shield volcanoes, and they're randomly scattered—that indicates that Venus does not have moving tectonic plates, and that's a big difference compared to Earth. [03:11.45]Here on Earth, moving tectonic plates are a major geological element...just crucial for the whole surface dynamic, right?
[03:19.08] So why doesn't Venus have them? [03:21.61]Well, there are a few theories—[03:24.70]uh, one of them is that this has to do with the fact that Venus has no surface water that's needed to kinda lubricate the movement of the plates. You know, like oceans on Earth? [03:35.33]Yeah, I forgot to spell that out, uh, [03:37.20]Venus has no surface water...
[03:39.81]FEMALE STUDENT:Wait a second, [03:41.57]did you say we have shield volcanoes on Earth? [03:44.19]Can you give an example?
[03:45.73]MALE PROFESSOR: Sure...the volcanoes in the Hawaiian Islands, in the Pacific Ocean, are shield volcanoes; [03:51.82]they've formed over a hot spot of magma. [03:54.64]So while on Earth we have several types of volcanoes, on Venus, there is mostly the one type. [04:02.16]Uh, Eric?
[04:03.77]FEMALE STUDENT: Uh, are the volcanoes on Venus still active?[04:12.15]MALE PROFESSOR: Well, that's an interesting question. [04:14.10] There's still some discussion on that point. [04:16.91]But here's what we do know. [04:18.92]First, the level of sulfur dioxide gas above Venus' clouds shows large, and uh, very frequent fluctuations. [04:28.09]It's quite possible that these fluctuations, the huge increase and decrease of sulfur dioxide, happening again and again, [04:37.10]it's quite possible that this is due to volcanic eruptions because volcanic eruptions often emit gases.
[04:44.52] If that's the case, volcanism could very well be the root cause of Venus' thick cloud cover. [04:51.37]And also, we have observed bursts of radio energy from the planet's surface. [04:57.20]These bursts are similar to what we see when volcanoes erupt on Earth. [05:01.99]So this too suggests ongoing volcanic activity. [05:06.31]But, although this is intriguing evidence, no one's actually observed a Venus volcano erupting yet, so we can't be positive.